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The Nitrogen Phosphorus Detector (NPD)   The NPD, is a highly sensitive but very specific detector. It gives a strong response to organic compounds containing nitrogen and/or phosphorus. Despite its appearance it operates on an entirely different principle to that of the FID. A diagram of an NPD detector is shown in figure 14. The sensor of the NPD is a small rubidium or cesium bead contained inside a small heater coil. The helium carrier gas is mixed with hydrogen and passes into the detector through a small jet. The bead, which is heated by passing a current through the coil, is situated above the jet, and the helium-hydrogen mixture (produced by mixing the column carrier gas, helium with a separate stream of hydrogen) passes over it. If the detector is to respond to both nitrogen and phosphorus, then a minimum hydrogen flow is employed to ensure that the gas does not ignite at the jet. In contrast, if the detector is to respond to phosphorus only, a large flow of hydrogen

Mixture   The Nitrogen Phosphorus Detector (NPD) The nitrogen phosphorus detector (NPD), is a highly sensitive but specific detector and evolved directly from the FID. It gives a strong response to organic compounds containing nitrogen and/or phosphorus. Although it appears to function in a very similar manner to the FID, in fact, it operates on an entirely different principle. A diagram of an NP detector is shown in figure 24.   Figure 24. The Nitrogen Phosphorus Detector  

detector is to respond to both nitrogen and phosphorus, then a minimum hydrogen flow is employed to ensure that the gas does not ignite at the jet. In contrast, if the detector is to respond to phosphorus only, a large flow of hydrogen can be used and the mixture burned at the jet. A potential is applied between the bead and the anode. The heated alkali bead emits electrons by thermionic emission which are collected at the anode and thus produce an ion current. When a solute containing nitrogen or phosphorus is eluted, the partially combusted nitrogen and phosphorus materials are adsorbed on the surface of the bead. This adsorbed material reduces the work function of the surface and, as consequence, the emission of electrons is increased which raises the anode current. The sensitivity of the NPD is about 10-12 g/ml for phosphorus and 10-11 g/ml for nitrogen). Unfortunately, the performance deteriorates with time. Reese (10) examined the function of the NPD in

hydrogen converting the alkali silicate to the hydroxide and free silica. At the normal operating temperature of the bead, the alkali hydroxide has a significant vapor pressure and consequently, the rubidium or cesium is continually lost during the operation of the detector. Eventually all the alkali is evaporated, leaving a bead of inactive silica. This is an inherent problem with all NP detectors and as a result the bead needs to be replaced regularly if the detector is in continuous use. Thedetector can be made "linear" over three orders of magnitude although no values for the response index appear to have been reported. Like the FID it is relatively insensitive to pressure, flow rate and temperature changes but is usually thermostatted at 260oC or above. The specific response of the NPD to nitrogen and phosphorus, coupled with its relatively high sensitivity, makes it especially useful for the analysis of many pharmaceuticals and in environmental samples containing

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. Figure 22.  The Nitrogen Phosphorus Detector

(e.g., halogenated substances by the electron capture detector). Conversely, those detectors with a catholic response, although highly sensitive compared to LC detectors (e.g. the flame ionization detector) are significantly less sensitive than the specific detectors. The detectors with a catholic response are the most popular and the majority of GC separations are monitored by the flame ionization detector (FID). The most commonly used specific detectors are the nitrogen phosphorus detector (NPD) and the electron capture detector (ECD) The katharometer detector, although having relatively poor sensitivity is widely used in gas analysis. The Flame Ionization Detector The FID, invented by Harley and Pretorious (7), and separately by McWilliams and Dewer (8), evolved from the Heat of Combustion Detector developed by Scott (9). The FID detector employs hydrogen as the combustion gas which is mixed with the column eluent (helium, nitrogen or other