Introduction Sulphur-containing metabolites play an important role in metabolism and homeostasis. Determination of these metabolites is challenging owing to their low concentrations and the interference in mass spectrometry analysis. Objective To develop a sensitive and accurate method based on liquid chromatography, electrospray ionisation, tandem mass spectrometry (LC-ESI-MS/MS) and 34S-metabolic labelling for quantification of methionine, reduced glutathione, oxidised glutathione in Arabidopsis thaliana. Methodology A hydroponic set-up was used for the in vivo 34S-metabolic labelling of A. thaliana. The 34S-labelled metabolites biosynthesised in plant were extracted and used as internal standards. Tissue was extracted with perchloric acid (PCA) or PCA containing a known amount of the analytes for recovery analysis. Tissue extract mixed with extract of 34S-labelled A. thaliana in an appropriate ratio was subjected to a LC system and electrospray ionisation-mass spectrometric (ESI-MS) analysis. Quantification of metabolites was measured by comparing the 32S/34S ratios obtained for samples with the calibration curves. Results Calibration curves showed linearity with regression coefficients in the range of 0.9994-0.9999. Analyte recoveries were approximately 100%. The coefficients of variation of intra-assay and inter-assay were less than 4.2% and 5%, respectively. The ranges for the limits of detection determined for Met, GSSG and GSH were 10?fmol
