Optimisation of ICP-dynamic reaction cell-MS as specific detector for the speciation analysis of vanadium at therapeutic levels in serum

Dynamic reaction cell (DRC) technology was applied in the context of the speciation analysis of vanadium at therapeutic levels in serum. This technology was necessary in order to detect vanadium on-line after size-exclusion chromatography with a buffer at physiological salinity (0.15 M NaCl). This salinity was compulsory to assure the stability of the vanadium compounds during chromatography, in other words to prevent interspecies conversion. In fact, the DRC allowed the detection of vanadium without adapting the conditions of the chromatographic separation to ICP-MS. First, the merits of various reaction gases were compared: methane, carbon monoxide, ammonia, oxygen and the combination of argon ( collision gas) and hydrogen ( reaction gas). In each instance, the reaction cell parameters were optimised in order to obtain the lowest detection limit for V-51 (as V-51(+) or as (VO+)-V-51-O-16 with O-2 as the reaction gas) in chlorine-rich solution, Cl being the parent element of the (ClO+)-Cl-35-O-16 interference. Ammonia was found to offer the best detection limit (3s criterion, 10 ng L-1 with pneumatic nebulisation as the sample introduction system). The detection limit with size-exclusion chromatography-ICP-DRC-MS for vanadate, expected to be the worst among all vanadium chemical species, was found to be 40 ng L-1 serum (or 4 pg V, 100 muL serum injected) and the repeatability 7%. This on-line separation method was used in order to speciate vanadium in serum after incubation with vanadate, at a concentration level that is representative of the pharmacological concentration range of vanadium when used as an insulin-like agent.