method development of gas analysis with mass …kröger, t. & paaso, n. 2006. method development...

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T imo Kröger

N ina Paaso

October 2006

Work ing Repor t 2006 -41

Method Development of Gas Analysiswith Mass Spectrometer

October 2006

Working Reports contain information on work in progress

or pending completion.

The conclusions and viewpoints presented in the report

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coincide with those of Posiva.

T imo Kröger

N ina Paaso

Teo l l i suuden Vo ima Oy

Work ing Repor t 2006 -41

Method Development of Gas Analysiswith Mass Spectrometer

Kröger, T. & Paaso, N. 2006. Method development of gas analysis with Mass Spectrometer. Working Report 2006-41. Posiva Oy, Eurajoki. 94 p. ABSTRACT

Dissolved gas content in deep saline groundwater is an important factor, which has to be known and taken into account when planning the deep repository for the spent nuclear fuel. Posiva has investigated dissolved gases in deep groundwaters since the 1990's. In 2002 Posiva started a project that focused on developing the mass spectrometric method for measuring the dissolved gas content in deep saline groundwater. The main idea of the project was to analyse the dissolved gas content of both the gas phase and the water phase by a mass spectrometer. The development of the method started in 2003 (in the autumn). One of the aims was to create a parallel method for gas analysis with the gas chromatographic method. The starting point of this project was to test if gases could be analysed directly from water using a membrane inlet in the mass spectrometer. The main objective was to develop mass spectrometric methods for gas analysis with direct and membrane inlets. An analysis method for dissolved gases was developed for direct gas inlet mass spectrometry. The accuracy of the analysis method is tested with parallel real PAVE samples analysed in the laboratory of Insinööritoimisto Paavo Ristola Oy. The results were good. The development of the membrane inlet mass spectrometric method still continues. Two different membrane materials (silicone and teflon) were tested. Some basic tests (linearity, repeatability and detection limits for different gases) will be done by this method. Keywords: Dissolved gases, groundwater sampling, MS, PAVE

Kröger, T. & Paaso, N. 2006. Liuenneiden kaasujen massaspektrometrisen määritysmene-telmän kehitystyö. Työraportti 2006-41. Posiva Oy, Eurajoki. 94 s. TIIVISTELMÄ Pohjavesiin liuenneiden kaasujen koostumus tulee tietää ja ottaa huomioon, kun suunnitel-laan käytetyn polttoaineen loppusijoitustilaa. Posiva on tutkinut pohjavesiin liuenneiden kaasujen koostumusta jo vuodesta 1990 lähtien. Vuonna 2002 Posiva käynnisti kehitystyön, jonka päätarkoituksena on kehittää veteen liuenneiden kaasujen massaspektrometrinen määritysmenetelmä. Tavoitteena on analysoida veteen liuenneiden kaasujen koostumus sekä vesi- että kaasufaasista massaspektrometrillä. Tarkoituksena oli kehittää rinnakkainen analysointimenetelmä kaasujen kaasukromatografiselle analyysimenetelmälle. Yhtenä ta-voitteena oli testata voidaanko liuenneet kaasut analysoida suoraan membraani massaspekt-rometrillä. Työn tavoitteena oli kehittää kaasujen massaspektrometrinen analyysimenetelmä suo-rasyöttö- ja membraanitekniikoilla. Liuenneiden kaasujen analysointimenetelmä suorasyöt-tötekniikalla on kehitetty. Tämä kaasumaisten näytteiden analyysimenetelmä on testattu ja todettu luotettavaksi rinnakkaisnäytteillä. Rinnakkaisnäytteet analysoitiin Insinööritoimisto Paavo Ristola Oy:ssä. Nämä tulokset olivat hyviä. Vesinäytteiden menetelmäkehitystyö membraanisisäänviennillä jatkuu edelleen. Kaksi membraania (silikoni ja teflon) on tes-tattu. Membraanisisäänvientimenetelmän toteamisrajat, lineaarisuus ja toistettavuus tullaan testaamaan. Avainsanat: Liuenneet kaasut, pohjavesinäytteenotto, MS, PAVE

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TABLE OF CONTENTS ABSTRACT TIIVISTELMÄ 1 BACKGROUND AND OBJECTIVES OF WORK.........................................5 2 INTRODUCTION TO MASS SPECTROMETRY.........................................7

2.1 Basic terminology.................................................................................7 2.2 Mass spectrometer...............................................................................8 2.3 Electron impact ionization (EI) .............................................................9 2.4 Quadrupole mass filter .......................................................................12 2.5 Detection of small ion currents...........................................................13 2.6 Vacuum..................................................................................................

.........................................................................................................14 2.7 Background gas .................................................................................14

3 MEMBRANE INLET MASS SPECTROMETRY (MIMS)............................17

3.1 General concepts ...............................................................................17 3.2 Membranes ........................................................................................18 3.3 Membrane inlet design.......................................................................19

4 THE SAMPLER AND THE OPERATIONAL PRINCIPLE..........................21 5 SAMPLING................................................................................................23

5.1 The sampling procedure ....................................................................23 5.2 Evolution of the sampler.....................................................................24

6 MS-INSTRUMENT TESTING AND METHOD DEVELOPMENT...............25

6.1 Gas components ................................................................................25 6.2 Instrument parameters .......................................................................26 6.3 Calibration constants used in gas analyses .......................................27

7 DIRECT GAS INLET TESTS.....................................................................29

7.1 Effects of sample pressure.................................................................29 7.2 Detection limits...................................................................................29 7.3 Effects of mass spectrometric pressure .............................................30 7.4 Effect of sample humidity ...................................................................32 7.5 Determination of calibration constants ...............................................33 7.6 Repeatability of gas analysis..............................................................34 7.7 Linearity of the signals .......................................................................35

8 DIRECT GAS ANALYSIS RESULTS OF GROUNDWATER SAMPLES...41

8.1 Sample OL-KR22/390-392 m.............................................................41 8.2 Sample OL-KR2/876-1050 m.............................................................41 8.3 Sample OL-KR6/422-425 m...............................................................44 8.4 Sample OL-KR6/135-137 m...............................................................46 8.5 Sample OL-KR6/125-130 m...............................................................47

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8.6 Sample OL-KR6/98-100 m.................................................................48 8.7 Summary of PAVE sample analyses..................................................49

9 MEMBRANE INLET TESTS......................................................................51

9.1 Measurement parameters in membrane measurements....................51 9.2 Effect of temperature on the properties of a silicone membrane........52 9.3 Linearity of signals measured with membrane inlet and gas samples53 9.4 Gas components analysed directly from water with a membrane

inlet ................................................................................................ 56 9.4.1 Calibration constants with a silicon membrane ........................... 56 9.4.2 Production of water samples with a known gas composition ...... 59 9.4.3 Linearity of signal intensities versus gas concentrations analysed

directly from water....................................................................... 60 9.4.4 Repeatability of membrane measurements ................................ 68

10 SUMMARY................................................................................................71

10.1 The advantages and weaknesses of direct gas inlet mass spectrometry ....................................................................................71

10.2 Advantages and weaknesses of membrane inlet mass spectrometry72 10.3 Comparison of membrane and direct gas inlet methods ....................73

REFERENCES.................................................................................................75 APPENDICES ..................................................................................................77

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Used symbols and abbreviations CI = Chemical Ionization EI = Electron Impact Ionization FI= Field Ionization MCD = Multiple Concentration detection, same as MID with concentration calculation MID= Multiple Ion Detection. Measures only intensity of selected peaks (mass

or m/z ratios) MIMS = Membrane Inlet Mass Spectrometry MS= Mass Spectrometry SIM = Selected Ion-Monitoring (mass or m/z ratios) mbar = Unit of pressure A = Unit of current, Ampere RSD = Relative Standard Deviation (standard deviation / average) IPROY= Insinööritoimisto Paavo Ristola Oy

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1 BACKGROUND AND OBJECTIVES OF WORK

Dissolved gas content in deep saline groundwater is an important factor, which has to be known and taken into account when planning the deep repository for spent nuclear fuel. Dissolved gases will be one important aspect when considering the long-term safety of the repository. One of the aims of dissolved gas analysis was to get information to be used as a basis in the monitoring program during the construction of ONKALO, and to clarify the origin of different water types. In addition, redox circumstances in deep groundwater can be confirmed. The high methane values analysed at Olkiluoto may also affect working safety in the underground tunnels. Posiva has investigated dissolved gases in deep groundwaters since the 1990's. The first gas samples were taken from the surface and analysed by gas chromatography in the laboratory. Since 1997 gas samples have been taken at in-situ pressures using PAVE down-hole sampling equipment (1) developed in collaboration with Lapela Oy and several other consultants. The analysing of gases in pressurized water samples has been difficult and time consuming and some problems have also been encountered with air contamination over the years. In 2002 Posiva started a project that focused on developing a mass spetrometric method for measurement of dissolved gas content in deep saline groundwater. The main idea of the project was to analyse the dissolved gas content of both the gas phase and the water phase by a mass spectrometer. The main objective of the work was to develop mass spectrometric methods for gas analyses with direct and membrane inlets. The development of the method started in 2003 (in the autumn). The aim was to simplify and speed up the gas analysis process. Another aim was to create a parallel method for gas analysis with the gas chromatographic method. One of the starting points of this project was to test if gases could be analysed directly from water using a membrane inlet in the mass spectrometer. The three objectives defined for the development of the method were:

1 To test and develop method get gas sample out of the pressure vessels for subsequent gas analysis. For analysis gas samples from pressure vessels.

2 To test and develop an analysis method for gaseous samples using a direct

gas inlet mass spectrometer. 3 To test and develop an analysis method for aqueous samples using a

membrane inlet mass spectrometer.

A method was developed for the emptying of pressurized gas samples from pressure vessels. An analysis method was developed for dissolved gases using a direct gas inlet mass spectrometer. Some basic tests, such as detection limits,

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repeatability and linearity, were checked. In addition, real PAVE samples were tested by this method and the results were good. The development of the membrane inlet mass spectrometric method still continues. Two different membrane materials (silicone and teflon) were tested. Some basic tests (linearity, repeatability and detection limits for different gases and mixtures) will be carried out. Repeatability will be measured during one day and from day to day. The main idea is to develop an analysis method for dissolved gases in water phase using a membrane inlet MS.

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2 INTRODUCTION TO MASS SPECTROMETRY

2.1 Basic terminology

This chapter provides a short introduction to mass spectrometry and the techniques used with a typical membrane inlet mass spectrometer. A mass spectrometer is a device used for mass spectrometry, and it produces the mass spectrum of a sample for determination of its composition. This is normally achieved by ionizing the sample and separating ions of differing masses, and recording their relative abundance by measuring the intensities of the ion flux. Mass spectrometry uses the difference in the mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other. Mass spectrometry is therefore useful for quantitation of atoms or molecules and also for determining chemical and structural information about molecules. Molecules have distinctive fragmentation patterns that provide structural information to identify structural components. The general operating sequence of a mass spectrometer is:

1. create gas-phase ions 2. separate the ions in space or time based on their mass-to-charge ratio 3. measure the quantity of ions for each mass-to-charge ratio

A mass spectrometer consists of the following parts: 1. A sample handling unit used to introduce the sample into the mass spectrometer 2. An ion source 3. A mass-selective analyser and 4. An ion detector.

Figure. 2.1. Schematic picture of a quadrupole MS system. (Ref. http://ull.chemistry.uakron.edu/gcms/MS_detector/index.html)

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The sample must first be introduced into the mass spectrometer (MS). These techniques vary a lot. The sample handling technique used in this study, described in chapter 5, is based on a direct gas inlet. The ion source is the part of the mass spectrometer that ionizes the material under analysis (the analyte). Magnetic or electrical fields (in this study is used only electrical fields) then are used for seperation the ions to the mass analyzer. Ionization can be carried out in many different ways, and this is where the principal difference between different mass spectrometric techniques is observed. The ionization techniques have been the key in determining what types of samples can be analyzed by mass spectrometry. Electron impact ionization (EI) cf. chapter 2.3 and chemical ionization (CI) are used for gases and vapors. In chemical ionization sources the analyte is ionized by ion-molecule reactions. Other techniques include field ionization (FI), field desorption (FD), fast atom bombardment (FAB), thermospray, atmospheric pressure chemical ionisation (APCI), secondary ion mass spectrometry (SIMS) and thermal ionisation (Ref. http://en.wikipedia.org/wiki/ Mass spectrometry#Instrumentation). The mass analyser separates the ions according to their mass per charge (m/z). There are many types of mass analysers. Two common mass analysers are time of flight mass analyser and quadrupole mass analyser. The quadrupole mass analyser is discussed in detail in chapter 2.4. A Faraday cup and a channeltron are commonly used as detectors (cf. chapter 2.5). The method of choice for ionization depends on the analytical circumstances. A problem common to all the techniques is the transfer of the components to be analysed from atmospheric pressure into vacuum (< 10 -5 mbar). Once the mass analysis has taken place, the detection of ion currents that can occasionally be very low (< 10-15A) in individual components of the sample poses yet another challenge. (2,3) With membrane inlet mass spectrometry (MIMS) the problems connected with the introduction and vaporization of the sample are solved simply by allowing the components of interest to evaporate through a thin polymer membrane directly into the mass spectrometer, a process called pervaporation. The ionization of the vaporized molecules normally happens through radiation with energetic electrons and the mass analysis is commonly conducted using a quadrupole mass filter. The principles and the theory of membrane inlets are discussed in Chapter 3.

