Some abbreviations used: A = mass number,    A_r = relative atomic mass,   Z = atomic number (NOT z)

m/z = relative molecular mass or isotopic mass / electric charge for ions formed in a mass spectrometer

Mass spectrometry gives accurate information on the relative masses of isotopes and their relative abundance (proportions).

Mass spectrometry is an important method of analysis in chemistry and can be used to identify elements and compounds by their characteristic mass spectrum pattern - the technique is used in planetary space probes e.g. mass spectrometer instrumentation is incorporated in the Mars explorer vehicles, mass spectrometers can monitor the concentration of air pollution molecules and detect traces of illegal drugs in the urine of athletes.

What is Mass Spectroscopy and a mass spectrum?

A mass spectrometer is an instrument of analysing particles of different relative mass.

The instrument used is called a mass spectrometer, of which there are several types.

All types of mass spectrometers involve vaporising atoms or molecules in high vacuum and subjecting the vapourised particles to electron bombardment to generate a beam of positive ions, a process called ionisation.

The mass spectrometer, by several different means, separates and counts the numbers of different positive ion particles produced.

The resulting data from the detector is called a mass spectrum (plural mass spectra) which gives you lots of data including:

the accurate relative masses (based on ¹²C = 12.0000) of all the positive ions generated from individual atoms (isotopes), whole molecules and fragments of molecules,

the relative numbers of each particle (listed above) generated by the electron bombardment of the original atoms or molecules.

 

Uses of mass spectrometry include:

the determination of

very accurate relative isotopic masses (these days to at least 9 significant figures with high resolution mass spectrometers,

the relative abundances of the isotopes for a specific element - from this you can calculate the relative atomic mass of an element (which can also be measured from chemical analysis),

identifying molecular formula using a high resolution mass spectrometer

identification of organic molecules from fragmentation patterns (each has a mass spectra fingerprint)

See separate page for more on details on uses and applications of mass spectrometry

Advantages of using mass spectrometry as an analytical technique

Like other modern instrumental analytical techniques used in chemistry in the 20th-21st centuries, mass spectroscopy has several advantages over traditional methods of chemical analysis e.g.

It is a very sensitive technique, only requiring tiny amounts of material for analysis and only tiny amounts might be available e.g. in forensic analysis of a crime scheme.

It is a very accurate technique, but the mass spectrometer does require careful calibration e.g. relative to carbon-12 isotope given a value of 12.0000 atomic mass units and quality instruments rarely make a mistake.

The analysis can be done quickly AND continuously. A sampling plus a mass spectrometer system could monitor pollution or a chemical production process 24/7!

A mass spectrometer can be linked to other analytical instruments e.g. you can set up a mass spectrometer to sample the separate molecules exiting from a gas/liquid chromatograph column.

See separate page for more on details on uses and applications of mass spectrometry

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2. Method (1) Magnetic field DEFLECTION MASS SPECTROMETER

(NOTE: Most mass spectrometers these days are of the TOF type, and students now, and in the future, should be expected know how a TOF works, the results are ultimately the same, but I've described the older type of mass spectrometer as an introduction to mass spectroscopy of the atoms and molecules of elements and compounds)

My use of the word 'deflection' in describing this type of mass spectrometer is NOT official, but I couldn't think of any other way of distinguishing it from a 'time of flight' mass spectrometer described in method (2) at the end of this page!

The substance to be analysed is introduced/injected into a high vacuum (extremely low pressure) tube system (at K left diagram) where the particles are ionised by colliding with beam of high speed electrons (at Q in left diagram).

Note: If the sample is not already a gas, then a liquid or solid substance must be vapourised, i.e. the material must be in the gaseous state to be analysed in a mass spectrometer. The material being analysed must in the form of free moving gaseous atoms or molecules which can be then bombarded with electrons to produce equally free moving positive ions which can rapidly be accelerated in a powerful electric field. It is the manipulation of the stream of gaseous ions that forms the basis of mass spectrometry.

You cannot analyse any liquid or solid material in this way unless it is vapourised.

The resulting (+) ions are accelerated down a tube (from + to - plates, P in left diagram) and then through a powerful magnetic field.

The charged or ionised particles are deflected by this powerful magnetic field (R in left diagram).

How much they are deflected depends on the particle mass and the speed of the particle and the strength of a magnetic field i.e. lighter particles of lower mass (and momentum) are deflected more than heavier particles of bigger mass (see right diagram below) for a given set of conditions.

