Characteristics of Mass Fragmentation of Steroids by Atmospheric Pressure Chemical Ionization-Mass Spectrometry

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The characteristics of the mass spectra of sixty steroids were investigated using atmospheric pressure chemical ionization-mass spectrometry (APCI-MS). In APCI-MS, the drift voltage and nebulizer temperature affected the appearance of molecular ions. Solvent adducted ions [M+H+CH₃CN]⁺ and [M+H+H₂O]⁺ were decreased with an increase in drift voltage. The optimum drift voltage differed slightly for each steroid.These sixty steroids were divided into two groups according to their mass spectra profiles : one having a carbonyl group at position 3 together with a double bond at position 4 (group A) and the other bearing a hydroxyl group at the 3 position (group B). In group A, the predominant peak observed corresponded to the protonated molecular ion [M+H]⁺. The fragment ion corresponding to the elimination of CH₂OH, COCH₃ and/or COCH₂OH from the steroid skeleton appeared as a base peak in some steroids of group A. In group B, predominantly [M+H-H₂O]⁺ and/or [M+H-2H₂O]⁺ ion(s), originating from the loss of water molecules, were observed. Other major ions in this group were the protonated molecular ions. Like other chemical ionization mass spectrometry, the mass spectrum of each steroid was very simple. These results suggest that APCI-MS is a suitable tool for the determination of the mass number of polar, nonvolatile and thermolabile steroids without derivatization.