Ion selective sensor with modulated ionization

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On the basis of the research results of the ongoing project towards a new sensor for fast and sensitive detection of volatile organic trace compounds in air the renewal proposal aims for an increased sensor selectivity. The sensor principle is based on a pulsed atmospheric pressure chemical ionization of the substances to be detected. After ionization ion specific ion-ion-recombination as well as charge transfer reactions lead to an ion specific life time of the ion species present in the ionization region. With an increasing delay between the end of the ionization and measuring the remaining total ion charge in the ionization region ion species with shorter life time are increasingly discriminated. Thus, selectivity is based on the ion specific ion-ion-recombination rate and charge transfer rate, which allows identifying different gas mixtures by their characteristic decay curves. Furthermore, defining a threshold of the remaining ion charge long living ions at a certain concentration can be identified and detected with high reliability. However, for better selectivity the ion specific ion generation rate and its effect on the decay curve will be investigated in the renewal proposal. Short ionization will mainly lead to ion species with fast generation rates and therefore change the initial ion distribution affecting the ion-ion recombination and charge transfer reactions. This concept can be also used in ion mobility spectrometry to direct ion generation towards low proton affine and electron affine substances, which are usually hard to ionize in the presence of high proton affine and electron affine substances and therefore difficult to detect. However, the investigation of the ion specific ion formation and its application in ion mobility spectrometry and gas sensing requires a pulsed ionization source with pulse widths from 0.1 µs to 10 ms at constant ion generation rates. The commercially available electron source (Optimare Analytik) with a minimum pulse width of about 10 µs and a rather uncontrolled ion generation rate is not feasible to investigate the ion formation processes. Thus, an own non-radioactive electron source and a modified corona discharge source with pulsed UV-LED will be developed in the renewal project, both providing suitable pulse widths and controlled ion generation rates to manipulate the ion distribution in the ionization region by simply varying the duration of ionization. This can be employed in two ways: First, to discriminate ion species with low ion formation rates by using short ionization pluses, and second, to generate a characteristic set of decay curves of a gas mixture due varying initial ion distributions in the ionization region when varying the duration of ionization. Both effects can be used to significantly improve selectivity and sensitivity in ion mobility spectrometry. Furthermore, the ion selective sensor will be improved by modulating the ionization.