Development of a compact and efficient FAIMS ionization source based on UV-LED photoelectric effect

Abstract

A compact photoelectric effect ionization source for high-field asymmetric waveform ion mobility spectrometry (FAIMS) based on UV-LED was proposed in this paper. Ambient air and three electronegative compounds (SF₆, NO₂, and SO₂) were selected as samples to verify the effectiveness of the proposed method, and the effects of the emission electrode material, light source, relative humidity, repeller voltage, and flow rate of carrier gas were investigated. Distinctive fingerprint spectra were successfully obtained for the air background and the three compounds, exhibiting the method's discrimination capability. The silver (Ag) was identified as the optimal material for the emission electrodes, capable of generating an ion signal intensity of -77.8 pA in an air background. The repeller voltage and flow rate have a significant impact on ionization efficiency. An increased flow rate was associated with enhanced ionization efficiency, necessitating a corresponding increase in the repeller voltage. When the flow rate of carrier gas is 5 L/min, the optimal repeller voltages for the three samples (SF₆, NO₂, and SO₂) were -2 V, -0.5 V, and -1 V, respectively, which roughly corresponded to their ion mobility. With increasing concentration of the samples, the ion signal intensity linearly increases at first and then levels off. Different samples show significant differences in the growth rates in the linear region, and the detection limits for the three substances, SF₆, NO₂, and SO₂, were 0.028 ppm, 0.014 ppm, and 0.033 ppm, respectively.