Mobility-Based Ionization Detector with a Soft X-ray Source

This work concerns an analytical instrument intended for the detection of chemical substances characterized by the ability to form stable ions with mobilities lower than those generated in clean air. The operation of the detector is based on the ionization of gas and the measurement of the ionic current flowing at alternating electric field. The detector's electrode geometry is cylindrical, with the ion collector incorporated into the external electrode. An integral part of the detector is an ionization source emitting low-energy X-ray radiation. The effect of adding two test substances (2-heptanone and methyl salicylate) to air on ionic current values was studied in various detector operating modes. With a constant voltage power supply, the changes in current induced by the introduction of the test substance were small. Significantly higher analytical signal values were obtained when the detector was powered by a square-wave alternating voltage. The ionic current waveform under these conditions provides information about the presence of substances in the air with high proton affinity, which form stable positive ions, as well as the presence of electrophilic substances. It is also possible to estimate the minimum mobilities of the ions formed by the sample molecules. Particularly favorable detection conditions are obtained in the operating mode in which the ionization source is periodically switched on. In this mode, the analytical signal values are significantly higher than in other operating modes. A theoretical analysis of the detector's operation was conducted, based on ion balance equations.