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Introduction to quantitative gas analysis. Practical aspects of mass-spectrometry (Theory, Manual/Tutorial, Examples)*
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Andrey Tarasov
1. Is intended for senior students and postdocs of natural science faculties of universities and technical institutes closely working with mass-spectrometry as a main tool for gas phase analysis.
2. The target audience is AC FHI PhD students and Postdocs.
3. Implies reader/user to have first experience working with MS as well as to have a notion about ionization and fragmentation process.
4. Points to stress: -general understanding-practical aspects, limitations-examples of relevance to AC FHI research-practical tips, functional faults-precision and accuracy
5. Information, examples of practical application, advices are drawn from author´s personal practical experience and does not cover all possible cases (no GC/MS calibration) as well does not provide complete explanation on. In this cases the reader is referred to other sources from the web or AC library.This manual should also not be considered a panacea for solution of analytical tasks.
Purpose & requirements
Please read the comments.
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 2
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Outline1. General considerations1.1. MS calibration: before you start
2. Theoretical background• 2.1. Linearity of partial pressure and intensity• 2.2. Approaches (Isolated spectra. Overlapping spectra. Internal standard)• 2.3. Calibration models and design
3. Description of the procedure• 3.1. Internal Standard Calibration. Overlapping pattern A. 12CH3OH/13CH3OH.• 3.2. Internal Standard Calibration. Overlapping pattern B. C3H8, C3H6. • 3.3. Internal Standard Calibration. Isolated spectra. Example with Pfeiffer Vacuum software
(CO2/CH4/CO/H2)• 3.4. Preparation for the calibration. Offset. Mass scale. Background.
4. Calibration techniques• 4.1. Calibration of liquids (water)• 4.2. Calibration of TA-MS setup, peak area calibration (decomposition, pulses)• 4.3. Direct calibration of solids
5. Practical examples• 5.1. Operation in BESSY • 5.2. Oxygen Evolution Reaction calibration with O2 pulses
5. Trace gas analysis, calibration PTR-MS 6. Appendix. • 6.A. Isotope distribution and Isotope scrambling.• 6.B. List of books for quantitative mass-spectrometry• 6.C. List of errors and pitfalls
3
Why?- no GC available- isotope effects- time resolution, fast reaction (transient state or switching experiments)
When?- good knowledge of the (gas phase) composition(additional components or reaction products which were not considered upon calibration may falsify the quantitative results)
- the less overlapping components the easier is the calibration and calculation
How?- grate care should be taken for background subtraction- pure calibration substances, - no overlapping peaks within 1 calibration mixture - the calibration components should not chemically react with each other and the experimental environment under calibration conditions
- it is necessary to use all fragments of the pattern, chose intensive well separated peak or a pair of peaks.
- use internal standard- always monitor temperature and pressure (crucial for the analysis)- permanent check background and calibration after measurement or series of
measurements stability of the calibration
1.1.MS calibration: before you start
4Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
1. General considerations
5
- Assess costs and time for the calibration (expenses and delivery time of gas mixtures, additional valves, gas tubeconnections, heaters, pumps, saturator, pressure reducer). Crucial upon planning the isotope exchangeexperiments
- Working with any MS software try to analyze the raw data manually with excel or other program package.Important for verification of results when relying on the software quantitative analysis.
€
- Consider the recommendations of the manufacturer given in the manual
FHI Database record for Pfeiffer Vacuum Manuals: 38760
- Take into account whether the setup is operated as single user or multi user. Knowledge of the chemical andtechnical history may help with identification of contaminants in the background. Consult with the previous users.
- Useful online sources for the properties of main compounds
MS spectra: http://webbook.nist.gov/chemistry/Isotope Distribution of substances: http://www.sisweb.com/mstools/isotope.htmCalculator of vapor-liquid and liquid-liquid phase equilibria: http://vle-calc.com/phase_diagram.htmlPhysical properties of gases and liquids: https://www.fluidat.com/default.asp(Registration required)
% - Try to calculate what concentration of educts is optimal for the experiment or the amount of productswhich has to be expected i.e. asses possible conversion, selectivity and yield. The quantitative data are moreprecise when calibration is performed in the range of concentration close to the gas composition used in theexperiment. Prepare calibration mixtures respectively.
1. General considerations
1.1 MS calibration: before you start
- We strongly recommend making yourself familiar with the handbooks on general and quantitative mass-spectrometry . Original papers are referred in the presentation. Some books might be found in the Appendix.
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
6
Separation of components in the gas phase by mass-to-charge ratio
Detection of ion currents
The method is not inherently quantitative
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
2 Theoretical Background
2.1 Linearity of partial pressure and intensity
7
• Small fraction of sample is converted in ionized state (0.1%)• 20% of all ions are extracted from the ionization chamber• Collision energy influences the number and type of ions (50-150eV)
𝐼𝑖𝑜𝑛
𝑠𝑒𝑐= α
𝑖𝑜𝑛
𝑚𝑜𝑙𝑒𝑐∙ 𝜌
𝑚𝑜𝑙𝑒𝑐
𝑐𝑚2𝑠𝑒𝑐∙ 𝑆𝑒𝑓 𝑐𝑚2
I – full ion current, ion flux reached the collectorα – fraction of the molecules which has been undergone ionization and reached collector i.e.α = ω1·ω2·ω3
ω1- probability that the molecule reaches the ionization zoneω2- probability of the ionization of the molecule in the ionization zoneω3- probability that the formed ion reaches collectorρ – molecules flux through the effective area Sef
I = k ·n ·σ
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Ionization efficiency
𝑝 =𝐼 ∙ 𝑅 ∙ 𝑇
𝑘 ∙ 𝜎
2.1 Linearity of partial pressure and intensity
k – Instrument sensitivity, [ion molec sec-1 cm-2]n – molecules/cm3
σ – ionization cross section, cm2 (Å2)p – partial pressure, Pa
p=nRT
8
14N2+
Multicomponent analysis of Mass-spectra becomes complicated:• spectra overlapping of the single components • Fragmentation due to dissociative ionization• For solids there are several molecular forms in the gas phase
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
𝑝 =𝐼 ∙ 𝑅 ∙ 𝑇
𝑘 ∙ 𝜎
𝑝𝑗 =𝑅
𝑘𝜎𝑖𝑗𝐼𝑖𝑗𝑇 = 𝑘´𝑖𝑗𝐼𝑖𝑗𝑇
𝑝𝑗 =𝑅
𝑘𝜎𝑗
𝑖
𝐼𝑖𝑗𝑇 = 𝑘 �́�
𝑖
𝐼𝑖𝑗𝑇
for one fragmentation ion
for all ions of the molecules formed
2.1 Linearity of partial pressure and intensity
i – ion type and j the is the origin moleculeσij – partial cross section of the molecule j with formation of ion iσj – full molecule j ionization cross sectionIij – ion current i, formed by the ionization of the molecule j
- sum ion current from all ions from the molecule jpj – partial pressure of the molecule j
𝑖
𝐼𝑖𝑗
Example: equilibrated vapor of NaF at 1100 K
At 1100K the solid phase is equilibrated with the gas phase which consists of the molecules NaF and Na2F2
By ionization the following ions are formed:
Then partial pressure of NaF according to (1):
𝑝𝑁𝑎𝐹 = 𝑘´𝜎𝑁𝑎+𝑁𝑎𝐹
∙93𝑇=𝑘´
𝜎𝑁𝑎𝐹+𝑁𝑎𝐹∙2𝑇
𝑝𝑗 =𝑅
𝑘𝜎𝑖𝑗𝐼𝑖𝑗𝑇 = 𝑘´𝑖𝑗𝐼𝑖𝑗𝑇
𝑝𝑗 =𝑅
𝑘𝜎𝑗
𝑖
𝐼𝑖𝑗𝑇 = 𝑘 �́�
𝑖
𝐼𝑖𝑗𝑇
(1)
(2)
Then partial pressure of NaF according to (2):
𝑝𝑁𝑎𝐹 = 𝑘´𝜎𝑁𝑎𝐹
∙95𝑇
9Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Full ionization cross section of molecule j is a sum of partial cross 𝜎𝑖 = 𝜔𝑖 ∙ 𝜎0
𝜎0 = 𝜔𝑖
𝑖
𝜎𝑖 𝑎𝑛𝑑
𝑖
𝜔𝑖 = 1
Where wi is a formation probability of ion i.
Practical difficulties:• Determination of origin molecule (Na2F2 or NaF)• Intensities overlapping e.g. INa+,NaF, INa+,Na2F2, m/z = 23
• Calibration of the instrument, determination of k´j or k´ij
2.1 Linearity of partial pressure and intensity
10
I28 = K[a11P(N2)+a12P(CO)]
I14= K[a21P(N2)+a22P(CO)]
𝐼1
𝐼2
⋮𝐼𝑖
=
𝑎11 … 𝑎1𝑗
𝑎12 … 𝑎2𝑗
⋮𝑎𝑖1
⋮…
⋮𝑎𝑖𝑗
∙
𝑝1
𝑝2
⋮𝑝𝑗
Ii – ion current of the mass MK - instrument constant, related to the setup settingsσij – ionization cross section of component (molecule) j with formationof ion i with mass M aij ~ σij/RT– component of cracking pattern, calibration factorPj – pressure of the j component.
