Quantification of amino acids, including tryptophan, in human sweat via high-performance liquid chromatography with fluorescence derivatization using naphthalene-2,3-dicarboxaldehyde.

As sweat is a biofluid that can be collected noninvasively, it has attracted attention as an alternative analytical target for conventional techniques that typically employ blood or urine. Among the components of sweat, amino acids are promising biomarker candidates as they can reflect physiological and metabolic states. Sweat amino-acid analysis using fluorescence derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) and reversed-phase high-performance liquid chromatography (HPLC) has been reported previously. However, sensitive fluorescence detection of tryptophan is difficult using this method, and quantitative information on tryptophan in sweat is not available. Tryptophan is involved in multiple metabolic pathways related to neurotransmission, fatigue and immune regulation; therefore, the absence of tryptophan data limits the information that can be obtained from sweat amino-acid profiles. In this study, we developed a sweat amino-acid analysis method using fluorescence derivatization with naphthalene-2,3-dicarboxaldehyde (NDA) and reversed-phase HPLC. Using this method, 18 amino acids, including tryptophan, were detected in trace amounts in fingertip sweat collected under resting conditions. Of these, 17 amino acids (excluding methionine) were successfully quantified. The proposed method exhibited satisfactory linearity, sensitivity, precision, and accuracy. In addition, the quantitative results agreed well with those obtained using the NBD-F method. The method expands sweat amino-acid profiling to include tryptophan and provides an analytical basis for future physiological and metabolic investigations.