Acetonitrile/water gas-phase reaction products observed by use of atmospheric pressure chemical ionization: adducts formed with sulfonamides

Electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) are the two most common mass spectrometric ionization methods used in the pharmaceutical industry. However, APCI analysis can sometimes lead to ambiguity in compound characterization and quantitation due to gas-phase reactions occurring between acetonitrile and water in the plasma, and between these plasma-generated compounds and the analyte. During the analysis of various sultams and sulfonamides we observed signals corresponding to m/z M+44](+) and M+60](+). Various solvent conditions and collisionally activated dissociation MS(n) experiments revealed that under the high-energy plasma conditions of APCI, the acetonitrile/water solvent mixture reacts undergoing acid-catalyzed hydrolysis producing acetamide, 59 Da. Further, the highly reactive 43 Da species ethanimine is also produced. These two compounds, normally not observed in APCI analysis, are stabilized by the sulfonamide and appear as adduct species in the mass spectra. The sulfone oxygens and the lone pair of electrons on the amide nitrogen play a role in stabilizing this adduct.