Ion discrimination during ion accumulation in a quadrupole interface external to a Fourier transform ion cyclotron resonance mass spectrometer

When coupled with high-performance capillary separations, Fourier transform ion cyclotron (FTICR) mass spectrometry provides powerful new capabilities for proteomic studies. Selective ion accumulation in a two-dimensional quadrupole device external to an FTICR mass spectrometer has been shown to increase its sensitivity, dynamic range, and duty cycle. In this article, we have evaluated the operation of a linear quadrupole ion trap coupled to a 3.5-tesla FTICR mass spectrometer in the presence of high space charge. On increasing the ion population in the linear quadrupole trap, pronounced m/z discrimination was observed. We have found that the superposition of the applied quadrupole rf and dc fields, the effective dc field from space charge within the quadrupole, and the quadrupole fringing fields at the quadrupole entrance and exit result in ion instability and m/z discrimination. After optimizing experimental parameters responsible for m/z discrimination, capillary LC-separated tryptic peptide ions (from a bovine serum albumin digest) were shown to be externally accumulated and detected using the FTICR mass spectrometer without evident m/z discrimination.