An Improved Design of Electron Ionization Time-of-Fight Mass Spectrometry with Collisional Focusing Ion Guiding

Electron ionization time-of-flight mass spectrometry (EI-TOF-MS) has been widely used in the fields of chemical, environmental, food, medical and pharmaceutical, etc. Coupling gas chromatography (GC) with EI-TOF-MS yields promising qualitative and quantitative results, and the performance is closely related to with the ion guide devices. A good ion guide device for EI ion source can generate a narrow beam of low-energy ions, resulting in high ion transition efficiency. In this study, an ion transition system for EI ion source with radio frequency quadrupole (RFQ) as the collisional focusing ion guide was developed. The ions in the RFQ could be cooled by collisions with buffer gas molecules and focused to near the axis by a RF field. To determine the parameters of the RFQ, including rod length, rod radius, field radius, RFQ exit orifice diameter, RF frequency, RF amplitude, and other transmission electrode voltages, vacuum design theory, RF quadrupole field theory and SIMION software simulations were applied. The performance of a GC-EI-TOFMS instrument was significantly improved using RFQ as collisional focusing ion guide, and the evaluation results showed that the mass resolution was up to 6500 FWHM (m/z 502), the limit of detection (LOD) was 1 pg of OFN (S/N > 30:1), the mass range was 15-968 amu, the mass accuracy was below 0.01 amu, and the relative standard deviation (RSD) was below 1.5% within one week. This study provided an example to build an ion transition system for mass spectrometry, which had important theoretical and practical significance in design and development of new mass spectrometric instrument.