Overtone mobility spectrometry

Abstract

Mobility of ions through a static buffer gas as a means of separating components in complex mixtures was introduced in 1970 by Cohen and Karasek and later named as ion mobility spectrometry (IMS). Currently, there are two other methods that are based on this method; differential mobility analysis (DMA), in which ions migrate through a flowing gas under the influence of a field, and, field asymmetric ion mobility spectrometry (FAIMS) which separates ions based on differences between high- and low-field mobilities. IMS and its variations are extensively used in various applications as a means of characterizing a range of analytes as well as simplifying the analysis of complex mixtures through the removal of chemical noise.

The work presented here introduces a new separation technique called overtone mobility spectrometry (OMS) based on mobilities of ions in gas phase. OMS approach uses multiple segmented drift regions with modulated drift fields to produce conditions that allow only ions with appropriate mobilities to pass through the instrument. Therefore, the instrument acts as a mobility-filter for continuous ion sources. By changing the frequency of the drift field application, it is possible to tune this instrument to transmit ions having different mobilities. Also, a scan over a wide range of field application frequencies for a single ion species shows a peak corresponding to the expected resonance time of the ions in one drift segment as well as a series of peaks at higher overtone frequencies. The measured resolving power increases for higher overtones making it possible to resolve structures that were unresolved in the region of the fundamental frequency. Because of the ability to select ions in different frequency regions, including those that are associated with higher overtones, we refer to the approach as Overtone Mobility Spectrometry (OMS).

The majority of the work presented here involves the development of this technique and its characterization using different systems such as simple isobaric carbohydrate mixtures and complex protein digests. Also presented here is the theoretical evaluation of the resolving power and ion simulation data to understand the basis of the peak formation in the frequency domain.