A Radio Frequency Ion Guide for Transporting Cooled Ions Through Differential Vacuum Stages: Design and Simulation

In order to transport ions from an intermediate-pressure collisional cooling device to a planar electrostatic ion trap (PEIT) mass analyzer in an ultrahigh-vacuum chamber, a segmented radio frequency (RF) quadrupole system was designed and investigated by using an ion optical simulation. Two gas-flow-limiting units were employed in the system, allowing the differential pumping to establish a pressure difference of 7 orders of magnitude. Flow-limiting units including a plain aperture lens and 3 small quadrupoles in different shapes were investigated, and the transmission efficiency and energy distribution of the transported ions were evaluated. The small quadrupoles with conical front-ends inserted between the multiple segments gave the best transmission efficiency, and the collisional cooled ions can maintain their low energy distribution after being transported to the ultrahigh-vacuum stage. With an RF voltage amplitude of 200 V, the maximum transmission efficiency reached above 90%. After ions are cooled in the high-pressure quadrupole segment, the extraction voltage difference in high-pressure region needs to be low enough to avoid reheating the ions during the transport. It was found that an extraction voltage difference below 1.2 V can restrict the half width of the energy spread within 0.5 eV, which is required by PEIT to get ultrahigh mass resolution.