Nitrogen phosphorus detector application

The final stage of the residue analysis procedures involves the chromatographic separation and instrumental determination. Where chromatographic properties of some food residues are affected by sample matrix, calibration solutions should be prepared in sample matrix. The choice of instrument depends on the physicochemical properties of the analyte(s) and the sensitivity required. As the majority of residues are relatively volatile, GC has proved to be an excellent technique for pesticides and drug residues determination and is by far the most widely used. Thermal conductivity, flame ionization, and, in certain applications, electron capture and nitrogen phosphorus detectors (NPD) were popular in GC analysis. In current residue GC methods, the universality, selectivity, and specificity of the mass spectrometer (MS) in combination with electron-impact ionization (El) is by far preferred. 

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. 

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... 

Other detectors besides the BCD may lead to the design of detector-oriented derivatization schemes, particularly where, as with the BCD, they improve the sensitivity or specificity of detection. One of these is the thermionic nitrogen/phosphorus detector (NPD), for which specific derivatives have been proposed, either in the nitrogen mode [5-8] or in the phosphorus mode [9,10]. More recently, the availability of the electrochemical detector has occasioned the application of specific derivatives such as the 0-acetylsalicyloyl derivatives for the HPLC of amines [11]. 

Element selective detectors Element selective detectors applicable in pesticide residue analysis include electron capture detector (ECD), electrolytic conductivity detector (ELCD), halogen-specific detector (XSD), nitrogen phosphorus detector (NPD), flame photometric detector (FPD), pulsed flame photometric detector (PEPD), sulfur chemiluminescence detector (SCD), and atomic emission detector (AED). To cover a wider range of pesticide residues, a halogen-selective detector (ECD, ELCD, XSD) in conjvmction with a phosphorus- (NPD, FPD), nitrogen- (NPD), and/or sulfur-selective detector (FPD, SCD) is commonly used. A practical approach is to spht the column flow to two detectors that reduces the number of injections however, the reduced amoimt of analyte that reaches the detector must be considered. 

Since the introduction of the Mark V latroscan model, there have been no further technical improvements to the TLC-FID system. However, the development of vapour phase detectors (such as the flame thermionic ionization detector for compounds containing nitrogen and halogens, and the flame emission photometric detector which detects compounds containing sulphur and phosphorus) should extend the range of applications of the TLC-FID system. 

Paradoxically, the gas-density detector, which can supply the molecular mass of an unknown, can be considered to be a selective detector in gas chromatography (58). This comment effectively illustrates the dearth of convenient selective detectors. At best such detectors help to identify compounds that contain halogen atoms (electron-capture or thermoionic detectors), sulfur, nitrogen, or phosphorus (flame-photometric or thermoionic detectors). Physiological detectors have also been used in certain rare cases (insects that react to sexual pheromones, for example, or the chemist s nose, a dangerous and hazardous application). 

Some recent developments involving the use of novel vapor phase detectors, such as the flame thermionic ionization detector (FTID), which responds to compounds containing nitrogen and halogen atoms, and the flame emission photometric detector (33), which detects substances containing sulfur and/or phosphorus as well as chemiluminescent nitrogen detector, coupled on-line with FID (33a), should be able to widen the range of possible applications of the coated rod TLC-FID systems. 

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