2.2 Mass spectrometer

The mass spectrometer used in this study is MIMS 2200P. It is a quadrupole mass spectrometer equipped with an electron ionization source (EI). The instrument has two possible sample inlets, a membrane and a direct gas inlet. The direct gas inlet (UVD 146) is pressure-controlled by a specific unit (RGV 050C). The instrument has two detectors, a Faraday cup and an electron multiplier (Channeltron) detector. The mass range of the instrument is up to 200 amu. The vacuum for the system is produced by two vacuum pumps: A membrane pump and a turbo pump. The membrane pump is for rough pumping and the turbo pump for high vacuum pumping up to 1x10-8 mbar. The

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mass spectrometer is connected to computer via a serial port. The software used for measurements and service purposes is QuadstarTM version 422. 2.3 Electron impact ionization (EI)

The individual components in a sample have to be ionized before they can be separated in the mass analyser. This is usually done by radiation with energetic electrons (70 eV). Here (cf. Figure 2.1) it is important that all the ions are created at the same potential in order to be transmitted through an ion optical system with high efficiency. The electrons emitted from the filament enter the ionization chamber through a small aperture and ionize the vaporized sample molecules inside the chamber. The ionized sample molecules are then extracted from the ion source by help of an ion optical system. In contrast to most other mass analysers the performance of the quadrupole mass spectrometer is not critically dependent on the distribution of ion energies. In the MIMS 2200P ion source, the ions are formed in a cylindric vessel with two filaments for electron production placed on the outside. Only one is used at a time, while the other is a spare one. Three parameters are important for the performance of the ion source: the ionization energy, the potential of the ionization chamber and the filament current. These values are preset in the MIMS 2200P system and should only be adjusted by skilled personnel. (2)

Filament = typical made of Re. Source of 70 eV electrons. Target = anode used in association with the filament to produre electrons.

Repeller = positively charged electrode use to "push" positive ions out of the ionzation source Lens stack = series of increasingly more negative electrodes used to accelerateions to constant kinetic energy

Figure 2.2. Schematics of electron ionisation. (Ref. http://ull.chemistry.uakron.edu/gcms/MS_detector/index.html)

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When the vaporized sample molecules interact with the electrons, ions can be formed in a number of ways. In the most common process an electron is knocked out of the sample molecule and a positive ion, i.e., a molecular ion, is formed (see Eq. 2.1). Very often the molecular ion is formed with a lot of excess energy and it falls apart into fragments (see. Eq. 2.2). Sometimes the excess energy causes a restructuring of the molecule before it falls apart and the fragments are no longer simple breakdown pieces of the original molecule (see. eq. 2.3). Sometimes, doubly charged ions are observed, in which case the molecule has lost two electrons in the interactions with the electrons. These ions are observed at half m/z of the original molecular ion. (see Eq. 2.4).(4,5) ABCD + e- ABCD+ + 2 e- (2.1) ABCD + e- ABCD+ + 2 e- ABC+ + D (2.2) AC+ + BD (2.3) ABCD + e- ABCD++ + 3 e- (2.4) When the ionization process takes place under standard conditions (as in the case with MIMS 2200P) the relative distribution of ions in the spectrum of a compound is characteristic for the compound and the spectrum can be regarded as a fingerprint useful for identification. The mass spectrum of carbon dioxide is shown in Figure 2.2.

Figure 2.3. Typical electron impact ionization mass spectrum of carbon dioxide. Even a simple compound such as carbon dioxide (CO2) gives six different ions: The molecular ion [CO2]+ m/z 44 100% intensity [13CO2]+ m/z 45 1% intensity, isotope peak Fragments [C]+ m/z 12 10% intensity

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[O]+ m/z 16 12% intensity [CO]+ m/z 28 12% intensity Doubly charged ion [CO2]2+ m/z 22 2% intensity The gases, which is used in this study, and their m/z peak and relative abundance is shown in Table 2.1. Table 2.1. The m/z peak of common gases and their relative abundance Gas : u (relative abundance, %) H2 : 2(100) He : 4(100) CH4 : 12(2.4), 13(7.7), 14(15.6), 15(85.8), 16(100), 17(1.2) H2O : 16(1.1), 17(23), 18(100), 19(0.1), 20(0.3) CO : 12(4.5), 14(0.6), 19(0.9), 28(100), 29(1.1), 30(0.2) N2 : 14(7.2), 28(100), 29(0.8) O2 : 16(11.4), 32(100), 33(0.1), 34(0.4) Ar : 20(20), 36(0.3), 40(100) CO2 : 12(6), 13(0.1), 16(8.5), 22(1.2), 28(11.4), 29(0.1), 44(100), 45(1.3), 46(0.4) Larger organic compounds can produce very complicated spectra. Good reproducibility of EI mass spectra makes it possible to use a standard database (literature spectra) to identify the components of a sample. However, with membrane inlets where all volatile compounds are introduced into the mass spectrometry simultaneously, composite spectra arise and identification can be difficult. In order to obtain the lowest possible detection limits it is important to ionize as many gas molecules as possible. The ionization efficiency will depend on the total interaction cross-section between the electrons and the gas molecule, and on operational characteristics such as electron current and ionization path length. The total ion current, Im, can be calculated as Im = Ie Q n l (2.5) where, Ie is the electron current, Q the total interaction cross-section, n the concentration of molecules per cm3 and l the ionization path length. (4,5) Whereas the total interaction cross-section is characteristic of the individual compound, the electron current and the ionization path length can be used to increase the intensities in general. However, the maximum number of electrons that can be emitted from the filament before it burns out is restricted, and mechanical dimensions limit the path length.

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2.4 Quadrupole mass filter

Quadrupole mass filters (Figure 2.4) are very simple in construction. The quadrupole consists of four parallel metal rods (a quadrupole). Each opposing rod pair is connected together electrically and a radio frequency voltage is applied between one pair of rods, and the other. The combination of these two potentials creates a mass filter that allows ions below a certain mass to pass through in one of the quadrupole planes (the negative rods), while ions above a certain mass pass through in the other quadrupole plane (the positive rods). When the potentials are correctly chosen, the two mass filters overlap and form a small window, where the ions are stable in both directions. The stable ions will pass all the way through the quadrupole mass filter and hit a detector at the end. The ion current measured by the detector gives a signal. (6-8)

Figure 2.4. The quadrupole mass filter (Ref. http://elchem.kaist.ac.kr./vt/chem-ed/ms/detector/detector.htm) The mass spectrum is recorded simply by increasing the amplitude of the electrical potentials. In this fashion, successively higher masses will pass through the quadrupole mass filter and the spectrum of the ion current versus the mass is recorded by the computer. With MIMS 2200P, this is done simply by choosing the first mass and the width of the spectrum (9). The quality, i.e., the signal-to-noise (S/N) ratio of the resulting spectrum will depend on the scan rate. The slower the scan rate the more time for averaging and the better the signal-to-noise ratio. The scan rate is therefore chosen as a compromise between a desire for a short analysis time and the quality of the spectrum. In cases where the identity of the individual compounds in the sample is known in advance, it is often an advantage to use a so-called selected ion-monitoring (SIM) or multiple ion detection (MID) mode. Instead of scanning the instrument, the mass spectrometer shifts between preselected masses characteristic of the individual compounds in the sample. In this fashion, no time is wasted in measuring at masses

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where no signal appears. As a result, lower detection limits and a shorter analysis time are achieved in the SIM mode. With the MIMS 2200P instrument up to 64 different masses can be selected at once, but it is generally not recommended to use more than 6 at a time. 2.5 Detection of small ion currents

The final element of the mass spectrometer is the detector. The detector records the charge induced or the current produced when an ion passes by or hits a surface. Typically, some type of an electron multiplier is used, though other detectors (such as Faraduy cup) have been used. Because the number of ions leaving the mass analyzer at a particular instant is typically quite small, significant amplification is often necessary to get a signal (Ref. http://en.wikipedia.org/ wiki/Mass_spectrometry#Detector).

Most mass spectrometers have a very wide dynamic range. The MIMS 2200P instrument detects ion currents that vary from 1×10-6 A down to 1×10-14 A. High currents, i.e., 1×10-10 A up to 1×10-6 A are detected using a so-called Faraday cup detector, which in the case of MIMS 2200P refers to just a direct detection of the current of the ions hitting a metal plate at the end of the mass filter. A Faraday cup is a metal cup that is placed in the path of the ion beam. It is attached to an electrometer, which measures the ion-beam current. Since a Faraday cup can only be used in an analog mode it is not as sensitive as other detectors that are capable of operating in pulse-counting mode (Ref. cf. below). Faraday cup detectors are the most stable detectors, but their usefulness is limited to the detection of compounds present at high concentrations. In the case of compounds present at a low concentration, the ion current coming out from the mass filter must be pre-amplified prior to its detection. In secondary electron multipliers (SEM), the ion current coming out from the mass filter is converted into an electron current, which is then amplified. The electron multiplier might consist of a narrow funnel or a series of discrete dynodes. A channeltron is a horn-shaped continuous dynode structure that is coated on the inside with an electron emissive material. An ion striking the channeltron creates secondary electrons that have an avalanche effect to create more secondary electrons and finally a current pulse. (Ref. http://elchem.kaist.ac.kr/vt/chem-ed/ms/detector/detector.htm). In both cases the principle of amplification is the same. The ions leaving the mass filter hit the top of the funnel/the first dynode and release a number of electrons. The electrons are accelerated and hit the inside surface of the funnel or dynode number two with a relatively high energy. Upon collisions, new electrons are released, accelerated and hit further down the funnel or the next dynode. This process is repeated 15-20 times and each time approximately two electrons are released from the funnel or the dynode surface for each hitting electron. In this fashion, the electron current is amplified by a factor of approximately 100 000, but the background noise is amplified simultaneously and the actual amplification is just round a factor of 1000. MIMS 2200P uses an electron multiplier of the Channeltron type. This type of multiplier has the advantage of being very small as compared to the dynode type, but the surface coating in the funnel wears out in the process and, if not handled with care, the multiplier has to be replaced

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regularly. In normal operation with a MIMS system they should last several years before replacement is needed. (2,3)

Figure 2.5. Electron multipliers (2). (a) Channeltron, (b) Discrete dynode multiplier. The choice between the electron multiplier or the Faraday cup detector depends on the analytical conditions. Small signals (low concentrations) are only detectable with a electron multiplier, and on the other hand, large signals (high concentrations) deteriorate the surfaces of the electron multiplier. In between these two limits the choice depends on a number of factors: (1) The electron multiplier might saturate and the signal will no longer be linearly dependent on the concentration of the analyte, (2) the deterioration of the surfaces in the multiplier reduces its lifetime, (3) the S/N-ratio will improve with sample concentration and favour the use of a Faraday cup detector and (4) the Faraday cup detector gives the most stable signal. (2)

2.6 Vacuum

Most mass spectrometers, including quadrupole mass filters, require a low pressure (< 1×10 -5 mbar) for optimal performance. At such low pressures ions created in the ion source can pass through the mass spectrometer without any collisions with the molecules of the background gas. This collision free flight is important because ions might be lost or modified as a result of the collisions. To obtain the low pressure, special vacuum pumps are needed. The most common pump is a so-called turbopump. This pump in many respects works like a turbine in a jet engine. The turbine blades rotate at a frequency of 1000-1500 Hz and gas molecules are simply smashed down through the pump to the exhaust. The turbopump (and most other high vacuum pumps) need to be used in conjunction with an ordinary vacuum pump, like a diaphragm pump or a rotary pump that pre-pumps the mass spectrometer and keeps the pressure at the exhaust of the turbopump at a regular vacuum. Otherwise the friction created when the gas molecules are pumped out will generate excessive heat that could cause fatal damage to the turbopump. (2,3) 2.7 Background gas

During the operation of the mass spectrometer a steady state total pressure will be established where the flow of molecules entering the instrument equals the flow of

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molecules leaving it via the vacuum pumps. Here it is important to realize that the flow of molecules into the instrument has three origins: (1) Sample molecules introduced via the inlet system, (2) atmospheric air flowing into the system via leaks in the vacuum system, and (3) degassing from the walls inside the vacuum system. The detected signal will be proportionally influenced by all three sources. The flow through leaks and the degassing cause a background signal that sets the sensitivity limit for the gases to be analysed. For interference free analysis of the sample molecules it is important that the flow of molecules into the vacuum system via leaks as well as degassing are at least 100 times lower than the flow of sample molecules introduced via the inlet system. This requires the use of special gaskets in all assemblies in the vacuum system. In assemblies that are only taken apart occasionally copper gaskets are recommended because they make very tight seals. Degassing (liberation) of gases from the surfaces in the vacuum system is the major cause of background signals in a well-sealed instrument. All surfaces have 1-2 atomic layers of gas molecules attached to them and these gases are liberated from the surface at low pressures. In order to understand the importance of this gas liberation one has to realize that 1 cm2 of a metal surface with just a single-atomic layer of a bound gas contains enough molecules to create a fairly high mass spectrometric signal for an hour of analysis. The surface area of a typical MIMS system is more than 1000 cm2. Normally degassing should reach a level of 10% of the introduced sample in one hour. After that degassing decreases with time and reaches 1% in approximately 10 hours. Heating of the vacuum system speeds up the degassing process and the limiting of the exposure time of the vacuum system to atmospheric air during inlet replacements etc. also reduces the degassing time. In the MIMS 2200P system a special design based on direct interfacing of a so-called closed ion source to the inlet flanges considerably reduces the problem of background signals caused by leaks and degassing. In this design advantage is taken of the special rules that apply to gas transport at low pressures (< 1×10 -4 mbar). At such low pressures gas transport is no longer the result of collisions between gas molecules, but transport is limited by collisions between gas molecules and the vacuum surfaces. When a gas molecule hits the surface, it forgets its origin and is released “randomly” in any direction. As a result of this phenomenon it is possible to ionize the sample molecules in a defined volume, where the sample pressure is 100-1000 times higher than in the surrounding vacuum. (2)

