Determination of soil amino acids by high performance liquid chromatography-electro spray ionization-mass spectrometry derivatized with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate

Determination of amino acids by mass spectrometry (MS) is an important technique to investigate soil nitrogen transformation and cycling as amino acids being the major nitrogen-containing compounds in soil organic matter. However, researchers have long faced a critical problem in coupling an efficient separation technique to a sensitive MS detection system simultaneously. In this context, we established a new method of liquid chromatography coupled to mass spectrometry based on the 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) derivatization method for convenient and accurate quantification of amino acids in soil samples. Baseline separation of 17 amino acid AQC-derivatives was achieved on an XTerraR MS C18 column using ammonium formate as a mobile phase modifier. The concentration of ammonium formate and the pH of the mobile phase were optimized in order to obtain sensitive MS signals. The response curves were linear over the range of 50–800 μmol L−1 amino acids. The detection limits were 0.20–0.60 pmol μL−1 on column and 0.07–0.24 μg g−1 soil under the optimized conditions. The method has been applied successfully for the first time to determine amino acids in 4 types of soil samples, in which 15 amino acids were quantified by MS detector but methionine and cystine were below the detection limits. Both the recovery and the precision were satisfactory. Hence, this proposed technique shows a potential for the identification of amino acids in soil as well as tracing the transformation of soil amino acids with isotope dilution technique in nitrogen cycling investigation.