Headspace analysis is a method to determine the concentration of volatile substances of a solution in the vapour above this solution. It is used, among other things, to analyse taste, odour and aroma substances, but also to determine the alcohol content in blood. The method uses the property that the partial pressure of a substance from the vapour is proportional to the concentration of the substance in the solution. The substances in the vapour are analysed with the help of a gas chromatograph. During the procedure, the liquid to solid sample (e.g. blood, honey, piece of textile), which often has a complex composition, is placed in a gas-tight vessel and heated. After the phase equilibria have been set, a needle of the autosampler draws a certain volume out of the gas space and feeds it to the GC column.

(https://de.wikipedia.org/wiki/Dampfraumanalyse)