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  

analysis frequently carried by the hydrocarbonindustryistheparaffin,  isoparaffin,  aromatic,   naphthalene and olefin estimation(the so-called PIANO analysis), an example of which is shown in figure 21. The column was fused silica, 50 m long and 0.5 mm I.D., and the stationary phase was also Petrocol DH 50.2. The column temperature was held at 35oC for 5 minutes and then programmed up to 200oC at 2o/min. The carrier gas was helium and its velocity through the column 20 cm/sec. 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.  

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

nbsp; Virtually all the basic drugs contain nitrogen and thus can be specifically detected among a large number of other unresolved compounds not containing nitrogen. The Emissivity or Photometric Detector The emissivity detector or, the Flame Photometric detector (FPD), was described by Grant (10) in 1958 but as it could not compete in sensitivity with the ionization detectors, did not raise any commercial interest. The emissivity detector, however, has some unique properties that could make its response quite specific and giving it certain unique areas of application. It was originally used to differentiate aromatic from paraffinic hydrocarbons by measuring the luminosity that the aromatic nucleus imparted to the flame. Contemporary photometric detectors do not usually monitor the total light emitted only light emitted at specific wavelengths. For example, phosphorus and sulfur containing hydrocarbons generate chemi-

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