Analysis of hydrolysable neutral sugars in mineral soils: Improvement of alditol acetylation for gas chromatographic separation and measurement

To understand plant-microbe relationships, a simple method is required for identification of the nature of soil polysaccharides. Acid hydrolysis, reduction of sugar monomers to the corresponding alcohols and subsequent derivatisation with acetic anhydride prior to gas chromatography has often been used for identification and quantification of hydrolysable sugars in plant and soil samples. In mineral soil samples, precipitation of iron hydroxides and dissolved organic substances after addition of ammonia may lead to co-precipitation of the analytes, leading to an underestimation of the neutral sugar content. The aim of this study was to adapt the derivatisation procedure for soil samples with large iron (hydr)oxide contents. This was done by omission of ammonia and addition of ethylenediaminetetraacetic acid (EDTA) to keep iron in solution and to avoid co-precipitation. Standard addition approaches show that the recovery of all sugars is enhanced with the modified method. Application of the EDTA method improves the recovery of added internal standard, increases yields of sugars in mineral soils and reduces the observed standard error compared to the ammonia method. This was shown for a set of various soil samples with different iron (hydr)oxide contents. The EDTA method is also applicable for mineral free samples and therefore suitable for routine use.