Question

MCAT Mini-Tutorial: Mass Spectrometry

Answer 1

\({\bf{Basic~Mechanism:}}\) molecules are bombarded with electrons to break bonds and produce a series of fragments that are characteristic to the molecule. These fragments are then sorted by a magnetic field, where charged fragments are sorted by their m/z ratio.

The fragments are usually +1 charged so their m/z ratios are typically representative of the relative masses of the fragments.

Definitions:

base peak: the tallest peak
M peak: the peak with the largest m/z ratio, the same MW as the compound
M + 1 peak: indicates the presence of carbon-13 isotope

helpful tip: calculating the difference in the m/z ratios between the peaks can help determine what functional groups/atoms might have been involved in the fragment

Answer 2

Here are some example problems for you to try:

https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/questions/Spectroscopy/masspec5.htm

example: M peak 79 with smaller peak at 52

79 - 52 = 27

27 = 14 + 12 + 1 indicating that there may be an HCN fragment on there somewhere

the very high % abundance of the M peak relative to all other fragments suggests an aromatic compound

propose: pyridine



double check the MW to be approx 79g/mol

Answer 3

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT