The Nitrogen-Phosphorus Detector

Water samples are acidified and extracted with solvent (Kawamura and Kaplan 1983 Muir et al. 1981). Clean-up steps may be used (Kawamura and Kaplan 1983). Methylene chloride is often used as the extracting solvent, and it may interfere with the nitrogen-phosphorus detector. In this case, a solvent-exchange step is used (Muir et al. 1981). Analysis by GC/NPD or GC/MS provides specificity (Kawamura and Kaplan 1983 Muir et al. 1981). Accuracy is acceptable (>80%), but precision has not been reported. Detection limits were not reported, but are estimated to be 0.05-0.1 pg/L (Muir et al. 1981). Detection limits at the low ppt level (ng/L) were achieved by concentrating organophosphate esters on XAD-2 resin. The analytes were solvent extracted from the resin and analyzed by GC/NPD and GC/MS. Recovery was acceptable (>70%) and precision was good (<10% RSD) (LeBel et al. 1981). 

The nitrogen-phosphorus detector (NPD) is, as the name implies, preferentially sensitive to nitrogen and phosphorus atoms. The detector contains a ceramic bead formed with an alkali salt, such as rubidium or cesium. The bead is heated in the presence of hydrogen, but the flow of gas is limited so that no flame is present. As the column effluent passes around the bead, nitrogen- and phosphorus-containing compounds produce electronegative moieties, which are treated as an increase in current and recorded as peaks (Carlsson et al., 2001). 

The most popular thermionic detector (TID) is the nitrogen-phosphorus detector (NPD). The NPD is specific for compounds containing nitrogen or phosphorus. The detector uses a thermionic emission source in the form of a bead or cylinder composed of a ceramic material impregnated with an alkyl-metal. The sample impinges on the electrically heated and now molten potassium and rubidium metal salts of the active element. Samples which contain N or P are ionized and the resulting current measured. In this mode, the detector is usually operated at 600-800°C with hydrogen flows about 10 times less than those used for flame-ionization detection (FID). 

TCD) detector or the flame-ionization (FID) detector, which are the two most common detectors in gas chromatography, respond to all (organic) compounds except the carrier gas. On the contrary, a selective detector responds to a range of compounds with a common physical or chemical property. Representatives of the latter group of detectors are the nitrogen-phosphorus detector (NPD), the electron capture detector (ECD), the mass selective detector (MSD) and - last, but not least - the tandem mass spectrometer (MS/MS). 

Note The nitrogen-phosphorus detector responds to nitrogen-phosphorus compounds about 100 000 times more strongly than normal hydrocarbons. Due to this high degree of selectivity, the NPD is commonly used to detect pesticides, herbicides, and drags. 

The mass-selective detector is more specific and allows a lower limit of quantification than the nitrogen-phosphorus detector (NPD). 

The problem arises in choosing the most appropriate solute with which to measure the concentration sensitivity of all detectors. Obviously toluene could be used to define the sensitivity of the FDD, UV detector and the RI detector but would be useless for measuring the sensitivity of the electrical conductivity detector or the nitrogen phosphorus detector (NPD) in GC. In a similar way, if the reference solute was chosen to have high refractive index and low UV absorption and contain no phosphorus or nitrogen then the RI detector... 

The response of a GC detector can be general or specific. A detector with a catholic response such as the FID is used widely in routine analysis. The specific detector, such as the nitrogen-phosphorus detector (NPD), is extremely useful for measuring particular types of compounds such as herbicides and pesticides, where the compounds of interest are not eluted discretely but mixed with a number of other contaminating compounds. Examples of this type of application will be given when the NPD is discussed in detail. 

The nitrogen phosphorus detector (NPD) (sometimes called the thermionic detector) is another sensitive, but, in this case, a specific detector, that is based on the FID. Physically the sensor appears to be similar to the FID but, in fact, operates on an entirely different principle. A diagram of an NPD detector is shown in figure 9. 

Two other types of element-specific detector for nitrogen currently in use coupled to SFCs are the nitrogen phosphorus detector (NPD) and the thermal energy analyzer (TEA). The NPD uses a hot, catalytically active solid surface immersed in a layer of dissociated H2 and O2 to form electronegative N and P ions which are detected on a nearby electrode [2]. NPD has been shown to have broad application in SFC, especially in the agrochemical industry [3]. The TEA, as described by Fine et al. [4], uses low-temperature pyrolysis, followed by ozone-induced chemiluminescence, for the detection of compounds containing NO2 groups. The TEA has been used for the determination of tobacco-specific nitrosamines and explosives [5]. Both of these detectors require spedlic standards of the analytes of interest for quantitation... 

Thermionic Seiective Detector. The TSD, also known as the nitrogen-phosphorus detector (NPD), is a modification of the FID in which an alkali bead is heated electrically in the area above the jet. In the presence of alkali atoms in the flame, nitrogen-containing compounds give a 15 times greater, and phosphorus-containing compounds a 300 times greater, response. 

Gas chromatography with either sulfur chemiluminescence detection or atomic emission detection has been used for sulfur-selective detection. Selective sulfur and nitrogen gas chromatographic detectors, exemplified by the flame photometric detector (FPD) and the nitrogen-phosphorus detector (NPD), have been available for many years. However, these detectors have limited selectivity for the element over carbon, exhibit nonuniform response, and have other problems that limit their usefulness. 

There are a large number of GC detectors available but the majority of GC separations are monitored by the flame ionization detector (FID), the nitrogen phosphorus detector (NPD), the electron capture detector (BCD) or the katherometer detector (or Hot Wire Detector). Tlie latter is almost exclusively used in gas analysis and rarely used in chiral chromatography and so will only be briefly described here. Furthermore, the FID is used in probably 90% of all chiral analyses. However, before describing the construction and function of each detector the subject of detector specifications needs to be discussed. 

The most common detection method used in gas chromatography is FID. The nitrogen-phosphorus detector (NPD) can be used to identify nitrogen-containing compounds (Stashenko et al., 1996). Another possibiUty is the use of oxygen flame ionization detection (O-FID) for the selective determination of oxygenates (Betts, 1994 Schneider et al., 1982). Mass spectrometry is a very useful tool for detecting complex terpene mixtures. 

Pesticides and herbicides have been trapped out of water and detected with various GC-like detectors such as the nitrogen phosphorus detector and the electron capture detector as well as UV photodiode array spectra. SFC is fully compatible with mass spectrometry and many other GC and HPLC detectors. 

In cyanomethyl derivatization, particularly suited for use with the nitrogen-phosphorus detector, cyanomethyl esters are formed by alkylation of the carboxyl group (R-COO-CH2-CN). The method is rapid, inexpensive, and is resistant to contaminants frequently found during the chromatographic separation of very-long-chain FAs. ... 

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