Soil lipid analysis by chromatography: A critical review of the current state in sample preparation

The slow formation and renewal of soil, coupled with the multitude of services it provides to humanity, render this resource a critical component of the biosphere. The maintenance of soil structure and the accumulation of carbon, as well as the maintenance of biodiversity in soils, rely on the contribution of different fractions of soil organic matter. Despite its minor component in soils, the lipid fraction is of particular importance. Its functional diversity offers an inherent diagnostic value regarding soil organic matter, microbial biodiversity and the pedological processes to which the soil is subjected. Soil lipids are comprised of diverse groups of organic compounds, exhibiting structural variations from derivatives of complex organic components (e.g., phosphoglycerides or phospholipids, sphingomyelins, glycosphingolipids) to simpler functional classes which can combine to form other compounds such as wax esters, acylglycerols, sterols, terpenoids, and fatty acids. The analytical strategy for determining lipids in soils commonly involves extracting the lipids, then fractionating, hydrolyzing, derivatizating, identifying, and/or measuring them by chromatography. These sample treatment procedures for lipid analysis in this complex matrix are typically traditional. While sustainable sample preparation procedures are not yet fully implemented, the analysis of intact lipids by liquid chromatography coupled to mass spectrometry detection is becoming more common to avoid fractionation and derivatization. This paper reviews current sample preparation strategies for the analysis of soil lipids and presents some alternatives to the traditional methods used for soil lipid extraction, fractionation, and derivatization.