Mass spectrometry Mass spectrometry is used widely in science. It is used How a mass spectrometer works to determine the structure of chemical compounds, to A mass spectrometer at its simplest level records what identify the components of mixtures – even as happens when a sample is bombarded with high complex as blood – and to search for chemicals in energy electrons. Compounds fragment under the remote places such as outer space. bombardment to give positively charged ions. These The range of uses for this technique is vast. ions are then fed into an analyser tube that is However, you can get a flavour when you realise that surrounded by a magnet. The ions are deflected by mass spectrometry has been used to date geological the magnetic field and then follow a circular path, samples, identify the organic compounds in oil and the radius of which depends on what is called their hence the source of the oil, characterise the tar from mass/charge ratio (m/z). Ions with a smaller m/z – ie the Mary Rose, check for contaminants in foodstuff, lighter ions – are deflected more than those with a detect traces of metal ions in seawater, examine dyes larger m/z – ie heavier ions. The ions are focused into in art treasures, study large fragements of DNA, and a detector and recorded. The detector records the determine the presence of hallucinogens in blood. mass spectrum which is a measure of the number of Many chemical laboratories have a mass spectrometer ions of a particular mass. Every sample has its own and data are collected routinely and used by many characteristic mass spectrum. chemists and other scientists as they go about their There are two ways of determining the m/z values work. of the ions produced. By scanning the magnetic field The scientific principles behind mass – single focusing – and by scanning both the spectrometry are simple (Fig. 1). The heart of the magnetic field and the electric field – double technique involves the generation of ions, which can focusing. then be detected. The sophistication comes in the methods that are used to generate the ions and the Producing the ions ways of detecting them. Until several years ago mass The most common ionisation method is electron spectrometers were expensive and large, but new ionisation (EI). This is a very simple method where instruments have been developed recently which sit the electrons used in the bombardment come from a on bench tops and can provide data for many heated filament (Fig. 2). The filament is made the purposes. cathode so that the electrons that are produced Sample Sample Heated Repeller molecules filament (+) Plate set at –ve potential Repeller plate To ion Ion source or analyser ionisation chamber Electron gun Slit system Magnetic Anode field Heavier Figure 2 particles Particles of accelerate towards the anode. As they do so they intermediate interact with the vaporised sample. The high energy mass Ion detector electrons knock out electrons from the sample to Lighter form ions. particles – → + – – M(g) + e M (g) + e + e The quadrupole approach To vacuum pump As well as conventional single and double focusing instruments there are also quadrupole spectrometers Recorder (Fig. 3). As in other spectrometers the sample is Amplifier introduced as a gas or a vapour at low pressure. A small number of the atoms or molecules that comprise the sample is ionised and the ions are Figure 1 accelerated into the quadrupole section. 1 Some formulae corresponding to nominal m/z = 122 Formulae Actual mass C4H4N5 122.046668 C4H10O4 122.057903 C6H4NO2 122.024201 C6H6N2O 122.048010 C6H8N3 122.071819 C7H6O2 122.036776 C7H8NO 122.060585 C7H10N2 122.084394 C8H10O 122.073161 C8H12N 122.096970 C H 122.109545 Figure 3 9 14 These are based on the following relative The quadrupole section is made up of four rods atomic masses: across which an electric field is applied. By altering this field only those ions within a specific m/z range, C = 12.0000000 so called ‘stable’ ions, are allowed to pass through the H = 1.0078246 quadrupole and these ions are collected by the ion N = 14.0030738 detector. In this way the quadrupole instrument acts O = 15.9949141 as a mass filter. Figure 4b High resolution High resolution is needed for the precise peak in the mass spectrum of the unknown compound determination of atomic masses in isotopic abundance shown is 122.036776 (Fig. 4a). This can be assigned studies, in the identification of organic compounds the molecular formula C7H6O2 if it is assumed that and in the mass spectroscopic analysis of solids. The only carbon, hydrogen, nitrogen and oxygen are most common use of high