Design and Performance Investigation of Octa-electrode Linear Ion Trap Based on an Electrospray Ionization Mass Spectrometer

The octa-electrode linear ion trap (OeLIT) had achieved great analytical performance in our previous theoretical simulation studies. In order to verify the accuracy of the simulation results and to test its actual mass analysis capacity in mass spectrometer, OeLIT was designed, fabricated and assembled using the optimal structure parameters achieved by the previous simulation. It was mainly composed by eight identical columnar electrodes with bevel shaped and two planar electrodes, which were fabricated from stainless steel. The external dimension of OeLIT was 62 mm×36 mm×44 mm and its internal volume was about 3.5 cm3. Unlike the majority of simplified ion traps with “stretched” structures, the OeLIT had a completely centrosymmetric structure, which was much simpler and had larger mechanical tolerances. The whole OeLIT could be assembled by hands and its total mechanical error was measured less than 10 μm. Based on the structure of OeLIT, a mass spectrometer was developed with three-stage vacuum, and the OeLIT was placed in the third chamber with the optimized pressure of 0.002 Pa. Besides, the instrument also included an electrospray ionization source, an ion guide, an ion detector, and electronics. Here, the two electrodes on the same side were defined as a group, so there were four groups of electrodes in the OeLIT. The width between the two electrodes in one group was 0.8 mm for ion ejection, which replaced the electrode-slit design of traditional linear ion traps and was beneficial to the mass resolution. The traditional radio frequency (RF) voltage application mode was used that two balanced RF signals with 180° phase shift were applied to alternate groups of electrodes, and the performance of OeLIT was experimentally characterized with the reserpine solution. When the scan rate was 756 u/s, mass peak with FWHM of 0.3 u was achieved for ions with m/z 609, which corresponded to a mass resolution of 2 030. The signal to noise ratio was up to 45.8 when analyzing the reserpine solution with a concentration of 10 mg/m3. The influence of scan rate on mass resolution and sensitivity was explored in detail. Two-stage tandem mass spectrometry was successfully performed in OeLIT, and the collision-induced dissociation efficiency was 36% without buffer gas. The 25 hydroxyvitamin D2 standard samples at different concentrations were quantitatively analyzed, and the linear dynamic range was 4 orders of magnitude. The experimental results show that OeLIT has good analytical performance, which confirms to the simulation results. It can provide a brand new idea for the research of ion traps with simplified structure, which will promote the development of miniaturized ion trap mass spectrometers.