Study on Distribution Characteristics of n-Alkanes in Loess-paleosol Lipids by Different Extraction Methods

Two extraction methods, ultrasonic-assisted solvent extraction (UASE) and accelerated solvent extraction (ASE), were used to study the distribution characteristics of n-alkanes in loess-paleosol lipids. A suite of loess-paleosol lipid samples were collected from Liuwan profile located in Luonan county, Shaanxi province. The results showed that no significant differences for the distribution pattern and relative abundance of n-alkanes were observed when using UASE either increasing the ultrasonic frequency to 100 Hz or tripling the extraction times under ultrasonic frequency of 60 Hz. The n-alkane abundance in 7 paleosol samples from Liuwan profile is rather low when UASE is used and increasing with depth of the profile. The carbon preference index (CPI) of n-alkanes for 2 paleosol samples extracted with UASE were 5.4 and 4.6 with isotopic composition (average value) of -31.80‰ and -30.52‰. CPI of n-alkanes from the same samples extracted with ASE was 5.3 and 4.3 with isotopic composition of -31.46‰ and -30.56‰. The results showed that there is no significant difference in selectivity between UASE and ASE, but ASE is more efficient in extracting n-alkanes from paleosols than UASE. For geologically older loess-paleosol samples with lower content of n-alkanes, ASE is obviously a preferable extraction method with better recovery of n-alkanes for further compositional and isotopic analysis. While for geologically younger samples with higher organic content, UASE seems a preferable extraction method with advantages of less expensive and easier-to-operation.