Effects of drift-gas polarizability on glycine peptides in ion mobility spectrometry

This investigation is a continuation of our previous work on the feasibility of utilizing ultra-high resolution electrospray ionization/ion mobility spectrometry (ESI/IMS) for in situ analysis of biomolecular compounds. The compounds we studied, in this investigation, were glycine, the smallest amino acid and four of its oligomers, namely triglycine, tetraglycine, pentaglycine, and hexaglycine. Experimental effects of drift-gas polarizability on target ions in IMS were explored by utilizing four different drift-gases with differing polarizability values (He, Ar, N-2, and CO2). The gas-phase ion radii for all five compounds were calculated from the reduced ion mobilities, K-0(m), and the effective drift-gas radii employing a simple hard-sphere model. When ion radii were plotted against the polarizabilities of the drift-gases, linear plots with different slopes were produced. This empirical observation indicated that the polarizing of drift-gas can change the calculated ion radii in a linear fashion over a limited range of polarizability values and does not affect all ions equally. This effect can be exploited in order to alter the separation factors between different ions since all ions that yield different slopes can, theoretically, be separated with IMS using different drift-gases. We demonstrated that the separation factor (alpha) is highly dependent on the drift-gas. The maximum separability and, hence, unique identification of target ions was achieved when He and CO2 were used. (C) 2002 Published by Elsevier Science B.V.