Questions: Mass Spectrometry in Organic Chemistry

Question 1

A mass spectrum shows a molecular ion at m/z = 136 with an M+2 peak approximately one-third the height of M+. Which element is most likely present?

• A. Carbon (due to 13C isotope)
• B. Chlorine (35Cl/37Cl)
• C. Bromine (79Br/81Br)
• D. Nitrogen

Answer: (B)

Chlorine has two major isotopes: 35Cl (75%) and 37Cl (25%), giving M:M+2 intensity ratio of approximately 3:1, so M+2 is about one-third of M+. Bromine's isotopes (79Br and 81Br) are nearly equal in abundance, giving M:M+2 ≈ 1:1. Carbon-13 produces a small M+1 peak, not M+2. The distinctive M+2 isotope pattern is one of the most reliable indicators of halogen presence in a mass spectrum.

Question 2

The base peak in a mass spectrum generally corresponds to the molecular ion (M+).

• T. True
• F. False

Answer: False

The base peak is simply the most abundant (tallest) peak in the spectrum and is used as the 100% reference. It can be any fragment — in many compounds the molecular ion fragments so readily that M+ has very low abundance or is absent entirely. The base peak must be identified by its m/z value. For example, in many branched alkanes the molecular ion is weak and a stable carbenium ion fragment is the base peak.

Question 3

A compound produces a molecular ion at m/z = 57 (odd mass). What does this tell you about the compound's nitrogen content, and why does this rule hold?

Model answer: An odd molecular mass indicates the compound contains an odd number of nitrogen atoms (one, three, etc.). This is the nitrogen rule: nitrogen is the only common organic element whose most abundant isotope has an even mass (14) but an odd valence (3), causing each nitrogen to shift the molecular mass by an odd net amount.

For molecules containing only C, H, O, S, and halogens, all of which have even-mass isotopes and even valences, the molecular mass is always even. Each nitrogen (mass 14, valence 3) behaves differently: adding a nitrogen to a molecular formula adds 14 (even) in mass but also forces the H count to adjust by an odd amount due to valence bookkeeping, producing an overall odd shift. The rule is simple and reliable: odd M+ = odd number of nitrogens.