Numerical investigation of interface region flows in mass spectrometers: neutral gas transport

Abstract

The supersonic jet flows of neutral gas from atmospheric to near-vacuum conditions in the interface region of mass-spectrometer systems is investigated by continuum-based (fluid) numerical simulations. An enhanced understanding of the neutral gas transport is of paramount importance to fully understand flows in the interface region of mass spectrometers, for it is the neutral dynamics that governs and drives the ions from the high pressure ion source through the interface orifice towards the ion optics and detector subsystems. The key features and structure of the complex neutral gas flow are examined and the influence of large pressure differences imposed across the interface region, orifice geometry, and gas skimmer configuration used for flow control are assessed. The flow structure is shown to be that of a classical under-expanded free jet for 'skimmer-absent' cases and very good agreement between the numerical predictions and empirical and experimental values is demonstrated. For the 'skimmer-present' cases, the shock structure downstream of the orifice and skimmer is identified and fully described and its influences on the flow skimming and focusing processes are discussed.

Numerical Investigation of Ion Transport in Under-Expanded Jet Flows

“…Comparisons at other radial and axial stations yield similar agreement. 46 Although, there is some disagreement between measured and predicted values of the pitot pressure close to the orifice (this difference is most likely due to probe interference effects), overall the agreement between the experimental data and numerical results is very good, providing strong support for the validity and reliability of the neutral gas calculations. In particular, the excellent agreement in the pressure profiles would imply that the boundaries and extent of the free jet are well predicted by the neutral gas flow solver.…”

“…46 In the absence of an applied electric field (E = |E| = 0 V/m), the ion-neutral collision frequency is sufficient to maintain equilibrium conditions between ions and neutral particles such that v i = v n and T i = T n . Thus differences between the ion and neutral trajectories are negligible for all of the ions considered here when the electric field is absent.…”

Coupled gas and ion transport in quadrupole interfaces

“…However, as discussed by Jugroot et al [20], the overall structure of the neutral jet with a gas skimmer present is quite different from that of a classical under-expanded free jet. The presence of the skimmer disrupts and prevents the formation of the free jet structure.…”

Argon gas flow through glass nanopipette

“…This gas flow geometry resembles a skimmer for obtaining stable molecular beams. 28) On the other hand, in the case of the gas flow from the wider end to the top, the distribution did not change markedly at the entrance; however, the pressure rapidly changed around the exit top of the nanopipette, which might have caused turbulence in the nanopipette. Note that the frequency of clogging the nanopipette is more than ten times when the gas flows from the wider end to the top than the opposite flow, owing to the hardly unavoidable dust in the vacuum chamber.…”