ORGANIC MASS SPECTROMETRY, VOL. 28, 176-184 (1993) Chemical Reactions at Interfaces: Hydrogen-Deuterium Exchange Using a Solid ND4C1 Matrix in Molecular Secondary Ion Mass Spectrometry? Owen W. Hand, Asoka Ranasinghe and R. Graham Cooks* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA Organic compounds containing active hydrogens (e.g. imino, amino, carboxylic acid and hydroxy functional groups) undergo hydrogetdeuterium (H-D) exchange reactions in the course of argon ion bombardment in molec- ular secondary in mass spectrometry (SIMS) when examined as solids in solid ND,CI matrix. The positive-ion SIMS spectra of non-ionic compounds (M) display ions resulting from exchange of active hydrogens in both the deuterated ((M + D)+) and cationized ((Ag + M)+ and (C’ + M)’) forms, while ionic compounds display exchange products in the intact cation (C’, where C = cation). Analogous H-D exchange is observed in the negative SIMS spectra of compounds bearing active hydrogen atoms. The fact that the exchange reactions are dependent on the primary ion dose demonstrates that they occur in the energized sample and not prior to ion bombardment. Information on the mechanisms of interfacial reactions, insights into the SIMS mechanism and confirmation of the number of acidic hydrogens in non-volatile organic ompounds are revealed by this experiment. INTRODUCTION The chemical reactions which take place between gaseous ions and neutral molecules provide a rich source of analytical information in addition to fasci- nating mechanistic problems.’-4 Deuterated organic compounds have long been used to help elucidate reac- tion mechanisms in the gas phase.’ Hydrogen- deuterium exchange, observed for gas-phase organic molecules under chemical ionization (Cl) conditions, has been used to confirm the number of active hydrogens in analyte although exchange of inactive hydrogens has also been observed in other C1 In a recent study, the number of active hydrogens present in polyfunctional aromatic com- pounds was determined by performing selective isotopic exchange with deuterated gases in the collision cell of a tandem mass spectrometer.’ ’ Hydrogen-deuterium exchange is also widely used in flowing afterglow and other types of mass spectrometers for the study of ion- molecule reaction mechanisms. ” Several recent studies have focused on the use of deuterated glycerol as a hydrogendeuterium exchange agent for liquid second- ary ion mass spectrometry (LSIMS) and fast atom bom- bardment (FAB).13-” For example, the LSIMS spectra of organic dyes in deuterated glycerol display ions resulting from the exchange of up to six active hydro- gens.” The results of FAB analysis of biomolecules 7 Dedicated to Professor Dr Herbert Budzikiewicz, whose career in mass spectrometry has covered so rich a period in which his defining influence was so strong. with as many as 28 active hydrogens show that exchange can be up to 97% e f i ~ i e n t . ’ ~ In this paper beam-induced hydrogendeuterium exchange using solid samples in which an ND,Cl matrix acts as the deuterium source is discussed. The exchange reactions are dependent on the primary ion dose, demonstrating that they are indeed induced by the ion beam. Exchange of active hydrogens in =NH, NH, , COOH and OH functional groups produces =ND, NHD, ND,, COOD and OD products which are observed as both the deuterated (M - nH + (n + 1)D)+ and cationized (Ag + M - nH + nD)’ molecules. Beam-induced H-D exchange is a useful method for the analysis of non-volatile and thermally labile organic compounds since there is no interference from the ND4Cl matrix. This is because ND4Cl is a transparent matrixI6 which does not contribute any ions to the sec- ondary ion mass spectra, in contrast to the behavior of glycerol and other commonly used liquid matrices.” Various examples of beam-induced hydrogen- deuterium exchange were examined using N- and S- containing heteroaromatics, dyes, aromatic amides, alcohols and amines as model compounds. The value of this experiment is shown as a source of information on the mechanisms of beam-induced chemical reactions and on the SIMS mechanism itself. In addition, the pro- cedure used here allows confirmation of the number of active hydrogens in organic compounds. EXPERIMENTAL Secondary ion mass spectra were recorded using a com- mercial Riber secondary ion mass spectrometer which 0030-493X/93/030176-09 $09.50 0 1993 by John Wiley & Sons, Ltd. Received I0 September 1992 Accepted 9 October 1992