Analysis of free amino acids in natural waters by liquid chromatography–tandem mass spectrometry

This paper reports a new analytical method for the analysis of 18 amino acids in natural waters using solid-phase extraction (SPE) followed by liquid chromatography-electrospray tandem mass spectrometry (LC–MS/MS) operated in multiple reaction monitoring mode. Two different preconcentration methods, solid-phase extraction and concentration under reduced pressure, were tested in development of this method. Although concentration under reduced pressure provided better recoveries and method limits of detection for amino acids in ultrapure water, SPE was a more suitable extraction method for real samples due to the lower matrix effects for this method. Even though the strong cation exchange resin used in SPE method introduced exogenous matrix interferences into the sample extracts (inorganic salt originating from the acid–base reaction during the elution step), the SPE method still incorporates a broad sample clean-up and minimised endogenous matrix effects by reducing interferences originating from real water samples. The method limits of quantification (MLQ) for the SPE LC–MS/MS method in ultrapure water ranged from 0.1 to 100 μg L−1 as N for the different amino acids. The MLQs of the early eluting amino acids were limited by the presence of matrix interfering species, such as inorganic salts in natural water samples. The SPE LC–MS/MS method was successfully applied to the analysis of amino acids in 3 different drinking water source waters: the average total free amino acid content in these waters was found to be 19 μg L−1 as N, while among the 18 amino acids analysed, the most abundant amino acids were found to be tyrosine, leucine and isoleucine.