Role of ion-ion recombination for alkali chloride cluster formation in liquid secondary ion mass spectrometry.

Liquid secondary ionization mass spectra of solutions of alkali chlorides in glycerol were studied as a function of salt concentration. The experimental abundances of glycerol ions and of Cs(+)(CsCl) n cluster ions were successfully reproduced by assuming that most of the randomly distributed ions pair up with counterions shortly after impact. Further, it is considered that clustering (or proton transfer) reactions occur mainly between an ion that survives the pairing process and ion pairs (or basic analytes) in the immediate vicinity; however, some mixing undoubtedly occurs in the later stages of the desorption process. At the density of the original matrix, the range of proton transfer is calculated to be 5-15 Å, and that of clustering approximately 25% shorter. These reaction distances are inversely correlated with the internal energy of the ejected ions. In general, liquid secondary ionization mass spectra of alkali chloride solutions can be seen to result from competitive ion-ion recombination reactions in the decaying matrix. Finally, from the abundances of cluster ions containing [glycerol - H](-) ions, it is estimated that approximately 1% of the glycerol molecules in the ejected volume are ionized in the collision cascade.