Method and device for detecting substances in an ambient substance, in particular for detecting chemical warfare agents

A method of detecting a gaseous warfare agent in the presence of an interfering compound in ambient air, comprising the steps of:

• providing a measuring chamber having spaced apart electrodes for generating therein an electrical field, at least one of said electrodes containing radioactive material and the separation between said electrodes being substantially greater than the half-life range of the radioactive material, said electrical field being developed by applying selected voltages across said electrodes;
• developing a first plurality of characteristic curve data by plotting the quasi-molecular ion current flowing from one of said electrodes to the other versus the applied voltage when first and second asymmetric time varying voltages are alternately selected and applied across said electrodes and various known concentrations of said gaseous warfare agent are caused to flow through said measuring chamber;
• developing a second plurality of characteristic curve data by plotting the quasi-molecular ion current flowing from one of said electrodes to the other versus the applied voltage when first and second asymmetric time varying voltages are alternately selected and applied across said electrodes and various known concentrations of said interfering compound are caused to flow through said measuring chamber;
• causing a sample of ambient air including a reactive compound and believed to include at least one of said gaseous warfare agent and said second interfering compound to flow through said chamber and in said electric field, the radiation from said radioactive material having an ionizing effect on said reactive compound thereby generating negatively charged ions and positively charged ions that will attach to molecules of said gaseous warfare agent and said interfering compound present in said sample thereby forming electrically charged quasi-molecular ions;
• applying said asymmetric first time varying voltage across said electrodes;
• measuring any resulting first quasi-molecular ion current flowing from one of said electrodes to the other;
• comparing the measured first quasi-molecular ion current to each of said first plurality of characteristic curves and developing a first detection signal indicating whether or not a match to one of said first characteristic curves is detected;
• comparing the measured first quasi-molecular ion current to each of said second plurality of said characteristic curves and developing a second detection signal indicating whether or not a match to one of said second characteristic curves is detected;
• applying said asymmetric second time varying voltage across said electrodes;
• measuring any resulting second quasi-molecular ion current flowing from one of said electrodes to the other;
• comparing the measured second quasi-molecular ion current to each of said first plurality of characteristic curves and developing a third detection signal indicating whether or not a match to one of said first characteristic curves is detected;
• comparing the measured second quasi-molecular ion current to each of said second plurality of characteristic curves and developing a fourth detection signal indicating whether or not a match to one of said second characteristic curves is detected;
• and
• logically combining said first, second, third and fourth detection signals to generate an output signal indicative of the presence of at least one of said gaseous warfare agent and said interfering compound, or the lack thereof, in said sample of ambient air.