The total content of hydrocarbon compounds (THC) is determined using gas chromatography (GC) coupled with mass spectrometry (MS). The compounds of the impurities are separated on the basis of their chemical properties, the individual “molecule packages” are ionized and detected on the basis of their specific mass-to-charge ratio. Prior calibration with a hydrocarbon mixture of known concentration enables precise semi-quantification of the substances to be determined. As the mass-to-charge ratio is an excellent characteristic of the chemical composition, the GC/MS method is ideal for characterizing the underlying substances with the aid of a comparison with reference substances or a database. In particular, this facilitates research into the causes of chemical contamination.

Prior to GC/MS analysis, the filmic residues are first extracted with a volatile organic solvent. A range of solvents can be used for this, which can differ greatly in their solvation capacity. These are frequently used: Short-chain alkanes C5-C7 (pentane, hexane, heptane), ethers (diethyl ether, tert-butyl methyl ether, tetrahydrofuran = THF), ketones (acetone, methyl ethyl ketone = MEK), esters (ethyl acetate, butyl acetate), dichloromethane, alcohols C1-C3 (methanol, ethanol, iso-propanol). The selection should always be based on aspects of occupational safety and material compatibility.