Chiral-GC - The Nitrogen Phosphorus Detector (NPD) from Chiral Gas Chromatography.

The Nitrogen Phosphorus Detector (NPD)

The nitrogen phosphorus detector (NPD) (which appears physically very similar to the FID) is a highly sensitive but specific detector that gives a strong response to compounds that contain nitrogen and/or phosphorus. Although its construction is similar to that of the FID, it operates on an entirely different principle. A diagram of an NPD detector is shown in figure 62.

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Figure 62. The Nitrogen Phosphorus Detector

The essential component of the NPD sensor is a rubidium or cesium bead contained inside a small heater coil. Helium is used as the carrier gas that after leaving the column, is mixed with hydrogen and passed into the detector through a small jet. The cesium or rubidium bead is situated in a wire coil that is heated by a current passing through it. The coil, containing the bead, is situated above the jet, and the helium-hydrogen mixture 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 can be used and the mixture is burned at the jet. A potential is applied between the bead and the anode. During normal operation, when there is no solute being eluted from the column, the heated alkali bead emits electrons by thermionic emission that are collected at the anode and, thus, produce a constant ion current. When a nitrogen or phosphorus containing solute is eluted from the column, the partially combusted nitrogen and phosphorus materials, in the combustion gas round the bead, are adsorbed on the surface of the bead. The adsorbed material on the bead surface reduces the work function of the surface and, as consequence, the emission of electrons is increased and the current collected at the anode is increased. The general sensitivity of the NPD is about 10-12 g/ml for phosphorus and 10-11 g/ml for nitrogen). Unfortunately, the performance of the NPD deteriorates with time. Reese (39) examined the function of the NPD in great detail. Reese pointed out that the alkali salt that is used for the bead is usually a silicate and demonstrated that the reduced response was due to water vapor from the burning hydrogen, converting the alkali silicate to the hydroxide. At the 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 replaced fairly regularly if the detector is in continuous use.