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3 MEMBRANE INLET MASS SPECTROMETRY (MIMS)

Membrane inlet mass spectrometry is a method for direct analysis of gases and volatile organic compounds in air, water and soil. A polymer membrane is used as the sole interface between the sample to be analysed and the vacuum of the mass spectrometer. Volatile compounds in the sample dissolve in the membrane, diffuse through it and evaporate into the mass spectrometer, where they are ionized and analysed according to their m/z-ratios. In this fashion, changes in the chemical environment of the sample can be monitored continuously, a property that has made the MIMS method useful for on-line monitoring of chemical and biological processes. (10-12) The transport of compounds through the membrane is a highly selective process that favours the transport of small hydrophobic compounds such as gases, small halogenated compounds and benzenes. Water, on the other hand, does not penetrate the membrane very well and in aqueous samples extreme enrichment of atmospheric gases and small hydrophobic compounds is obtained. The response time for gas analysis will typically be a few seconds, while for small hydrophobic compounds it is 30 – 60 seconds. The MIMS method is often compared to a combined gas chromatographic/mass spectrometric analysis. Both the advantages and the disadvantages of the method are discussed here. The most important advantages include extremely simple sample handling and the possibility for continuous monitoring, whereas the most important disadvantages are due to poorer selectivity and lower sensitivity for polar compounds. The MIMS method is particularly suited for gas analysis and for monitoring processes, where the chemical composition of the sample is known. 3.1 General concepts

The transport of gases through the membrane is a three-step process where the molecules first dissolve in the membrane, diffuse through it and finally evaporate from the membrane surface into the vacuum of the mass spectrometer. The whole transport process is quite selective. Some gases dissolve very well in the membrane while others do not and the rate at which the molecules diffuse through the membrane depends on the physical dimension of the molecule. Only a few equations are needed to describe the basic behaviour of a membrane inlet system. For gases, the steady state flow through the membrane is given by:

lC K D A = ISS ××× (3.1)

where A is the surface area of the membrane, C is the analyte concentration in the sample, D is the diffusion constant, K is the distribution ratio of the analyte between the concentration in the membrane and in the sample, and l is the thickness of the membrane. In other words, the flow through the membrane is proportional to the concentration of the analyte in the sample, the surface area of the membrane and the product between the

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diffusion constant and the distribution ratio (a parameter called permeability). The flow through the membrane is inversely proportional to the thickness of the membrane. (2) The response time (t10-90%), defined as the time it takes the signal to increase from 10% to 90% following a step change in concentration, is given by:

Dl 0.237 = t2

90%-10 × . (3.2)

The response time increases with the square of the membrane thickness and decreases proportionally to the diffusion constant. The response time only depends on the physical dimensions of the gas molecule and not on the solubility of the gas in the membrane. The larger the molecule the slower it diffuses. In connection with a gas analysis the membrane response time is very short, less than 1 s with silicone membranes and about 1 s when a Teflon membrane is used. Membrane temperature is a very important factor to the performance of the membrane inlet. Both the diffusion constant, the distribution ratio and the permeability (equal to the product of the diffusion constant and the distribution ratio) depend on the temperature according to the Arrhenius equations. For the permeability (decisive for the detection limit) the equation is

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−××

0P0 TR

11 E-exp P = PRT

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−×∆××

0SD00 TR

11 H + E - exp K D =RT

(3.3)

where D0 , K0 and P0 represent the diffusion constant, the distribution ratio and the permeability at an initial temperature T0. ED is the energy of activation for diffusion, ∆HS the difference in heats of solution between the membrane and the sample matrix and EP the activation energy for permeation. (10,11) For a gas analysis the diffusion constant will normally increase with the temperature, whereas the solubility decreases. The increase in the diffusion constant is normally larger than the decrease in solubility and the result is that the signal increases with the temperature until a certain limit, where the gas pressure in the mass spectrometer becomes too high for optimal mass spectrometry. 3.2 Membranes

The membrane is the most important part of a membrane inlet system and two materials are commonly used in connection with gas analyses, polydimethylsiloxane (silicone rubber) and Teflon. Both membranes are highly hydrophobic, which is a major advantage since they discriminate against water. As a result the relative gas content in

19

the flow of molecules through the membrane is highly enriched and better detection limits for the gases can be obtained. The two membrane types differ in the way the polymer chains are packed together. In the Teflon membrane the polymer chains are packed very tight, whereas the structure in the silicone membrane is quite loose and Teflon is also more hydrophobic than silicone. As a result of these, different gases diffuse faster through a silicone membrane (response times less than 1 s) than through at Teflon membrane (response times a few seconds) and the permeability of a silicone membrane is higher than that of a Teflon membrane. The big advantage offered by the tight packing of the polymer chains in a Teflon membrane is that it prevents penetration of small organic molecules that might interfere with the gas signals. Teflon membranes are therefore often the membranes of choice in connection with gas analyses. (10,11) 3.3 Membrane inlet design

Membrane inlets exist in many different designs, each of them adapted to a special application. The three most common principles are shown in Figure 3.1. (2)

Figure 3.1. Common membrane inlet designs: (a) Membrane probe to be inserted into the sample. (b) Flow through membrane inlet. (c) Measurement cell. The membrane probe (Figure 3.1a) is a very practical inlet that can be produced in many forms and dimensions, and inserted directly into most environments. The common probe is simply a steel tube, which is perforated in one end and covered by a membrane,

20

whereas the other end is attached to the mass spectrometer. These probes can be inserted into most media and have been used with success for in-vivo studies of blood gases, for on-line monitoring of bioreactors and studies of the gas composition of sediments. The drawback of the membrane probe is that it is applicable only to gas analyses or and to applications that involve extremely volatile organic compounds, because the transport of most compounds through the evacuated steel tube is very slow. The flow through the membrane inlet (Figure 3.1b) can be used to analyse less volatile organic compounds (boiling point up to 200 oC) and it is quite practical for the analysis of many liquid and gas samples. This inlet requires withdrawal of the sample (continuously if required) from the environment to be analysed, before it is flushed through a membrane inlet that has the membrane attached directly to the ionization region of the mass spectrometer. After passage through the inlet the sample can either be returned to its source or disposed of. The major advantage of this inlet is the elimination of any transport of sample molecules through vacuum tubes. In addition to the expanded application area, the form of the inlet gives an advantage with calibration, since it is quite easy to shift between the sample and a standard. (2) Correctly designed measurement cells (Figure 3.1c) have the same advantages as the flow through a membrane inlet, but they can also be used to analyse samples that are quite viscous and/or contain particles. The inlet simply consists of a measurement cell (0.5 mL and upwards) where the membrane at the same time constitutes a part of the wall in the measurement cell and of the ion source. The sample is transported directly to the measurement cell and after a few minutes a MIMS spectrum of the volatiles can be recorded. This type of inlet has found widespread use in kinetic studies of chemical and biological reactions, analysis of volatile compounds in microbial cultures and analysis of soil samples.

21

4 THE SAMPLER AND THE OPERATIONAL PRINCIPLE

The gas sample is taken with PAVE down-hole sampling equipment (1), which consists of a membrane pump, the PAVE unit and two inflatable rubber packers. The PAVE unit contains three pressure vessels and a valve for operating the equipment. The pressurized water sample in a pressure/sampling vessel, which is still at pressure prevailing at the samplings depth. The vessel is attached to the equipment (Figure 4.1), where the gases will be released from the groundwater to another sample vessel, which is in a vacuum. The valve between the PAVE-sample (pressure vessel) and the sampler is opened. The pressure from the PAVE sample is released to the sampler and will find gas-liquid equilibrium state. The gases, which are dissolved in pressurized water, will be released to the sampler and further to the mass spectrometer. The sampler is connected to the vacuum pump and to the direct gas inlet in the mass spectrometer. In the upper part of the pressure vessel of the sampler a pressure gauge and connections to the mass spectrometer and the vacuum pump are provided. A flush gas connection and another pressure gauge are provided at the bottom, near the connection to the PAVE-sample. There is also a metallic sinter which is after a psessure gauge. The aim of sinter is to prevent water drops from entering the sampler and further the direct gas inlet. All the connections are equipped with valves. The sampler is presented in Figure 4.1.

22

Figure 4.1. The equipment for emptying the groundwater samples from the pressure vessels used in PAVE sampling.

Pressure gauges Pressure vessel

23

5 SAMPLING

5.1 The sampling procedure

The gases, which are dissolved in water at a high pressure, are released from water by means of a special sampler. The pressurized PAVE-sample is opened and depressurized with a sampler, which allows control of the sample depressurizing process. The released gases are analysed with a mass spectrometer The pressure vessels of PAVE contain two compartments, which are separated with a piston. The sample part is in the upper part of the vessel and the lower part of the pressure vessel is for the backpressure gases used during sampling. The purpose of backpressure gas is to retain the sampling pressure and slow down the movements of the piston during sampling. The backpressure gas is released by opening the valve at the bottom of the PAVE-vessel. This and the metallic sinter in the tube prevent water splashes and a flow to the sampler’s pressure tank. To prevent sample contamination, the sampler has to be carefully cleaned. In the sampling procedure the sampler is cleaned and flushed with pure nitrogen and pumped to a vacuum with a vacuum pump. The PAVE-sample has to be connected to the sampler. The exact sampling and cleaning procedures are shown in the instructions for direct gas inlet mass spectrometry measurements (13). The sampling procedure is as follows:

- Connect the PAVE-sample to the sampler. - Release counter pressure from the PAVE-vessel by opening

the valve at the bottom. - Evacuate the sampler with a vacuum pump. The pumping

time should be around 20-30 s. - Fill the sampler with pure nitrogen (N2, 99.999%) to a slight

over pressure (1-3 bar). - Measure the mass spectrum and check the purity of the

sampler by monitoring the oxygen content (Oxygen level <200 ppm, 5x10-11 A).

- Start the procedure again if oxygen levels decrease. When the oxygen level no longer changes a background signal is detected.

- Save the spectral data. - Evacuate the sampler with a vacuum pump. - Open the valve between the high-pressure PAVE-sample and

the pressure tank. The released gases will flow into the pressure tank.

- When the pressure in the pressure tank stops rising, close the valve between the PAVE-sample and the sampler.

- The sample is ready to be measured.

24

5.2 Evolution of the sampler.

The sampler was modified during the development of the method. The modifications were quite small, concerning mainly the attachments of connections and valves. The modifications were mostly based on recommendations of F. Lauritsen, MIMS SYSTEMS (14, appendix 2). The problems found in the sampler system and the improvements enhancements made: Problem: The sampler was difficult to clean with flush gas. Solution: An extra connection for flush gas is added near the PAVE-sample connection. Problem: Water rose to the pressure tank during sampling. Solution: A metal sinter was attached to the tubing near the PAVE-sample connection. Problem: The pressure gauge in the sampler was inaccurate. Solution: A more accurate digital pressure gauge was attached to the sampler. Problem: The sampler was difficult to use in an upright position. Solution: The sampler was modified and supports were attached.

25

6 MS-INSTRUMENT TESTING AND METHOD DEVELOPMENT

The objective of the work was to develop mass spectrometric methods for gas analyses with direct and membrane inlets. The instrument was installed and tested, and some recommendations for the future were made. A summary of the installation process and preliminary implementation of the MS system is provided in memo by F. Lauritsen (14, appendix 2). During the work, several parameters were defined. These parameters included: Detection limits, linearity, calibration constants, repeatability, optimal instrument parameters and possible sources of errors. 6.1 Gas components

The gas components that were tested were H2, He, CH4, C2H6, N2, O2, Ar and CO2. The gases that were used for the development of the method , were all of analytical grade (Appendix 1). The gas compositions of every gas/gas mixture are well known. The concentration of gases in the gas mixtures was analysed and reported by the gas manufacturers. A gas calibrator (mass flow calibrator) was used to create specific gas mixtures. The mass flow calibrator was a Teledyne API Model 700 calibrator. The dilution range for this calibrator is 1/2 to 1 / 100 and the flow rate is 1 to 10 litres/min depending on the dilution range. The dilution gas was pure nitrogen (N2, 99.999%) and the source gases were also pure gases or gas mixtures of a well-known composition. The mass spectrometric measurements of the gas components were mainly carried out with molecular ions (often peaks with the highest intensities) in the spectra. A few gas components were detected from the fragment peaks. Methane was measured from a peak appearing at m/z 15. The highest peak for methane, however, appeared at m/z 16, but the overlapping of the oxygen fragment (oxygen, m/z 16) makes the use of this peak unfavourable. Other fragments / isotope peaks (m/z) peaks of measured peaks are shown in Table 2.1. Table 6.1. Gas components and measured peaks. Component Molecular weight

(g/mol) Measured peak (m/z)

Hydrogen, H2 2 2 Helium, He 4 4 Methane, CH4 16 15 Ethane, C2H6 30 26 Nitrogen, N2 28 28 Oxygen, O2 32 32 Argon, Ar 40 40 Carbon dioxide, CO2 44 44

Hydrogen sulphide was one of the most interesting gases. Hydrogen sulphide is a very difficult component to analyse, because it is a reactive and corrosive gas. Also a few

26

other difficulties were encountered when measuring gas samples that contained hydrogen sulphide.