By varying the strength of the magnetic field, it is possible to bring into focus onto an ion detector (N in diagram above right) at the end of the tube (effectively an electrical event is detected), every possible mass in turn and a measure the strength of the ion current, which is a measure of how much of that ion has been formed from the sample under analysis.

Simplified diagrams of a mass spectrometer tube system are shown above and below with further explanation as to what is going on and an extra diagram to show the relative paths of light to heavy ions for a given strength of magnetic field.

KEY TO DIAGRAM and more detail of each component's function

K = sample injection point, it must be a gas, so a liquid/solid must be vaporised at the injection point.

IONISATION

Q = high voltage (high +/- p.d.) electron gun which fires a beam of high speed/energy electrons from a heated 'metal element' into the vaporised sample under analysis and causes ionization of the atoms (or molecules) forming positive ions (mainly monopositive in charge).

The collision of high KE electrons with atoms or molecules causes another electron to be knocked off the particle leaving a negative deficit i.e. a positively charged particle is formed e.g.

M_(g) + e^- ==> M⁺_(g) + 2e^-, usually written as just

M_(g) ==> M⁺_(g) + e^- (M might represent e.g. a metal atom or a molecule)

The ions formed should be written as [M]⁺, a notation that is handy if you are dealing with ionised molecule fragments with an overall single positive charge e.g. [CH₃]⁺ is seen in the mass spectrum of methane gas, CH₄.

The low pressure (~vacuum) is needed to prevent the ions from colliding with air particles which would stop them reaching the ion detector system.

ACCELERATION

P = are negative plates which accelerate the positive ions down the tube (there are positive plates at the start of the tube). A moving beam of charged particles creates a magnetic field around itself, and this 'ion beam' magnetic field interacts with the magnetic field at R.

DEFLECTION-SEPARATION

R = the magnetic field that causes deflection of ions, this is can be varied to change the extent of deflection for a given mass and to focus a beam of ions of particular mass down onto the detector. Hence, by programming the mass spectrometer to 'sweep' through all likely particle masses, in terms of the right hand diagram, you can increase the strength of the magnetic field to bring into focus onto the ion detector monopositive ions of increasing mass.

DETECTION

N = an ion detection system which essentially generates a tiny electrical current when the ions hit it. The minute electric current which can be amplified. The strengths of the 'electronic' signals from the various ion peaks are sent to a computer for analysis, computation and display. They tell you the particle masses present and their relative abundance (see the mass spectrum diagram for the element strontium below). The data is then presented as an m/z versus peak height.

m/z means the relative mass of the ion over its charge, which for our purposes the electric charge is +1 (lower case z) and the mass (lower case m) is the relative atomic/formula mass of the particle ionised. You should write the structure of the ion in square brackets and put the charge on the outside of them in the top right - this is an important and universally accepted notation in mass spectrometry.

Examples of m/z values (mass/charge ratio) m/z values apply to ALL methods of mass spectrometry (see TOF later)

Other terms used in mass spectroscopy:

Monatomic (mononuclear ions) are derived from single atoms eg [³⁵Cl]⁺ or [⁸⁸Sr]⁺ and a molecular ion (polynuclear ion) is derived from when the molecule is more than one atom i.e. a complete but ionised molecule (molecular ion) e.g. the complete molecules minus one electron to give a singly charged positive ion OR the positive residue left when one of more electrons are broken off to leave a molecular fragment ion)

Molecular ions: [Cl₂]⁺ from chlorine molecules, [C₆H₅COOH]⁺ from benzoic acid molecules

Fragment ions: [CH₃CH₂]⁺, an ethyl fragment from the fragmentation of a hydrocarbon in a mass spectrometer.

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See also Spectroscopy indexes: IR, mass, H-NMR & C-13 NMR spectra of organic compounds

3. Examples of a MASS SPECTRUM explained

The resulting record of the ion peaks is called the mass spectrum or mass spectra. The highest peak is called the base peak and is often given the relative and arbitrary value of 100, particularly in the mass spectra of organic compounds.

MASS SPECTRA

For elements you get a series of signals or ion peaks for each isotope present and the ratio of peak heights gives you the relative proportion of each isotope in the element so that you can calculate the relative atomic mass of an element. This 'simple' spectra of mononuclear ions like [Sr]⁺ is only true for non-molecular elements like metals (see mass spectrum of strontium diagram below) or noble gases, but for molecular elements like nitrogen or the halogens things are not so simple (see chlorine example below).