resolution mass present (Fig. 4b). However, which isomer is present spectrometry is for the direct determination of cannot be identified without further information. elemental compositions. These high resolution measurements are done in double focusing mass Identifying compounds spectrometers with high resolving power. The The mass spectrum that is produced is characteristic principles of these spectrometers are identical to of that particular sample. The mass peaks provide the those of single focusing machines, but they include an information that enables identification of the sample electric focusing sector before separation by the from the molecular ion peak and from the magnetic field. characteristic fragmentation pattern. These instruments can record accurate relative Mass spectra give characteristic isotope patterns masses which enable the molecular formulae to be according to mass and natural abundance. For assigned to compounds. For example, the molecular example, the presence of chlorine in a sample is Unknown 100 105 100 122 77 35 % 50 % 50 51 37 28 39 57 65 94 0 0 20 40 60 80 100 120 20 40 60 m/z m/z Figure 4a Figure 5 2 + usually easy to determine (Fig. 5). The two isotopes of a peak is expected at m/z =77 corresponding to C6H5 . chlorine have mass numbers of 35 and 37 respectively This peak is often observed and it is present in the and they occur naturally in the ratio 76:24. Thus the spectrum of both benzoic acid and methyl benzoate appearance of two peaks differing by two mass units (Figs. 6a and b). However, bond breaking occurs more and in the approximate ratio 3:1 suggests the presence frequently one bond away from the benzene ring (see of a chlorine atom in the compound. fragment at 105). Methyl benzoate and benzoic acid Some characteristic fragmentation patterns can have similar fragmentation patterns and mass spectra. be illustrated by using the example of two The compounds in this case can be identified from monosubstituted aromatic compounds – benzoic acid the mass of the molecular ion (136 for methyl and methyl benzoate. For monosubstituted aromatics benzoate and 122 for benzoic acid). m/z Ion 100 105 + + OH 122 M O C 122 105 (M-OH)+ +O C 77 + 77 (M-OH-CO)+ C6H5 % 50 + C H + 51 (M-OH-CO-C2H2) 4 3 51 28 39 57 65 94 0 20 40 60 80 100 120 m/z Figure 6a m/z Ion 136 M+ +O O—CH C 3 105 105 (M-OCH )+ +O 100 3 C + + 77 (M-OCH3-CO) C6H5 + + 51 (M-OCH3-CO-C2H2) C4H3 77 136 51 % 50 15 39 92 0 20 40 60 80 100 120 140 m/z Figure 6b 3 Other ionisation techniques masses of whole proteins. This is due to advances in There are cases where EI cannot be used to form ions ionisation methods and by applying analyses that can because the molecule breaks up into smaller fragments cope with massive ions. Pushing back the upper mass and no molecular ion is seen. In these cases it is limits is just one example of the rapid advances taking possible to use a ‘soft’ ionisation technique. Two of place in mass spectrometry. The detection limits of the most popular techniques are chemical ionisation modern mass spectrometers continue to improve to (CI) and fast atom bombardment (FAB). These such an extent that very small amounts can be alternative techniques give more molecular mass detected – ie 10–15 mol. information and the spectra recorded from each Mass spectrometry is often combined with other technique give different fragmentation patterns. techniques – eg gas chromatrography and high In CI a reagent gas such as methane, performance liquid chromatography, hence mass methylpropane or ammonia is ionised by electron spectrometry can be regarded as a family of methods bombardment and is then allowed to react with a with the common aim of measuring the masses of ions. neutral molecule to produce a molecular ion. The choice of reagent gas depends on the ease of Abundantly obvious? fragmenting the sample. The following example shows how the information in a mass spectrum enables the identification of a CH + e– → CH + + e– + e– 4 4 compound (Fig. 7). then The abundance of the peaks at m/z= 96 and 98 raises the question of whether the peak at 100 is the CH + + CH → CH + + CH • 4 4 5 3 molecular ion peak, rather than an isotopic peak. A M + CH + → MH+ + CH 5 4 glance at m/z= 35 and m/z= 37 shows the presence of Fast atom bombardment is useful if the sample is peaks in the ratio 3:1, strongly suggesting chlorine. susceptible to decomposition when heated. This The abundance of the peaks at m/z= 96, 98 and 100 technique involves no heating. An atom beam with in the ratio of 9:6:1 suggests that two chlorine atoms high kinetic energy, usually xenon atoms, is used to are present.