- The oxygen base peak (m/z 32) and one of main fragments S+ (m/z 32) of hydrogen sulphide will overlap. The S+ intensity is 22% of the intensity of hydrogen sulphide H2S (m/z 34).

- 16O18O (m/z 34) and H2S (m/z 34) peaks will overlap. - Hydrogen sulphide, H2S, will react in the gas bottle or in the ion

source and form several products. These products include: - SO2

+ (m/z 64, 66) (m/z 66 is 34S16O16O and 32S16O18O) - SO+ (m/z 48, 50) - HSO2

+ (m/z 65) - SO3

+ (m/z 80) - Hydrogen sulphide, H2S, reacts with oxygen and decreases the

oxygen content as well as the hydrogen sulphide content itself. Overlapping and reactivity disturbs the gas analysis by changing the gas composition.

For these reasons, hydrogen sulphide, H2S, was not analysed. The hydrogen sulphide content of real gas samples is usually low with only few exceptions. 6.2 Instrument parameters

The direct gas inlet measurements were performed with a Channeltron-detector. The direct gas inlet was adjusted to a pressure of 4.0×10-6 mbar. The spectra were measured using several amplification ranges from 10-6 to 10-12A. The direct gas inlet was used at an elevated temperature to prevent contamination. The temperature was 60°C and it was monitored and controlled by a thermostated cap and a temperature control unit. The scanning speed in mass spectrometric measurements was kept unchanged in all measurements (1s /amu). The reason for the slow scan speed was that the intensity of the peak depends on the scanning speed and at a high scan speed the weak intensities are not detected properly. The instrument parameters are as follows: Detector: CH-TRON Mass mode: SCAN-F SEM Voltage 1200 (Detector voltage) First mass 0 Speed 1 s/amu (amu= Atomic mass unit) Width 50 (Spectral width) Resolution 30 Amp. Mode FIX (Amplification mode) Amp. Range E-6 E-12 (Amplification range) Offset ON (Detector OFFSET-correction) Parameters in MCD and MID-measurements (SIM):

27

Detector: CH-TRON SEM Voltage 1200 (Detector voltage) Mass 0 Dwell 1 s (Dwell time for the measurement on this channel) Resolution 25 (Mass resolution) Amp. Mode AUTO (Amplification mode) Offset ON (Detector OFFSET-correction) 6.3 Calibration constants used in gas analyses

It is possible to determine the correlation between the mass spectrometric signals (peak intensities) and the gas composition. Because signal intensities are labile, the use of absolute intensities is not realistic. However, relative intensities can be used in the calculations. The calibration constant determines the relation the intensities of nitrogen and intensities of other gas components. The calibration constants are measured and calculated from the data obtained from measurements of known gas samples (gas mixture, etc.). A nitrogen signal is used to normalize the signal/concentration factors. (9)

A

AA C

IS = , (6.1)

SA= Component A sensitivity [A %-1] IA= Component A peak intensity [A] CA= Component A concentration in sample [%] The component sensitivity factor has to be normalized with the sensitivity of another gas component. Normalization of the peak intensities is carried out with nitrogen sensitivity.

N

AA S

SK = , (6.2)

SN = Sensitivity of nitrogen [A%-1] KA = Calibration constant. When measuring an unknown sample, the peak intensity of component A is normalized with the calibration constant.

28

A

AAnorm K

II = , (6.3)

IAnorm = Normalized sensitivity of component A [A] When component intensities have been normalized, they are summed. With the summed total intensity, the concentrations of the components can be calculated. The total intensity equals 100 % when the concentrations are calculated by this method. IKok= IAnorm+IBnorm+ICnorm+… (6.4)

%100×=kok

AnormA I

IC (6.5)

29

7 DIRECT GAS INLET TESTS

The direct gas inlet was tested carefully and several properties and several parameters were determined. The following properties and parameters were determined: The effect of sample pressure, the effect of sample humidity, linear dynamic range, repeatability of the measurements and detection limits. 7.1 Effects of sample pressure

The effects of the sample pressure were tested in a pressure range of 1.2-4.2 bar. The gas bottle was connected to a direct gas inlet and automatic pressure control was used. The automatic pressure control adjusted the pressure in 30 s. It was noticed that the sample pressure did not effect the nitrogen and oxygen signals in any way, while an increase in pressure decreased the hydrogen signal but only very fractionally. This test has been reported in memo by F. Lauritsen (14, appendix 2). 7.2 Detection limits

The detection limits for the direct gas inlet method were determined with gas mixture 5 (AGA) and a Channeltron detector. The detection limits were calculated when the peak intensity versus concentration was stabilized. The background of the measurement was not stable and for this reason the determinated detection limits are quite high compared to the sensitivity of the instrument, which is up to 10-14 A, about 10 ppm. The intensity level (10-14 A) is near the detector’s (Channeltron) detection limit. The detection limits are calculated with a noise signal (S/N is about 3). Noise signal was nitrogen. The measurements were done at least three parallel. In addition, it was studied have watered gas effect on the detection limits. It can be concluded based on the new experiments that the moisturing has not much effect on the limits.The new measurements ( in year 2006) was done in the three different concentration (50, 100 and 250 ppm). The parallel analysis was done at least three. Table 7.1. Measured detection limits. Component Peak intensity

(A) Concentration, (ppm)

Additional data

Hydrogen 2,64E-11 250 Dependent on background signals. Helium 3.37E-13 20 Methane 2.28E-11 500 Peak 15 overlapped with large peaks 14 and 16. Ethane 9.53E-13 50 Peak 26 overlapped with large peak 28. Oxygen 7.94E-12 75 Dependent on background signals. Argon 3.92E-12 50 Carbon Dioxide 5.64E-12 100 Background signal was around 5x10-12

30

7.3 Effects of mass spectrometric pressure

The mass spectrometric pressure can be controlled with the direct gas inlet and the total pressure control unit. The effects of the mass spectrometric pressure on the gas analysis results were tested in a pressure range of 1-6×10-6 mbar. All other parameters were kept constant, the pressure in the mass spectrometer being the only factor that changed during these measurements. The tested pressure range was optimal for the mass spectrometer. At higher pressures, the intensity increases and the instrument also becomes more unstable (direct gas inlet pressure regulation) while at lower pressures the intensities are smaller and the effect of the background increases, the detection limits are also higher. The measurements were made in MID-mode, which allows collection of data on preselected components as a function of time. The test procedure was as follows:

- The direct gas inlet was used and the pressure was controlled by a total pressure controller, and the pressure range was 1-6x10-6 mbar

- The gas sample was taken directly from the gas bottle at an overpressure of 1 bar, and connected directly to the direct gas control. The total pressure controller was set to the desired pressure.

- The direct gas inlet was heated to 60 °C. - The peak intensities were measured as a function of the pressure in

MID-mode. - Two parallel measurements were done in each concentration.

Table 7.2. Peak intensities as a function of pressure in mass spectrometer. Gas Concentration % 1x10-6 mbar 2x10-6 mbar 4x10-6 mbar 6x10-6 mbar Nitrogen 88.94 1.06x10-11 4.14x10-11 1.84x10-10 3.94x10-10 Oxygen 2.01 5.42x10-12 2.36x10-11 9.10x10-11 2.94x10-10 Argon 1.02 1.03x10-11 4.41x10-11 2.09x10-10 4.80x10-10 Helium 1 2.60x10-12 6.60x10-12 3.75x10-11 1.50x10-10 Carbon dioxide 2.01 7.96x10-12 3.28x10-11 1.63x10-10 3.48x10-10

Hydrogen 1 1.70x10-11 4.50x10-11 2.17x10-10 8.40x10-10 Methane 2.03 1.13x10-11 4.04x10-11 2.09x10-10 5.42x10-10

31

0,0E+00

5,0E-09

1,0E-08

1,5E-08

2,0E-08

2,5E-08

3,0E-08

3,5E-08

4,0E-08

0,E+00 1,E-06 2,E-06 3,E-06 4,E-06 5,E-06 6,E-06 7,E-06

Pressure (mbar)

Inte

nsity

(A)

Figure 7.1. Intensity of the nitrogen peak (m/z 28) as a function of pressure.

0,0E+00

2,0E-10

4,0E-10

6,0E-10

8,0E-10

1,0E-09

1,2E-09

0,E+00 1,E-06 2,E-06 3,E-06 4,E-06 5,E-06 6,E-06 7,E-06

Pressure (mbar)

Inte

nsity

(A)

Oxygen Argon Helium Carbon dioxide Hydrogen Hydrogen sulphide methane

Figure 7.2. Oxygen, argon, helium, carbon dioxide, hydrogen, hydrogen sulphide and methane peak intensities measured as a function of pressure.

32

Table 7.3. Calculated calibration constants as a function of mass spectrometric pressure. Gas Concentration

(%) 1x10-6 mbar 2x10-6 mbar 4x10-6 mbar 6x10-6 mbar Nitrogen 88.94 1.00 1.00 1.00 1.00 Oxygen 2.01 0.51 0.57 0.49 0.75 Argon 1.02 0.98 1.07 1.13 1.22 Helium 1 0.25 0.16 0.20 0.38 Carbon dioxide 2.01 0.75 0.79 0.88 0.88 Hydrogen 1 1.61 1.09 1.18 2.13 Hydrogen sulphide 1.99 0.04 0.03 0.05 0.06 Methane 2.03 1.07 0.98 1.14 1.38 As noticed in the table above, the calibration constants were dependent on the pressure in the mass spectrometer. The calculations of the calibration constants are shown in chapter 6.3. The sensitivity of the gas components and the further calibration constants are pressure dependent. The results of these tests are that pressure is a crucial component when performing a qualitative analysis with a mass spectrometer. To achieve good and precise results the pressure in the mass spectrometer has to be stable and the determination of calibration constants has to be done at the same pressure as the sample analysis. The pressure in the mass spectrometer is kept at 4x10-6 mbar, which is also the recommendation of F. Lauritsen, MIMS Systems. 7.4 Effect of sample humidity

The effects of the humidity of the samples was tested by bubbling air at two different temperatures (20 and 50o). The test was carried out with the MCD-macro. The MCD-macro measures only preselected peaks and the macro calculates the gas composition. The testing procedure was as follows:

- The direct gas inlet was used, and the pressure was set at 4x10-6 mbar

- The airflow was bubbled through distilled water with the water temperature at 20°C or 50°C.

- The sample was at atmospheric pressure and the bubble flow was controlled by a small under pressure created by the direct gas inlet. In other words, the direct gas inlet sucked the air sample through the washing bottle.

- The results were compared with the results of the atmospheric air sample.

- The measurements were carried out with MCD-macro.

33

Table 7.4. Effects of sample humidity on peak intensities and MCD-macro results. Gas Atmospheric air,

measurement (%) Air bubbled through water at 20 °C Measurement result (%)

Air bubbled through water at 50 °CMeasurement result (%)

Nitrogen (28) 77.85 78.19 78.27

Oxygen (32) 21.3 21.0 20.9

Argon (40) 0.86 0.84 0.84 CO2 (44) 0.03 0.03 0.03

Atmospheric air Peak intensity (A)

Air bubbled through water at 20 °C Peak Intensity (A)

Air bubbled through water at 50 °C Peak Intensity (A)

Nitrogen (28) 1.051x10-8 9.81x10-9 9.53x10-9

Oxygen (32) 2.77x10-9 2.535x10-9 2.45x10-9

Argon (40) 1.56x10-10 1.41x10-10 1.39x10-10 CO2 (44) 9.59x10-12 9.42x10-12 9.37x10-12

The result of this test was that humidity has no effect on gas composition. The only effect is that all (except water) peak intensities decrease by the same magnitude but this has no effect on the gas composition.

7.5 Determination of calibration constants

The calibration constant was determined using the direct gas inlet and the Channeltron-detector. The mass spectrometric pressure was 4×10-6 mbar in all measurements. When analysing gas components with a mass spectrometer, the correlation between a gas composition and the peak intensities has to be known. The simplest way is to calculate the relative intensities and normalize these values with nitrogen. The exact calculation is shown in Chapter 6.3. The correlation between concentrations and peak intensities was measured from gas mixtures with a gas composition of more than 2000 ppm. The measurements of the calibration factors were made by measuring the same sample several times (5 times) on different days (>2) and in a wide concentration range. For example, the calibration constant for oxygen was measured from several samples, in which the oxygen concentration varied in a range of 1-25%. The calibration constants are averages of measurements made at several different concentrations. The used gas mixtures are listed in Table 7.5. The calibration constants and the standard errors are shown in Table 7.6.

