Determination of amino acid enantiomers in soils

Determination of amino acid enantiomers in environmental samples is difficult, because metals and organic impurities interfere with the analyses. We developed a new gas chromatographic method to assess amino acid enantiomer concentrations in complex soil matrices following hot HCl hydrolysis (6 M, 12 h, 105°C). The crucial focus was the establishment of a simple, reliable sample clean-up procedure. The presented method involved the adsorption of the enantiomers on a Dowex 50 W X8 cation exchange resin and the removal of interfering compounds with 0.1 M oxalic acid prior to amino acid elution with 2.5 M NH4OH. After conversion to N-pentafluoropropionyl-amino acid iso-propyl esters, the diastereomers were separated by a Chirasil l-Val capillary column and quantified by a flame ionization or mass selective detector. The lower limit of quantification tested here was ≤1 pg injection amount. The recovery of amino acid enantiomers averaged 99±11% for pure standards and 97±11% for spiked soil hydrolysates. The general applicability of the method was demonstrated by determination of amino acid enantiomers in taxonomically different soils from different geographic regions. The coefficients of variation presented by d/l ratios were 12% for alanine and <10% for the other amino acids.