A mass spectrometer is used to find the relative molecular mass of an organic compound and to give information about its structure.Inside the instrument, molecules of the sample are ionised. An electron is usually removed, forming a positive ion called the molecular ion, M+:M(g) → M+(g) + e− The ions are separated according to their mass-to-charge ratio, m/z, and detected. Almost all ions have a charge of 1+, so m/z is equal to the mass of the ion.The result is a mass spectrum: a chart of relative abundance (vertical axis) against m/z (horizontal axis). Each line shows an ion of a particular mass, and its height shows how many of those ions reached the detector. A mass spectrum of an organic compound has three features:
• the molecular ion peak, which gives the relative molecular mass
• a small M+1 peak just above it
• fragment peaks at lower m/z, from ions formed when molecular ions break apart
Mass spectrometry needs only a tiny sample. It is used to identify unknown compounds in forensic science, to test athletes for banned drugs, and on space probes to analyse other planets.Worked example: what is the m/z value of the fragment ion C2H5+?mass = (2 × 12) + (5 × 1) = 29, charge = 1+, so m/z = 29
• the molecular ion peak, which gives the relative molecular mass
• a small M+1 peak just above it
• fragment peaks at lower m/z, from ions formed when molecular ions break apart
Mass spectrometry needs only a tiny sample. It is used to identify unknown compounds in forensic science, to test athletes for banned drugs, and on space probes to analyse other planets.Worked example: what is the m/z value of the fragment ion C2H5+?