Pulsed-nESI Gateless Ion Mobility Spectrometry

In this work, we demonstrate a novel pulsed approach to sample introduction in Ion Mobility Spectrometry (IMS). Through the application of a custom 100-1000 V Field-Effect-Transistor (FET) based pulser, ionization energy is rapidly pulsed above the onset threshold required to generate ions in nano-electrospray ionization (nESI) with a sub-millisecond rise time. Unlike conventional sample introduction in IMS, where ions are continuously generated from a source and an electrostatic or mechanical ion gate is used to initiate mobility separation, this pulsed method eliminates the gate through directly pulsing the ion source, avoiding prevalent gating deficiencies such as the gate depletion effect.

Pulsed n-ESI demonstrates mobility dependence across the range of common positive mode IMS standards tested, including tetraalkylammonium salts, chemical warfare agent simulants, and narcotic standards such as cocaine, oxycodone, and heroin. As the ions were not continuously generated during the pulsed experiment, the sample consumption of analytes was greatly reduced. In addition, pulsed IMS also exhibited an improved signal-to-noise ratio (SNR) and greater signal intensity while maintaining fidelity to mobility measurements and peak characteristics demonstrated by gated systems. Large, slow ions are more favorably introduced using this pulsed system when compared to gated cells due to the elimination of the gate-depletion effect. The elimination of gate depletion resulted in additional large ion peaks not present in the gated experiment. It is hypothesized that these additional ions are previously unreported dimers and Mass Spectral (MS) analysis will confirm the mass identity of these additional ions.