Fringing Field Effect of Orbitrap and Its Compensation Method

Since it was introduced, the Orbitrap has been proved to be a powerful ion analyzer whose resolution is affected by the fringing field in the electrode structure. Orbitrap is an ultra-high precision instrument. The gap between two outer half electrodes and the design of ion perforation on one side of the outer electrode will produce fringing field effect, resulting in the distortion of the ideal electrostatic field. The distorted field formed in the internal space of Orbitrap will have a serious impact on the resolution. Thermo Fisher Scientific is the only company that has a patent on Orbitrap mass spectrometers and related technologies, however, it has not provided relevant materials to explain the problem in detail and specific solutions, and few researchers have conducted in-depth research. Therefore, it is necessary to study the influence of the distortion field caused by the ion perforation and the outer electrode gap on the resolution, and propose a solution to the fringing field effect. In this work, ion optical simulation software (SIMION) was used to establish ideal, distorted and corrected models, in order to measure the limiting resolution of the orbital trap by measuring the axial vibration period dispersion corresponding to different initial axial amplitudes and different orbital radius. By comparing the experimental and theoretical results of the ideal model, the reliability of the selection of simulation parameters and the calculation method of ion motion were verified. In the distortion model experiment, the ion axial motion period dispersion caused by the fringing field effect arising from the outer electrode gap and ion perforation and its influence on the resolution was investigated. In the correction model experiment, it was proved that the distortion field can be corrected by adjusting the voltage of the two compensating electrodes, so as to overcome the fringing field effect caused by the ion perforation and the outer electrode gap. In addition, it was also found that the axial flight period error caused by radial dispersion in the corrected model is smaller than that in the ideal model, and the difference between the two is small in the axial dispersion. The comprehensive result of the corrected model is better than that of the ideal model. Therefore, the error caused by scratches on the surface of the electrodes can be corrected by adjusting the voltage of the compensating electrodes. The final simulation results showed that the limiting mass resolution for m/z 100 ion can reach to 2 500 000.