Characterization of a Modulated X-ray Source for Ion Mobility Spectrometry

Abstract

As a highly deployed field instrument for the detection of narcotics, explosives, and chemical warfare agents, drift tube ion mobility spectrometry relies heavily upon the performance of the ionization source and mechanism of ion beam modulation. For this instrumental platform, ion chemistry plays a critical role in the performance of the instrument from a sensitivity and selectivity perspective; however, a range of instrumental components also occupy pivotal roles. Most notably, the mechanism of ion modulation or ion gating is a primary contributor to peak width in a drift tube ion mobility experiment. Unfortunately, physical ion gates rarely perform perfectly, and in addition to serving as physical impediments to ion transmission, their modulation also has undesirable field effects. Using a recently developed modulated, non-radioactive X-ray source, we detail the performance of an ion mobility spectrometry (IMS) system that is free of a gating structure and utilizes the pulsed nature of the modulated X-ray source (MXS) for both ion generation and initiation of the IMS experiment. After investigating the influence of pulse duration and spatial X-ray beam width on the analytical performance of the instrument, the possibility of using multiplexing with a shutterless system is explored. By increasing ion throughput, the observed multiplexing gain compared to a signal-averaged spectrum approaches the theoretical maximum and illustrates the capability of the MXS−IMS system to realize significant signal to noise improvements.

Implementation of an Open-Source Multiplexing Ion Gate Control for High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

“…However, multiplexing involves multiple gate pulses during the course of the experiment while multiple ion packets move through the drift tube, the gate pulse significantly interferes with the ion signal (Figure B, Figure S1). A recent effort has addressed a way to mitigate this effect by changing the ion source to an orthogonal pulsed X-ray source and eliminating the ion gate altogether to reduce the effect of the capacitive coupling between the ion gate and Faraday plate . However, for our purposes, we cannot change the ion source or eliminate the ion gate, a blank spectrum (i.e., without ions) must be taken in addition to the Hadamard spectra and subtracted from the Hadamard spectra with ions present (Figure B).…”

“…A recent effort has addressed a way to mitigate this effect by changing the ion source to an orthogonal pulsed X-ray source and eliminating the ion gate altogether to reduce the effect of the capacitive coupling between the ion gate and Faraday plate. 44 However, for our purposes, we cannot change the ion source or eliminate the ion gate, a blank spectrum (i.e., without ions) must be taken in addition to the Hadamard spectra and subtracted from the Hadamard spectra with ions present (Figure 2B). This treatment of the data partially mitigates the noise, but the noise from the ion gate will be addressed in more detail later.…”

Development of an Ion Mobility Spectrometer Based on a Pulsed Photoelectric Effect Ionization Source

“…Nevertheless, the vacuum conditions, high voltage supply, and control circuitry required for the ionization source add to the complexity of the instrument. In 2022, Reinecke et al 33 designed an IMS based on a pulsed X-ray source. The X-rays were emitted by accelerating photoelectrons generated by a UV-LED, which was modulated by a TTL waveform.…”