High-frequency AC electrospray ionization source for mass spectrometry of biomolecules

Abstract

A novel high-frequency alternating current (AC) electrospray ionization (ESI) source has been developed for applications in mass spectrometry. The AC ESI source operates in a conical meniscus mode, analogous to the cone-jet mode of direct current (DC) electrosprays but with significant physical and mechanistic differences. In this stable conical-meniscus mode at frequencies greater than 50 kHz, the low mobility ions, which can either be cations or anions, are entrained within the liquid cone and ejected as droplets that eventually form molecular ions, thus making AC ESI a viable tool for both negative and positive mode mass spectrometry. The performance of the AC ESI source is qualitatively shown to be frequency-dependent and, for larger bio-molecules, the AC ESI source produced an ion signal intensity that is an order of magnitude higher than its DC counterpart. in mass spectrometry (MS) has revolutionized the study of proteins and peptides and contributed significantly to biological and biomedical research. In contrast to ionization techniques such as fast atom bombardment (FAB), electron impact (EI), and thermal ionization that tend to fragment large molecules, ESI and MALDI minimize fragmentation, hence making them appropriate for analysis of biomolecules [3,4], polymeric macromolecules [5], and the study of the environmental implications of hazardous materials [6]. The operating principle of ESI is that it generates charged droplets from the tip of a static conical liquid structure [7] known as Taylor's cone [8]. These charged droplets, containing analyte molecules present in the liquid, eventually form molecular ions by either multiple Raleigh fission events or, alternatively, via the charge residue mechanism [9]. In this study, we introduce another variant of ESI that is obtained by application of high-frequency alternating current (AC) potential to generate charged droplets for mass spectrometry.In previous studies by the authors' group, it has been shown that in a certain window of frequency-voltage phase space, a steady, slender conical meniscus [10 -12] is obtained using a high-frequency AC electrospray. As shown in Figure 1b, these AC cones, which have a half cone angle of ϳ11°and exhibit a continuous longitudinal growth in time are significantly different from static DC Taylor cones (with half cone angle ϳ49°), pulsating electrosprays [13], or low-frequency (Ͻ10 kHz) AC electrosprays [14] (Supplementary Material showing an AC cone of ethanol solvent at an applied AC potential of frequency 80 kHz and magnitude ϳ 5.5 kV (peak to peak) along with the preliminary sensitivity curve and positive mode mass spectra of cytochrome c, which can be found in the electronic version of this article). The formation of the AC cone is due to the difference in the mobility of liquid phase anions and cations that leads to a build up of the low mobility species in the cone because of insufficient time to relax onto the surface of the meniscus when the AC signal switches polarity-a condition induced when the inverse r...

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