mass spectrometry

Imagine you could weigh a single molecule on an unimaginably precise scale, and then deliberately smash it and weigh each piece. That, in essence, is mass spectrometry. Unlike infrared or NMR, which bathe the molecule in light or radio waves, mass spectrometry actually weighs the molecule and its broken pieces. The reward is the molecule's mass — and from a very precise mass, its molecular formula — plus a wealth of clues about its structure from how it falls apart.

Here is the mechanism in plain steps. The sample is vaporized and zapped, classically by a beam of electrons, which knocks one electron off a molecule to make a positively charged radical cation called the molecular ion. Most molecular ions, being unstable, then break apart into smaller charged fragments. All these ions are accelerated and sent through a magnetic or electric field that sorts them by their mass-to-charge ratio (written m/z); since most carry a single charge, m/z is effectively the mass. A detector counts how many ions arrive at each mass, producing a spectrum: a forest of vertical lines, one per fragment mass, the tallest one called the base peak.

Mass spectrometry is extraordinarily sensitive — it can detect tiny amounts, which is why it underpins drug testing, doping control, forensic toxicology, and detecting pollutants in water. With a high-resolution instrument, the mass is measured so precisely (to four or more decimal places) that the exact molecular formula can be read off directly, because, say, C2H4 and CO have very different exact masses even though both are near 28. Coupled to a chromatograph that first separates a mixture, it identifies the components of complex samples one by one.