Kinetics of small ion evaporation from the charge and mass distribution ofmultiply charged clusters in electrosprays

The distribution of charge z and radii Ii in clusters electrosprayed from f ormamide solutions of tetraheptylammonium bromide was investigated by selec ting those within a narrow range of electrical mobilities Z(1) in a first d ifferential mobility analyzer (DMA), reducing their charge to unity by pass age through a neutralizing chamber containing a radioactive (alpha) source, and measuring the mobilities Z(z) of the resulting discrete set of singly charged clusters in a second DMA, After correcting for the polarization con tribution to cluster drag, the tandem DMA data yield the range of radii pre sent at detectable levels for each charge state up to z = 9. Because small ion evaporation from electrospray drops leads to charge loss when a drop re aches a certain critical radius R-crit(z), the measured maximum and minimum cluster radii associated with a given z can be used to infer the activatio n energy A for ion evaporation as a function of drop charge and curvature, These results confirm the Iribarne-Thomson ion-evaporation mechanism, and s upport earlier theoretical expressions for the functional form of Delta(z,R ), The different phenomenon of ion evaporation from metastable multiply cha rged dry clusters is also observed at characteristic times of 1 s. Its acti vation energy is estimated as similar to 0.3 eV larger than For ion evapora tion from the drops. This new process complicates the interpretation of the present measurements in terms of ion evaporation from liquid surfaces, but introduces no radical change in the picture. It helps understand why salt clusters with more than two or three charges are harder to see in mass spec trometers than in mobility studies under ambient conditions, Copyright (C) 2000 John Wiley & Sons, Ltd.