Exploration of Some Problems in the Separation and Purification of n-alkanes in Geological Samples

Adsorption characteristics of silica gel and alumina for n-alkanes were tested by column chromatography under equivalent solvent elution. The results showed that alumina could adsorb long chain n-alkanes in certain extent, while silica gel exhibited the opposite characteristic. The further experiments using plant samples and chromatographic columns filled with different materials were carried out. The experiment results indicated whether the columns filled with alumina and silica gel or with alumina alone, it exhibited adsorption characteristics for long chain n-alkanes and the adsorption effect became explicit with increasing of length of carbon chain. Under the proposed experiment condition, when the chain length reached C36, adsorbed amounts by the columns filled with silica gel-alumina and with alumina alone reached up to 20% and 50%, respectively. Therefore, silica gel chromatographic column could be the better choice when long chain n-alkanes in geological samples were analyzed. Moreover, the experiment results also indicated that urea was an effective purification agent in pretreatment of some very "dirty" samples. The average recovery for n-alkanes in 18 loess, paleosoil and plant samples was about 50% and relative abundance for each component of n-alkanes in samples kept unchanged and without distinct isotopic fractionation after sample purification with urea. Therefore, urea could be used in purification of complex geological samples to avoid the serious interference from non-n-alkanes species in determination of relative abundance and isotope composition of n-alkanes.