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Thermal gas jet probes, including post-plasma desorption/ionization sources, have not been studied using computational fluid dynamics (CFD) models, as have other ambient mass spectrometry sampling techniques. Two systems were constructed: a heated nitrogen jet probe to establish practical bounds for a sampling/transmission experiment and a CFD model to study trajectories of particles desorbed from a surface through optimization of streamlines and temperatures. The physical model configuration as tested using CFD revealed large losses, transmitting less than 10% of desorbed particles. Different distances between the desorption probe and the transport tube and from the sample surface were studied. The transmission improved when the system was very close to the sample, because the gas jet otherwise creates a region of low pressure that guides the streamlines below the inlet. A baffle positioned to increase pressure in the sample region improves collection efficiency. A Lagrangian particle tracking approach confirms the optimal design leading to a transmission of almost 100%.

Toward greater separation techniques for ions, a differential mobility analyzer (DMA) has been coupled with field asymmetric waveform ion mobility spectrometry (FAIMS) to take advantage of two mobility-related but different methods of separation. The filtering effect of the DMA allows ions to be selected individually based on low-field mobility and studied in FAIMS at variable electric field, yielding mobility separations in two dimensions. Because spectra fully describe ion mobility at variable field strength, results are then compared with a twotemperature theory-predicted mobility up to the fourth-order approximation. The comparison yields excellent results up to at least 100 Td, beyond which the theory deviates from experiments. This is attributed to two effects, the enlargement of the structure due to ion heating and the inelasticity of the collisions with the nitrogen bath gas. The corrected mobility can then be used to predict the dispersion plot through a newly developed implicit equation that circumvents the possible issues related to the more elaborate Buryakov equation. Our results simultaneously show that the DMA−FAIMS coupling yields complete information on ion mobility versus the field-strength to gas-density ratio and works toward predicting such spectra from ion structures and gas properties.

A linearly decreasing electric field has been previously proven to be effective for diffusional correction of ions in a varying field drift tube (VFDT) system, leading to higher resolving powers compared to a conventional drift tube due to its capacity to narrow distributions midflight. However, the theoretical predictions in resolving power of the VFDT were much higher than what was observed experimentally. The reason behind this discrepancy has been identified as the difference between the theoretically calculated resolving power (spatial) and the experimental one (time). To match the high spatial resolving power experimentally, a secondary high voltage pulse (HVP) at a properly adjusted time is used to provide the ions with enough momentum to increase their drift velocity and hence their time-resolving power. A series of systematic numerical simulations and experimental tests have been designed to corroborate our theoretical findings. The HVP-VFDT atmospheric pressure portable system improves the resolving power from the maximum expected of 60−80 for a regular drift tube to 250 in just 21 cm in length and 7kV, an unprecedent accomplishment.