Using mass spectrometry to investigate the structure of molecules

Question 1

on a mass spectrometry, what does the peak furthest to the right represent?

Answer 1

the Mr of the compound

Question 2

what do you look for on the mass spectrum of a molecule?

Answer 2

1. the molecular ion peak M+ (furthest right = Mr of molecule
2. the M+1 peak, next to the M+ peak, represents the 1% of C atoms that are 13C
3. the fragmentation pattern,
   note:
   - anything giving a peak is positive
   - each fragment is given by one single bond in the molecule breaking
   - taller peaks are given by more stable fragments
   - if asked to give the formula, include +ve charge

Question 3

what are some common fragment peaks?

Answer 3

15 = CH3+
29 = CH2CH3+ or an aldehyde
77 = C6H5+ (benzene)

Question 4

if two molecules have the same molecular formula, will the M+ peak help distinguish them?

Answer 4

no, since they have the same Mr