About: nitrogen phosphorous detector   Goto Sponge  NotDistinct  Permalink

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  • nitrogen phosphorous detector
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  • PERSON:Daniel Schober
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  • 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. The nitrogen phosphorous detector (sometimes called the thermionic detector) is a very sensitive but specific detector that responds almost exclusively to nitrogen and phosphorous compounds. It is based on the flame ionization detector but differs in that it contains a rubidium or cesium silicate (glass) bead situated in a heater coil, a little distance from the hydrogen flame. If the detector is to respond to both nitrogen and phosphorous then the hydrogen flow should be minimal so that the gas does not ignite at the jet. If the detector is to respond to phosphorous only, a large flow of hydrogen is used which is burnt at the jet. The heated bead emits electrons by thermionic emission. These electrons are collected under a potential of a few volts by an appropriately placed anode, and provides a background current. When a solute containing nitrogen or phosphorous is eluted from the column, the partially combusted nitrogen and phosphorous materials are adsorbed on the surface of the bead. The adsorbed material reduces the work function of the surface and, as consequence, the emission of electrons is increased which raises the current collected at the electrode. The sensitivity of the detector to phosphorous is about 10-12 gram per ml and for nitrogen about 10-11 gram per ml at a signal to nose ratio of 2. The alkali bead as a finite life and needs regular replacement.
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