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3.8.6    Mass spectrometry

The principal types of mass analysis in current use are magnetic sector, quadrupole, ion trap and time of flight.

In the magnetic sector instrument ions with a mass to charge ratio m/z are transmitted through the instrument such that:

Where B is the magnetic field, r the radius of curvature of the path through the magnetic field and V is the voltage with which the ions are accelerated towards the source exit slit. The mass range is normally scanned by varying the magnetic field. The double focusing instrument also contains an electric sector in addition to a magnetic sector, which provides energy focusing of the ions either before (conventional geometry) or after (reverse geometry) the magnetic sector. Such instruments are capable of high resolution (up to 200 000), where resolution (R) is defined as:

Where M1 and M2 are the masses of two overlapping ion peaks. The overlap between the two ion peaks is normally defined as 10% of the ion peak intensities, which for definition purposes, are equal.
   The quadrupole mass analyser (of which the ion trap is a three dimensional variant) consists of four hyperbolic rods arranged in a parallel radial array. Each pair of opposite rods are electrically connected to a d.c. voltage on which an oscillating radio-frequency voltage is superimposed. Ions introduced into the quadrupole field undergo oscillations and for a certain value of these fields a stable trajectory will result for ions of a particular m/z value resulting in their transmission to the detector. The mass range is scanned by varying the d.c. and RF fields whilst keeping the voltage ratio and oscillator frequency constant. This produces a low resolution spectrum. In the ion trap ions are stored within the trap in stable trajectories. Raising the RF potential renders successive m/z values unstable; they are then ejected from the trap and detected.
   In the time of flight mass spectrometer, which is generally of low resolution, ions of different masses are separated by virtue of their different velocities after acceleration through a potential (V). The velocity v of an ion of mass m is given by the equation:

Following acceleration of a pulsed beam of ions, those of different mass to charge ratio will arrive at different times, the lightest ions being the first to arrive.

The most common mode of ionisation is electron impact ionisation. Molecules in the gas phase (at a pressure of typically 10⁻⁴ Pa) are subjected to bombardment by electrons, normally at an energy of 70 eV, generating a radical cation.

This form of ionisation normally imparts considerable energy to the molecular ion; this causes decomposition with the formation of fragment ions which may themselves fragment to produce a characteristic fragmentation pattern, the mass spectrum. Compounds may be identified by their mass spectra and libraries of such electron impact spectra are available, NIST (1992), Eight Peak Index (1991), McLafferty and Stauffer (1989), to enable identification of unknown compounds to be carried out.

Chemical ionisation is a less energetic (soft) form of ionisation producing much less fragmentation. A reactant gas (R) at a relatively high pressure (100 Pa) is ionised by electron impact as shown:

R + e.  R+ ·

(R + H)+

An ion molecule reaction then produces ionisation of compound molecule M:

The technique of fast atom bombardment is a particularly soft form of ionisation. A beam of heavy ions such as Ar^+. is produced by ionising argon atoms and passing them through an electric field to accelerate them. These fast ions are then passed through a chamber containing argon where charge exchange occurs:

The beam of fast atoms then bombards a metal plate coated with the sample and the high kinetic energy of the atoms is transferred to the sample molecules on impact. This energy is dissipated in various ways, some of which leads to volatilisation and ionisation of the sample.

Sample introduction into the mass spectrometer may be accomplished by a variety of means depending on the physical state of the sample. For pure compounds the gas inlet, heated inlet (for liquids) and direct introducton probe (for solids) are used. For mixtures of volatile compounds the mass spectrometer is linked to a gas chromatograph (GC). The use of packed GC columns, with high carrier gas flow rates, requires the use of a separator (McFadden, 1979, 2–16) to preferentially remove the majority of the carrier gas whilst allowing the majority of the sample component to be transferred to the mass spectrometer. Capillary GC columns have sufficiently low flow rates to permit them to be directly coupled to the mass spectrometer.

The analysis of thermally labile or involatile compounds may be achieved using a liquid chromatograph (LC) linked to the mass spectrometer. Various types of separator are available to separate the LC mobile phase from the analyte compound, some of which enable additional ionisation techniques to be carried out, normally leading to soft forms of ionisation. Principal types of separation include moving belt, direct liquid introduction, thermospray, particle beam and electrospray (Niessen and van der Greef, 1992).

The table below is for guidance only since at any given mass isobaric ions of many different elemental compositions can occur.