𝑝1
𝑝2
⋮𝑝𝑗
=
𝑎11 … 𝑎1𝑗
𝑎12 … 𝑎2𝑗
⋮𝑎𝑖1
⋮…
⋮𝑎𝑖𝑗
∙
𝐼1
𝐼2
⋮𝐼𝑖
-1
1. Direct calibration. For Isolated spectra
𝑃 𝑗 =𝑡𝑜𝑡𝑎𝑙 𝑖𝑜𝑛 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 (𝐼)
𝐾
𝐾– instrument sensitivity to the specific component, A/mbar** Has to be determined by calibration procedure. For the rough estimation the sensitivity provided by the manufacturer for nitrogen could be used.
2. Direct calibration. For overlapping spectra
Ii = K*Σ σij*nj ~ K*Σ aij*Pj
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
2.2 Calibration Approaches
J.F. O´ Hhanlon, A User´s Guide to Vacuum Technology,, WILEY, 2003 pp. 172-176
𝑅𝑇
𝑘𝜎𝑗
−1
K is Analogues to
For j components and i ions the system of j linear equations could be written as follows:
11
𝑎11 … 𝑎1𝑗
𝑎12 … 𝑎2𝑗
⋮𝑎𝑖1
⋮…
⋮𝑎𝑖𝑗
∙
𝑝1
𝑝2
⋮𝑝𝑗
=
𝐼1
𝐼2
⋮𝐼𝑖
ak + bq = X
ar +bs = Y
𝑘 𝑞𝑟 𝑠
∙𝑎𝑏
=𝑋𝑌
ExperimentalIon currents
Calibrationfactors
Unknownconcentrations
𝑎𝑏
=𝑘 𝑞𝑟 𝑠
∙𝑋𝑌
=1
𝑘𝑠 − 𝑞𝑟∙
𝑠 −𝑞−𝑟 𝑘
∙𝑋𝑌
-1
ks – qr = D, matrix determinant
Adjugate matrixInverse matrix
𝑎 =𝑠𝑥 − 𝑞𝑦
𝑘𝑠 − 𝑞𝑟𝑏 =
𝑘𝑦 − 𝑟𝑥
𝑘𝑠 − 𝑞𝑟𝑏/𝑎 =
𝑘𝑦 − 𝑟𝑥
𝑠𝑥 − 𝑞𝑦
XY
m2/em1/e
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
2.2 Approaches
𝑝1
𝑝2
⋮𝑝𝑗
=
𝑎11 … 𝑎1𝑗
𝑎12 … 𝑎2𝑗
⋮𝑎𝑖1
⋮…
⋮𝑎𝑖𝑗
∙
𝐼1
𝐼2
⋮𝐼𝑖
-1
Calculation example:
12
γij – response coefficient of species i of component j with respect to Ar
IjMi – MS intensities (ion currents i of mass M of component j)
xj – the mole fraction of the component jPtotal – total pressure inside the MS chamber
i28 = γ28CO [CO] Ptotal+ γ28
CO2[CO2]Ptotal
i44= γ44CO2[CO2]Ptotal
𝛾𝑖𝑘 = ൙
𝐼𝑘𝑀𝑖
𝑥𝑘∙𝑃𝑡𝑜𝑡𝑎𝑙
𝐼𝐴𝑟𝑥𝑎𝑟∙𝑃𝑡𝑜𝑡𝑎𝑙
- determined by calibration with the known gas composition
3. Internal standard calibration
For mixture of CO and CO2 with Ar as internal standard.For ideal gas the concentration could be used instead of partial pressure
IMi = PtotalΣ γij*Cj
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
2.2 Approaches
Note: This approach is widely used in continues gas phase analysis and is most useful for catalysis research when measuring catalytic activities versus time on stream . It enables relatively simple calibration with respect to total pressure and Intensity of inert component as Ar, He or N2. Such a normalization keeps the calibration factors constant by changing the setup settings or experimental conditions. This removes possible source of error arising through inefficient and variable inlet conditions. The ratio of compound to internal standard will remain constant even though the inlet conditions may be changed inadvertently. For internal standard the calibration factor is always constant and equals 1. (See examples part 3.2 and 3.3)
Tip: it might be useful to have two inert components in the calibrated gas mixture (Ar and He). One serves as internal standard the other serves as additional unchanged component. It helps to recognize possible drift of massanalizer.
2.3 Calibration design and models
It is noteworthy to outline that the calibration design used for the estimation of traces or contaminantscan be quite different from that used for quantitative analysis. In former case the known concentrations ofthe calibration mixtures must be in range of hypothesized detection limit or possible trace gasconcentration, whereas in the latter the standard calibrating mixtures must embrace the unknownconcentrations. The number of suitably spaced calibration point can range between seven and ten toobtain a reliable model for the calibration line. One point has to be measured several times, at least eightto ten replicates to gain information on about the overall variances of the responses at variousconcentrations.The more popular model is always the straight line independent of scedasticity. Sometimes nonlinearcalibration relationships have to be used, in particular when a wide dynamic range is explored. In this casesimple quadratic functions are completely satisfactory. Depending on the test of scedasticity theunweighted or the weighted regression procedure is followed.
13Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Model for linear regression with constant variance, showing distribution of measurements around the true calibration line
Examples of multipoint calibration without statistics see Section 3.1 and 5.1
Source: I. Lavagnini, F. Magno, R. Seraglia, P. Traldi , Quantitative application of mass spectrometry, WILEY, 2006 pp. 107-133
14
3.1. Internal Standard Calibration. Overlapping patterns. A
Direct calibration, mass matrix of 13CH3OH/12CH3OH mixtures
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
13CH3OH+
12CH3OH+/13CH3O+
12CH3O+/13CH2O+
12CH2O+/13CHO+
1. Effect of different carrier gases
2. Flow calibration3. Effect of diluent gas
(concentration variation study)
4. Background determination5. Concentration determination
of pure methanol6. Concentration determination
of pure methanol and water7. Concentration determination
of mixed 12C, 13C methanol and water
Goal: Clarifying the source of methanol from syn gas: CO or CO2
RWGS
Fixed BedReactor
To GC
To MS
Bypass valve
Saturator with MeOH/H2O
Gas supply system12CO2, 13CO2, CO, H2,
Ar
1. Effect of different carrier gases
2. Flow calibration3. Effect of diluent gas
(concentration variation study)
4. Background determination5. Concentration determination
of pure methanol6. Concentration determination
of pure methanol and water7. Concentration determination
of mixed 12C, 13C methanol and water
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
T-const
Done By E. Kunkes and N. Thomas in ChemCatChem 2015, 7, 1105-1111
15
3.1. Internal Standard Calibration. Overlapping patterns. A
16
y = 8,1950E-16xR² = 9,9839E-01
0
2E-11
4E-11
6E-11
8E-11
1E-10
1,2E-10
0 20000 40000 60000 80000 100000 120000 140000
MS
sign
al (
bac
kgro
un
d
sub
trac
ted
12CH3OH vapor, ppm
m/z = 29
y = 1,5782E-16xR² = 9,8563E-01
0
5E-12
1E-11
1,5E-11
2E-11
2,5E-11
0 50000 100000 150000
MS
sign
al (
bac
kgro
un
d
sub
trac
ted
)
12CH3OH vapor, ppm
m/z = 30
y = 1,4581E-15xR² = 9,9985E-01
0
2E-11
4E-11
6E-11
8E-11
1E-10
1,2E-10
1,4E-10
1,6E-10
1,8E-10
2E-10
0 50000 100000 150000M
S si
gnal
(b
ackg
rou
nd
su
btr
act
ed12CH3OH vapor, ppm
m/z = 31
y = 1,0551E-15xR² = 9,9980E-01
0
2E-11
4E-11
6E-11
8E-11
1E-10
1,2E-10
1,4E-10
0 50000 100000 150000
MS
sign
al (
bac
kgro
un
d s
ub
tra
cted
)
12CH3OH vapor, ppm
m/z = 32
y = 1,7146E-17xR² = 9,9781E-01
0
5E-13
1E-12
1,5E-12
2E-12
2,5E-12
0 50000 100000 150000
MS
sign
al (
bac
kgro
un
d s
ub
tra
cted
m/z = 33
12 CH3OH vapor, ppm
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.1. Internal Standard Calibration. Overlapping patterns. A
0
1E-12
2E-12
3E-12
4E-12
5E-12
6E-12
7E-12
8E-12
9E-12
1E-11
2930
3132
3334
Series1
Series2
13CH3OH12CH3OH
Ion
cu
rre
nt F
rom
MS
Calibration curves were obtained for CO2, CO and Ar
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 17
3.1. Internal Standard Calibration. Overlapping patterns. A
Peak Designation A (CH2O) B (CH3O) C (CH3OH)13C-labeled feed: m/e =
natC feed: m/e =3130
3231
3332
Sens. 13CH3OH (MS signal/ppm) 1.87 × 10-6 9.07 × 10-6 6.32 × 10-6
Sens. 12CH3OH (MS signal/ppm) 1.97 × 10-6 8.77 × 10-6 6.94 × 10-6
% Difference 4.0 % -3.5 % 8.8 %
Sensitivities of the MS signals of selected fragments of MeOH.
