Liquid chromatography-mass spectrometry (revision 6)

Liquid chromatography-mass spectrometry couples a chromatographic separation to a mass spectrometer, so that each eluting peak is characterised by both its retention time and its mass. It converts a weak identification into a strong one.^[1]

The interface is almost always electrospray, which works directly from flowing solution. This compatibility is what made the technique routine, and it constrains the chromatography: mobile phases must be volatile and free of non-volatile buffers.^[2]

For peptide work its principal use is identity confirmation — establishing that the peak dominating a chromatogram has the expected mass — and characterisation of impurities, since each minor peak carries a mass that often identifies what it is.^[3]

Chromatography alone establishes that one species dominates; mass spectrometry alone establishes that a species of the expected mass is present. Together they establish that the dominant species has the expected mass, which is a materially stronger statement than either.^[1]

Impurity identification is where the combination is most valuable. A deletion sequence appears as a peak of lower mass by one residue; an oxidised species by 16 Da more; an incompletely deprotected species by the mass of the retained group. See Resin cleavage and Solid-phase peptide synthesis.^[3]

Tandem operation — selecting a precursor, fragmenting it, and measuring the fragments — adds sequence information and can locate a modification to a residue rather than merely detecting it.^[1]