Collisional focusing effects in radio frequency quadrupoles

TLDR

Collisional focusing in RF quadrupoles is analogous to effects observed in 3‑D ion traps. The study examined ion transmission through a 2‑D RF quadrupole at pressures 5×10⁻⁴–1×10⁻² torr and used a Monte Carlo simulation of energy loss to match observed axial kinetic energy losses. Ion transmission through the RF quadrupole increases with pressure up to a peak at 8×10⁻³ torr for low‑energy ions, then declines, contradicting classical scattering; the effect grows with ion mass (up to 16,950 u), is accompanied by axial kinetic energy loss, and suggests high‑pressure operation could be useful for sampling ions from atmospheric‑pressure sources.

Abstract

The transmission of ions through a conventional two-dimensional radiofrequency-only (rf) quadrupole has been studied for comparatively high operating pressures between 5 × 10−4 and 1 × 10−2 torr. Measurements of signals from mass-resolved analyte ions and total ion currents show that, provided the initial injection ion energy is low (1–30 eV), the ion transmission observed through a small aperture at the exit of the rf quadrupole first increases as the gas pressure increases, reaching a maximum at − 8 × 10−3 torr before decreasing at higher pressures. This is in direct contrast to the expectations of classical scattering. This "collisional focusing" appears to be analogous to effects seen in three-dimensional ion traps. The collisional focusing increases with the mass of the ion (not mass-to-charge ratio) for masses up to at least 16,950 u. The collisional focusing of the ions is found to be accompanied by significant losses of axial kinetic energy. A Monte Carlo simulation of the energy loss process is reported that can provide agreement with the observed losses for reasonable collision cross-sections. The results suggest that operation of rf quadrupoles at relatively high pressure may find practical application in sampling ions from high (e.g., atmospheric) pressure ion sources.