When O2 is used as a reagent gas for negative‐ion
chemical ionization (NCI) of phenothiazines and related
compounds, the majority of the ions observed are formed by
surface‐catalyzed reactions prior to ionization. Both
oxidation of the sulfur atom and formation of quinoid systems are
observed. Alkyl derivatives also show degradation in addition to loss
of the substituent. Deuterium labeling studies allowed several of the
reaction paths to be elucidated. Copyright © 1999 John Wiley
& Sons, Ltd.