Direct species-selective determination of cobalamins by ionspray mass spectrometry and ionspray tandem mass spectrometry
Résumé
A direct method for the simultaneous compound-specific determination of vitamin B12 (cyanocobalamin), its analogues methylcobalamin, hydroxocobalamin, adenosylcobalamin (coenzyme B12) and cobinamides by ionspray MS was developed. Single and tandem MS modes were compared. For all the compounds except hydroxocobalamin, the most intense ion formation was observed as a result of protonation of the original molecular cation leading to an analytically useful doubly charged ion. For hydroxocobalamin a similar process leads to the loss of the functional group leading to the [Cbl + H]2+ ion, which is analytically useful only in the absence of other cobalamins. Hydroxocobalamin can be determined in the presenee of other compounds using the minor [OH-Cbl]+ ion. The calibration curves are linear in the range 0.01-1 μg ml-1, which can be extended up to 10 μg ml-1 if alternative ions are chosen. Detection limits of 5-40 ng ml-1 are obtained. Collision-induced dissociation leads first to the loss of the Co substituent (Me, CS, OH, adenosyl) and thus to a decrease in selectivity. The method was applied to the analysis of a synthetic mixture of the cobalamins and to the determination of hydroxocobalamin in a commercial pharmaceutical preparation.