A conservation scientist wants to identify the chemical compounds present in a sample of ancient plant resin found at an archaeological site. Explain how gas chromatography–mass spectrometry (GC-MS) could be used to identify the compounds in the resin sample.

The resin sample is first heated in the injector so that it vapourises and the vapour is carried by an inert carrier gas (e.g. helium) through the GC column. As the vapour travels, the different compounds interact with the stationary phase to different extents and therefore move at different speeds, giving distinct retention times. When each separated compound reaches the detector it is ionised and enters the mass spectrometer. The mass spectrometer fragments the ions and records a mass spectrum – a plot of ion intensity versus mass‑to‑charge ratio – which is characteristic of that compound. The obtained mass spectra are then matched against a reference library of known spectra, allowing the identity of each compound in the resin to be determined.