34

Table 7.5. Gas mixtures used in the determination of calibration constants. Hydrogen

(%) Helium (%)

Methane (%)

Nitrogen (%)

Oxygen (%)

Argon (%)

Carbon dioxide (%)

Ethane (%)

Air - - - 78.2 20.9 0.93 - - Mixture 1* 2 - - 88 10 - - - Mixture 2* 1 - - 98 1 - - - Mixture 3* 1 - 1 94 - 1 1 - Mixture 4* 1 1 1 94 1 1 1 -

0.5 2 5 - 5 5 5 5 - 3 49 - 25 - 2 3 - - 25 - 10 - 1 2

Gas mixtures, which are made with gas calibrator.

- - 10 - 5 - - 1

* = More information on these gas mixtures is provided in Appendix 1. Table 7.6. Average calibration constants and determined standard errors. Oxygen Hydrogen Helium Methane Argon Carbon dioxide Ethane Average 0.84 1.13 0.364 1.06 0.98 0.99 0.146 RSD % 6.5 6.5 10 14 9.7 8.6 9.5 The calibration constants listed in Table 7.6 are normalized with nitrogen intensity, with the calibration constant for nitrogen being 1. The calibration constants for hydrogen, argon, carbon dioxide and methane are close to nitrogen (1). The value for oxygen is a bit lower and the values for helium and ethane are much lower. A low calibration constant means that the sensitivity of the component is much lower than the sensitivity of nitrogen. 7.6 Repeatability of gas analysis

Estimation of the uncertainty of an analysis is very important when developing new analysis methods. These measurements were started measuring the repeatability of pure gas components. The measurements were made directly from the gas bottle and all the parameters (ion source settings, quadrupole and detector settings, used pressures in inlet and MS, detector in MS ) were the same in all the measurements (see chapter 6.2). Repeatability in one day was measured by repeating the measurement 6 times. The long time repeatability was measured by repeating the measurements 3 times a day on 3 days, and after that once a day during the next 3 days. The best way to control the repeatability of pure gases is to control two different fragments / ions of the same gas. All gases do not have suitable fragments and for this reason, isotopic peaks were monitored. The results are listed/presented in Tables 7.7 and 7.8.

35

Table 7.7. Repeatability of pure gases Component Parameter to

follow In-day Repeatability (RSD%) n = 6

Day-to-day repeatability (RSD%) 3/day on 4 days

Hydrogen 2/3 Ratio 3.9 5.8 Helium Peak 4

intensity 5.6 7.8

Methane 16 / 15 Ratio 0.7 0.9 Ethane 26 / 28 Ratio 1.8 1.5 Nitrogen 14 / 28 Ratio 1.5 6.6 Oxygen 16 /32 Ratio 4.1 5.9 Argon 36 / 40 Ratio 1.5 2.4 Carbon dioxide

28 / 44 Ratio 4.5 4.1

Gas mixture 4 was measured every day using a direct gas inlet. As the repeatability results of the in-day repeatability versus day-to-day repeatability show, the repeatability of the analysis method was quite good in the in-day measurements. Long term repeatability (i.e., day-to-day) suffers more from the instrument’s instability, the background signals and the reactivity of the gas component. Table 7.8. Repeatability of gas mixture 4. Component Parameter In-day Repeatability

(RSD%)(n=6) Day-to-day Repeatability (RSD%) (n=38)

Hydrogen 2 / 28 Ratio 2.0 7.5 Helium 4 / 28 Ratio 1.8 10.1 Methane 15 / 28 Ratio 2.8 10.6 Oxygen 32 / 28 Ratio 9.7 16.3 Argon 40 / 28 Ratio 2.3 4.6 Carbon dioxide 44 / 28 Ratio 5.5 7.0

7.7 Linearity of the signals

The linearity of the concentration and the peak intensity is one of the most important parameters to be determined. Unknown samples were usually made in a linear range. The linear dynamic range of the method was measured using commercial gas mixtures and mixtures made with the gas calibrator instrument. Figure 7.3 shows gas mixture 4 diluted with pure nitrogen. Seven different dilutions were made and measured with the direct gas inlet MS.

36

0,0E+00

5,0E-11

1,0E-10

1,5E-10

2,0E-10

2,5E-10

3,0E-10

3,5E-10

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

Concentration (%)

Inte

nsity

(A)

Hydrogen Helium Methane Oxygen Argon Carbon Dioxide

Figure 7.3. Gas mixture 4 diluted with nitrogen. If calibration constants are calculated for every dilution, the correlation between intensity and concentration can be clearly seen. The intensities and the concentrations were in very good linear correlations. The linearities of the gas components are shown in Table 7.9 Table 7.9. Linearity R2 of gas components measured from gas mixture 4 in the range 100-5000 ppm. Component Linearity, R2 Hydrogen 0.998 Helium 0.999 Methane 0.999 Ethane 0.999 Nitrogen 0.999 Oxygen 0.987 Argon 0.999 Carbon dioxide

0.999

37

0

0,5

1

1,5

2

2,5

3

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

0,55

0,60

0,65

0,70

0,75

0,80

0,85

0,90

0,95

1,00

1,05

Concentration (%)

Cal

ibra

tion

Con

stan

t

Hydrogen Helium Methane Oxygen Argon Carbon Dioxide

Figure 7.4. Calibration constants as a function of gas composition. As the figure above shows (Figure 7.4), the gas calibrator produced excellent dilutions and the MS response to the gas composition was linear. The linear dynamic ranges are 500ppm 100% for helium, argon and carbon dioxide, and 2000 ppm 100% for methane and oxygen, and 4000ppm 100% for hydrogen. The linear dynamic range is affected by the background signal. If background correction is used the linear range will be much broader. Pure gases were diluted with pure nitrogen using the gas calibrator and the calibration constants were calculated. For example, the peak intensity of ethane as a function of the ethane concentration was linear. The same true also to other gases.

38

0 , 0 E + 0 0

2 , 0 E - 1 0

4 , 0 E - 1 0

6 , 0 E - 1 0

8 , 0 E - 1 0

1 , 0 E - 0 9

1 , 2 E - 0 9

0 1 2 3 4 5 6 7 8 9 1 0C o n s e n t r a t i o n ( % )

Inte

nsity

(A)

Figure 7.5. Peak intensity of ethane as a function of concentration.

39

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

0 5 10 15 20 25 30 35 40 45 50Methane Concentration (%)

Cal

ibra

tion

cons

tant

0,0

0,1

0,1

0,2

0,2

0,3

0,3

0,4

0,4

0,5

0,5

0 2 4 6 8 10 12

Ethane Concentration (%)

Cal

ibra

tion

cons

tant

a) Methane b) Ethane

40

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

0 5 10 15 20 25

Oxygen concentration %

Cal

ibra

tion

cons

tant

0

0 ,2

0 ,4

0 ,6

0 ,8

1

1 ,2

1 ,4

1 ,6

1 ,8

2

0 1 2 3 4 5 6

A rg o n co n cen tra tio n (% )

Cal

ibra

tion

cons

tant

c) Oxygen d) Argon Figure 7.6. Calibration constants for a) methane, b) ethane c) oxygen, d) argon as a function of the component's concentration the mixture.

In small concentrations, the calibration constants increased significantly. The reason for this is that small concentrations are measured directly from the gas mixture (mixture 4) while larger concentrations are made with the gas calibrator. The gas calibrator is not as precise as the gas bottles, but the dilution is still highly linear with increasing gas concentration.

41

8 DIRECT GAS ANALYSIS RESULTS OF GROUNDWATER SAMPLES

The direct gas inlet was tested with several PAVE samples. The samples were analysed during the development process of the method and the used calibration constant values have changed during the measurement process. 8.1 Sample OL-KR22/390-392 m

The sample was measured in connection with the preliminary tests of the instrument. The results are shown in Table 8.1. Other measurement data are reported by F. Lauritsen, MIMS SYSTEMS (14, appendix 2). Two samples were analysed (middle and lower). One sample (upper) was analysed in the laboratory of IPROY (Insinööritoimisto Paavo Ristola Oy). The samples were parallel samples (Table 8.1). Table 8.1. OL-KR22/390-392 m gas composition (%). OL-KR22/ 390-392 m

Lower vessel (%) Middle vessel (%) Upper Vessel (%) (IPROY)

Helium 2.7 1.3 1.4 Argon 2.6 0.7 0.81 Nitrogen 85 93 86.9 Oxygen <1 <1 1.2 Carbon dioxide

3.5 1.9 4.1

Methane 3.9 2.7 4.9 The measurements were made during preliminary testing and the calibration constants were determined at only one concentration, and still the results are quite good. 8.2 Sample OL-KR2/876-1050 m

The sample from OL-KR2/876-1050 m was measured with MCD-macro (Multiple Concentration Detection) and a mass spectrum was also measured and interpreted. The calibration constants used in MCD-measurements were adjusted with gas mixture 1 (Appendix 1). The determination of calibration constants is based on one measurement. The calibration constants were tested and approved by measuring gas mixture 1. The results for OL-KR2/876-1050 m are shown in Tables 8.2 and 8.3.

42

Table 8.2. Result for sample OL-KR2/876-1050 m measured with MCD-macro. OL-KR2/ 876-1050 m

Gas composition of calibration gas (%)

MCD-macro calibration constants

Peak intensity (A)

Corrected intensity (A)

Results Corrected results

Nitrogen,(28 m/z) 95 1 5.78E-09 5.78E-09 37 % 10.48 % Oxygen, (32 m/z) 1 0.7883 0 0 0 0.00 % Hydrogen , (2 m/z) 1 1.323 8.43E-10 1.12E-09 4 % 2.02 % Carbon dioxide, (44 m/z)

1 1.9924 2.78E-12 5.53E-12 100ppm 0.01 %

Carbon monoxide, (12 m/z)

1 0.2238 5.31E-10 -2.12E-11 8 % 0.00 %

Methane, (15 m/z) 1 2.445 1.98E-08 4.83E-08 51 % 87.49 %

The intensity of peak m/z 12 (carbon monoxide) was partly due to the fragmentation of methane. This phenomenon was eliminated by calculating the effect of methane fragmentation and subtracting this effect from the peak intensity. The oxygen peak was missing because the mass calibration of the instrument and the MCD-macro did not recognise this peak. The MCD-macro has one crucial limitation. The spectral data cannot be seen and recorded with the measurement. Only preselected peak data are recorded and used in calculations. In a spectral analysis, the calibration constants are average values of several analyses. The calibration constant of ethane was unknown and therefore the value 1 was used. The calibration constants were calculated from the results of several analyses. The relative standard deviations were calculated for these analyses. The effect of the uncertainty of the gas analysis results is shown in Table 8.3. The errors in the determination of calibration constants are converted into concentration. The MCD-macro results and the results calculated from the spectral data were analysed for the same sample. The spectral data measurements were made straight after the MCD-macro measurements.

43

Table 8.3. Result for sample OL-KR2/876-1050 m analysed from spectral data. OL-KR22/ 390-392 m

Intensity (A)

Calibration constant

Normalized intensity (A)

Gas Composition(%)

Calibration constants RSD (%)

RSD converted into concentration percent (%)

Nitrogen 5.17E-09 1 5.17E-09 29.94 - - Oxygen 2.70E-12 0.843 3.20E-12 0.02 14.8 ± 0.003 Hydrogen 1.21E-09 1.77 6.84E-10 3.96 32.5 ± 1.3 Carbon dioxide 5.63E-12 0.992 5.68E-12 0.03 8.6 ± 0.003

Methane 1.42E-08 1.299 1.09E-08 63.31 14.5 ± 9.2 Argon 1.74E-11 1.055 1.65E-11 0.10 18.9 ± 0.02 Helium 9.21E-11 0.2567 3.59E-10 2.08 21.5 ± 0.45 Ethane 9.73E-11 1 9.73E-11 0.56 The quite high oxygen concentration was probably due to small leaks in the capillary connections. When comparing the results of two different types of analysis, significant differences can be seen. When using the MCD-macro, a couple of facts have to be known.

- The MCD-macro measures only preselected peaks/gases and the spectral data cannot be seen or measured.

- It is crucially important that the calibration constant is measured properly because the calibration constants have a direct effect on the gas composition.

- The mass range calibration also has to be precise. If the mass range calibration is not precise, the macro cannot measure peak intensities at the top of the peak.

For comparison, two PAVE-samples were analysed by IPROY (Insinööritoimisto Paavo Ristola Oy). The comparison of results is shown in Table 8.4.

