COLLISION-INDUCED DISSOCIATION STUDIES OF PROTONATED ALCOHOL AND ALCOHOL-WATER CLUSTERS BY ATMOSPHERIC-PRESSURE IONIZATION TANDEM MASS-SPECTROMETRY .1. METHANOL

Cluster size distribution and collision-induced dissociation (CID) stu dies of protonated methanol and protonated methanol-water clusters yie ld information on the structure and energetics of such ions. Ions were formed at atmospheric pressure in a corona discharge source, and were subjected to CID in the center quadrupole of a triple quadrupole mass spectrometer. Cluster ions containing up to 13 molecules of methanol and/or water were observed and examined using CID experiments. The CID of all (CH3OH)n . H2O . H+ clusters, where n less-than-or-equal-to 8, showed that water loss was statistically favored over methanol loss a nd that the preferred dissociation channel involved loss of water with methanol molecules. These results support a model employing a chain o f hydrogen-bonded solvent molecules rather than one in which fused rin gs of ligands surround a central hydronium ion. However, CID of larger clusters, where n greater-than-or-equal-to 9, showed that loss of one methanol was equal to or less than loss of water, reflecting a change in structure.