Quick questions on Mass spectrometry of organic compounds explained: HSC Chemistry Module 8

<!-- Diagram: schematic mass spectrum | reviewed 2026-05-21 --> <svg class="fig" viewBox="0 0 540 280" role="img" aria-labelledby="ms-t ms-d"> <title id="ms-t">Schematic mass spectrum with molecular ion and base peak</title> <desc id="ms-d">Relative abundance on the y axis against mass to charge ratio on the x axis. Bars rise from zero to one hundred percent. The base peak at fragment forty three is at one hundred percent. The molecular ion M plus is at the highest m over z value, often shorter than the base peak.

So a loss of 17 ($OH$) suggests an alcohol; a loss of 45 ($COOH$) suggests a carboxylic acid; a loss of 29 ($CHO$) suggests an aldehyde.

The peak at the highest $m/z$ (excluding the small isotope satellites) is the molecular ion. Its $m/z$ is the molecular mass of the compound.

The tallest peak, set to 100% relative abundance. Often a fragment, not the molecular ion. The base peak indicates the most stable cation formed in fragmentation.

Other peaks. The difference between $M^+$ and a fragment is the mass of the neutral radical lost.

$^{35}Cl$ : $^{37}Cl$ = 3 : 1. A compound with one $Cl$ shows M : M+2 = 3 : 1. Two chlorines give M : M+2 : M+4 = 9 : 6 : 1.

$^{79}Br$ : $^{81}Br$ $\approx$ 1 : 1. A compound with one $Br$ shows M : M+2 of roughly equal height. Two bromines give M : M+2 : M+4 = 1 : 2 : 1.

$^{12}C$ : $^{13}C$ = 98.9 : 1.1, so the M+1 peak is about 1.1% per carbon atom. A 10-carbon molecule shows an M+1 peak about 11% of M; this can be used to count carbons.

Gives the exact molecular mass and structural fragments. Picks out chlorine, bromine and sulfur by isotope pattern. Sensitive to nanograms of sample.

Destroys the sample during analysis. Cannot distinguish stereoisomers. Cannot always distinguish structural isomers if they fragment similarly (propan-1-ol and propan-2-ol have very similar spectra).

Describe the five stages of a mass spectrometer in order. [3 marks]

A compound shows a molecular ion at $m/z = 88$, a base peak at $m/z = 73$ and another peak at $m/z = 43$. Identify the fragment losses and deduce the compound (a saturated organic ester of $C_4H_8O_2$). [3 marks]

A mass spectrum shows a 3:1 isotope cluster at $m/z = 78$ and 80. (a) Identify which element is present. (b) Calculate the molecular mass of a hypothetical methyl halide containing this element.