Measurement of sulfate concentrations and tracer/tracee ratios in biological fluids by electrospray tandem mass spectrometry

A reproducible and very sensitive method is described for the quantitation of inorganic sulfate in biological fluids by negative electrospray ionization tandem mass spectrometry. After addition to the sample of S-34-labeled sodium sulfate internal standard and deproteinization with methanol, interfering bicarbonate anions are removed by acidification and chloride and phosphate by means of a single filtration step. The tandem mass spectrometer is used in neutral loss mode to detect HSO4- ions free of interference from residual isobaric H2PO4- ions. Organic sulfates do not interfere with the measurement. Serum and urinary inorganic sulfate concentrations measured with this technique agree closely with determinations by ion-exchange chromatography with conductivity detection. Unlike the latter method, this technique does not require dedicated equipment. The method is also suitable for measuring the ratio of S-34-labeled sulfate to unlabeled sulfate in serum and hence represents an attractive alternative for the use of the radioactive S-35 isotope in human studies of body composition and oxidation of sulfur-containing substrates to sulfate. (C) 1998 Academic Press.