Gas Chromatography - GC Detectors > The Nitrogen Phosphorus Detector (NPD) > Page 40
The actual NPD sensor is a 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 is heated by a current passing through the coil which 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 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 great detail. The alkali salt employed as the bead is usually a silicate and Reese showed 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.