44

Table 8.4. The comparison of results for the mass spectrometric method and the gas chromatographic method used by the laboratory of IPROY. OL-KR22/ 390-392 m

Sample 1 (%) (IPROY)

Sample 2 (%) (IPROY)

MCD-macro result (%)

Results for spectral data (%)

Nitrogen 12.04 22.65 10.48 29.94 Oxygen 0.002 0.003 0* 0.02 Hydrogen 2.56 2.21 2.2 3.96 Carbon Dioxide 0.05 0.02 0.01 0.03 Methane 81.54 72.05 87.49 63.31 Argon 0.26 0.07 - 0.10 Helium 2.50 2.06 - 2.08 Ethane 1.04 0.94 - 0.56 * = MCD-macro did not find the peak. The results show quite good correlation and, as a whole, the accuracy is good. The accuracy could be improved by measuring the calibration constants more precisely. The MCD-macro should not be used in gas analyses, because the calibration constants determined with MCD-macro are not as precise as the calibration constants determined from spectral data. Also, when using MCD-macro mass scale calibration has to be precise and adjusted regularly. 8.3 Sample OL-KR6/422-425 m

Vessel Size: Small Sample size: 68 g Pressure in sampler during sampling Pressure in sampler before sampling: -960 mbar (underpressure) Pressure in sampler before sampling to pressure vessel: -910 mbar Pressure when sample is in sampler: -900 mbar Pressure equipment is calibrated so that the normal athosphere pressure is 0 bar. The sample was measured twice. The interval between these two measurements was 10-15 minutes. The first measurement was made just after the pressure reached its maximum (-910 mbar). The connection between the PAVE-sample and the sampler was open during the measurements. The second measurements were done immediately after the first measurements were completed.

45

Table 8.5. Comparison of results of two sequential mass spectrometric measurements on the same sample. OL-KR6/ 422-425 m

Measurement 1 (A)

Measurement 2 (A)

Calibration Constant

Measurement 1 (%)

Measurement 2 (%)

Hydrogen (2) 6.29E-11 8.09E-11 1.77 0.13 0.48 Helium (4) 6.54E-11 8.92E-11 0.2667 0.91 3.51 Methane (15) 1.35E-09 2.05E-09 1.299 3.84 16.57 Nitrogen (28) 2.55E-08 7.40E-09 1 94.47 77.76 Ethane (30) 1.88E-11 1.07E-11 1.299 0.05 0.09 Oxygen (32) 1.86E-11 1.22E-11 0.84 0.08 0.15 Argon (40) 7.37E-11 8.12E-11 1.055 0.26 0.81 Carbon dioxide (44) 6.63E-11 5.94E-11 0.992 0.25 0.63

The results of the two measurements differ significantly. One reason for this is that gases are liberated from water at different rates. It can take time before the equilibrium state is reached. Table 8.6. Comparison of results of the mass spectrometric method and the gas chromatographic method used by the laboratory of IPROY. OL-KR6/ 422-425 m

Measurement 1 (%)

Measurement 2 (%)

Upper Vessel High-N2 (IPROY)

Lower vessel Low-Ar (IPROY)

Argon 0.3 0.8 1.1 1.6 Helium 0.9 3.5 6.2 5.4 Nitrogen 94.5 77.8 67.0 67.6 Carbon dioxide

0.3 0.6 0.2 0.2

Methane 3.8 16.6 25.0 22.6 Oxygen 0.1 0.2 0.3 2.5 Hydrogen 0.1 0.5 - - Ethane 0.1 0.1 0.1 0.1

46

8.4 Sample OL-KR6/135-137 m

Sample taken on: 14.06.2004 Sample measured on: 15.06.2004 Vessel Size: Small Vessel weight: 1540g Vessel weight before sampling: 1636g Sample size: 96 g Pressure in sampler during sampling Pressure in sampler before sampling: -970 mbar Pressure in sampler before sampling to pressure vessel: -940 mbar Pressure when sample is in sampler (after 30 min): -900 mbar The sample was measured twice. The interval between the first (measurement 1) and the second (measurement 2) measurement was around 10-15 min. The first measurements were made when pressure had achieved its maximum (-900 mbar). The results for the OL-KR6/135-137 m PAVE-sample analysis are shown in Table 8.7 and the comparison with the results of IPROY (Insinööritoimisto Paavo Ristola Oy) are shown in Table 8.8. Table 8.7. OL-KR6/135-137 m PAVE-sample analysis results. OL-KR6/ 135-137 m

Measurement 1 (A)

Measurement 2(A)

Calibration constants

Measurement 1 (%)

Measurement 2(%)

Hydrogen 1.973E-10 2.12E-10 1.770 0.33 0.36 Helium 4.257E-11 3.38E-11 0.267 0.47 0.38 Methane 5.768E-11 4.91E-11 1.299 0.13 0.11 Nitrogen 3.199E-08 3.11E-08 1.000 93.42 94.07 Oxygen 6.868E-11 7.85E-11 0.840 0.24 0.28 Argon 5.021E-10 4.31E-10 1.055 1.39 1.24 Carbon dioxide 1.339E-09 1.14E-09 0.992 3.94 3.47 Ethane 3.778E-11 3.42E-11 1.299 0.08 0.08 Table 8.8. Comparison of results of the mass spectrometric method and the gas chromatographic method used by the laboratory of IPROY. OL-KR6/ 135-137 m

Measurement 1 (%)

Measurement 2 (%)

Upper vessel/high-N2 (%) (IPROY)

Lower vessel /low-Ar (%) (IPROY)

Hydrogen 0.33 0.36 Helium 0.47 0.38 0.37 0.32 Methane 0.13 0.11 0.09 0.11 Nitrogen 93.42 94.07 93.20 92.20 Oxygen 0.24 0.28 1.12 0.22 Argon 1.39 1.24 1.49 2.54 Carbon dioxide

3.94 3.47 3.73 4.61

Ethane 0.08 0.08

47

The results of these parallel samples are all of same magnitude and the deviation between all three samples is small. The mass spectrometric results are close to the results of IPROY `s gas chromatographic method. 8.5 Sample OL-KR6/125-130 m

Sample measured on: 30.07.2004 Vessel Size: Small Vessel weight: 1498g Vessel weight before sampling: 1592g Sample size: 94 g Pressure in sampler during sampling Pressure in sampler before sampling: -880 mbar Pressure in sampler before sampling to pressure vessel: -850 mbar Pressure when sample is in sampler (20 min): -850 mbar The sample was measured twice from same sample vessel. The interval between the first (measurement 1) and the second (measurement 2) measurement was around 10-15 min. The first measurement was made when pressure had achieved its maximum (-850 mbar). The results are not background corrected. Table 8.9. OL-KR6/125-130 m PAVE-sample analysis results. OL.KR6/ 125-130 m

Measurement 1 (A)

Measurement 2 (A)

Calibration constants

Measurement 1 (%)

Measurement 2 (%)

Hydrogen 1.97E-10 1.60E-10 1.77 0.52 0.34 Helium 2.24E-11 2.42E-11 0.2667 0.40 0.34 Methane 1.37E-10 1.44E-10 1.299 0.49 0.41 Nitrogen 1.90E-08 2.45E-08 1 89.5 91.7 Oxygen 4.97E-11 8.39E-11 0.84 0.28 0.37 Argon 2.76E-10 2.98E-10 1.055 1.23 1.06 Carbon Dioxide 1.58E-09 1.53E-09 0.992 7.49 5.74 Ethane 2.23E-11 2.58E-11 1.229 0.081 0.074 The comparison of the results of the mass spectrometric method and the gas chromatographic method used by the laboratory of IPROY are shown in Table 8.10. As can be seen in Table 8.10, the measurements show is good correlation and the differences in hydrogen are due to background gas, and ethane was measured from peak 30.

48

Table 8.10. Comparison of results of the mass spectrometric method and the gas chromatographic method used by the laboratory of IPROY. OL.KR6/ 125-130 m

Measurement 1 (%)

Measurement 2 (%)

Upper-N2/high (%) (IPROY)

Lower-Ar/low (%) (IPROY)

Hydrogen 0.52 0.34 0.00 0.00 Helium 0.40 0.34 0.16 0.15 Methane 0.49 0.41 0.12 0.20 Nitrogen 89.5 91.7 91.2 91.0 Oxygen 0.28 0.37 0.10 0.41 Argon 1.23 1.06 1.57 2.24 Carbon Dioxide

7.49 5.74 6.86 5.95

Ethane 0.081 0.074 0.000 0.003 The results of the three samples show very good correlation. Ethane was measured from peak 30. 8.6 Sample OL-KR6/98-100 m

Sample taken on: 06.09.2004 Sample measured on: 13.09.2004 Vessel Size: Small Vessel weight: 1550g Vessel weight before sampling: 1627g Sample size: 77 g Pressure in sampler during sampling Pressure in sampler before sampling: -930 mbar Pressure in sampler before sampling to pressure vessel: -670 mbar Pressure when sample is in sampler (40 min): -820 mbar The sample vessel was measured three times. The interval between the first (measurement 1) and the third (measurement 3) measurement was around 40 min. Water raised to the sampler pressure tank.

49

Table 8.11. Comparison of results of the mass spectrometric method and the gas chromatographic method used by the laboratory of IPROY.

OL-KR6/ 98-100 m

Measurement 1 (%)

Measurement 2 (%)

Measurement 3 (%)

Middle vessel N2/high (IPROY) (%)

Lower vessel Ar/low (IPROY)(%)

Hydrogen 0.10 0.12 0.13 0 0 Helium 0.19 0.12 0.12 0.16 0.14 Methane 0.08 0.07 0.07 0.12 0.20 Nitrogen 94.09 93.70 93.63 90.50 87.50 Oxygen 0.22 0.21 0.22 0.10 0.39 Argon 0.67 0.72 0.74 1.60 2.20 Carbon dioxide

4.59 4.98 5.01 6.80 8.20

Ethane 0.07 0.07 0.08 0 0 The three measurements of the sample were quite repeatable. The differences found in the gas composition during the measurements appear at small gas concentrations. In comparison with the results of IPROY, the accuracy of the analysis is good. 8.7 Summary of PAVE sample analyses

The results of the PAVE-sample analyses are good. The results obtained with the first samples analysed with the direct gas inlet were of the same magnitude as IPROY's results. The biggest differences in the results were in the MCD-measurements. The MCD-macro is difficult to use and sensitive to disruptions such as small mass scale errors. The development of the method continued later, and several issues were noticed. The ethane analysed from peak 30 was problematic. Peak 30 could be also NO+. Ethane is now determined from peak 26 and the calibration constants are calculated with it. The high hydrogen concentrations are due to background signals. The accuracy will be improved if background correction is used. First, nitrogen gas is measured. The samples are after that measured. The sample concentration = measured sample values – nitrogen amount. A gas analysis with a direct gas inlet should be performed without the MCD-macro. Spectral data provides more useful data and the accuracy of the analysis is better. The calibration constants are more accurate and determined over a large concentration range. The direct gas inlet analysis with spectral data is accurate, the measurement procedure is fast and easy to use.

51

9 MEMBRANE INLET TESTS

Membrane inlet mass spectrometry is quite a different method versus direct gas inlet mass spectrometry. The testing procedure in membrane inlet mass spectrometry was challenging. When using the membrane inlet system for measurements, several issues have to be known. The properties of the membrane depend on several factors. The flux through the membrane changes with temperature, area, thickness, material, sample pressure, and mass spectrometer pressure. The permeability properties of the membrane have to be determined for all gas components and all membrane materials, thickness, temperatures and sample pressures. 9.1 Measurement parameters in membrane measurements

The membrane inlet was tested with two different membrane materials. These were a Silicone membrane, thickness 250 µm, (Sil Tec 500–3, Technical Products inc., USA) and a Teflon membrane (Radiometer, Denmark). The flow through the membrane inlet allowed water to flow by the membrane surface and the sample flow was controlled by a peristaltic pump. The sample flow was 1-1.5 ml/min (15 rpm). The pressure in the sample was atmospheric pressure. The MIMS measurements were made using the Channeltron detector. Spectral data were recorded with several amplification ranges from 10-6 to 10-12A. The membrane was at an elevated temperature, 100°C in the gas analysis and 30°C when the water samples were measured. The scanning speed in the mass spectrometric measurements was kept the same in all the measurements (1s /amu). The reason for the low scan speed is that intensity data depends on the scanning speed and at a higher scan speed weak intensities are not detected properly. Dissolved gases are best determined when the water is pumped through the inlet. If suction is used, a risk of bubble formation in front of the membrane exists, with highly unstable signals. The problem increases with higher inlet temperatures. The used membrane temperatures comply with the recommendations of the instrument manufacturer/supplier. The instrument parameters are as follows: Detector: CH-TRON Mass mode: SCAN-F SEM Voltage 1200 (Detector voltage) First mass 0 Speed 1 s/amu (amu= Atomic mass unit) Width 50 (Spectral width) Resolution 30 Amp. Mode FIX (Amplification mode) Amp. Range E-6 E-12 (Amplification range) Offset ON (Detector OFFSET-correction) Sample flow 0.75-1 ml/min (15 rpm)

52

Membrane temperature 30 °C (aqueous) 100 °C (gas) 9.2 Effect of temperature on the properties of a silicone membrane.

Temperature affects the permeability of the silicone membrane (Sil Tec 500–3, Technical Products inc., USA) and further the pressure in the mass spectrometer. The samples were moisturized gas samples. The peak intensities were also measured as a function of membrane temperature. The temperature was controlled with a thermostated cover cap. The sample gas (air) was pumped through water in a washing bottle and further to the membrane inlet. A peristaltic pump controlled the sample flow, which was 10 ml/min. Spectral data were measured after pressure and temperature had stabilised The permeability of the membrane achieved the maximum at a temperature of 70°C. Figure 9.1 shows the mass spectrometric pressure as a function of membrane temperature. Figure 9.2 shows the peak intensities as a function of membrane temperature. At the same temperature, the intensities of the peaks were at the maximum level. Figure 9.1 indicates clearly that the permeability properties of the membrane are dependent on the temperature used. Peak intensities changed and the intensity rations between the gas components changed. The biggest changes occurred with water intensity. This means that when membranes are used in the inlet systems, the temperature should be stable.