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Done By E. Kunkes and N. Thomas in ChemCatChem 2015, 7, 1105-1111
18
3.1. Internal Standard Calibration. Overlapping patterns. A
19
XY
We have 2 species a and b. a = 12C methanol and b = 13C methanol
gax = Contribution of species a(12C methanol) to the peak X }
gby = Contribution of species b(13C methanol) to the peak Y }
Sensitivity factors
X
Y
gbx
gby
=g
ax
gay
a
bX = a* g
ax + b*
gbx Y= a*ga
y + b*gb
y
Mass Matrix Calculation
𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 13𝐶 𝑚𝑒𝑡ℎ𝑎𝑛𝑜𝑙 =𝑏
𝑎 + 𝑏=
gax ∙ 𝑦 −g
ay ∙ 𝑥
𝑦 ∙ gax −g
bx + 𝑥 ∙ (gb
y −gax)
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Done By E. Kunkes and N. Thomas in ChemCatChem 2015, 7, 1105-1111
M/Z
I
3.1. Internal Standard Calibration. Overlapping patterns. A
Step-1 Steady state in 12C Syngas by GC
Step-2 Switch to MS wait for steady signal
Step-3 Depressurize and flush the 12C syngas with Ar
Step-4 Pressurize with 13C Syngas
Step-5 Switch to MS wait for steady signal (50-60min)
Step-6 Switch to GC and take one point
Step-7 Flush the 13C Syngas in Helium
Step-8 Determine 12C Background
Step-9 Determine 13C Background
Strategy
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 20
3.1. Internal Standard Calibration. Overlapping patterns. A
Done By E. Kunkes and N. Thomas in ChemCatChem 2015, 7, 1105-1111
21Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Sample
% of C-13 Methanol
GC-MS , 33 m/e
MS
31 and 33 m/e
MS
32 and 33 m/e
NGM STD 93 97 92
Cu/Al2O3 94 87 87
Cu/MgO 91 88 86
Cu/MgO/Zn 96 94 91
All Experiments performed at 2% total CO2 conversion, as measured by GC
3.1. Internal Standard Calibration. Overlapping patterns. A
3.2. Internal Standard Calibration. Overlapping patterns. B
C3H6
C3H8 C3H8 + 0.5O2 →C3H6 + H2O
C3H8 + 5O2 → 3CO2 + 4H2O
C3H8 + 7/2O2 → 3CO + 4H2O
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 22
Performed By P. Kube
C3H8
C3H6He Internal Standard
Measured Ion currents of the relevant fragments at certain time, IM
Response coefficients of the mass M of the component k , Ii/Cj
Calibration factors of the mass M of the component k with respect to He γji=(Ii/Cj)/(IHe/CHe)
Same procedure performed for calibration of CO and CO2
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 23
3.2. Internal Standard Calibration. Overlapping patterns. B
0,0E+00
1,0E-08
2,0E-08
3,0E-08
4,0E-08
5,0E-08
4 12 13 14 15 16 19 20 22 25 26 27 28 29 30 36 37 38 39 40 41 42 43 44 45
CO2
CO
C3H6
C3H8
He
Performed By P. Kube
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Calibration factors of the fragment i for the component j
Total pressure in MS, mbar. Has to be measured for calculation of concentration from the ion currents or for simulation of MS pattern from the known concentration, Ii=Cj ·γi
j·ptotal
Measured Ion currents of the relevant fragments at certain time during experiment, I i
Calculated concentrations , Ii/(γi
j·ptotal )
24
3.2. Internal Standard Calibration. Overlapping patterns. B
25
Calibration protocol (Pfeiffer Software), CO, CO2, CH4, H2, Ar for dry reforming of methane
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
DRM: CO2 + CH4 ⇌ 2 CO + 2 H2
Boudouard reaction: 2 CO ⇌C + CO2
Methane pyrolysis: CH4 ⇌C + 2H2
Calibration gases used: 1) CH4/CO2/Ar_32/40/28,2) CO/H2/Ar_10/60/30 With Argon as internal standard.
Procedure with the QuadStar software:
1. Tuning of the ion source parameters
2. Offset calibration, Electronic zero point3. Mass scale adjustment
4. Background determination / Zero gas
5. Gas specific calibration with gases of known concentration
6. Control measurement with the known gas composition
Has to be performed once before the measurement session and SHOULD NOT BE CHANGED. See section 3.4
Has to be cheked before every measurement. Accasionaly also after. See section. 3.4
Has to be peformed permanetly with the control gas mixture, See section 3.6
3.3. Internal Standard Calibration. PV Software Example.
Current section
See Pfeiffer Manual
Read me.
The procedure is described in details, however some moments are omitted for the sake of simplicity.
Working with the software one have to use Pfeiffer Vacuum Manuals. Database ID #38760.
Some steps are skipped, one has to apply intuition and feel of the software
26Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
CO2/CH4/Ar
CO/H2/Ar
CO2
CH4
Ar
MFC 2
MFC 3
MFC 4
MFC 1
Reactor Thermobalance
MS
exhaust1 MFC Flow calibration
2 MS Background determination
3 MS calibration with gas mixtures
4 Check calibration , Control measurement with different gas ratios
Calibration gases: 1) CH4/CO2/Ar_32/40/28,2) CO/H2/Ar_10/10/30
Note: In some cases the background correction have less effect on the result. Operation of the experiment with very high concentrations of products and educts. If the catalyst is very active and one runs the experiment near equilibrium conditions.
Done By A. Tarasov and K. Mette in Catal.Today 2015, 242, 101-110
27Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
Edit >> Insert from spectra library
Export
Note: The library consists of the EI spectra of the simple molecules at 60-70eVWhen working with higher concentrations and If there is no requirement of a very precise measurement, it is not necessary to consider all fragments. The most intensive and not overlapping lines are usually used.
Configuration of the calibration matrix
Open any existing file and modify it
28Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
File >> save as
Note: The masses chosen for the calibration should be for one component only. Every component of the gas mixture must have unique masses, thus no overlapping of the masses should be present in the calibration matrix.
29Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
-Chose not overlapping fragments within one gas mixture-m/e 13/12/16 may only be used either for CO2 or CH4 but not for both components.-for calibration with full pattern use separate gas mixtures CO2/Ar and CH4/Ar
-Highlight mass 40 (Ar) and set as internal standart
Note: The calibration of the components which have similar fragmentation patterns and thus causing strong overlapping must be calibrated SEPPARATELY and NOT WITHIN ONE GAS MIXTURE.
30Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
Configuration of calibration parameters
Detector Bookmark:Detector type is channeltron. The voltage applied between the dynodes of the secondary electron multiplier is 1kV.
31Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
Mass bookmark:Dwell time: time the MS stays at peak maximum. For calibration 0.5-1sec dwell time are recommendedResolution: internal value between 20-55. Must be optimized during ion source tuning.
32Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
Amplifier bookmark:Pause-cal: internal parameter representing the time the MS sweeps from one peak maximum to the other. For calibration longer values are recommended.Offset: Electronic zero. Will be subtracted from the measured ion currents. Must be determined before calibration.
33Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
For the sake of simplicity we chose only main, most intense not overlapping peaks.
File >> save as
34Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
„Single“ point calibration implies on the linearity in the whole concentration range
Calibration1. introduce pure carrier gas (Ar). Wait until stabilization (60min). Record background of relevant signals (12/14/15/20/28/40/44)2. introduce the gas mixture and flush the system until all relevant signals stabilize (50-60min)3. monitor the signals in MID mode (see Pfeiffer Vacuum Manual)4. start calibration
Note: The error may occur if the matrix is unmodified e.g. one mass ascribed to several components or there is a mass which does not have any ascription to the component. The calibration fails if the library file with calibration factors is full. It is limited to 24 components. In that case substitute the file Q_CALFAC.LIB with the one from the backup.
5. When calibration is accomplished the intensity and calib. factor columns will be filled out.
35Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
𝐼𝑖𝑗(𝑐𝑎𝑙)=𝐼𝑖
𝑗𝑡𝑜𝑡𝑎𝑙 − 𝐼𝑖
𝑗(𝑟𝑒𝑠𝑡)
Calibration ion current i of the mass M component j, I(cal) equals total measured current on the mass M I(total) minus background current on the mass M, I(rest).
3.3. Internal Standard Calibration. Software Example.
The same procedure has to be performed for H2/CO/Ar mixture
Note: one could use less fragments for CO
possible maximum matrix for the calibration of CO/H2/Ar
File >> save as
Note: Check detector, mass, amplifier bookmark. The parameters should be the same as for calibration of CO2/CH4
Calibration>>Gas specific sensitivity>>open the previously saved *.gsp file
36Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
Note: the Pfeiffer Vacuum software performs two point calibration. Zero value is automatically ascribed to zero current. Manual direct multipoint calibration is demonstrated in sections 3.1 and 5.1.
Configuring the measurement file
-Open any existing file and edit it.-Insert components and masses. The calibration factors will be drawn automatically form the library
Note: there should be no undefined calibration factors. All factors should be a positive value or zero . Undefined calibration factors “?” should be set on zero through Options>>Set Calib. Factors Zero. The calibration factors could also be given manually.
37Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
-Check channel parameters: SEM voltage, dwell time, resolution, offset must be set as during calibration measurement.-Pause-Cal. Might be set on 1 instead of 5
Note: Additional free channels might be used to monitor other fragments which are not present in the matrix e.g. oxygen 32, water 18 etc. This signals will be also saved and plotted together with ion currents of matrix fragments and calculated concentrations of the matrix components.
38Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
Moment of truth: Measurement
Switch to from Ar to CO2/CH4/Ar
DRM on 50%Ni/MgAlOx catalyst
CO2
CH4
Ar
CO
H2
Switch back to Ar
MCD>>Versus Time>>open the previously saved *.msp file
39Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
Moment of truth: Measurement
Switch to from Ar to CO2/CH4/Ar
DRM on 50%Ni/MgAlOx catalyst
CO2
CH4
Ar
CO
H2
Switch back to Ar
40Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
-it is recommended to monitor the concentrations and ion currents of the relevant and additional components
File>>Save Cycle Data!!! Note: Save data before starting the experiment.By default setting the data will not be saved.
41Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
Done By A. Tarasov and K. Mette in Catal.Today 2015, 242, 101-110
DRM in thermobalance coupled with mass-spectrometer
42Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.3. Internal Standard Calibration. Software Example.
3.4. Preparations before calibration(refers to the use of QuadStar software
and Pfeiffer OmniStar/ThermoStar QMS)
Frank Girgsdies
43Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Prerequisites and Steps
It is assumed here that some essential procedures for the proper operation of the QMS have already been performed. Such procedures are typically necessary only after hardware changes, or if there are reasons to assume that the state of the hardware has changed with time.Such steps involve:• General calibration of the mass scale
(slope and intercept of the linear relation between quadrupole radio frequency and mass scale)• Ion source tuningThese procedures should be reserved to the QMS responsible person ("super user") and not be altered by the general user.
Other procedures need to be repeated every time the QMS was switched off and vented (chamber exposed to atmospheric pressure):• Baking of the QMS chamber• Degassing of the filamentDepending on the circumstances and policy, these steps may either be within the responsibility of the "super user" or the general user.
The preparation steps which should be done by users who want to perform quantitative calibration and will be explained in more detail here are the following:• Offset calibration• Mass scale adjust• Zero gas measurement
3.4. Preparations before calibration
44Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Offset calibration, Id
Mass scale adjust, ∆mi
Zero gas measurement, I0
45Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Ion current, A
time
Measured signal Itotal,(mi±∆mi)/z
Dark current, Id
Background I0,(mi±∆mi)/z
Ion current, A
time
True signal, (mi±∆mi)/zZero gas and offset subtraction
I true=Itotal-I0-Id
3.4. Preparations before calibration
Offset calibration
What is it?„Offset“ in this context means the electronic background or „dark current“ of the amplifier measuring the ion current.The offset depends on the measurement conditions, i.e. it may be different for different amplifier ranges and measurement speeds.It needs to be known (measured) and accounted for (subtracted) in order to get quantitatively correct ion currents.
When to do it?Offset correction is a prerequisite for any quantitative ion current measurement.Thus, it needs to be done in the calibration sequence already before the zero gas measurement.Note:The matrix of offset values is not stored in any file, but only kept electronically in the QMS memory.Thus, it needs to be re-measured at least every time the QMS was switched off, including after power failures.Note 2:Measuring the offsets will only help if the offset correction is actually applied during measurement.When you setup a measurement file (e.g. using the Parset module) and add a new channel, be aware that the default state of using the offset is “OFF”, i.e. you need to change it to “ON” for every channel!Note 3:Since the offset values also depend on your detector settings, you need to re-measure them every time you change your detector settings (stick with the same settings for the whole calibration process!).
3.4. Preparations before calibration
46Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Offset calibration
How to do it?Please refer to your QMS manual for details. In brief:• The measurement should be performed either with the gas inlet closed or with the emission
switched off.• Start the “Measure” software module > “Calibration” menu > “QMS Offset”
(alternatively: “TuneUp” software module > “Tune” menu > “QMS Offset”).
47Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Offset calibration
• The program window will display the currently stored offset value matrix (only zero values in case no offsets were measured after the last switch off).
• In the menu “Parameters” > “Setup”, make sure that you select detector type (and SEM voltage) which you are going to use for your calibrated measurements.
• Make sure that the Zero Mass is set to “5.5”, which is considered to be a mass which shouldn’t be present at all and thus yield zero intensity (except for the offset to be measured).
• Close the popup window with the “OK” button.• Chose “Operation” > “Remeasure” to initiate the measurement• Close the “Tune Up” module when the measurement is done
48Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Note: subtraction of the offset values might cause the negative values or strong fluctuations of ion current displayed. This happens when the signal measured has very low intensity, in the same order of magnitude as offset measurement.
49Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Offset calibration
50Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Mass scale adjust
What is it?Quantification relies on the linear relation (proportionality) between ion current and concentration. In order to ensure a linear response, the ion current has to be measured at the tip of the mass peak.Remember that in Multiple Ion Detection (MID) or quantification (MCD) mode, the detection “jumps” from peak to peak instead of recording a full mass scale scan (analog scan). Thus, individual deviations of the actual peak positions from the nominal mass scale have to be accounted for, so that the “jumps” hit the actual peak maxima.
When to do it?A general calibration of the mass scale, i.e. determination of slope and intercept of the linear relation between quadrupole radio frequency and mass scale, is usually done only after changes to the QMS hardware.In contrast, the mass scale adjust, which is added on top of this general calibration, should be performed for each new system or set of calibrated measurements.
51Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Mass scale adjust
How to do it?Please refer to your QMS manual for details. In brief:• In order to measure the relevant peaks, a corresponding template file*.msp has to be created
(or modified) with the Parset software module (Menu: “Calibration” > “Mass Scale…”).
52Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Mass scale adjust
• Make sure that all measurement parameters, like e.g. detector settings, match those which you are using in your actual measurements.
53Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Mass scale adjust
• The measurement is performed with the Measure module (Menu: “Calibration” > “Mass Scale…”).
54Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Mass scale adjust
• A popup window will give you the choice of Calibration Mode (Coarse or Fine) and Save Mode (Reset or Append).
• For a new calibration, you typically start with “Coarse” to locate the peaks roughly and repeat the process with “Fine” to find the exact positions.
• If you intend to delete all previous entries in the calibration file and start from scratch, chose “Reset” for the very first (Coarse) run, but switch the to “Append” from the second run on.
• You may build your peak library step by step, using different gas mixtures and several corresponding *.msp files, adding new peaks with “Append”.
55Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Mass scale adjust
• Or you may try to catch all peaks in one go, provided you find a suitable gas mixture.Hint:Many of the usually interesting peaks can be captured by bubbling helium through an open acetone vessel, and sniffing the air/helium/acetone vapor mixture with the MS capillary.To see which peaks this mixture will give you, simply perform an analog scan first.
• The actual mass scale values for all measured peaks will be used in all MID or MCD measurements from now on.
• Note:In order to actually use the adjusted mass scale values, you should make sure that during your measurements, the check box “Mass Scale Correction” is marked in the menu “Parameters” > “Setup”!
56Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Mass scale adjust
• The contents of the mass scale adjust file “q_mascal.dat” can be inspected (and edited) with the DipSav module (Menu: “Auxiliary” > “Mass Scale Calibration…”).
57Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Mass scale adjust
• If masses are measured in a regular measurement for which no mass scale adjust values have been determined previously, a value will be interpolated by the software between the two closest calibrated values left and right.
• For non-calibrated masses outside the calibrated mass range, the value is extrapolated from the lowest and highest calibrated mass values. Note that this procedure may sometimes deliver unreasonably deviating values, because the very low masses like 2 and 4 may often show a deviation behavior which is different from the other masses!
interpolationextrapolation
58Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
2.4. Preparations before calibration
Mass scale adjust
• Hint:To counter such effects in absence of an actual peak calibration (not recommended in the context of quantification!), you may use DispSav to enter a dummy mass at high mass values and give it a reasonable deviation from the nominal value by guessing it from the typical behavior of the calibrated peaks.
59Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibrationMass scale adjust
dummy mass
inserting "corrected dummy mass"199.97 (for 200)
"typical" constant deviation of e.g. -0.03
60Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Zero gas measurement
What is it?At every point of the mass scale, a certain background value can be measured. While the electronic background of the detector is already accounted for in the QMS offset calibration step, residual gases in the QMS chamber will also contribute to the background and disturb the linearity of the gas specific calibration.The zero gas measurement determines the residual ion currents of the masses to be used in the calibration, which will then be subtracted during the actual calibration and quantification measurements.
When to do it?Since the zero gas signal depends both from the detector settings used and the cleanliness (history) of your QMS chamber, a new zero gas determination should be performed before every new gas specific sensitivity calibration.
Tipp: Typical gases present in background are oxygen, nitrogen, water, carbon dioxide. In the inert flow the ratio of the I(28)/I(32) is approximately ¼, which corresponds to residual pressure of air. If this ratio is higher then there must be same additional source of fragment 28 present. Possible reasons, oil diffusion from pre-vacuum oil pump, purge gas contaminations, high CO2 concentration in room, filament contamination through previous measurements.
