Proton-bound cluster ions in ion mobility spectrometry

Gaseous oxygen and nitrogen bases, both singly and as binary mixtures, have been introduced into ion mobility spectrometers to study the appearance of protonated molecules, and proton-bound dimers and trimers. At ambient temp erature it was possible to simultaneously observe, following the introducti on of molecule A, comparable intensities of peaks ascribable to the reactan t ion (H2O)(n)H+, the protonated molecule AH(+) and AH(+) . H2O, and the sy mmetrical proton bound dimer A(2)H(+). Mass spectral identification confirm ed the identifications and also showed that the majority of the protonated molecules were hydrated and that the proton-bound dimers were hydrated to a much lesser extent. No significant peaks ascribable to proton-bound trimer s were obtained no matter how high the sample concentration. Binary mixture s containing molecules A and B, in some cases gave not only the peaks uniqu e to the individual compounds but also peaks due to asymmetrical proton bou nd dimers AHB(+). Such ions were always present in the spectra of mixtures of oxygen bases but were not observed for several mixtures of oxygen and ni trogen bases. The dimers, which were not observable, notable for their low hydrogen bond strengths, must have decomposed in their passage from the ion source to the detector, i.e. in a time less than similar to 5 ms. When the temperature was lowered to -20 degrees C, trimers, both homogeneous and mi xed, were observed with mixtures of alcohols. The importance of hydrogen bo nd energy, and hence operating temperature, in determining the degree of so lvation of the ions that will be observed in an ion mobility spectrometer i s stressed. The possibility is discussed that a displacement reaction invol ving ambient water plays a role in the dissociation. (Int J Mass Spectrom 1 93 (1999) 57-68) (C) 1999 Elsevier Science B.V.