Mass Spectrometry

Dr. Karin Habermehl-CwirzenDr. Karin Habermehl-Cwirzen

Mass Spectrometry

measures individual molecules

Definition

Mass spectrometry is a method in whichMass spectrometry is a method in which

• a sample is ionizeda sample is ionized

• the ionized ions are seperated the ionized ions are seperated according to their mass difference (mass-according to their mass difference (mass-

to-charge)to-charge)

• ions/weight are registered / identifiedions/weight are registered / identified

Ion Source

Mass analyser

Detector

Mass Spectrometer set-up

System System InletInlet

Ion Ion SourceSource

Mass Mass AnalyzerAnalyzer

DetectorDetector

Signal Signal processorprocessor

m/zm/z

1010-5-5 to 10 to 10-8 -8 TorrTorr

All mass spectrometers have the same set-up

Ion source (for ionizing and accelerating the sample)Ion separationIon detection

Vacuum needed: avoid ions hitting air molecules

The analytical data are shown as plots m/z versus detection intensity

Mass Spectrometry

A heavy ”ball” movingA heavy ”ball” moving

A light ”ball” movingA light ”ball” moving

A sideway force acting on the ballsA sideway force acting on the balls

WHAT WILL HAPPEN???WHAT WILL HAPPEN???

How does a mass spectrum look like?• horizontal axis : m/z

most of the ions formed have a single charge m/1~m

atomic mass It is defined as one twelfth of the rest mass of an unbound atom of carbon-12 in its nuclear and electronic ground state, and has a value of 1.660538782(83)x10-27 kg, e.g.

C= 12, H=1, O = 16

• vertical axis : abundance

molecular ion: most of the time the highest-mass ion in the spectrum

base peak: the highest abundace peak in the spectrum

Carbon dioxide and fragmented ions CO and O

Isotopes

What are isotopes?

Since a mass spectrometer seperates and detects ions of slightly different masses it can easily distinguish between different isotopes

bromine : Br-Br

Bromine:   50.50% 79Br and 49.50% 81Br

80 Da is assumed for Br.

BUT natural bromine consists of a nearly 50:50 mixture of isotopes having atomic masses of 79 and 81 respectively.

Thus, the bromine molecule may be composed of two 79Br atoms (mass 158 Da), two 81Br atoms (mass 162 Da) or the more probable combination of 79Br-81Br (mass 160 Da).

molybdenum: 7 stable isotopes

Isotope Half Life

Mo-91 15.5 minutes

Mo-92 Stable

Mo-93 3500.0 years

Mo-94 Stable

Mo-95 Stable

Mo-96 Stable

Mo-97 Stable

Mo-98 Stable

Mo-99 2.74 days

Mo-100 Stable

Mo-101 14.6 minutes

Mass Spectrometry

Areas of usage:

Applications in building technology:•organic compounds of high-performance water-reducing agents•hydrogen detection•trace elements in clinker

Other application: environmental polutants, pesticide residues, forensics (cocaine)

Mass Spectrometry

Mass spectrometers are classiefied by their ionisation source and analyser type;e.g. electron spray source (ESI) with a time-of-flight analyser (TOF) = ESI-TOF MS

Mass Spectrometry

Ion Sources

A mass spectrometer can only analyse charged particles, therefore neutral elements or molecules have to be ionised before they can be detected.

Ion Sources

1) by electron ionization (EI) (gas-phase)• most widely used method• ionization chamber neutral gas• thermoelectrons knock out electrons from sample atoms

– positive ions• Most ions will carry +1charge

Disadvantages:• excessivefragmentation• loss of molecularpeak

2) chemical ionization (CI) (gas-phase)

Some molecules prefer [M+H]+ and not M+

analyte – reagentreagent: methane, ammonia or isobutanereagent gas amount much higher than analyteElectrons entering the source will preferebly ionize the reagent gasCollision of the ions with other reagent gas atoims will create secondary ionssecondary ions react with analyte molecules in a specific manner

Formation of a new ion in the gas phase by the reaction of a neutral species with an ion. The process may involve transfer of an electron, a proton or other charged species between the reactants.

Chemical ionization is a lower energy process than electron ionization. The lower energy yields less fragmentation, and usually a simpler spectrum.

Used. e.g. for drugs, morphine

3) fast atom bombardment (FAB)

In FAB a high-energy beam of neutral atoms, typically Xe or Ar, strikes a solid sample causing desorption and ionization. It is used for large biological molecules that are difficult to get into the gas phase. FAB causes little fragmentation and usually gives a large molecular ion peak.

4) electrospray ionization (ESI)A solution containing the sample molecules is sprayed out the end of a fine capillary into a heated chamber. Small charged droplets are expelled in the ionization chamber. The charged droplets are subjected to a counterflow of a drying gas. Thus the charge density of each droplet increases until the electrostatic repulsive forces exceed the surface tension and it breaks in smaller droplets. This process continues until small enough fractions are obtained.

Mass Spectrometry

4 different kinds of mass spectrometers:

•simple focussing•double focusing•time-of flight•Quadrupol mass specrometer

a) simple focussing mass spectrometer

Sector instruments have higher resolution and greater mass range thanquadrupole instruments, but they require larger vacuum pumps and often scan more slowly. The typical mass range is to m/z 5000

Mass Spectrometry

b) Double converging magnetic field type mass spectrometer

ions have a ”thermic velocity”equalize the energy of the ions by using an electric field for improving the resolving power

Mass Spectrometry

b) Double focusing mass spectrometer

c) time- of flight spectrometer

• utilizes the different flying velocity of ions in the electric field according to m/z (lighter ions arrive first)

This is a very simple mass spectrometer that uses fixed voltages and does not require a magnetic field. The greatest drawback is that TOF instruments have poor mass resolution, usually less than 500.

d) Quadrupole type mass spectrometer

Only ions of a certain mass-to-charge ratio m/z will reach the detector for a given ratio of voltages: other ions have unstable trajectories and will collide with the rods The quadrupole mass spectrometer is the most common mass analyzer.

+ Its compact size, fast scan rate, and modest vacuum requirementsare ideal for small inexpensive instruments.

- Most quadrupole instruments are limited to unit m/z resolution** and have a mass range of m/z 1000.

Secondary Ion Mass Spectrometry

A primary ion beam; such as 3He+,16O+, or 40Ar+ is accelerated and focused onto the surface of a sample and sputters material into the gas phase.

Due to the sputtering, neutral atoms, molecules, X-rays, SECONDARY IONS and electrons are emitted from the sample surface.

Approximately 1% of the sputtered material comes off as ions, which can then be analyzed by a mass spectrometer.

SIMS has the advantage that material can be continually sputtered from a surface to determine analyte concentrations as a function of distance from the original surface (depth profiling).

• seldom in building mat.res.• demanding technique• expensive• few labs• surface concentration• depth concentration• quantification difficult• very sensitive• also light elements H, Li

Secondary Ion Mass Spectrometry

He+, O+, Ar+, Cs+

small quantities 100ppm of ANY element can be analyzed

area hundreds of nm or smaller

depth 1-10nm

Secondary Ion Mass Spectrometry

in building technology

• comparison of bulk/surface composition

•Where are certain elements like Co, Ce, Sr?

•the cement manufacturing plant can be identified from the kind and contents of the trace elements

SIMS (historic mortars)

Petalite LiAlSi4O10

Mass Spectrometry

Advantages:

possibility to measure all elements (even hydrogen)

Detection sensitivity 10-12 g

Disadvantages:

Vacuum needed

gas state ions

2 spectra can only be directly compared if measured under the same cicumstances

Mass Spectrometry