HPLC-MS/MS identification of tryptophan-derived tetrahydro-β-carboline derivatives in food

Abstract

Based on electrospray ionization – tandem mass spectrometry coupled to liquid chromatography (HPLC-ESI-MS/MS) a method for separation and selective detection of 1,2,3,4-tetrahydro-β-carboline derivatives (THC) was developed. Retro-Diels Alder (RDA) fragmentation of the tetrahydropyrido moiety resulted in the characteristic neutral loss of 73 amu for tryptophan-derived THC-3-carboxylates. Accordingly, Pictet-Spengler condensation products of tryptamin exhibited product ions formed by loss of 29 amu. However, THC-1-carboxylates as obtained by reaction of tryptamin with α-oxo acids also yielded product ions [M+H-73]⁺, apparently originating from the combination of RDA-cleavage plus subsequent decarboxylation. As result, one had to consider the possibility of false-positive identification of THC-3-carboxylates in presence of isomeric THC-1-carboxylates. In order to overcome these analytical pitfalls, the unequivocal identification of trace amounts of THC-3-carboxylates by HPLC-MS/MS required the chromatographic separation of isomeric THC prior to selected reaction monitoring (SRM). Utilizing SRM, limits of detection for various THC were established in the 10 ng mL^–1 range. Subsequent analysis of food samples like seasoning sauce and yeast extract by HPLC-ESI-MS/MS revealed the presence of tryptamin-derived 1,2,3,4-tetrahydro-β-carboline-1-carboxylic acid, 1-methyl-tetrahydro-β-carboline-1-carboxylic acid, 1-carboxyethyl-tetrahydro-β-carboline and 1-carboxyethyl-tetrahydro-β-carboline-1-carboxylic acid beside established THC-3-carboxylates and -1,3-dicarboxylates.