Ambient Pressure Inverse Ion Mobility Spectrometry Coupled to Mass Spectrometry

Although higher resolving powers are often achieved using ambient pressure drift tube ion mobility mass spectrometry (DT-IMMS) systems, lower duty cycles are often required which directly impacts sensitivity. Moreover, the mechanism of ion gating using Bradbury-Nielsen or Tyndall-Gate configurations routinely results in ion gate depletion effects which discriminate against low mobility ions. This paper reports a new method of ambient pressure ion mobility operation in which inverse ion mobility spectrometry is coupled to a time-of-flight mass spectrometer to improve sensitivity and minimize the effects of ion gate depletion. In this mode of operation, the duty cycle is improved to approximate 99% from a typical value of less than 1%, improving the signal intensity by over 2 orders of magnitude. Another advantage of inverse ion mobility mass spectrometry is a reduction of the impact of ion gate depletion on low mobility molecules that translates into higher sensitivity for this class of analytes. To demonstrate these benefits afforded by this instrumental mode of operation differences in sensitivity, resolving power, and ion discrimination are compared between the inverse and normal modes of operation using tetraalkylammonium standards. These results show that the ion throughput is significantly increased for analytes with a broad range of mobilities with little impact on resolving power. While the mobility-based discrimination is minimized using the inverse mode of operation, the noise level in the inverse mode is highly dependent upon the stability of ionization source.