Mass Spectrometry Questions and Answers

Answer: A

The base peak is simply the tallest, and all other intensities are quoted relative to it as a hundred per cent. It is often a stable fragment rather than the molecular ion, which is why the two must not be confused. Normalising a spectrum to its own largest peak makes spectra comparable between instruments and concentrations, which is what library matching requires.

Answer: B

For a compound of carbon, hydrogen, oxygen and nitrogen, an odd nominal molecular mass implies an odd number of nitrogen atoms, because nitrogen is the only common element whose valency and mass have opposite parity. Molecular ion strength and fragmentation are unrelated. A rule that extracts an element count from a single number costs nothing to apply and immediately narrows the possible formulae. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: C

Applying the nitrogen rule to an odd nominal mass:

\[ 121 \ \text{odd} \Rightarrow \text{odd number of N} \]

so an odd number, most likely one. An even mass would imply zero or an even number. The rule cannot distinguish one nitrogen from three, but narrowing an unknown to odd or even nitrogen content from a single integer is a remarkable return for no work.

Answer: D

Chlorine's two isotopes occur in roughly a three to one ratio, so a compound with one chlorine shows an M plus two peak at about a third the height of the molecular ion. Bromine's isotopes are nearly equal and give an M plus two of almost the same height, sulphur gives about four per cent, and oxygen about two tenths. Reading the isotope pattern's height is what identifies the halogen without any further evidence. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: A

Bromine's two isotopes are almost equally abundant, so the molecular ion and the M plus two peak are nearly the same height - an unmistakable signature. One chlorine gives a third, two chlorines give a three-peak pattern in roughly nine to six to one, and sulphur gives four per cent. That each halogen leaves a distinctive fingerprint in the isotope cluster makes the pattern one of the most informative features of a spectrum. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: B

A loss of eighteen is water, which alcohols and many other oxygen compounds lose readily. Methyl is fifteen, carbon monoxide twenty-eight, and ammonia seventeen. Recognising common neutral losses is how a fragment's identity is inferred from a mass difference rather than from its absolute value, and a short list of them accounts for most of what a spectrum shows.

Answer: C

Fifteen is a methyl radical, lost readily from any compound carrying one. Water is eighteen, carbon monoxide twenty-eight, and hydroxyl seventeen. A small table of common losses - fifteen, eighteen, twenty-eight, twenty-nine, thirty-one, forty-five - covers a large fraction of the fragmentations seen in practice, and reading differences rather than absolute masses is the habit that makes it usable. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: D

Benzylic cleavage gives an ion at ninety-one that rearranges to the very stable seven-membered tropylium ring, which is why the peak is so often the base peak in alkylbenzenes. Phenyl is seventy-seven, toluene's molecular ion is ninety-two, and phenol loses carbon monoxide to give sixty-six. A fragment whose stability makes it dominate a spectrum is worth recognising on sight, since it identifies a structural class immediately. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

Fragmentation is a competition, and an ion that is thermodynamically stable and does not decompose further accumulates while less stable ones break down before detection. The lowest energy pathway matters too, which is a related but distinct point, and neither mass nor charge decides it. That the peaks seen are the survivors rather than simply the products is what makes fragmentation patterns reproducible. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

Resolution is the ratio of a mass to the smallest difference distinguishable there, so a figure of ten thousand separates 500.00 from 500.05. The absolute difference and the peak width are the quantities from which it is computed rather than the definition, and peak count is unrelated. A dimensionless ratio lets instruments be compared across the whole mass range, which an absolute figure could not do. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.