Differentiation between dexamethasone and betamethasone in a mixture using multiple mass spectrometry

The objective of this study was to provide LC and GC–multiple mass spectrometry (MSn) data in positive and negative ion modes to prove the distinction between dexamethasone and betamethasone in a mixture of both components. Using GC–MS, the differentiation was based on a difference in the ratio of the ion traces of the two chromatographic peaks of the α and β epimer with m/z 310 and 330. A minimum of 15% dexamethasone should be present in a mixture of both to detect it as present with a probability of 95%. In the same way betamethasone can be detected from 15% on. Because of the very similar structures of the dexamethasone and betamethasone epimers, no reversed-phase (RP) separations have been reported. Normal-phase separations have been reported in other studies. However because of the compatibility of RP mobile phases in the coupling with MS, the latter was the method of choice. In LC–MSn positive ion mode the product ion 355 was plotted against the sum of 337 and 319. With this combination dexamethasone and betamethasone could be discriminated in a mixture of 20 to 80% of each combination of analytes. In negative ion mode only two product ions were formed from the fragmentation of the acetate adduct, [M-H]− and [M-H-CH2O]−. The intensity of the fragment 391 ([M-H]−) was determined in the discrimination of the two epimers.