Acidic chemical modifier to increase the selective detection of steroids by LC-DMS-MSMS

摘要

The addition of formic acid to the transport gas of a DMS cell clearly shows benefit for the analyses of steroids. In the few cases studied, steroid seems to be preferentially shifted in toward negative CoV values when many other endogenous species seem to be essentially unaffected by the presence of formic acid and their CoV typically stay positive. Though the addition of formic acid may reduce the signal intensity by 20-50% when compared to no modifier added in the transport cell, this approach offers the ability to detect the steroid with a degree of selectivity with SIM detection that can rival MRM detection. The exact nature of the interaction between formic acid and the steroids in the DMS cell is likely complex, but it is clearly unique and it is likely associated with the site of protonation of the analyte. When compared to species where the charge is localized on nitrogen groups (whether primary, secondary or tertiary amines), the addition of formic acid in the transport gas did not show any shift in CoV values for given SV. For these nitrogen containing species, addition of alcohol in the transport gas causes their CoV to shift towards negative values [2]. The addition of alcohol, even low proton affinity ones such as methanol, essentially eliminates all of the signal associated with steroids, which is not the case with formic acid. This combination of DMS with formic acid in the transport gas opens up the ability to simplify the MS detection part for the analysis of steroids while still providing a high degree of selectivity.