Note: As in the case of offset correction the subtraction of the background may cause signals displayed as negative ion currents. This appears when the background during the measurement is for some reason lower or very close to background determined prior the measurement as zero gas.
61Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Zero gas measurement
How to do it?Please refer to your QMS manual for details. In brief:• The template file needed for the zero gas measurement is identical to that used for MID
measurements (*.mip) and can be created/edited using the Parset module (Menu: “Measure” > “MID…”).
• For the actual zero gas measurement, you may either use an extremely pure gas which has none of the peaks you are interested in, or measure the residual gas with a closed inlet valve.
• The measurement is performed with the Measure module (Menu: “Calibration” > “Zero Gas…”).• A popup window lets you chose between “Reset” and “Append”.
“Reset” will delete all previous entries, while “Append” will only add or update the currently measured masses and leave the rest untouched.
• Using the “Append” feature allows you to use different zero gases consecutively.For example, you could measure the background of all masses except 2 and 4 with pure helium, then append the background for 2 and 4 by measuring pure argon.
• As usual, the measurement is specific for the detector settings, i.e. make sure you use that same settings consistently.
• DispSav allows you to review and even edit the zero gas values stored in the file “q_zero.dat” (Menu: “Auxiliary” > “Zero Gas…”).
• Note:In order to actually use the zero gas correction, you should make sure that during your measurements, the check box “Zero Gas Subtraction” is marked in the menu “Parameters” > “Setup”!
62Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Zero gas measurement
How to do it?Please refer to your QMS manual for details. In brief:• The template file needed for the zero gas measurement is identical to that used for MID
measurements (*.mip) and can be created/edited using the Parset module (Menu: “Measure” > “MID…”).
• For the actual zero gas measurement, you may either use an extremely pure gas which has none of the peaks you are interested in, or measure the residual gas with a closed inlet valve.
• The measurement is performed with the Measure module (Menu: “Calibration” > “Zero Gas…”).
63Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Zero gas measurement
• A popup window lets you chose between “Reset” and “Append”.“Reset” will delete all previous entries, while “Append” will only add or update the currently measured masses and leave the rest untouched.
• Using the “Append” feature allows you to use different zero gases consecutively.For example, you could measure the background of all masses except 2 and 4 with pure helium, then append the background for 2 and 4 by measuring pure argon.
• As usual, the measurement is specific for the detector settings, i.e. make sure you use that same settings consistently.
64Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Zero gas measurement
65Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Zero gas measurement
• DispSav allows you to review and even edit the zero gas values stored in the file “q_zero.dat” (Menu: “Auxiliary” > “Zero Gas…”).
66Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Zero gas measurement
• DispSav allows you to review and even edit the zero gas values stored in the file “q_zero.dat” (Menu: “Auxiliary” > “Zero Gas…”).
67Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.4. Preparations before calibration
Zero gas measurement
• Note:In order to actually use the zero gas correction, you should make sure that during your measurements, the check box “Zero Gas Subtraction” is marked in the menu “Parameters” > “Setup”!
3.5. Calibration check box
Optimization of ion source and detector parameters.
Use analog scan to get qualitative knowledge of the system background and possible contaminations.
General mass scale adjustment (tunemass scale)
Offset calibration
Mass scale calibration (individual masses)
Long purging, observation of background intensities till stabilization
Background determination (Zero gas)
Typical formal steps prior the quantitative measurement.
Inlet valve is open, vacuum 10-6 -10-5 mbar, Carrier gas flowing (1-1.5bar), the system is baked out
Introducing calibration mixture, purging
Create calibration file, ParSet
Calibrate/ Determination of calibration factors
Repeat steps for the other components
Control measurement: Introduce several premixed calibrated components with different known ratios
Check the calibration, the nominal premixed composition should be in agreement with measured values
68Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Pitfalls and possible reasons of deviations from nominal values are listed in the appendix of this manual
Step 1
Step 2
Step 3
Step 4
Perform measurement
Control calibration after and before each measurement
Step 5
69Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
3.6. Control measurement
The control measurement has to be performed at least once per day, before the experiment. The control measurement has its aim verification of quantitative analysis. One has to set fast and simple method which enables examination of the calibration adequacy by introducing an multicomponent gas mixture with known composition.
When plotting on y axis the set or nominal concentration values and on x axis the measured values all points has to lay on the line. If it is not the case one could assume that the quantitative analysis has a systematic error. This error embraces not only the instrument sensitivity error but also contribution from contaminations, fluctuations of gas pressure and pure resolution. The control measurement may be performed with one composition. One has to verify, that the measured value is within tolerance of the calibration line.
nominal
measured
a
a/2
4 Calibration techniques4.1. Calibration of a mass spectrometer for the direct measurement of water
Li et al. Meas. Sci. Technol. 23 (2012)
80%N2, 20%O2
Response coefficient of H2O and N2 to ArAr 1
N2 0.71H2O 0.68H2 7.56
Note: it is recommended heating the line, valves and connectors all the way to the MS inlet in order to maintain condensation free flow.Calibrating the liquid multi component solutions consider the deviation from the ideal solution (Raults Law). The composition of the vapor may differ strongly from the liquid phase.
Mixtures of H2O-vapor-saturated air is obtained by the equilibrium of air with liquid water using a constant temperature water bath maintained at different temperatures, and these were used to measure the H2O MS response coefficient with respect to N2
Stable pressure range
70Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 71
Vessel used for calibration of liquid components
thermostat
saturator
water in
water out
gas in
gas + vapor out
heating
Liquid mixture
4.2. Direct calibration of MS signal measured in coupled TA-MS system
1. by decomposing solids via stoichiometric reaction2. by injection of known amounts of calibration gas into the carrier gas stream flowing with a constant rate through the system3. by introducing the continues gas flow of known composition (standard way)
M. Maciejewski, A. Baiker, TCA 295 (1997) 95-105.
@745°C
4 Kpm
Peak: 745°C
1.97mg
1ml
72Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Calibration by decomposition of solids.
Advantages =)• does not require any changes or additional reconstruction of the
commercial thermoanalytical system, • Very high accuracy of the dosing of the analyzed species into the
system • Calibration and quantitative determination of water.
Restrictions =(• the method is time consuming,• does not offer the simple possibility of investigating the influence
of temperature on the calibration procedure, • can be applied only for the species evolved during decomposition
of solids.
4.2 Direct calibration of MS signal measured in coupled TA-MS system
73Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Calibration by injection.
Advantages =)• It is very fast• easy for reproducibility check• can be performed at any temperature in both operation modes
(iso- or non-isothermally)• can be used with any gas (except water due to the
condensation problems)• the shape of the MS response depends only on mass transfer in
the investigated TA-MS system and is independent of the chemical reaction.
Restrictions =(• the amount of the injected gas can not be changed arbitrarily
and is determined by the volume of the injection loops, • the method requires some reconstructions of the gas supply
system.
74Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
4.2 Direct calibration of MS signal measured in coupled TA-MS system
↑ to MS
←to outlet
Example: Pulse calibration.
Sample Injection
LOAD INJECT
75Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
←gas in
sample
sample holder
6 port external sample injector
4.2 Direct calibration of MS signal measured in coupled TA-MS system
Example: Pulse calibration.
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 76
m/z 28
m/z 44 m/z 28
500µl
A B
Pulses of 500 µl CO2 (A) and CO (B) in 21%O2 in Ar flow at RT. The area under the peak is proportional to the injection volume.
Done by A. Tarasov for ACS Catal 2016,
4.2 Direct calibration of MS signal measured in coupled TA-MS system
Peak area S under m/z 44 and m/z 28 from slide 76total pressure P in MS cell
Flow SynAir
Flow SynAir
Flow Ar
Note: the peak area of CO2 measured in Synair is smaller than measured in Ar flow due to different diffusion coefficients of CO2 D CO2-Ar 0.144 cm2/s, D CO2-SynAir 0.130 cm2/s
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 77
Example: Pulse calibration.
Calibration factor for each mass γi = S[A*s]/ν[µmol]/P[mbar]
Small CO2 signal due to CO oxidation might be neglected Calibration factor for each mass at every t: γi /t(dwell)
t(dwell)
ν
γ44(CO2)
γ28(CO2)
γ28(CO)
4.2 Direct calibration of MS signal measured in coupled TA-MS system
Example: Pulse calibration.
Results of oxidation experiments of different carbon soots, temperature range of 400-600°C
[CO]= (S28- γ28(CO2) ·[CO2]·P)/γ28(CO)/P
[CO2]= S44/γ44(CO2)/PS44 = [CO2]·γ44(CO2)·P
S28=[CO]·γ28(CO) ·P + [CO2]·γ28(CO2)·P
Analogous to slide 12
S44 S28 P
Note: carbon oxidation takes place around 500°C but the calibration factors were determined at 40°C. There were additional test performed demonstrating that for the current setup configuration the peak area at different temperatures remains similar (see fig right ) however the profile is different (see slide 72). Hence the temperature has a larger impact on differential calibration factor rather than on integral one.
Effect of temperature on CO2 peaks.