The proportions or percentages of all the isotopes of an element is often called the isotopic abundance.

For larger e.g. organic molecules, things can be very complex indeed, as molecules fragment and many different ions can be formed BUT you can get the relative molecular mass of a molecule by identifying what is called the molecular ion peak, that is, when one electron is knocked of the molecule but the molecule retains its full molecular structure.

e.g. benzoic acid (M_r = 122) gives a molecular ion peak of m/z = 122, due to [C₆H₅COOH]⁺

but you also get fragments such as [C₆H₅]⁺ with an m/z of 77 as the molecule breaks up from further electron impacts on the molecular ion and larger fragments formed, so you get quite a complex degradation of the original molecule (diagram below showing many of the fragments formed).

So thing get very complicated with organic molecules! (full details of mass spectrum of benzoic acid)

 The fragmentation pattern is characteristic for a particular molecule (and can be used for identification), BUT the fragmentation pattern is also dependent on experimental conditions e.g. lower/higher laser or electron beam ionisation energy results in lesser/more fragmentation.

'Soft ionisation' is where use a low ionisation energy to give a greater chance of measuring, M, the mass of the molecular ion peak, and a very accurate value of M, to 3-4 decimal places can itself be used to identify the molecule.

For more on this see: section 8. Introduction to more details on the mass spectra of organic compounds and the molecular ion peak and fragmentation - use in identification of organic molecules

and 9. Isotopic masses and accurate molecular ion peaks to identify molecules and molecular formulae

More highly charged ions can show up in mass spectra

You can get multiple ionisation e.g.³⁵Cl²⁺(m/z = 35/2 = 17.5), ¹⁶O²⁺(m/z = 16/2 = 8), ³²S²⁺(m/z = 32/2 = 16) etc. These more highly charged ions would be deflected or accelerated more in the mass spectrometer than the monopositive ions. In the mass spectrometer the monopositive ions are selected to produce the mass spectrum.

You should note that e.g. the m/z for ³²S²⁺(m/z = 32/2 = 16) is identical to the m/z for ¹⁶O⁺ (m/z = 16/1 = 16). In a low resolution mass spectrometer they would not be distinguishable, but in a very modern high resolution mass spectrometer they would be.

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4. CHLORINE EXAMPLE

The mass spectrum of chlorine is a good example of a molecular element whose mass spectra can be a bit tricky when first encountered.

Chlorine consists of two principal stable isotopes, chlorine-37 (~25% is ³⁷Cl) and chlorine-35 (~75% is ³⁵Cl).

A_r(Cl) is ~35.5 using the above percentages

from A_r(Cl) = [(75 x 35) + (25 x 37)] / 100

BUT, chlorine consists of Cl₂ diatomic molecules, which may or may not split on ionisation, so how can we explain the presence of five peaks and not just two for the two isotopes?

The result of the ionisation process and subsequent fragmentation of chlorine molecules is a series of 5 different mass peaks from the various isotopic monatomic or molecular ion possibilities.

So, in order of decreasing mass (m/z for a monopositive charge)

1. [³⁷Cl³⁷Cl]⁺ or [³⁷Cl₂]⁺ m/z = 74  (1-3 are molecular ions)

2. [³⁷Cl³⁵Cl]⁺ m/z = 72 (note that you must show the two isotopes separately in this molecular ion)

3. [³⁵Cl³⁵Cl]⁺ or [³⁵Cl₂]⁺ m/z = 70  (molecular ion)

4. [³⁷Cl]⁺ m/z = 37  (mononuclear ion, monatomic fragment)

5. [³⁵Cl]⁺ m/z =35  (mononuclear ion, monatomic fragment)

Reminder: (i) m/z means the relative mass of the ion over its charge (m/z explained), (ii) monatomic/mononuclear ions are derived from one atom, (iii) a molecular ion is derived from more than one atom.

So, the presence of five peaks is explained and the ratio of the peak heights can be explained by considering a simple probability table of all the permutations possible for the monatomic or molecular ions - remember in a mass spectrometer you are dealing with millions of 'randomised' particles.

m/z	35Cl	35Cl	35Cl	37Cl
35Cl	70	70	70	72
35Cl	70	70	70	72
35Cl	70	70	70	72
37Cl	72	72	72	74

The ratio of heights for peaks 4 and 5 of the monatomic ions is 1 : 3, the ratio of the isotopic abundance in the original naturally occurring sample of chlorine atoms in compounds..

