Cyclohexane is the solvent of choice for the extraction of n-alkanes from environmental matrices. This solvent contains traces of bicyclohexyl. This impurity does not interfere with analyses at the ppm level, but is a potential problem at the ppb* level. Erroneous concentrations of n-tridecane could be obtained using conventional packed gas chromatographic columns. However, this problem is obviated by using capillary columns or combined gas chromatography - mass spectrometry.