Mass Spectroscopy

MASS SPECTROSCOPY

R W Grime Ripon Grammar School

strong electromagnet

detector

electric field toaccelerate and focus

electron gun

sample

HIGH RESOLUTION MASS SPECTROSCOPY

• Molecular ion formed: M → [M]+. + e–

• m/z value for [M]+. gives the Mr.

• High resolution measures m/z to enough accuracy to find the molecular formula.

e.g. Mr = 60

CH3COOH 60.02112 molecular formula = C2H4O2

NH2CONH2 60.03235 molecular formula = CH4N2O

CH3CH2CH2OH 60.05751 molecular formula = C3H8O

CH3CH(OH)CH3 60.05751 molecular formula = C3H8O

FRAGMENTATION

• Molecular ion fragments due to covalent bonds breaking: [M]+. → X+ + Y.

• Much information from fragmentation patterns – can often work out structure.

• Relatively stable ions such as carbocations R+ and acylium ions [R-C=O]+ are

common.

• The more stable the ion, the greater the peak intensity.

m/z 15 29 31 43 57

fragment CH3+ CH3CH2

+ CH2OH+ CH3CH2CH2+

(CH3)2CH+

CH3CO+

CH3CH2CO+

butane

CH3CH2CH2CH3

m/z

43

10 20 30 40 50 60 70 80 90 100 110

58

29

C4H10 [C4H10]+. + e–

m/z 58

[C4H10]+. [CH3CH2CH2]+ + .CH3

m/z 43

[C4H10]+. [CH3CH2]+ + .CH2CH3

m/z 29

[C4H10]+.

[CH3CH2CH2]+

[CH3CH2]+

ISOTOPES

• Ions with different isotopes produce different signals.

• Most important for Br and Cl.

Br: 50% 79Br 50% 81Br

Cl: 75% 35Cl 25% 37Cl

• If two or more Br/Cl atoms, then spectrum is more complicated.

• Often small peaks at m/z + 1 due to 2H and 13C.

bromomethane

CH3Br

CH381Br [CH3

81Br]+. + e–

m/z 96

[CH3Br]+. [CH3]+ + .Br m/z 15

m/z

96

10 20 30 40 50 60 70 80 90 100 110

15

94

CH379Br [CH3

79Br]+. + e–

m/z 94

[CH381Br]+.[CH3

79Br]+.[CH3]+

chloroethane

CH3CH2Cl

CH3CH237Cl [CH3CH2

37Cl]+. + e–

m/z 66

[CH3CH237Cl]+. [CH2

37Cl]+ + .CH3

m/z 51

m/z

66

10 20 30 40 50 60 70 80 90 100 110

28

64

51

49

29

CH3CH235Cl [CH3CH2

35Cl]+. + e–

m/z 64

[CH3CH235Cl]+. [CH2

35Cl]+ + .CH3

m/z 49

[CH3CH2Cl]+. [CH3CH2]+ + .Cl m/z 29

[CH3CH237Cl]+.

[CH3CH235Cl]+.

[CH237Cl]+

[CH235Cl]+

[CH3CH2]+

dichloromethane

CH2Cl2

[CH237Cl35Cl]+.[CH2

37Cl]+

[CH235Cl]+

m/z 10 20 30 40 50 60 70 80 90 100 110

51 84

88

86

49

[CH235Cl2]+.

[CH237Cl2]+.

37Cl : 35Cl = 1 : 3

37Cl2 : 35Cl37Cl : 35Cl2 = 1 : 6 : 9

dibromomethane

CH2Br2

81Br : 79Br = 1 : 1

81Br2 : 81Cl79Br : 79Br2 = 1 : 2 : 1

176 [CH281Br2]+.

174 [CH281Br79Br]+.

172 [CH279Br2]+.

95 [CH281Br]+93 [CH2

79Br]+

butanone

CH3CH2COCH3

CH3CH2COCH3 [CH3CH2COCH3]+. + e–

m/z 72

[CH3CH2COCH3]+. [CH3CH2CO]+ + .CH3

m/z 57

m/z

72

10 20 30 40 50 60 70 80 90 100 110

57

43

29

[CH3CH2COCH3]+. [CH3CO]+ + .CH2CH3

m/z 43

[CH3CH2COCH3]+. [CH3CH2]+ + .COCH3

m/z 29

[CH3COCH2CH3]+.

[COCH2CH3]+

[CH3CO]+

[CH3CH2]+

methyl butanoate

CH3CH2CH2COOCH3

CH3CH2CH2COOCH3 [CH3CH2CH2COOCH3]+. + e–

m/z 102

[CH3CH2CH2COOCH3]+. [CH3CH2CH2COO]+ + .CH3

m/z 87

[CH3CH2CH2COOCH3]+.

m/z

102

10 20 30 40 50 60 70 80 90 100 110

59

43

2987

71

[CH3CH2CH2COOCH3]+. [CH3CH2CH2CO]+ + .OCH3

m/z 71

[CH3CH2CH2COOCH3]+. [COOCH3]+ + .CH2CH2CH3

m/z 59

[CH3CH2CH2COOCH3]+. [CH3CH2CH2]+ + .COOCH3

m/z 43

[CH3CH2CH2COOCH3]+. [CH3CH2]+ + .CH2COOCH3

m/z 29

[CH3CH2CH2COO]+

[CH3CH2CH2CO]+

[COOCH3]+.

[CH3CH2CH2]+

[CH3CH2]+

H C

O

O CH2 CH2 CH3

H C

O

O CH CH3

CH3

CH3 C

O

O CH2 CH3

CH2 C

O

O CH3CH3

propyl methanoate

methylethyl methanoate

ethyl ethanoate

methyl propanoate

H C

O

O CH2 CH2 CH3

propyl methanoate

m/z 59[OCH2CH2CH3]+

m/z 43[CH2CH2CH3]+

m/z

10 20 30 40 50 60 70 80 90 100 110

59

29

88

57

15

m/z 29[HCO]+

m/z 45[HCOO]+

methylethyl methanoate

m/z 59[OCH2(CH3)2]+

m/z 43

H C

O

O CH CH3

CH3

[CH2(CH3)2]+

m/z

10 20 30 40 50 60 70 80 90 100 110

59

29

88

57

15

m/z 29[HCO]+

m/z 45[HCOO]+

ethyl ethanoate

m/z 45[OCH2CH3]+

m/z 29[CH2CH3]+

CH3 C

O

O CH2 CH3

m/z 43[CH3CO ]+

m/z 59[CH3COO ]+

m/z

10 20 30 40 50 60 70 80 90 100 110

59

29

88

57

15

methyl propanoate

m/z 31[OCH3]+

m/z 29[CH2CH3]+

m/z 57[CH3CH2CO ]+

m/z 59[CH3OCO ]+

CH2 C

O

O CH3CH3

m/z

10 20 30 40 50 60 70 80 90 100 110

59

29

88

57

15