Creating an evanescent ion/ion reaction region within a low-pressure linear ion trap

Abstract

We present a new environment for conducting ion/ion reactions within a linear ion trap (LIT) mass spectrometer. Normally, these reactions are performed in a dedicated high-pressure (P>mTorr) region that uses some form of radio frequency (RF) potentials to trap the reacting cations and anions in the axial and radial dimensions. Generally, a low-pressure (P≤10−5Torr) environment is not well suited for ion/ion reactions due to the low ion-neutral gas collision frequency that results in higher kinetic energies for the ions – a deleterious condition for efficient ion/ion reactions. However, with some minor modifications, a low-pressure LIT can be transformed into an evanescent (or temporary) ion/ion reaction region. This is done by adding DC and RF voltages to auxiliary electrodes and a pulse valve for the addition of cooling gas. These features serve to restrict axial and radial motion of the reacting ions. We compared the performance of this ion/ion reaction region to a static high-pressure region (∼10−3Torr) and a static low-pressure region (∼10−5Torr). In terms of reaction efficiency, cycle times, etc., the performance of the evanescent ion/ion reaction environment compares favorably with the dedicated static high-pressure ion/ion reaction region.