Mass spectrometry

Last time

Before we start: last time, in Infra-red spectroscopy and identifying compounds, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Weighing a molecule

How do you weigh a single molecule? A mass spectrometer ionises it, accelerates the ion, and sorts it by mass and charge. The result is a spectrum of peaks. The heaviest peak gives the relative molecular mass, the extra peaks reveal how many carbons and which halogens are present, and the fragment peaks show how the molecule is built.

The mass spectrum

The horizontal axis is the mass to charge ratio, m over e. For singly charged ions, it equals the relative mass of the ion. The vertical axis is the relative abundance: how many ions of that mass reach the detector. The molecular ion, M plus, is the whole molecule minus one electron. Its m over e equals the relative molecular mass. Smaller peaks come from fragment ions, formed when the molecular ion breaks up.

The M+1 peak

A small peak at one mass unit above the molecular ion arises from molecules containing carbon-13, which is about 1.1 percent of carbon. The more carbon atoms in the molecule, the taller the M+1 peak relative to M. The number of carbon atoms is the abundance of M+1, over the abundance of M, times 100, divided by 1.1. If M has abundance 34.7 and the molecule has 6 carbons, M+1 is 34.7 times 6 times 1.1 over 100: 2.29.