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 78
4.2 Direct calibration of MS signal measured in coupled TA-MS system
↑ to MS
←to outlet
Example: Calibration, decomposition of solids
79Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
gas in
sample
Sample
holder
4.2 Direct calibration of MS signal measured in coupled TA-MS system
m/z=18
Dehydration of CuSO4*5H2O in 21%O2iArEvery mass loss step corresponds to water evolution
Amount of water based on theory according to the sample mass
Experimental mass loss
Experimental peak area of m/z 18 and average pressure
Calibration factor water γi = S[A*s]/ν[µmol]/P[mbar]
Peak 1 and 2 Peak 3
↑ to MS
←to outlet
Example: Calibration, decomposition of solids
80Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
gas in
sample
sample holder
4.2 Direct calibration of MS signal measured in coupled TA-MS system
Dehydration of NaHCO3 in 21%O2inArThe mass loss corresponds to water to CO2 and water evolution
Theoretical mass loss
Experimental mass loss for different sample loading and amount of products according to theoretical ratio.
Different sample loadings correspond to difference of working concentration of products.
Peak areas of relevant fragments for different sample loading
Calibration factors for each fragment for two concentrations. Calculation to final concentration s. slide 78.
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 81
4.3 Direct calibration of solids
• When talking of solids we consider substances with the very low vapor pressure. Dynamic methods for determination of vapor pressure over solids imply operation under low pressures (p<10-4 atm) with continues sampling of the vapor phase. The following approach deals with Knudsen effusion chamber directly introduced into the cell of mass-spectrometer (direct inlet) or through skimmer inlet. The chamber is heated to temperature T and the substance flux evolving through effusion pinhole is registered. The effusion pinhole of the Knudsen chamber is significantly smaller than area of the chamber. Hence the probability of leaving the chamber is much lower than the probability of interaction with the chamber walls. Thus sampling could be neglected and the vapor over the solid is in equilibrium. When using other sample holders one has to consider that the composition measured does not correspond to equilibrated vapor but from technical point of view the specific instrument constants could be measured. Further we will consider the solid phase in equilibrium with its vapor.
• Working with solids implies frequently not only on lower operating pressure of the experiential setup but also on very high temperatures. The calibration therefore may become tedious and complex.
• We insist on dealing with high temperature mass-spectrometry only with experienced supervision and corresponding equipment.
L N Sidorov, M V Korobov, L V Zhuravleva 1985 Mass-spektral'nye Termodinamicheskie Issledovaniya (Mass Spectral Thermodynamic Research) (Moscow: Moscow State University)
• Using combination of Knudsen effusion method with mass-spectrometry one determines the instrument sensitivity constant: k´j or k´ij (see slide 9)
• The are two options for calibration: integral (absolute) and relative (differential)
1 – effusion orifice , 2r diameter of the orifice, l – channel length2 – top lid3 – sample4 – chamber body, 2R – inner diameter5 - thermocouple
Effusion chamber
The size of the orifice is ~30 times lower than inner diameter of the chamber
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 82
4.3 Direct calibration of solids
𝑑𝑞
𝑑𝑡= 𝜌
𝑚𝑜𝑙𝑒𝑐
𝑐𝑚2𝑠𝑒𝑐∙ 𝑆𝑒𝑓 𝑐𝑚2 = 𝑆𝑒𝑓 ∙ 𝑝 ∙ (2𝜋 ∙ 𝑀 ∙ 𝑅 ∙ 𝑇)−
12
ρ – molecules flux through the effective area Sef
q – amount of substancet – time, p – equilibrium vapor pressure, M – molecular massR – gas constant, T- temperature in K
The Hertz-Knudsen equation reflects the dependence of the pressure inside the cell with the amount of evaporated substance
4.3 Direct calibration of solids
V. L. Stolyarova, Russ. Chem. Rev. 2016, 85 (1) 60-80
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 83
Integral (absolute) method.
The known amount of the sample is loaded to Knudsen chamber. The complete evaporation is carried out. The ion current is continuously registered by MS thus the time of complete evaporation is detected. Substituting p by ion current in the previous equation the amount of evaporated substance is determined as integral under the curve.
4.3 Direct calibration of solids
84Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
q(mol) = 𝐵 ∙ 𝑘𝑖𝑗𝑀𝑗−
12 න
0
∞
𝐼𝑖𝑗𝑑𝑡 𝐵 = 𝑆𝑒𝑓 ∙2𝜋𝑅
𝑇
−12
where
q(mol) = 𝑘𝑖𝑗𝑆𝑒𝑓(2𝜋𝑀𝑅)−12 න
0
∞
𝑇12𝐼𝑖𝑗𝑑𝑡
Isothermal
Polythermal (Dynamic change of temperature)
After integration the instrument sensitivity constant kij could be determined.
Ion current of ZrF3+ vs. Time at 836K for evaporation of ZrF4
𝑝𝑍𝑟𝐹4= 𝑘´𝑍𝑟𝐹3
+𝑍𝑟𝐹4𝐼𝑍𝑟𝐹3
+𝑍𝑟𝐹4𝑇
85Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
4.3 Direct calibration of solids
Differential (relative) method.
In this method a standard is used, i.e. the substance with very well defined vapor pressure.The ion currents of the standard and the investigated component are registered simultaneously.
Vapor pressure of the of the analyzed component is expressed as follows:
Ratio of full ionization cross sections,
IA, Ix, - full ion currents of molecule
L N Sidorov, M V Korobov, L V Zhuravleva 1985 Mass-spektral'nye Termodinamicheskie Issledovaniya (Mass Spectral Thermodynamic Research) (Moscow: Moscow State University)
𝑝𝑥 = 𝑝𝐴
𝑘´𝑥
𝑘´𝐴∙
𝐼𝑥
𝐼𝐴= 𝑝𝐴
𝜎𝐴
𝜎𝑋∙
𝐼𝑥
𝐼𝐴
𝑝𝑥 = 𝑘´𝑥𝐼𝑥𝑇 𝑝𝐴 = 𝑘´𝐴𝐼𝐴𝑇
5.1. QMS Calibration at BESSY
Mark Greiner
86Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5. Examples from FHI practice
87Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5.1. QMS Calibration at BESSY
Performed by M. Greiner for Phys. Chem. Chem. Phys., 2015,17, 25073-25089
• The reaction being investigated is ethylene epoxidation:
• ½ O2 + C2H4 C2H4O
• The non-selective (total oxidation) reaction is:
• 3O2 + C2H4 2CO2 + 2H2O
• Another partial oxidation product is acetaldehyde: C2H4O
• All the carbon-containing products have mass 44
• Fortunately, the QMS is much more sensitive to CO2 than it is to C2H4O.
• Therefore, the C2H4O concentration is determined by PTRMS (or better yet, GC) and the CO2 concentration is determined by QMS.
• This situation makes the calibration of the QMS very simple. We only need to dose a known concentration of CO2 into the chamber and measure the corresponding M44 signal .
88Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5.1. QMS Calibration at BESSY
• Here we feed the same composition of C2H4 and O2 into the chamber as we do in the experiments, and then dose a small amount of CO2 from a test gas mixture that contains a known concentration of CO2 (2 %).
• The MFC used to dose the calibration gas had a maximum flow capacity of 0.7 mln/min.
• The reactant gases (C2H4 and O2) were supplied at 3 mln/min each (6 mln/min total).
• The reaction chamber pressure was kept constant at 0.3 mbar.
89Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5.1. QMS Calibration at BESSY
1
2
3
4
5
Example of calibration data
90Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5.1. QMS Calibration at BESSY
P(total)[mbar]
Flow(C2H4)[mln/min]
Flow(O2)[mln/min]
Flow (CO2) [mln/min]
CO2
concentration[ppm]
Mass 44 Signal (cts./sec.)
0.3 3 3 0 0 2E-13
0.3 3 3 0.07 173.482 6.82E-13
0.3 3 3 0.106 261.5347 8.15E-13
0.3 3 3 0.14 343.9803 9.76E-13
0.3 3 3 0.175 428.1346 1.13E-12
0.3 3 3 0.21 511.5713 1.27E-12
Calibration chart showing flow rates and measured QMS values
91Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5.1. QMS Calibration at BESSY
background level
92Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5.1. QMS Calibration at BESSY
MS-coupled sacrificial agent testing of water oxidation catalysts (WOC)
Cyriac Massué
93Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5.2. Oxygen Evolution Reaction calibration with O2 pulses
A wireless electrocatalytic test:
(NH4)2CeIV(NO3)6 („CAN“) mimics the oxygen evolution reaction (OER)[1]
Fast screening method of WOC without electrode preparation
Limited to pH<2 (above CAN is notstable).
