Polarity organophosphorus compounds

The use of t.l.c. for separating organophosphorus compounds has been reviewed. In a study of phosphoryl compounds it was shown that / m values i.e. log [(1 - Rv)lRy on silica gel using hexane-acetone as eluant, depend upon the polarity of the P=0 group.The migration order found was... 

The onset of symptoms depends on the particular organophosphorus compound, but is usually relatively rapid, occurring within a few minutes to a few hours, and the symptoms may last for several days. This depends on the metabolism and distribution of the particular compound and factors such as lipophilicity. Some of the organophosphorus insecticides such as malathion, for example (chap. 5, Fig. 12), are metabolized in mammals mainly by hydrolysis to polar metabolites, which are readily excreted, whereas in the insect, oxidative metabolism occurs, which produces the cholinesterase inhibitor. Metabolic differences between the target and nontarget species are exploited to maximize the selective toxicity. Consequently, malathion has a low toxicity to mammals such as the rat in which the LD50 is about 10 g kg-1. 

The stationary phases coated on the inert supports are similar in the two classes of columns. Liquids of different polarities are chosen for the separation of non-polar or polar compounds a relatively small number of solvents, chemically inert and heat resistant, are sufficient for most residue analyses. The most commonly used for organophosphorus compounds vary from low-polarity hydrocarbons, methylsilicones, phenylsilicones, phenylmethylsilicones or carboranesiloxanes 165 168 175 178 180 182 184 192,195,197,198,... 

In general, the nature of the analyte determines the choice of stationary phase. For example, for the separation of organochlorine and pyrethroid pesticides, a nonpolar stationary phase such as DB-1 (or OV-1) is recommended. For the separation of somewhat more polar compounds, such as organophosphorus compounds, OV-17 (or DB-1701) can be applied. In addition, for confirmation purposes, the use of two columns with distinct stationary-phase polarities (e.g., DB-1 and DB-1701) is certainly required. A polar stationary phase (e.g., DB-wax) is suitable for the more polar compounds such as methamidofos, but its application to some detection modes is limited due to stationary-phase bleeding. 

Organophosphorus compounds are irreversible inhibitors of acetylcholine esterase and butyrylcholine esterase (BuChe, EC 3.1.1.8) because the phosphate group is irreversibly bound by the enzyme. Therefore, organophosphorus pesticides can be detected by using the free enzyme. Since the activity of cholinesterases (ChE) in normal serum is rather large (800 UA), untreated serum pools may be employed for inhibitor determination. Gruss and Scheller (1987) have shown that the hydrolysis of butyrylthiocholine iodide can be directly indicated at a membrane-covered platinum electrode polarized to +470 mV. Twenty seconds after sample addition a steady value proportional to the enzyme activity was obtained in the differentiated current-time curve. Injection of an inhibitor decreased the rate of thiocholine formation, so that the residual activity could be evaluated after 30 s (Fig. 115). 

This section deals primarily with the mass spectrometric analysis of organophosphorus compounds via techniques other than traditional EI-MS. Newer ionization techniques have greatly expanded the realm of mass spectrometric analysis to include polar, thermally labile species while tandem mass spectrometric techniques are now more routinely used to gain further structural informationThe most commonly used tandem mass... 

As mentioned, nerve agents designed for use as CWAs are mostly derivatives of organophosphorus compounds that have high proton affinities. Thus, the sensitivity and specificity of response and the high speed of measurement make mobility spectrometers superb instruments for in-field or on-site determinations. In addition, blister agents and related breakdown products can be detected in the negative polarity. 

The strongly polar character of the phosphoryl group of the H-phosphonates is responsible to a great extent for the reactivity of this class of organophosphorus compounds. 

Second detector confirmation is another compound confirmation technique. Two detectors with selectivity to different functional groups are connected in series to one column or in parallel to two columns. For example, two detectors, a UV/VIS detector and a fluorometer, connected in series to the HPLC column are used in the EPA Method 8310 for analysis of PAH compounds. If the second detector is connected to a second column of a dissimilar polarity, then the confirmation becomes even more reliable. An example of such a configuration is organophosphorus pesticides analysis the samples may be initially analyzed with an NPD, and then confirmed on a different column with an ECD or a FPD. 

The first section of this book describes the application of LC/MS to the analysis of agricultural chemicals and their metabolites. Using LC/MS for residue analysis in agricultural chemistry has become routine in many laboratories. Many pesticides, such as the chlorophenoxy acid and sulfonyl urea herbicides or organophosphorus and methyl carbamate insecticides, are too polar or thermally labile for analysis via GC. The use of LC/MS for the identification of polar pesticide metabolites and conjugates, an area traditionally dominated by radiolabeled compounds, stands out as a particularly dramatic demonstration of the power of this technique. 

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