mass spec

what is mass spec used for

determine molecular weight of a substance

what happens in mass spec

molecule is bombarded with e- which forms a cation radical

what kind of particles are detected in mass spec

ONLY charged particles, cations, or cation radicals

base peak

tallest peak, normalized to 100%

M+ peak

usually the highest intensity peak within the cluster of peaks at the highest m/z (mass to charge) value (towards the right)

M+1 peak

due to C 13 isotope (1.1%)

which mass spec fragments are most likely to form

most stable cations and radicals

M - 15

loss of methyl

M - 29

loss of ethyl

M - 43

loss of propyl

M - 57

loss of butyl

M - 18

loss of H + OH (usually w alcohols and phenols)

M - any even number x

McLafferty rearrangement, in aldehydes/ketones

double-focusing mass spectrometry

determine m/z values to 4 decimal places

nitrogen rule

odd MW odd N, even MW even N or no N (on test assume no N is present)

chlorine rule

M+2 peak 1/3 height of M+ peak

bromine rule

M+ and M+2 peak same intensity

iodine rule

peak at 127 due to I+

eqn to calculated max # H for a saturated compound

CnH2n+2

find # C from mass spec

(normalize, then solve for #C) M+1 %/M+ % x 100; normalized %/ C 13 abundance

degrees unsat eqn

.5(2C+2+N-H-X)