Nitrogen/phosphorus pesticides

Table 4.13 MS/MS acquisition parameters for nitrogen/phosphorus pesticides.

Methyl parathion was determined in dog and human serum using a benzene extraction procedure followed by GC/FID detection (Braeckman et al. 1980, 1983 DePotter et al. 1978). An alkali flame FID (nitrogen-phosphorus) detector increased the specificity of FID for the organophosphorus pesticides. The detection limit was in the low ppb (pg/L). In a comparison of rat blood and brain tissue samples analyzed by both GC/FPD and GC/FID, Gabica et al. (1971) found that GC/FPD provided better specificity. The minimum detectable level for both techniques was 3.0 ppb, but GC/FPD was more selective. The EPA-recommended method for analysis of low levels (<0.1 ppm) of methyl parathion in tissue, blood, and urine is GC/FPD for phosphorus (EPA 1980d). Methyl parathion is not thermally stable above 120 °C (Keith and Walters 1985). 

For multi-analyte and/or multi-matrix methods, it is not possible to validate a method for all combinations of analyte, concentration and type of sample matrix that may be encountered in subsequent use of the method. On the other hand, the standards EN1528 andEN 12393 consist of a range of old multi-residue methods. The working principles of these methods are accepted not only in Europe, but all over the world. Most often these methods are based on extractions with acetone, acetonitrile, ethyl acetate or n-hexane. Subsequent cleanup steps are based on solvent partition steps and size exclusion or adsorption chromatography on Florisil, silica gel or alumina. Each solvent and each cleanup step has been successfully applied to hundreds of pesticides and tested in countless method validation studies. The selectivity and sensitivity of GC combined with electron capture, nitrogen-phosphorus, flame photometric or mass spectrometric detectors for a large number of pesticides are acceptable. 

Universal and selective detectors, linked to GC or LC systems, have remained the predominant choice of analysts for the past two decades for the determination of pesticide residues in food. Although the introduction of bench-top mass spectrometers has enabled analysts to produce more unequivocal residue data for most pesticides, in many laboratories the use of selective detection methods, such as flame photometric detection (FPD), electron capture detection (BCD) and alkali flame ionization detection (AFID) or nitrogen-phosphorus detection (NPD), continues. Many of the new technologies associated with the on-going development of instrumental methods are discussed. However, the main objective of this section is to describe modern techniques that have been demonstrated to be of use to the pesticide residue analyst. 

Pesticides may change the soil s element content. Some pesticides may increase plants micro- and macroelement content, such as nitrogen, phosphorus, calcium, potassium, magnesium, manganese, iron, copper, barium, aluminum, strontium and zinc, whereas others decrease these or other elements. Pesticides may cause ammoniac compounds to accumulate in the soil. Dimethoate and fluometuron increase nitrates in the soil, while DDT, carbaryl and HCH sharply decrease them. When prometrin was used, soil nitrate content decreased by 30-40% [3]. 

GC is coupled with many detectors for the analysis of pesticides in wastewater. At the present time the most popular is GC-MS, which will be discussed in more detail later in this section. The flame ionization detector (FID) is another nonselective detector that identifies compounds containing carbon but does not give specific information on chemical structure (but is often used for quantification because of the linear response and sensitivity). Other detectors are specific and only detect certain species or groups of pesticides. They include electron capture,nitrogen-phosphorus, thermionic specific, and flame photometric detectors. The electron capture detector (ECD) is very sensitive to chlorinated organic pesticides, such as the organochlorine compounds (OCs, DDT, dieldrin, etc.). It has a long history of use in many environmental methods,... 

Edgell KW, Jenkins EL, Lopez-Avila V, et al. 1991. Capillary column gas chromatography with nitrogen-phosphorus detection for determination of nitrogen- and phosphorus-containing pesticides in finished drinking waters Collaborative study. J Assoc Off Anal Chem 74 295-309. 

The pesticides included in this study were fenvalerate, chlordecone (kepone), chlorothalonil, and chlorpyrifos. Fenvalerate is a synthetic pyrethroid insecticide used, for example, for mites on chickens. Its chemical name is cyano(3-phenoxyphenyl)-methyl 4-chloro-alpha-(1-methylethyl)benzeneacetate. Chlordecone is an insecticide, no longer used, and has a chemical name decachloro-octahydro-l,3,4-metheno-2H-cyclobuta(cd)=pentalen-2-one. Chlorothalonil is fungicide used on tomatoes whose chemical name is 2,4,5,6-tetrachloroisophthalonitrile. Chlorpyrifos is an insecticide with a chemical name 0,0-diethyl 0-(3,5,6-trichloro-2-pyridyl)phosphorothioate. Chlorpyrifos is the U. S. Food and Drug Administration chromatographic reference standard since numerous specific detectors (electron capture, flame photometric in both sulfur and phosphorus modes, alkali flame, nitrogen phosphorus, and Hall detectors) are sensitive to it. 

Determination of Nitrogen- and Phosphorus-Containing Pesticides in Water by GC with a Nitrogen-Phosphorus... 

Organochlorine pesticides and OPPs have been determined mainly using GC, because of the stability and volatility that most of them show under chromatographic conditions and, particularly, the availability of element-selective detectors that display high selectivity for OCPs (electron-capture detector, ECD), and OPPs (flame photometric detector, FPD, and nitrogen phosphorus detector, NPD). Mass spectrometry-based detection is also more popular in GC than in HPLC (1,2,12,16). 

JJ Jimenez, JL Bernal, MJ del Nozal, JM Rivera. Determination of pesticide residues in waters from small loughs by solid-phase extraction and combined use of gas chromatography with electron-capture and nitrogen-phosphorus detection and high-performance liquid chromatography with diode array detection. J Chromatogr A 289-300, 1997. 

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. 

Garcia-Repetto, R., I. Garrido, and M. Repetto (1996). Determination of organochlorine, organophosphorus and triazine pesticide residues in wine by gas chromatography with electron capture and nitrogen-phosphorus detection. J. Assoc. Off. Anal. Chem. Int., 79(6) 1423-1427. 

Psathaki, M., E. Manoussaridou, and E.G. Stephanou (1994). Determination of organophosphorus and triazine pesticides in ground-and drinking water by solid-phase extraction and gas chromatography with nitrogen-phosphorus or mass spectrometric detection. /. Chromatogr. A, 667 241-248. 

Highly selective to compounds containing nitrogen and phosphorus Nitrogen-phosphorus detector Organophosphorus pesticides (EPA 8141) Acrylonitrile (EPA 8031) Acetonitrile (EPA 8033) Nitrosamines (EPA 8070) Hydrocarbons, fats, oils, waxes may interfere with organophosphorus pesticides. 

Organopho sphorus pesticides Nitrogen phosphorus and mass spectrometric detectors GC-MS [206]... 

As its name implies, nitrogen/phosphorus detector (NPD) is sensitive to nitrogen and phosphorus compounds. The detector is similar to an FID, but uses a heated rubidium silicate bead in place of a hydrogen/air flame to generate ions. Nitrogen-and phosphorus-containing compounds such as pesticides cause the bead to emit... 

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 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 organic pesticides contain carbon. They also contain hydrogen and often oxygen, nitrogen, phosphorus, sulfur, or other elements. Most pesticides used today are organic compounds. A few organic pesticides are either derived or extracted directly from plants. Most, however, are synthetic compounds. It is these compounds that have been responsible for the expanded use of pesticides since World War II. They are often extremely effective and easy to use, have been relatively lowcost and some are quite specific in their activity. They have, however, been the principal focus of health and environmental concerns and are the pesticides most commonly associated with problems of pesticide use and misuse. 

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