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Chapter: 3 Chemistry
    Section: 3.8 Molecular spectroscopy
        SubSection: 3.8.6 Mass spectrometry

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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:


 

m/z

 =

B2r2

2V


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:

 

R

 =  

M1

M1 − M2


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:


 

v = √2zV/m

 

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.


 

M + e.M+· + 2e.

 

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:


 

(R + H)+ + M   (M + H)+ + R

 

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:

 

Ar+· (fast) + Ar (thermal) → Ar (fast) + Ar+· (thermal)

 

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
.

Formula
 

m/z
 

Relative
intensity %

Formula
 

m/z
 

Relative
intensity %

           

CF

   30.9984

0.5

C11F19

492.9696

  1     

CF2

   49.9968

2    

C12F19

504.9696

  1     

CF2H

   51.0046

2    

C13F19

516.9696

 0.6 

CF3

   68.9952

100        

C11F21

530.9665

 0.7 

C2F3

   80.9952

0.5

C12F21

542.9665

 1    

C3F3

   92.9952

1   

C13F21

554.9665

0.7

C2F4

   99.9936

4   

C14F21

566.9665

0.7

C3F4

111.9936

0.1

C12F23

580.9633

0.7

C2F5

118.9920

25     

C13F23

592.9633

0.8

C3F5

130.9920

27     

C14F23

604.9633

0.7

C4F5

142.9920

1   

C15F23

616.9633

0.7

C3F6

149.9904

1   

C13F25

630.9601

0.6

C5F5

154.9920

2   

C14F25

642.9601

0.6

C4F6

161.9904

4   

C15F25

654.9601

0.7

C3F7

168.9888

15     

C16F25

666.9601

0.6

C4F7

180.9888

15     

C14F27

680.9569

0.5

C5F7

192.9888

3   

C15F27

692.9569

0.5

C6F7

204.9888

2   

C16F27

704.9569

0.6

C4F9

218.9856

7   

C17F27

716.9569

0.6

C5F9

230.9856

7   

C15F29

730.9537

0.5

C6F9

242.9856

5   

C16F29

742.9537

0.5

C7F9

254.9856

3   

C17F29

754.9537

0.5

C5F11

268.9824

4   

C18F29

766.9537

0.5

C6F11

280.9824

6   

C16F31

780.9505

0.4

C7F11

292.9824

4   

C17F31

792.9505

0.4

C8F11

304.9824

2   

C18F31

804.9505

0.4

C6F13

318.9792

1   

C19F31

816.9505

0.5

C7F13

330.9792

3   

C17F33

830.9473

0.4

C8F13

342.9792

3   

C18F33

842.9473

0.3

C9F13

354.9792

1   

C19F33

854.9473

0.3

C10F13

366.9792

0.7

C20F33

866.9473

0.3

C7F15

368.9760

1   

C18F35

880.9441

0.2

C8F15

380.9760

4   

C19F35

892.9441

0.2

C9F15

392.9760

2   

C20F35

904.9441

0.2

C10F15

404.9760

2   

C21F35

916.9441

0.2

C11F15

416.9760

0.5

C19F37

930.9409

0.2

C9F17

430.9728

3   

C20F37

942.9409

0.2

C10F17

442.9728

0.5

C21F37

954.9409

0.1

C11F17

454.9728

0.5

C22F37

966.9409

0.1

C12F17

466.9728

0.5

C20F39

980.9377

0.1

C10F19

480.9696

1   

C21F39

992.9377

0.1

           

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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