Design and Characterization of a Novel Traveling-wave Off-axis Ion Transmitter

To enhance the sensitivity, signal-to-noise ratio and durance of electrospray mass spectrometer, an off-axis ion guide combining a traveling-wave (TW) stacked-ring and a shaped ion funnel has been designed, which is capable of filtering out the neutral contaminants and charged droplets entrained in the electrospray plume. According to its structural characteristics, the corresponding TW and radio frequency (RF) coupling power supply and measurement as well as control system were designed. Customizable Ethernet communication protocol was used to control the field-programmable logic gate array (FPGA) for full-circuit signal control and generation, and an 8-channel traveling-wave amplitude monitoring was achieved through a 12-bit analog-to-digital converter (ADC) and a 16-bit digital-to-analog converter (DAC). In addition, dual-plane transformer coupling circuit was designed. The use of traveling wave changed the central reference frame of the RF, achieving the purpose of programmed RF and the effect of TW in the stacked-ring to provide axial kinetic energy. The stability and accuracy of the electrical parameters of the hardware system on the off-axis transmission structure were tested. The results showed that the controllable voltage range of TW power supply is 0.1-100 V and the controlled DC power supply’s full range detection and control absolute error are less than 0.04 V. The frequency of TW up to 400 kHz, both rising edge and falling edge are less than 30 ns. Furthermore, the waveforms of TW, RF and post-coupling waveform are sufficiently smooth. The test results also showed that the performance of the power supply and measurement and control system are stable and reliable, and their accuracy and range can satisfy the design requirements. Then, the performance of off-axis structure was tested under rough vacuum using reserpine ions. It was found that the reference voltage difference between the upper and lower traveling-waves (the voltage which the ions were extracted), the voltage of the upper traveling-waves, and the traveling-wave duty cycle of the off-axis ion transmitter have more significant effects on the off-axis transmission of ions than other electrical parameters. The experimental results provide a fundamental understanding of ion transport and off-axis separation of traveling-wave stacked-ring structures, which is valuable for the development of high signal-to-noise mass spectrometry system. In the future, this novel traveling-wave off-axis transmitter and its coupling power supply system will be applied to homemade mass spectrometry instruments. Besides, we will compare the developed device with the mainstream ion transmission devices in the market so as to further improve the device structure and power supply parameters, and deeply explore the role of traveling-wave off-axis transmitter in different ionization sources and different detectors.