m/z	Possible associated      group	Possible inference
15	CH3	—
18	H2O	—
26	C2H2	Hydrocarbon
27	C2H3	Hydrocarbon
28	CO	Carbonyl
28	C2H4	Ethyl
28	N2	Azo
29	CHO	Aldehyde
29	C2H5	Ethyl
30	CH2NH2	Primary amine
30	NO	Nitro or nitroso
31	CH2OH	Primary alcohols or methoxy
32	CH4O	—
35/37 (3:1)	35Cl, 37Cl	Chloro
36/38 (3:1)	35ClH, 37ClH	Chloro
39	C3H3	Hydrocarbon
40	Ar	Air constituent
40	C3H4	Hydrocarbon
41	C3H5	Hydrocarbon
42	C2H2O	Acetates or acetyl
42	C3H6	Hydrocarbon
43	CH3CO	CH3COX
43	C3H7	C3H7X
44	CO2	Background (air), carbonates or anhydrides
44	C2H6N	Some aliphatic amines
44	OCNH2	Primary amides
44	CH2CH(OH)	Some aldehydes
45	CH2OCH3	Some ethers
45	CH3CHOH	Some alcohols
45	OCH2CH3	Ethoxy
45	CO2H	Acids
46	NO2	Nitro
47	CH2SH	Aliphatic thiol
47	PO	Phosphoryl
49/51 (3:1)	CH2Cl	Chloromethyl
50	C4H2	Aromatic
51	C4H3	C6H5X
55	C4H7	Some hydrocarbons
55	C3H3O	Some cyclic ketones
56	C4H8	Hydrocarbon
57	C4H9	C4H9X
57	C2H5CO	Ethyl ketone or propionate ester
58	CH2C(OH)CH3	Some methyl ketones or dialkyl ketones
58	C3H8N	Some aliphatic amines
59	COOCH3	Methyl ester
59	CH2C(OH)NH2	Some primary amides
59	C2H5CHOH	C2H5CH(OH)—X
59	CH2O—C2H5	Some ethers
60	CH2C(OH)OH	Some carboxylic acids
61	CH3CO(OH2)	Acetate esters CH3COOCnH2n+1 (n >1)
61	CH2CH2SH	Aliphatic thiol
65	C5H5	Benzyl, phenols or anilines
66	C5H6	Aromatic
66	H2S2	Dialkyl disulphide
68	CH2CH2CH2CN	Some pyrroles
69	C5H9	Some hydrocarbons
69	CF3	Fluorinated alkanes
70	C5H10	Hydrocarbons
71	C5H11	C5H11X
71	C3H7CO	Propyl ketone or butanoate ester
72	CH2C(OH)C2H5	Some ethyl alkyl ketones
72	C3H7CHNH2	Some amines
73	C4H9O	Alcohols, ethers
73	COOC2H5	Ethyl esters
73	CH2CHC(OH)OH	Aliphatic acids
73	(CH3)3Si	(CH3)3SiX
74	CH2C(OH)OCH3	Some methyl esters
75	(CH3)2Si OH	(CH3)3SiOX
75	C2H5CO(OH2)	C2H5COOCnH2n + 1 (n > 1)
76	C6H4	C6H5X or XC6H4Y
77	C6H5	C6H5X
78	C6H6	C6H5X
78	C5H4N	Some pyridines
79	C6H7	C6H5X
79/81 (1:1)	Br	Bromo compounds
80	C5H6N	Pyrroles
80/82 (1:1)	HBr	Bromo compounds
81	C5H5O	Furans
83	C4H3S	Monosubstituted thiophenes
83/85/87	HCCl2	CHCl3 or X—CHCl2
(9:6:1)
85	C6H13	C6H13X
85	C4H9CO	C4H9COX
85
85
86	CH2C(OH)C3H7	Some propyl alkyl ketones
86	C4H9CHNH2	Some amines
87	CH2CHC(OH)OCH3	XCH2CH2COOCH3
88	CH3CH2CH2COOH	C3H7COOCnH2n+1 (n > 1)
89	C7H5	Heterocyclics containing N and O
90	C7H6	Heterocyclics containing N and O
91	C7H7	C6H5CH2X
91/93 (3:1)	C4H8Cl	n-alkyl chloride ( hexyl)
92	C7H8	C6H5CH2X
92	C6H6N	Monoalkylpyridines
93	C6H5O	Phenols or nitrobenzenes
93	C6H7N	C6H5NHX
93	C7H9	Mono and sesquiterpenes
93/95 (1:1)	CH2Br	—
94	C6H6O	C6H5O-alkyl (alkyl ≠ CH3)
95	C6H7O
95	C7H11	Mono and sesquiterpenes
96	C5H4NO
97	C5H5S	Methyl or mono-alkyl thiophenes
99	C7H15	C7H15X
103	C6H5CHCH	C6H5CHCHX
105	C6H5CO	C6H5COX
105	C8H9	CH3—C6H4CH2X
106	C7H8N
107	C7H7O
107/109 (1:1)	C2H4Br	BrCH2CH2-X
111	C5H3OS
121	C6H5CO2	C6H5CO2X
121	C8H9O	CH3OC6H4CH2X
122	C6H5COOH	Alkyl benzoates
123	C6H5COOH2	Alkyl benzoates
127	C10H7	Naphthyl
127	I	Iodo compounds
128	HI	Iodo compounds
130	C9H8N
131	C6H5CHCHCO	C6H5CHCHCOX
135/137 (1:1)		n-alkyl bromide ( > hexyl)
141	CH2I	CH2IX
147	(CH3)2SiOSi(CH3)3	[(CH3)3SiO]n derivatives, n > 1
149		Dialkyl phthalates
160	C10H10NO
190	C11H12NO2

The fluorinated alkane mixture perfluorokerosene (PFK) is used for the calibration of the mass spectrometer mass scale in both low and high resolution modes. The accurate masses of the principal reference ions of PFK (high boiling) are listed below together with typical relative intensity values.

The listed masses are based on ¹²C = 12.0000 daltons
.

References

Eight Peak Index of Mass Spectra (1991) 4th edn, Mass Spectrometry Data Centre, Royal Society of Chemistry, Cambridge.
W. H. McFadden (1979) J. Chrom. Sci, 17, 2–16.
F. W. McLafferty and D. B. Stauffer (1989) Wiley/NBS Registry of Mass Spectral Data, Wiley, Chichester.
W. M. A. Niessen and J. van der Greef (1992) Liquid Chromatography—Mass Spectrometry, Principles and
    Applications, Chromatographic Science Series, 58, Marcel Dekker, New York.
NIST/EPA/NIH Mass Spectral Data Base (1992) National Institute of Standards and Technology, Gaithersburg.

K.S. Webb

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