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conducting an electrical current, they may be collected on a charged plate The measured current will be directly proportional to the number of ionized molecules The ultraviolet photoionization process can be illustrated as RH + hnu --------------------> RH + + e 1 where RH is an organic or inorganic molecule and hnu represents a photon of UV light with energy ' equal to or greater than the ionization potential of that particular chemical compound or atomic species to cause the emission of electron e Units utilizing ultraviolet photoionization include the AID 580, the Photovac Model #10A10 ' (includes a gas chromatographic mode), and the HNU P101 that is described below ' 4.1.2 HNU P101 Ultraviolet Photoionization Detector The HNU P101 is typical of field ultraviolet photojonization units now available It consists of two modules connected via a signal-power cord 1 • A readout unit consisting of a 4 112-inch analog meter, a rechargeable battery, and power supplies for operation of the amplifier and the UV lamp, and ' • A sensor unit consisting of the UV light source, pump, ionization chamber, and a preamplifier ' An electrical pump pulls the gas sample past an UV light source Constituents of a sample g p ' are ionized, producing an instrument response, if their IP is equal to or less than the ionizing energy supplied by the instrument UV lamp being utilized The radiation produces an ion pair for each molecule of contaminant ionized The free electrons produce a current directly proportional to the number of ions produced The current is amplified, detected, and displayed on the meter Three probes are available with the HNU, containing either an 11 7, a 10 2, or a 9 5-eV UV light source Species that have IN greater than the lamp rating will display a poor instrument ' response, or no response at all Thus employing the 11 7 eV lamp will ensure the greatest range of O - 8 1