A rapid and precise method for the analysis of underivatized amino acids in natural samples using volatile-ion-pairing reverse-phase liquid chromatography–electrospray ionization tandem mass spectrometry

Abstract

Amino acids are effective tracers of organic carbon and nitrogen cycling in natural substrates. However, total analytical time can be long due to extraction, derivatization, and chromatography. Here, we present a liquid chromatographic separation of 19 naturally occurring amino acids requiring no derivatization using triple-quadrupole mass spectrometry. This method builds on recent advances in volatile ion pairing and was applied to natural soil samples collected from two distinct systems with highly variable organic C contents. Separation and quantification of amino acids were achieved with an 18 min sample-to-sample run time employing a one-point standard addition to account for variable matrix effects during ionization. Detection limits ranged from 5.9 to 187.5 fmol (mean 30.0 fmol) while instrumental precision averaged 5.8% and 12.0% for intra- and inter-day error, respectively. The highest yields (mean > 100 µmol/g C) in our natural substrates were observed for glycine, aspartic acid, alanine, and glutamic acid while low yields (mean < 20 µmol/g C) were observed for all non-proteinogenic amino acids as well as histidine and tyrosine. A typical hot acid hydrolysis in 6N HCl was used and hydrolyzates were diluted and filtered rather than being subjected to solid-phase extraction or similar techniques, which require significant investment of time during sample preparation. This method enables high throughput and reliable analysis of hydrolyzable amino acids, while also reducing user workload and instrument time compared to previous techniques. The Alaskan tundra system had significantly higher absolute yields of amino acids (mean 88.4 vs 25.6 µmol/g dry sediment), in part driven by higher organic C concentrations, while the Louisiana delta system had significantly higher organic C normalized yields of amino acids (mean 1492.5 vs 541.6 µmol/g C). Despite these differences, both systems exhibited broadly similar mole-percent amino acid compositions.