Methane mixed plasma—improved sensitivity of inductively coupled plasma mass spectrometry detection for selenium speciation analysis of wheat-based food

Abstract

This work shows how the increase of carbon load to the ICP, by on-line addition of methane gas, in combination with anion-exchange high performance liquid chromatography inductively coupled plasma mass spectrometry (HPLC-ICP-MS) provides a powerful tool for Se speciation analysis of food-type samples, resulting in improved detection limits down to low ng kg⁻¹ (0.35 and 0.49) for selenate and selenomethionine (SeMet), respectively. The effect of an increase of the carbon load to the ICP, using different carbon sources, on the sensitivity of the ICP-MS for Se detection was investigated; the addition of methane gas to the ICP improved the sensitivity of ICP-MS Se detection by approximately a factor of 9 (for Se dissolved in methanol-free medium), in comparison with the conventional Ar plasma. Moreover, the combined addition of 2% of methanol to the mobile phase and methane gas resulted in a 11-fold sensitivity enhancement of Se ICP-MS detection. Results showed significant differences between quantitative speciation results of Se-rich (natural American flour) and enriched wheat flour; most of the Se incorporated into the Se-enriched flour was found as SeMet (75% of the total Se in the solid sample). Verification of the presence of selenomethionine in an enzymatic wheat flour extract, on the basis of the accurate molecular mass data obtained by ion pairing reversed phase HPLC-electrospray ionisation quadrupole time-of-flight MS/MS (ESI-Q-TOFMS/MS) measurements of the [M + H]^{+ 80}Se ions and their associated product ions, without the necessity for any pre-treatment of the water extract, is reported here for the first time. Analysis of an enzymatic extract of bread made with Se-enriched wheat flour revealed selenate concentrations comparable to those observed for Se-enriched wheat flour (approximately 1%). Although SeMet was again found to be the predominant Se species, only 42% of the total Se concentration in bread was present as SeMet.