PROTONATED WATER AND PROTONATED METHANOL CLUSTER DECOMPOSITIONS IN A QUADRUPOLE ION TRAP

Electrospray mass spectra of water and methanol have been studied with a quadrupole ion trap operated with background gas pressures of 2 x 10(-5)-2 x 10(-3) Torr. In each case, the most abundant ions observed consist of protonated clusters of varying size, e.g. (H2O)nH+ in which n ranges up into the twenties. For clusters of n > 6 or so, rapid desolvation is observed. For intermediate values of n, the desolvation rate constants can be readily measured in the typical time frame of a quadrupole ion trap experiment. The internal temperatures of the cluster ions can be estimated from the known thermochemical parameters and the measured decomposition rate constants. In all cases, the temperature exceeds that of the bath gas but is significantly less than 1000 K. It is also noted that the ion temperature is inversely proportional to the logarithm of the bath gas pressure.