Interpret mass spectra to determine molar mass and identify fragments of organic molecules

What this dot point is asking

Mass spectrometry is an instrumental technique that measures the mass of molecules and their fragments. It is the most direct way to find the molar mass of an organic compound and provides clues to its structure.

How it works in outline

A sample is vaporised and ionised (usually by knocking off an electron to form a positive ion). The ions are accelerated and separated according to their mass-to-charge ratio (m/z). A detector records the abundance of each, producing a spectrum of peaks. Most ions carry a single positive charge, so m/z effectively reads as the mass.

Finding the molar mass

The molecular ion peak appears at the highest m/z (apart from minor isotope peaks). Its value is the molar mass of the compound. For example, a molecular ion peak at m/z 46 is consistent with ethanol, $\text{C}_2\text{H}_5\text{OH}$ ($M = 46$).

Fragmentation and structure

Inside the spectrometer the molecular ion breaks into smaller fragment ions. Each fragment gives a peak, and the difference in mass between peaks tells you what was lost. Common losses include 15 (a $\text{CH}_3$ group), 17 (an -OH group), 29 (a $\text{CHO}$ or $\text{C}_2\text{H}_5$ group) and 45 (a $\text{COOH}$ group). Identifying these losses lets you piece together the structure.

The base peak

The tallest peak in the spectrum is the base peak, corresponding to the most stable (most abundant) fragment ion. Its abundance is set to 100 percent and other peaks are scaled relative to it. A particularly stable fragment dominates the spectrum.

Why this matters

Mass spectrometry is usually the first technique applied when identifying an unknown, because the molar mass narrows the possibilities dramatically. Combined with infrared and NMR spectroscopy, it lets chemists determine a full structure, which is exactly the skill the analysis section of Unit 4 assesses.