Mass Spectrometry

Summary of Fragmentation Patterns

Alkanes good M+  
14-amu fragments  
Alkenes distinct M+  
m/e = 27 CH2=CH+
m/e = 41 CH2=CHCH2+
M-15, M-29, M-43, etc... loss of alkyl
Cycloalkanes strong M+  
M-28 loss of CH2=CH2
M-15, M-29, M-43, etc... loss of alkyl
Aromatics strong M+  
m/e = 105 C8H9+
m/e = 91 C7H7+
m/e = 77 C6H5+
m/e = 65 (weak) C5H5+
Halides M+ and M+2 Cl and Br
m/e = 49 or 51 CH2=Cl+
m/e = 93 or 95 CH2=Br+
M-36, M-38 loss of HCl
M-79, M-81 loss of Br·
M-127 loss of I·
Alcohols M+ weak or absent  
M-15, M-29, M-43, etc... loss of alkyl
m/e = 31 CH2=OH+
m/e = 45, 59, 73, ... RCH=OH+
m/e = 59, 73, 87, ... R2C=OH+
M-18 loss of H2O
M-46 loss of H2O and CH2=CH2
Phenols strong M+  
strong M-1 loss of H·
M-28 loss of CO
Ethers M+ stronger than alcohols  
M-15, M-29, M-43, etc... loss of alkyl
M-31, M-45, M-59, etc... loss of OR
m/e = 45, 59, 73, ... CH2=OR+
Amines M+ weak or absent Nitrogen rule
m/e = 30 CH2=NH2+ (base peak)
M-15, M-29, M-43, etc... loss of alkyl
Aldehydes weak M+  
m/e = 29 HCO+
M-29 loss of HCO
M-43 loss of CH2=CHO
m/e = 44, 58, 72, 86, ... McLafferty rearrangement
strong M+ aromatic aldehyde
M-1 aromatic aldehyde loss of H·
Ketones M+ intense  
M-15, M-29, M-43, etc... loss of alkyl
m/e = 43 CH3CO+
m/e = 55 +CH2CH=C=O
m/e = 42, 83 in cyclohexanone
m/e = 105, 120 in aryl ketones
Carboxylic Acids M+ weak but observable  
M-17 loss of OH
M-45 loss of CO2H
m/e = 45 CO2H+
m/e = 60 ·CH2C(OH)2+
M+ large aromatic acids
M-18 ortho effect
Esters M+ weak but observable methyl esters
M-31 methyl esters loss of OCH3
m/e = 59 methyl esters CO2CH3+
m/e = 74 methyl esters CH2C(OH)OCH3+
M+ weaker higher esters
M-45, M-59, M-73, etc... loss of OR
m/e = 73, 87, 101 CO2R+
m/e = 88, 102, 116 ·CH2C(OH)OR+
m/e = 61, 75, 89 RC(OH)2+ (long alkyl ester)
m/e = 108 loss of CH2=C=O (benzyl, acetate)
m/e = 105 C6H5CO+ (benzoate)
M-32, M-46, M-60 loss of ROH (ortho effect)

 

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