For the diatomic molecular ions, (left table of possibilities) we assume (for simplicity) that exactly 3/4 (75%) of the chlorine isotopes are ³⁵Cl and 1/4 (25%) of the isotopes are ³⁷Cl.

This gives an expected ratio of the molecular ions 70 : 72 : 74 of 9 : 6 : 1, and this is what you observe for peaks 1 to 3.

The ratio of the heights of the first set of peaks (1-3) to the heights of the 2nd set (4-5) depends on the energy and intensity of the ionising beam of electrons. The greater this is, the greater the fragmentation of the molecules i.e. peaks 1-2 would increase and peaks 3-5 would decrease relative to each other, BUT, the height ratios would stay the same in each set i.e. the monatomic/mononuclear ions and the diatomic molecular ions.

For identifying molecules from a fingerprint pattern you should operate the mass spectrometer under the same conditions i.e. standards and unknowns compared under the same operating conditions to give reproducible mass spectra.

Other examples and explanation of the calculation of the relative atomic mass of an element using % of isotopes is given in Part 1 of GCSE-AS (basic) calculations.

The simplest and best example on this page of calculating relative atomic mass from a mass spectrum is fully explained for the metallic element strontium.

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5. STRONTIUM EXAMPLE

Using mass spectra data to calculate relative atomic mass from the mass spectrum of strontium

A 'simple' element mass spectrum to interpret AND a subsequent relative atomic mass calculation based on the mass spectroscopy of the element strontium

The relative atomic mass of an element, A_r, is the weighted average mass of the isotopes present, compared to 1/12th of the relative mass of the carbon-12 isotope. [ ¹²C is given the relative mass value of 12.0000 ]

Quite often the highest m/e peak is arbitrarily given the relative value of 100, as in this case and referred to as the base peak, but the peak lines might well indicate % abundance of isotopes. The diagram of abundances is sometimes called a stick diagram.

Relative peak height = relative abundance as measured from the ion current detector signal.

The mass spectrum shows strontium consists of four isotopes giving rise to four positive ions - relative peak heights of

⁸⁴Sr (peak height = 0.68), ⁸⁶Sr (peak height = 12.0),⁸⁷Sr (peak height = 8.47) and ⁸⁸Sr (peak height = 100.0)

The sum of the heights = 0.68 + 12.0 + 8.47 + 100.0 = 121.15

So we can now calculate the weighted average mass of ALL the isotopes.

Therefore A_r = {(0.68 x 84) + (12.0 x 86) + (8.47 x 87) + (100.0 x 88)}/121.15 =  87.7

The book value is 87.62, BUT this calculation does NOT take into account the very accurate relative atomic masses based on the carbon-12 scale, it merely uses the mass numbers, which are always integer.

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6. Potassium - Another relative mass calculation from mass spectrometry

Potassium has three naturally occurring isotopes, stable ³⁹K and ⁴¹K, and the long-lived ⁴⁰K (half-life of millions of years!)

The mass spectrum of potassium generated the following data:

More on relative atomic mass calculations

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7. The mass spectrum of bromine Br₂

You get five peaks in the spectra of bromine molecules. For molecules completely atomised you get two peaks (m/z) of almost equal height from [⁷⁹Br]⁺ and [⁸¹Br]⁺ mononuclear ions.

Because its ~50% of each isotope, the relative atomic mass of bromine is ~80 and hence the equality of peaks 1 [⁷⁹Br]⁺ and 2 [⁸¹Br]⁺ from the monatomic ions from the fragmentation and ionisation of bromine molecules.

However, as with chlorine, molecular bromine is also ionised without fragmentation, giving rise to three more ion permutations (3 more m/z peaks).

[⁷⁹Br⁷⁹Br]⁺ (158), [⁷⁹Br⁸¹]Br⁺ (160) and [⁸¹Br⁸¹Br]⁺ (162)

So! the presence of all five peaks is explained in the mass spectrum of bromine, and, because you are dealing with millions of randomised ionised atoms, the ratio of the two monatomic peaks can be used to accurately determine the relative atomic mass of bromine.

The data book quotes for the stable isotopes: ⁷⁹Br (50.69%) and ⁸¹Br (49.31)

The ratio of the heights for the monatomic ions in the mass spectrum of bromine would 50.69 : 49.31 ~ 1 : 1 as observed.

A_r(Br) = (50.69 x 79) + (49.31 x 81) / 100 = 79.90

 and this is what you observe in the mass spectrum of bromine.