关键词: 离子化检测, 痕量气体检测, 气相色谱, 双极偏压

Abstract: Accurate quantitative measurement of trace gas concentrations is of critical importance in high-end manufacturing, aerospace, semiconductor fabrication, and high-purity gas analysis. In these fields, even minute impurities can significantly affect product quality, process stability, and device performance. This drives the continuous demand for highly sensitive and reliable gas detection techniques. Most gas detection methods are based on the measurement of ion signals generated after gas ionization. Common techniques include mass spectrometry, flame ionization detection （FID）, and discharge ionization detection. Among them, discharge ionization detection has attracted considerable attention because of its simple operation and good stability. It is also suitable for the detection of various gaseous species. However, detection performance is strongly affected by ion collection efficiency, which depends on the distribution of the electric field in the detection region. A stable and uniform electric field is essential for efficient ion transport to the collector electrode. It can reduce ion loss and improve detection sensitivity and quantitative accuracy. In this study, a bipolar bias electric field optimization method was proposed for ionization-based detection, and applied to a typical pulsed discharge ionization detection system. The purpose was to improve ion transport and signal collection efficiency. Two auxiliary bias electrodes were introduced into the ion transport region. Their voltages were independently controlled to regulate the axial electric potential distribution. This configuration allows the electric field to be adjusted at different stages of ion transport. It can enhance directional ion migration and reduce ion losses caused by diffusion and recombination. The influence of the bipolar bias electric field was first investigated using COMSOL Multiphysics. A two-dimensional axisymmetric electrostatic model was established according to the actual detection system. Appropriate electrical boundary conditions were applied to the electrode surfaces and model boundaries. The primary discharge conditions were kept constant during the simulation. The voltages of the two bias electrodes were varied systematically. The axial electric potential distributions were calculated under different voltage combinations. The calculated potential profiles were compared to evaluate the effect of the bipolar bias on electric-field continuity and ion transport. The simulation results showed that the bipolar bias configuration significantly modified the axial potential distribution. An appropriate combination of the two bias voltages produced a more continuous potential gradient in the ion transport region. This provided a more favorable electric-field environment for directional ion migration toward the collection electrode. Experimental validation was then performed using a 5 μmol/mol methane standard gas. A pulsed discharge ionization detector equipped with two independently controlled bias electrodes was used for the measurements. The carrier gas flow rate and discharge conditions were kept constant throughout the experiments. The two bias voltages were systematically varied within the selected operating range. Baseline current, baseline noise, baseline drift, and methane peak height were recorded for each condition. Each experimental condition was measured three times, and the average values were used for data analysis. The signal-to-noise ratio （SNR） and limit of detection （LOD） were calculated from the measured chromatographic responses and baseline noise. The experimental results were consistent with the simulation results. Increasing the bias voltages improved the axial electric-field distribution and enhanced ion transport toward the collection region. Compared with the no-bias condition and single-bias conditions, the bipolar bias configuration produced stronger detector responses. The optimized operating region was relatively broad. When both bias voltages were within the range of 100-200 V, the detector achieved a low detection limit and a high SNR simultaneously. The lowest detection limit was 2.50 nmol/mol, while the maximum SNR reached 4 004.8. These results demonstrate that the bipolar bias electric field can effectively improve ion transport and collection efficiency. The relatively broad optimal voltage range also makes the system less sensitive to small voltage fluctuations during operation. In summary, the proposed bipolar bias electric field provides an effective approach for improving the performance of pulsed discharge ionization detection systems. The combined simulation and experimental results demonstrate that appropriate electric-field regulation can enhance ion transport and ion collection without changing the fundamental ionization mechanism. The proposed method may provide useful guidance for electric-field optimization in ionization-based gas detection systems.

Key words: ionization detection, trace gas detection, gas chromatography （GC）, bipolar bias