Sacrificial agent testing of WOC
[1] Parent, A. et al., Chem. Soc. Rev. 2013, 42 (6), 2247-2252
94Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5.2. Oxygen Evolution Reaction calibration with O2 pulses
Setup
95Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5.2. Oxygen Evolution Reaction calibration with O2 pulses
Calibration
date sampleinteg 0,5mL
A*mininteg 0,4mL
A*mininteg 0,3mL
A*min
09.12.2013 Solvicore 3,89E-11 3,13E-11 2,27E-11
04.12.2013 17580 4,06E-11 3,10E-11 2,17E-11
13.01.2014 17580 3,65E-11 2,93E-11 2,10E-11
16.01.2014 17708 2,84E-11 2,19E-11 1,63E-11
20.01.2014 17711 1,83E-11 1,69E-11 1,31E-11
03.03.2014 18151 1,47E-11 1,22E-11 9,05E-12
04.03.2014 18151 1,39E-11 1,09E-11 8,16E-12
12.03.2014 18218 1,46E-11 1,12E-11 8,86E-12
13.03.2014 18218 1,41E-11 1,07E-11 7,84E-12
18.03.2014 18085 2,22E-11 1,67E-11 1,27E-11
0,00E+00
5,00E-12
1,00E-11
1,50E-11
2,00E-11
2,50E-11
3,00E-11
3,50E-11
4,00E-11
4,50E-11
integ 0,5mL
integ 0,3mL
integ 0,4mL
I / A
96Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5.2. Oxygen Evolution Reaction calibration with O2 pulses
Results
I / A
97Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
5.2. Oxygen Evolution Reaction calibration with O2 pulses
mol, O2
6. PTR MS. Quantitative Analysis
Thomas Lunkenbein, Jorge I. Salazar Gómez
98Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Source: Proton Transfer Reaction Mass Spectrometry: Principles and Applications, First Edition. Andrew M. Ellis and Christopher A. Mayhew. Published 2014 by John Wiley & Sons, Ltd.
R. S. Blake, P. S. Monks, A.M. Ellis, Proton-Transfer Reaction Mass Spectrometry, Chem.Rev. 2009, 109, 861-896.
Extracting Concentration from PTR-MS data
Drift tube:
Volume mixing ratio (VMR) is the same inside the drift tube and outside in theanalyte [ppbv]:
Normalized count rate per second
99Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
6. PTR MS. Quantitative Analysis
methanol
sensitivity
Why calibrate?• Above mention equation would require knowledge of
Extraction efficiencies into the mass spectrometer
Transmissione efficiencies through the mass spectometer
the degree of fragmentation (tunable by the E/N ratio)
Overlapping contributions of unrelated VOC
Isotopologues
Product ions resulting from reactions with impurity reagent ions
Change of ion mobilities for H3O+ versus MH+
Radial diffusion at the end of the drift tube
Error for the proton transfer coefficients (~25%)
Error using this equation ~20%-30%
100
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
6. PTR MS. Quantitative Analysis
Calibration TechniquesStatic Gas Calibration
Using normal MFC the error is usally to high when preparing dilution in the ppbv regime.This would correspond to have one controller with a flow of 0.001 L/min and the other has a flow of 1000 L/min.
Diluted mixture of VOCs with known concentration in a container
Cheap (do-it-yourself) Gas adhesion to walls
Purchased from gas suppliers Possibility of chemical decomposition
simple Single calibration point
Expensive when purchased
101
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
6. PTR MS. Quantitative Analysis
Calibration Techniques
Dynamic Gas CalibrationPermeation tube
No long term contact to metal wall(dynamic)
Initial investment for set-up(permeation tube + experimental
set-up)
Large range(gases, liquids and solids)
Possibility of chemical decomposition
Small release of calibrant makes it possible to achieve very low
concentrations
Long start-up time of several hours
Changing the flow rate of diluent gas,
Changes the concentration
102Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
6. PTR MS. Quantitative Analysis
Validation studies have been carried in particular with gas chromatography
PTR-MS and GC will not always agree PTR-MS can separate compounds with the same mass
Fragment ions from heavier compounds may contribute to the protonated parent signal of lighter molecules
For most molecules PTR-MS and GC-MS agreed within the measurement uncertainties
Validation for PTR-MS
103Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
6. PTR MS. Quantitative Analysis
Calibration PTR-QiTOF-MS
PTR-MS is a pseudo-absolute method
• Quantification is possible by calculation but accuracy is often less than ~50%.
Uncertainties:
• T- Temperature (T drift)
• [PI]- cps on primary ions
• [m+1]- cps on protonated compound
• Tr- Transmission at mass (m)
• t- reaction time
• K- reaction rate (calculated or from literature)
104Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
6. PTR MS. Quantitative Analysis
Prior calibration
Before calibration:
• Be sure you have done a mass scale calibration by using specific masses, which are always present in the spectra e.g. H3O+, NO+, acetone. It becomes difficult if a background is taken before, since some masses may disappear.
• If necessary carry out a tuning.
• Be sure the ion source has been optimized for the components you want to measure.
• Carry out a SEM operating voltage check.
• Set up your instrument with the parameters that will be used for measurements.
Source: IONICON
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 105
6. PTR MS. Quantitative Analysis
Calibration frequency
How often you need calibration:
• Individual and occasional measurements (hours – one day): a calibration should be made for each new application or measurement phase. This is especially necessary if your instrument is switched off between measurements and is only set in operation when you need to measure.
• Short-term measurements (days – one week): a single calibration should suffice here, provided that your PTR-MS instrument runs stably over this period. Make the calibration any time during your measurements, although usually before measurements begin or at the end, once they are finished, are the most convenient times to calibrate.
• Long-term measurements (several weeks – a few months): here it would be recommended to calibrate before actual measurements begin, plus again at the end. If time permits, a half-time calibration may also be beneficial. This also helps to provide an overview of the stability of your PTR-MS.
• Continuous measurements (several months or longer): calibrate your instrument before you start your measurements, then at approximately one-month intervals until measurements end, whereupon you should calibrate a final time. This allows you to keep track of instrument sensitivities and adjust your data accordingly. It is quite likely that you will see shifts in your calibration values over such long time scales (e.g. due to detector aging effects).
Source: IONICON
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 106
6. PTR MS. Quantitative Analysis
Additionally, a calibration is essential when the PTR-MS operational parameters have been modified. This can include:
• Alteration of the reaction chamber voltages (e.g. drift tube or extraction voltages)
• Exchange of the secondary electron multiplier (SEM)
• After transportation of the instrument
• When the instrument has newly been switched on, e.g. after a service operation
Calibration frequency
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 107
6. PTR MS. Quantitative Analysis
Steps for calibration:
• Carry out a background with zero gas.
• Use a calibration gas standard and dilute it in at least 5 different concentration steps.
• Keep each concentration until the signal of the „stickiest“ compound is stable or run at least 5 cycles for unproblematic samples.
Calibration
108Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
6. PTR MS. Quantitative Analysis
Steps for calibration:
• If the sample to be measured contains humidity, it is useful to carry out a humidity-dependent calibration.
• In order to make datasets comparable, the cps of the VOCs have to be normalized to the primary ion (and water clusters) and pressure in the drift tube.
109Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
6. PTR MS. Quantitative Analysis
Steps for calibration:
• After normalization of all masses of interest, calculate a mean value for each concentration step and the zero air measurements.
• Then subtract this zero-air mean from each calibration step to provide a net n-cps signal per mass, per calibration step.
• Now you can derive the sensitivity of the VOCs (slope). This can be used as correction factor in the peak table for quantification.
• After that you can calculate the LOD (but using the normal cps!)
110Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
6. PTR MS. Quantitative Analysis
111
Isotope scrambling
Theoretical pattern of CO2+
1) CO2 12CO2 + 13CO2
12C=0.99 13C=0.012) C16O2 C16O18O C18O2
3) 16O – x , 18O - (1-x)
n k01234567
16O =0.998, 18O = 0.002
0.99*x2
(x+(1-x))2=x2 + 2x*(1-x) + (1-x)2
С16O2
C16O18OC18O2
12С16O213С16O2
0.01*x2
12С16O18O
0.99*2x*(1-x) 0.01*2x*(1-x)
12С18O213С18O2
0.99*(1-x)2 0.01*(1-x)2
13С16O18O
44 45 46 47 48 49
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
100
99%
1%
0.4%
40ppm
4ppm
13C
18O
2
12C
18O
2
12C
16O
18O
13C
16O
18O
13C
16O
2
I / %
m/z
12C
16O
2
0.04ppm
• Random distribution of the isotopes is determined by binominal coefficients
n – number of equivalent positions, row number, 2a, b, abundance of isotopes, x , 1-x.
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Appendix 1
112
Fragmentation
Cracking pattern of natural CO2 (experimental)
Fragmentation distribution at 70 eV
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
100
1000ppm
100ppm
13C
+
12C
+
18O
+
16O
+
CO++
2
CO+
Rel. Intensity / %
m/z
13
C18
O2
12
C18
O2
13
C16
O18
O
12
C16
O18
O
13
C16
O2
12
C16
O2
CO2 CO
+
2
1ppm
Cracking pattern of natural CO2 (calculated)
Deviation of experimental and theoretical spectra caused by: Proximity to equilibrium (equilibrium is not reached) Kinetic hindrance Thermodynamically favorable configuration Isotopic composition
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Appendix 1
Prediction of the Mass-spectra with the definite isotopic composition.
113
Isotope scrambling
Theoretical pattern of NH3+
1) NH3 14NH3 + 15NH3
14N=0.996 15N=0.0042) NH3 NH2D NHD2 ND3
3) 1H – x , 2D - (1-x)
n k01234567
1H =0.9999, 2D = 0.0001
(x+(1-x))3=x3 + 3x2*(1-x) + 3x*(1-x)2 + (1-x)3
NH3 NH2D
• Distribution is determined by binominal coefficients
n – number of equivalent positions, row number, 3a, b, natural abundance of isotopes, x , 1-x.
