The Nitrogen Phosphorus Detector (NPD) The nitrogen phosphorus detector (NPD) (sometimes called the thermionic detector) is a very sensitive, specific detector the design of which, is based on the FID. Physically the sensor appears to be very similar to the FID but, in fact, operates on an entirely different principle. A diagram of an NPD detector is shown in figure 22. The NPD sensor differs from that of the FID by a rubidium or cesium chloride bead contained inside a heater coil situated close to the hydrogen jet. The bead is situated above a jet and heated by a coil, over which the nitrogen carrier gas mixed with hydrogen passes. If the detector is to respond to both nitrogen and phosphorus, then the hydrogen flow should be minimal so that the gas does not ignite at the jet. If the detector is to respond to phosphorus, only, however, a large flow of hydrogen can be used and the mixture burnt at the jet. The heated alkali bead emits electrons by thermionic emission which are collected at the anode and provides background current through the electrode system. When a solute that contains nitrogen or phosphorusiseluted, thepartiallycombustednitrogenandphosphorusmaterials are adsorbed on the surface of the bead.
The adsorbed material reduces the work function of the surface and, thus, electron emission is increased and the current collected at the anode rises. The NPD has a very high sensitivity, i.e., about an order of magnitude less than that of the electron capture detector (ca.10-12 g/ml for phosphorus and 10-11 g/ml for nitrogen).
The Electron Capture Detector Lovelock's work on ionization detectors culminated in the invention of the electron capture detector. However, the electron capture detector operates on an entirely different principle from that of the argon detector. A low energy b-ray source is used in the sensor to produce electrons and ions. The first source to be used was tritium absorbed into a silver foil but, due to its relative instability at high temperatures, this was quickly replaced by the far more thermally stable 63Ni source.
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