Mass Spectrometry

What happens in mass spectrometry?

charged ions pass through an electric field into a beam of energy and are separated based on their mass/charge ratio

a detector counts the ions and their abundance

M+ peak

matches weight of the original, pre-fragmented compound

M+1 peak

present if there is a C13 isotope in the spectrum

How to determine the number of carbons give M+ and M+1 peak

formula

the intensity of the peak

is its height

Rule of 13

divide the M+ peak by 13 to determine the number of C and H molecules

the quotient is the #C

the quotient + the remainder is #H

IHD formula

Lowest IHD

more likely formula

adding O to molecular formula

subtract one C and 4 H for every O added

adding N to molecular formula

subtract one C and 2 H for every N added

changing C to H

1 C is equivalent to 12 H

Nitrogen rule

MW even = #N even

MW odd = #N odd

Halogen prescence

M+2 peak indicates halogen

if M+ and M+2 peak exist in ratio of 3:1, Cl is present

if M+ and M+2 peak exist in ratio of 1:1, Br is present

Calculating molecular formula with halogen present

subtract the molecular weight of the halogen, add one H

then once the rule of 13 calculations are done, take out an H and put the X back in

fragmentation occurs to make what kind of products?

stable carbocations

Beta cleavage

Occurs for saturated heteroatoms (O, N, S, X)

The bond adjacent to the C attached to the fxn group breaks because an e- is fired at it

splits the molecule into a neutral radical and a stable cation

half arrow represents movement of single electrons

Example beta cleavage

M-15 and M-29 groups

show methyl and ethyl fragments

fragments only show up if they are

charged (cation or radical cation)

Carbonyl compounds can do

McLafferty RAR and alpha cleavage

McLafferty RAR

can occur if there is a gamma C (3 away from fxn group) with H

uses double headed arrows to move e- from H bond to next bond down and to alpha bond

results in neutral alkene

and radical cation, O grabs H

Alpha cleavage

break bond that is alpha to carbonyl

results in radical and resonance stabilized cation