0,E+001,E-062,E-063,E-064,E-065,E-066,E-067,E-068,E-069,E-061,E-05

20 40 60 80 100 120 140

Membrane temperature °C

Pres

sure

(mba

r)

Figure 9.1. Effect of silicone membrane temperature on mass spectrometer pressure.

53

1,0E-12

1,0E-11

1,0E-10

1,0E-09

1,0E-08

1,0E-0730 40 50 60 70 80 90 100 110 120 130

Temperature (°C)

Inte

nsity

(A)

Hydrogen (2) Water(18) Nitrogen (28) Oxygen (32) Argon(40) Carbon dioxide (44)

Figure 9.2. Effect of silicone membrane temperature on peak intensities. Attention! The argon measurements was not done the whole temperature area. 9.3 Linearity of signals measured with membrane inlet and gas samples

These results are based on the pretests. All linearity, repeatability and detection limits experiments will be done in yaer 2006. The linear dynamic ranges were determined with MIMS using a silicone and Teflon membrane. The samples were made from pure gases or by diluting gas mixture 4. The dilutions were made with a gas calibrator and the diluent gas was nitrogen (99.999%). Eight different gas dilutions were analysed. The concentration range, which was tested with gas mixture 4, is shown in Figure 9.3. The linearities of the membranes were tested and the results are shown in Table 9.1 below. Both membranes displayed good linearity.

54

1,0E-14

2,0E-10

4,0E-10

6,0E-10

8,0E-10

1,0E-09

1,2E-09

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1

Gas concentration (%)

Inte

nsity

(A)

Hydrogen Helium Methane Oxygen Argon Carbon dioxide

Figure 9.3. Gas mixture 4 diluted with a gas calibrator and measured with a silicone membrane. Table 9.1. Linearity of a silicone membrane at small gas concentrations from 25 ppm to 1 %. Component Silicon, Linearity R2 Hydrogen 0.9983 Helium 0.9995 Methane 0.9997 Oxygen 0.9982 Argon 0.9984 Carbon dioxide 0.9997 The detection limits (ppm) of the gas components measured with the silicone membrane were as follows: Hydrogen: < 100-200 ppm (Background signal disturbs detection under <200 ppm) Helium: < 25 ppm, tested Methane: 200 ppm (Strong overlapping with peaks 14 and 16 m/z) Oxygen: < 100 ppm (Background is around 100- 200 ppm) Argon: < 25 ppm, tested Carbon dioxide: < 25 ppm, tested The detection limits were influenced by the background signals of the instrument.

55

Table 9.2. Repeatability of gas mixture 4 measured with a silicone membrane. Gas mixture 4 , (n =6) RSD (%) Hydrogen 5.6 Helium 9.4 Methane 3.1 Water 17 Nitrogen 3.8 Oxygen 2.1 Argon 2.7 Carbon Dioxide 7.3

0,0E+00

1,0E-10

2,0E-10

3,0E-10

4,0E-10

5,0E-10

6,0E-10

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1,00

Concentration (%)

Inte

nsity

(A)

Hydrogen Helium Methane Oxygen Argon Carbon Dioxide Figure 9.4. Gas mixture 4 diluted with a gas calibrator and measured with a Teflon membrane. Table 9.3. Linearity of Teflon membrane at small gas concentrations from 100 ppm to 1%. Component Teflon, Linearity R2 Hydrogen 0.9998 Helium 0.9998 Methane 0.9993 Oxygen 0.9872 Argon 0.9994 Carbon Dioxide 0.9971

56

The detection limits of the gas components when measuring with a Teflon membrane are as follows: Hydrogen: < 100-200 ppm (Background signal disturbs detection under <200 ppm) Helium: < 100 ppm, tested Methane: 500 ppm (Strong overlapping between peaks 14 and 16 m/z) Oxygen: < 100-200 ppm (Background is around 100-200 ppm) Argon: < 100 ppm, tested Carbon dioxide: < 100 ppm, tested The smallest dilution used was around 1/100, i.e. concentrations were around 100 ppm. The detection limits were influenced by the background signals of the instrument. Water is not noticed in these experiments. Table 9.4. Repeatability of gas mixture 4 measured with a Teflon membrane. Gas mixture 4, (n =6) RSD (%) Hydrogen 2.2 Helium 2.8 Methane 2.9 Nitrogen 5.5 Oxygen 19 Argon 1.6 Carbon Dioxide 1.0

9.4 Gas components analysed directly from water with a membrane inlet

9.4.1 Calibration constants with a silicon membrane

There are several ways to describe the solubility of a gas in water. Usually the constant kH in Henry's Law is defined as gaH pCk = Where Ca is the concentration of the species in the aqueous phase and pg is the partial pressure of the species in the gas phase. For the determination of calibration constants, water samples of a known gas composition are required. The determination of these calibration constants is based on the use of water samples bubbled with a known gas composition. The gas concentrations produced by bubbling have been defined mathematically using Henry's Law (15). For bubbling, the gases under analysis have been used, diluted with nitrogen. The observations for the nitrogen used as the dilutant gas were derived from observations made in helium and hydrogen measurements.

57

The sensitivity of the gases under analysis was defined based on the presumption that the intensity determined in the MIMS measurement is directly proportional to the concentration of the gas in water. A straight line was drawn through the observation points (intensity on y axis and concentration on x axis). The angular coefficient of the line then corresponds to the sensitivity of the gas. Gas i calibration constant Ki was calculated by dividing sensitivity Si by the sensivity of nitrogen SN. An example of the calculation for nitrogen and hydrogen is shown below. The calibration constants of all gases are listed at the end. NITROGEN Intensity IN Concentration CN Sensitivity SN = IN /CN

0,0000 0,0001 0,0002 0,0003 0,0004 0,0005 0,0006 0,0007

2,00E-010

4,00E-010

6,00E-010

8,00E-010

1,00E-009

1,20E-009

1,40E-009

1,60E-009

1,80E-009

Observation Linear fit

Inte

nsity

(A)

Concentration (mol/l)

Figure 9.5. The intensity of nitrogen as a function of concentration. Linear regression: Y = A + BX, where Y corresponds to IN, X corresponds to CN, B corresponds to SN and A corresponds to background signal.

58

Table 9.5. Data on linear regression for nitrogen

Parameter Value Error A 2,12323E-10 8,49477E-11 B 1,82923E-6 1,8603E-7

HYDROGEN Intensity IH Concentration CH Sensitivity SH = IH /CH

0,0000 0,0001 0,0002 0,0003 0,0004 0,0005 0,0006 0,0007 0,0008

1,00E-010

2,00E-010

3,00E-010

4,00E-010

5,00E-010

6,00E-010

7,00E-010

8,00E-010

9,00E-010

Observation Linear fit

Inte

nsity

(A)


Figure 9.6. The intensity of hydrogen as a function of concentration. Linear regression: Y = A + BX, where Y corresponds to IH, X corresponds to CH, B corresponds to SH and A corresponds to background signal. Table 9.6. Data on linear regression for hydrogen

Parameter Value Error A 1,40867E-10 2,3026E-11 B 1,04929E-6 6,37287E-8

59

The calibration constant of hydrogen KH = SH /SN = 1,04929E-6 / 1,82923E-6 = 0,57 Table 9.7 shows the calibration constants of different gases measured with a silicon membrane inlet MS. Table 9.7. Sensitivities and calibration constants of different gases Gas Sensitivity (10-7A / mol/l) Calibration constant Hydrogen 10.5 ± 0.7 0.57 Helium 2.6 ± 0.2 0.14 Methane 14.9 ± 0.6 0.82 Ethane 4.9 ± 0.4 0.27 Nitrogen 18 ± 2 1.00 Oxygen 8.4 ± 0.6 0.46 Argon 21.2 ± 0.1 1.16 Carbon dioxide 8.6 ± 0.4 0.47 9.4.2 Production of water samples with a known gas composition

Vacuum distilled water was used to create water samples with a known gas composition. The procedure was as follows:

- Distil water with a rotavapor vacuum distiller (Heindolph Laborota 4000)

- Transfer water to the washing bottle. - Connect the gas bottle to the washing bottle. - Connect the intake pipe of the peristaltic pump to the

washing bottle. The pipe has to be at the bottom of the bottle.

- Start the gas flow. - Start pumping with the peristaltic pump (15 rpm). - Start measurement with the MID-macro. This is set to

measure bubbling gas. - When the equilibrium states have been reached,

concentrations in water should not change. This takes approx. 20-45 minutes.

- Stop the MID-measurement and save the data. - Measure the spectral data with several amplification

values.

The intake pipe should be set so that the bubble flow does not get to the membrane. Bubble formation is a problem when producing saturated water samples. Bubbles on

60

the membrane surface will disturb mass spectrometric measurement by increasing the mass spectrometric pressure. The gas composition in the water samples can be calculated as shown in chapter 9.4.2.

9.4.3 Linearity of signal intensities versus gas concentrations analysed directly

from water

The water samples were produced by the method described in the previous chapter. The gas mixtures were made with a gas calibrator and the dilution gas was nitrogen. The water was vacuum distilled. The bubbling process was followed by MID-macro (Multiple Ion Detection) and after the signals had stabilized and an equilibrium state reached the spectral data were measured. Pure gas mixture 4 and several dilutions were measured directly from water. The analyses of the water sample gave interesting results. The oxygen level in the samples was so low that the background disturbed the analysis. The linearity of higher gas concentrations in water has been tested by making the water samples with pure gases diluted with nitrogen. The linearity values of the used gas components are shown in Figures 9.5 and 9.6. The gases displayed quite good linearity. The biggest challenge is to produce the samples with a washing bottle and gas bottles. However, the reaching of an equilibrium state was followed by MID-macro.

0,0E+00

5,0E-12

1,0E-11

1,5E-11

2,0E-11

0,0E+00 2,0E-06 4,0E-06 6,0E-06 8,0E-06 1,0E-05 1,2E-05 1,4E-05

mol/l

Inte

nsity

(A)

Hydrogen Helium Methane Argon

a) Hydrogen, helium, methane and argon

61

0,0E+00

2,0E-11

4,0E-11

6,0E-11

8,0E-11

1,0E-10

1,2E-10

0,0E+00 5,0E-05 1,0E-04 1,5E-04 2,0E-04 2,5E-04 3,0E-04mol/l

Inte

nsity

(A)

b) Carbon dioxide

0,0E+002,0E-114,0E-116,0E-118,0E-111,0E-101,2E-101,4E-101,6E-101,8E-102,0E-10

0,E+00 2,E-06 4,E-06 6,E-06 8,E-06 1,E-05 1,E-05

mol/l

Inte

nsite

etti

(A)

c) Oxygen Figure 9.7. a) Hydrogen, helium, methane, argon b) carbon dioxide c) oxygen from gas mixture 4 diluted with pure nitrogen and analysed directly from water with a Teflon membrane.

62

0,E+00

2,E-10

4,E-10

6,E-10

8,E-10

1,E-09

1,E-09

0,0E+00 4,0E-04 8,0E-04 1,2E-03mol/l

Int.

(A)

0,0E+00

2,0E-11

4,0E-11

6,0E-11

8,0E-11

1,0E-10

1,2E-10

1,4E-10

1,6E-10

0,0E+00 5,0E-04 1,0E-03 1,5E-03 2,0E-03mol/l

Int.

(A)

A) Oxygen B) Ethane

63

0,0E+001,0E-112,0E-113,0E-114,0E-115,0E-116,0E-117,0E-118,0E-119,0E-11

0,0E+00 1,0E-04 2,0E-04 3,0E-04 4,0E-04mol/l

Int.

(A)

0,0E+00

2,0E-09

4,0E-09

6,0E-09

8,0E-09

1,0E-08

1,2E-08

0,0E+00

5,0E-03

1,0E-02

1,5E-02

2,0E-02

2,5E-02

3,0E-02

mol/l

Int.

(A)

c) Helium d) Carbon Dioxide

64

0,0E+00

1,0E-10

2,0E-10

3,0E-10

4,0E-10

5,0E-10

6,0E-10

7,0E-10

8,0E-10

0,0E+00

2,0E-04 4,0E-04 6,0E-04 8,0E-04

mol/l

Int.

(A)

0,0E+00

2,0E-10

4,0E-10

6,0E-10

8,0E-10

1,0E-09

1,2E-09

0,0E+00 5,0E-04 1,0E-03 1,5E-03mol/l

Int.

(A)

e) Hydrogen f) Argon Figure 9.8. a) Oxygen b) ethane c) helium d) carbon dioxide e) hydrogen and f) argon diluted with pure nitrogen and analysed directly from water with a Teflon membrane.