NHD2 ND3
A*x3
14NH315NH3
A*3x2(1-x)
14NH2D
A*(1-x)3
15NH2D 14ND2H 15ND2H 14ND315ND3
B*x3 B*3x(1-x)2A*3x(1-x)2
B*3x2(1-x) B*(1-x)3
A, B, abundance of 14N, 15N (0.996, 0.004)
17 18 19 20 21
1E-14
1E-12
1E-10
1E-8
1E-6
1E-4
0.01
1
100
0.1ppb0.03ppm
1.2ppm
0.03%
0.4%
15ND
2H
14ND
315
ND3
14ND
2H
15NH
2D
14NH
2D
I, %
m/z
14NH
3
15NH
3
99.5%
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Appendix 1
Isotope scrambling
Theoretical pattern of CH4+
1) CH4 12CH4 + 13CH4
12C=0.99 13C=0.012) CH4 CH3D CH2D2 CH3D CD4
3) 1H – x , 2D - (1-x)
n k01234567
1H =0.9999, 2D = 0.0001
(x+(1-x))4=x4 + 4x3*(1-x) + 6x2*(1-x)2 + 4x*(1-x)3 + (1-x)4
• Distribution is determined by binominal coefficients
n – number of equivalent positions, row number, 4a, b, abundance of isotopes, x , 1-x.
CH4 CH3D CH2D2 CH3D CH3D
12CH413CH4 12CH3D 13CH3D
12CH2D2 12CD3H12CD4
A, B, abundance of 12C, 13C (0.99, 0.01)
13CH2D2 13CD3H13CD4
B*(1-x)4A*(1-x)4
4Ax*(1-x)3 4Bx*(1-x)3
6Bx2*(1-x)26Ax2*(1-x)2
4Ax3*(1-x) 4Bx3*(1-x)
Ax4 Bx4
15 16 17 18 19 20 21 22 23 24 25 26
1E-18
1E-16
1E-14
1E-12
1E-10
1E-8
1E-6
1E-4
0.01
1
100
10-5 ppb
10-3 ppb
0.6 ppb
0.06 ppm
4 ppm
0.4 %
1 %
13CD
4
12CD
4
13CHD
3
12CHD
3
13CH
2D
2
12CH
2D
2
13CH
3D
12CH
3D
13CH
4
m/z
I, %
12CH
4
98.9 %
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Appendix 1
114
TcO4-, s=2 (16O, 18O), n=4, (4 + 2 – 1)!/4!(2 – 1)! = 5!/4!1! = 5
C2H4+, s=2 (1H, 2D), n=4, (4 + 2 – 1)!/4!(2 – 1)! = 5!/4!1! = 5
s=2 (12C, 13C), n=2, (2+2-1)!/2!(2-1)!=3
C24*C2
2=15 (15 possible lines in spectra for ethylene (C2H4+))
[Mo2O7]2-, s=7 (92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo, 100Mo), n=2, (2 + 7 – 1)!/2!(7 – 1)! = 28s=2 (16O, 18O), n=7, (7+2-1)!/7!(2-1)!=8
C72*C2
7=224 (224 possible lines in spectra of [Mo2O7 ]2-
115
Csn=
𝑛+𝑠−1 !
𝑛! 𝑠−1 !
s - type of particles (number of isotopes of one element)n – number of particlesCs
n – number of combinations of n particles with s isotopes (number of lines in spectra)
Isotope scrambling
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Appendix 1
Isotope scrambling
Theoretical pattern of C2H4+
1) C2H4 12C2H4 + 13C2H4 + 12C13CH4
12C=0.99; y 13C=0.01; 1-y
2) C2H4 C2H3D C2H2D2 C2H3D C2D41H =0.9999; x 2D = 0.0001; 1-x
n k01234567
• Distribution is determined by binominal coefficients
n – number of equivalent positions, row number,2 (C) and 4 (H)a, b, abundance of isotopes, y, 1-y and x , 1-x,
(x+(1-x))4=x4 + 4x3*(1-x) + 6x2*(1-x)2 + 4x*(1-x)3 + (1-x)4 for hydrogen
(y+(1-y))2=y2 + 2y*(1-y) + (1-y)2 for carbon
(y+(1-y))2 · (x+(1-x))4 Intensity distribution for 15 possible lines of (C2H4+)
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Appendix 1
116
Isotope scrambling
Theoretical pattern of [Mo2O7]2-
1) Mo92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo, 100Mo
a, b, c, d, e, f, g
2) 16O – x , 18O - (1-x)
n – number of equivalent positions, row number,2 (Mo) and 7 (O)a, b, abundance of isotopes, a, b, c, d, e, f, g (Mo); x, 1-x, (O)
(x+(1-x))7 · (a+b+c+d+e+f+g)2 Intensity distribution for 224 possible lines of (Mo2O7)
92Mo 61.5%94Mo 38.3%95Mo 66.0%96Mo 69.1%97Mo 39.6%98Mo 100%100Mo 39.9%
16O =0.998, 18O = 0.002
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry
Appendix 1
117
J.F. O´ Hanlon, A User´s Guide to Vacuum Technology, WILEY, 2003 pp. 172-176, FHI shelf mark: 6F33
• Approaches to simple quantitative analysis
SourcesAppendix 2
• Some useful statistics equations and tables
B. J. Millard, Quantitative mass spectrometry, HEYDEN, 1979 pp. 161-165, FHI shelf mark: 22K42
• On calibration design and calibration models
I. Lavagnini, F. Magno, R. Seraglia, P. Traldi , Quantitative application of mass spectrometry, WILEY, 2006 pp. 107-133, FHI shelf mark: 22K43
• Isotope distribution and Isotope Analysis
H. Birkenfeld, G. Haase, H. Zahn, Massenspektrometrische Isotopenanalyse, DVW, 1962, FHI shelf mark: 22K7
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 118
• High-Temperature MS, equilibrated vapor over solids
L. N. Sidorov, M. V. Korobov, L. V. Zhuravleva, 1985 Mass-spektral'nye TermodinamicheskieIssledovaniya (Mass Spectral Thermodynamic Research) (Moscow: Moscow State University)
Main errors and pitfallsAppendix 3
• Negative ion current.One of the reasons of detecting negative ion currents is the subtraction of the offset values for the very low ion currents. The other reason is the Zero gas subtraction if the measured intensity is lower or close to the earlier measured background. Physically not corrected values are positive. Fast scan rate with short dwell time sometimes causes the drop of the intensities. The above mentioned problems cold be resolved by changing offset (ON/OFF)or dwell time setting in the channels setup and switching off the zero gas subtraction in the setup bookmark.
• Signal fluctuation, Concentration deviation from nominal valueThe measured signal may not have stable values if the MFCs are wrong calibrated or are not able to maintain the given set value. Also the single components could interact with each other or initiate reaction on the parts of the experimental setup already at room temperature causing oscillations of the signal. The secondary reactions initiated on the hot filament is also a reason of distortion in the mass-spectrum and appearance of byproducts.
• Side processes. Reactions with filament The working W filament reaches extremely high temperatures (1800°C). Under this conditions molecules may interact with the cathode body. Even in the UHV the W filament initiate decomposition of hydrocarbons with formation of WC2 (see fig, slide 117). High CO2/CO background is expected when working with oxygen afterwards (chemical history). Yttrium oxide coated iridium filaments are more inert but posses shorter life span due to metal evaporation (see Table slide 117). The type of ion source and filament has to be chosen with respect to measurement task.
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 119
Main Errors and PitfallsAppendix 3
• Background interpretation-The traces of air are considered as residual gas in the system. Thus the key fragments on mass to charge ratio 18, 32, 28, 17, 14 and additional masses are always present. The H2+
fragments on m/z 2 is typically also very abundant. If for some reason no signal is detected check the ion current scale and detector settings. With closed inlet valve, baked out system, pressure 10-7-10-6 mbar, the intensities are in range of 10-12-10-13A. In fact the intensity of mass 18 is the highest.-Appearance of mass 19 is attributed to F+ fragment. The source of fluorine could be the aluminum parts of the experimental setup which are exposed to HV (extraction method). Also decomposition of fluoropolymers fitting like Viton leads to plenty of CxHyFz signals and F+ as a key a product of fragmentation. -The peaks m/z 43, 58 often show up when using acetone for cleaning, 31 and 35 for ethanol.-50, 69, Carbon tetrafluoride (CF4), etch gas for semiconductors processes-36, 38, 70, 72, 74 – HCl, product from PVC-89, 108, 127 – Sulfur hexafluoride, tracer gas, isolation gas for electron microscopes-41, 42, 43; 55, 56, 57; 69, 70, 71 (multiplets characterized by CH2 (14) steps) – Solvents, linear alkenes, oil from vacuum pump
Figures to these examples might be found in Lecture Series Heterogeneous Catalysis, AC FHI Database ID #38007
Andrey Tarasov, Manual quantitative gas analysis; Mass-spectrometry 121