Isolation of neutral and acidic lipid biomarker classes for compound-specific-carbon isotope analysis by means of solvent extraction and normal-phase high-performance liquid chromatography

An analytical method for the quantitative determination of neutral and acidic lipid biomarkers in particulate and sediment samples has been developed. The method involves a first step with solvent extraction to isolate the neutral from the acidic compounds and a second step using normal-phase HPLC on a Nucleosil silica column to separate four different classes of neutral compounds: (1) aliphatic hydrocarbons, (2) aromatic hydrocarbons, (3) ketone compounds and (4) sterol and alcohol compounds. Recoveries of the individual spiked lipid biomarkers for the whole analytical procedure ranged from 88 to 106% for fatty acids, from 50 to 60% for aliphatic hydrocarbons (≥ n-C17), from 50 to 60% for polycyclic aromatic hydrocarbons (PAHs) (≥3 rings), 83% for friedelin and 60–80% for the sterol and alcohol compounds. The isolated compound classes were analysed by gas chromatography–combustion–isotope ratio mass spectrometry to measure the stable carbon isotope ratios in the individual compounds. The method enables the isolation of compound classes without fractionation for compound-specific carbon isotope analysis (δ13C). This analytical protocol has been applied, and proved suitable, for the determination of lipid biomarkers (sterols, fatty alcohols and fatty acids) in marine particulate material and for the determination of PAHs in sediment samples.