65

0,00E+00

5,00E-10

1,00E-09

1,50E-09

2,00E-09

2,50E-09

3,00E-09

0,0E+00 5,0E-04 1,0E-03 1,5E-03

Concenteration (mol/l)

Inte

nsity

(A)

0,0E+00

1,0E-10

2,0E-10

3,0E-10

4,0E-10

5,0E-10

6,0E-10

7,0E-10

8,0E-10

9,0E-10

1,0E-09

0,0E+00 2,0E-04 4,0E-04 6,0E-04 8,0E-04


Inte

nsity

(A)

a) Argon b) Hydrogen

66

0,0E+00

2,0E-11

4,0E-11

6,0E-11

8,0E-11

1,0E-10

1,2E-10

0,0E+00 1,0E-04 2,0E-04 3,0E-04 4,0E-04


Inte

nsity

(A)

0,0E+00

5,0E-10

1,0E-09

1,5E-09

2,0E-09

2,5E-09

0,0E+00 5,0E-04 1,0E-03 1,5E-03Concentration (A)

Inte

nsity

(mol

/l)

c) Helium d) Methane

67

0,0E+00

2,0E-10

4,0E-10

6,0E-10

8,0E-10

1,0E-09

1,2E-09

0,0E+00 5,0E-04 1,0E-03 1,5E-03 2,0E-03


Inte

nsity

(A)

0,0E+00

2,0E-10

4,0E-10

6,0E-10

8,0E-10

1,0E-09

1,2E-09

0,0E+00 5,0E-04 1,0E-03 1,5E-03


Inte

nsity

(A)

e) Ethane f) Oxygen

0,0E+00

5,0E-09

1,0E-08

1,5E-08

2,0E-08

2,5E-08

3,0E-08

0 0,01 0,02 0,03 0,04


Inte

nsity

(A)

g) Carbon Dioxide Figure 9.9. a) argon, b) hydrogen, c) helium, d) methane, e) ethane, f) oxygen and carbon dioxide diluted with pure nitrogen and analysed directly from water with a silicone membrane.

68

9.4.4 Repeatability of membrane measurements

The repeatability of the silicone and the Teflon membrane measurements was determined by measuring pure gas from water. The water samples were made with the procedure described in the previous chapter. The measurements were made from 3 parallel samples. The repeatability of measurements of the pure gas components from water is presented in Tables 9.8 and 9.9. Table 9.8. The repeatability of measurements of pure gas components from water with a silicone membrane. Component (n=6) RSD (%) Additional data Hydrogen 11.8 Helium 13.5 Methane 8.6 Nitrogen 13.5 Oxygen 10.4 Argon 9.1 Carbon dioxide

5.1 The pressure reading on the mass spectrometer is almost 10 times higher than in other measurements

Ethane 16.2 Table 9.9. The repeatability of measurements of pure gas components from water with a Teflon membrane. Component (N=6) RSD (%) Additional data Hydrogen 7.3 Helium 8.0 Measurements made after bubbling were

stopped and stabilized for about 10 minutes.Methane 9.4 Nitrogen 6.7 Oxygen 10.8 Argon 8.6 Measurements made after bubbling were

stopped and stabilized for about 10 minutes.Carbon dioxide

3.4 The pressure reading on the mass spectrometer was almost 10 times higher than in other measurements

Ethane 21.3 The repeatability of measurements of pure gases is not as good as it they should be. The quite great deviation between the measurements is due to sample preparation. The effect of sample preparations on repeatibility is tested with the following test. The repeatability of Teflon membrane measurements was determined by making 10 samples of water and allowing them to stabilize with air for 3 days. The samples were measured one after the other. Between the measurements, the membrane is allowed to stabilize for

69

10 minutes before the spectra are measured. The results of the repeatability test are shown in Table 9.10. Table 9.10. Repeatability data for measurements of 10 samples of water. The repeatability is indicated for every component. Component (n=10) Average (A) Standard deviation (A) RSD % Hydrogen 1.01E-11 5.32E-13 5.3 % Water 9.34E-10 2.08E-11 2.2 % Nitrogen 9.89E-10 2.49E-11 2.5 % Oxygen 4.28E-10 1.31E-11 3.1 % Argon 1.80E-11 7.43E-13 4.1 % Carbon dioxide 7.11E-12 9.76E-14 1.4 % As shown by the two tables above, the repeatability of the method was less than 6% (Table 9.10) The main error of the repeatability was sample preparation. Sample preparation increased the errors by about 5-10%.

71

10 SUMMARY

The mass spectrometric method for gas analyses was developed and tested. The parameters, which were determined, included; linearity, detection limits, repeatability, effects of sample pressure and mass spectrometric pressure and effects of sample humidity. The calibration constants were determined. The method was tested with several PAVE-samples and the results were good and precise. The advantages and the weaknesses of the direct gas inlet method are discussed in the next chapter. Chapter 10.2 contains a summary of the membrane measurements.

10.1 The advantages and weaknesses of direct gas inlet mass

spectrometry

The development of the direct gas inlet based method for gas analyses was successful. The method was tested and the results were good. The sampling and the analysing method of PAVE-samples gave good results in comparison with the results of parallel samples analysed by IPROY. A an analytical method, the direct gas inlet displayed several advantages and weaknesses. Advantages:

- The analysing procedure was fast. One analysis takes about 2-3 hours.

- The success of the measurement was easily seen from the spectral data.

- The sensibility much more better than in MIMS - Peak intensity versus concentration was linear and the

calibration constants can be used. - Sample pressure has no effect, the only exception is

hydrogen that shows a minor effect. - Unknown compounds could be identified by means of

spectral data and fragment ions. - The overlapping of the peaks can be corrected

mathematically. - The precision of the method depends mainly on

sampling and on the determination of the calibration constants.

Weaknesses:

- The pressure in the mass spectrometer / the sample flow has to be stable (pressure regulated). Pressure changes during the measurements will have a direct effect on peak intensities and further on the results.

72

Note, that the gas inlet automatically regulates the pressure.

- To calculate the gas composition, several spectra are needed with different detector amplification powers.

- The peak intensity is determined manually from the spectral data.

- The results are calculated manually and normalization is used.

- The gas composition is always 100%, even if unknown gases appear. The reason for this is that the result is calculated with normalized intensities.

- The interpretation of spectral data can be complicated - The method is only suited to gaseous samples. - The direct gas inlet does not tolerate water. - Sample pressure has a minor effect on hydrogen

intensity. - Routine users are not familiar with the interpretation

of spectral data. This will be a problem, if unknown compounds are analysed. However, it is not a serious problem because the identified compounds are known before analysis.

However, the direct gas inlet method is in use and thoroughly tested. A few recommendations for future work are shown below.

- The connection with PAVE-sample should be redesigned. The connection is the weakest link in the sampling system and will easily leak.

- The sampler system should be simplified. Sampling with the sampler is difficult.

- The capillary connection to the mass spectrometer should be properly designed and rebuilt. The volume of the connections is quite large, which will increase sample contamination.

- Sample measurements

10.2 Advantages and weaknesses of membrane inlet mass spectrometry

The membrane inlet mass spectrometric method was proven to be linear over a large concentration range. The repeatability of the method is of the same magnitude as that of the direct gas inlet. The production of the calibration samples was challenging. As an analytical method, the membrane inlet mass spectrometry displayed several advantages and weaknesses. Advantages:

- The sample can be aqueous or gas.

73

- The flow of interfering compounds into the mass spectrometer can be minimized by changing the membrane material.

- Membrane temperature is adjustable. - Signal versus concentration is linear. - The sampling and controlling of the sample flow is

simple.

Weaknesses: - Calibrations have to be carried out with all membrane

materials, thicknesses, temperatures and sample pressures. The permablity of membrane depends on temperature, membrane’s material and tickness, pressure

- Pressure in the mass spectrometer is difficult to control. The changing of membrane area, the sample pressure or the membrane thickness and material are the only means to control the pressure.

- The stabilization time of the membrane measurements is longer than in direct gas inlet measurements.

- In pure gas measurements, the membrane can dry and change the permeability properties.

- Calculation of results is more difficult than in direct inlet MS.

- There is no sampler system for the membrane inlet. The development of the membrane inlet analysis method is incomplete. However, membrane tests have given valuable information of gas and water sample analyses. The testing procedure should be continued. Gas analysis from water was tested to be linear, but a few more linearity tests should be carried out and some old tests repeated.

- The water sample production method should be updated. The production method gives quite a large deviation between parallel samples (over 5%).

- A simple method for calculation of results should be created.

- A connection to the PAVE-sample should be developed. The sampler system should be simple and easy to use. Without a sampler, sample contamination is a problem.

10.3 Comparison of membrane and direct gas inlet methods

When comparing the two different inlet methods in mass spectrometry, big differences can be seen.

74

The large number of advantages offered by the direct gas inlet, are based on the total pressure control of the direct gas inlet. This pressure control increases the repeatability of the method, with samples at many different pressures. The direct gas inlet is a more precise method and the background effect is much smaller than with the membrane inlet. Membrane inlet mass spectrometry is good when measuring gases directly from water. Gas analyses can be made with a membrane inlet but the results are influenced by a high background signal and the instability of pressure in the mass spectrometer. The detection limits for the direct gas inlet and the membrane inlet are of the same magnitude. More tests should be made with the membrane inlet to verify the detection limits and linearity.

Table 10.1. Comparison of detection limits for direct gas inlet and for silicone and teflon membrane inlets. Component Direct gas inlets

Concentration, (mol, ppm)

Silicone membrane (mol, ppm)

Teflon membrane (mol, ppm)

Hydrogen* 250 100-200 100-200 Helium 20 < 25 <100 Methane# 500 200 500 Ethane 50 - - Oxygen* 75 100-200 100-200 Argon 50 <25 <100 Carbon Dioxide 100 25 <100

* Background signal disturbs detection under <200 ppm # Strong overlapping with peaks 14 and 16 m/z

75

REFERENCES

1 Ruotsalainen, P. (toim.), Alhonmäki-Aalonen, S., Aalto, E., Helenius, J. & Sellge, R. 1996. Paineellisten vesinäytteiden ottolaitteiston kehitys. Työraportti PATU-96-82. (In Finnish with an English abstract).

2 Lauritsen, F.R., MIMS 2200P User Manual, MIMS Systems, 2003

3 Partial Pressure measurement in vacuum technology, Pfeiffer Vacuum, Operation instructions, 2003

4 Massey, H.S.W., Burhop, E.H.S., Electronic and Ionic Impact Phenomena, Vol.1, University Press, Oxford, 1969

5 Reed, R.I., Ion Production by Electron Impact, Academic Press, London, 1962

6 Paul, W., Steinwedel, Z., Naturforschung,80, 1953,448

7 Paul, W., Reinhard, H.P., Zahn,U.V., Z. Physik,1958, 143

8 Dawson, P., Quadrupole Mass Spectrometry, Elsevier, 1976, Amsterdam

9 (2001) Software Quadstar 422, Documentation, Pfeiffer Vacuum, BG 805 994 BE (0111)

10 Kotiaho, T., Lauritsen F.R., Comprehensive Analytical Chemistry XXXVII, Elsevier Science B.V., 2002, 531-557

11 Lauritsen, F., Membrane inlet tandem mass spectrometry, Theory and praxis, Odense universitets trykkeri, 1994

12 Lenneman, F., Membrane inlet mass spectrometry for bioreactors, Fortschritt-Berichte VDI, Reihe 8, Nro 802, Hamburg.

13 Kröger, T., Veteen liuenneiden kaasujen analysointi suorasyötöllä varustetulla massaspektrometrillä (only in Finnish), TVONS-instructions (tunnus 108714), 2005.

14 Lauritsen, F., MIMS SYSTEMS, memo number (TU-M-30/03).

15 Sander, F., Complilation of Henry's law constants for inorganic and organic species of potential importance in environmental chemistry, http//mpch-mainz.mpg.de/~sander/res/henry.hmtl.

77

APPENDICES

1 Used gases and gas mixtures 2 Lauritsen, F., MIMS SYSTEMS, memo number (TU-M-30/03

79

APPENDIX 1 Used gases and gas mixtures Table 1. Gas mixtures Mixture H2

% He %

CH4 %

N2 %

O2 %

Ar %

CO2 % Additional data Manufacturer

1 2 - - ground gas 10 - - Special gas AGA 2 1 - - ground gas 1 - - Special gas AGA

3 1 - 1 ground gas 1 - 1 Also carbon monoxide (CO) in mixture

Scott Speciality Gases/ Sigma Aldrich

4 1 1 1 ground gas 1 1 1 Special gas Aga

5 0.05 0.05 0.05 ground gas 0.05 0.05 0.05 Ethane 0.05% in mixture Aga

Table 2 Pure gases PURE GAS

Concentration % Quality Additional data Manufacturer

Hydrogen 99,999 5.0 Vety, Detector AGA Helium 99,996 4.6 Laser helium AGA Methane 99,995 4.5 Linde Gas AG Ethane 99,98 3.8 Ethane, chemical AGA

Oxygen 99,999 5.0 Oxygen, Instrument AGA

Nitrogen 99,999 5.0 Nitrogen, Instrument AGA

Argon 99,999 5.0 Argon, Instrument AGA Carbon dioxide 99,999 5.0 Laser carbon

dioxide AGA

81 APPENDIX 2

method development of gas analysis with mass …kröger, t. & paaso, n. 2006. method development...

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