Capillary Chromatography - The Nitrogen Phosphorus Detector (NPD)
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 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 nitrogen or phosphorus containing solute 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 current collected at the anode. The sensitivity of the NPD is about 10-12 g/ml for phosphorus and 10-11 g/ml for nitrogen).
Unfortunately, the response of the detector seriously deteriorates with time. Reese (9) studied the response 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 caused by water vapor produced by the burning hydrogen, converting the alkali silicate to the hydroxide. At the elevated bead temperature, the alkali hydroxide has a significant vapor pressure and, thus, the rubidium or cesium is continually lost while the detector is in operation. Eventually, all the alkali is evaporated, leaving an inactive bead of silica. This problem is inherent to all NP detectors and, consequently, the bead must be replaced fairly regularly if the detector is in continuous use.