Organophosphate determination

Individuals with hereditary low plasma cholinesterase levels (Kalow 1956 Lehman and Ryan 1956) and those with paroxysmal nocturnal hemoglobinuria, which is related to abnormally low levels of erythrocyte acetylcholinesterase (Auditore and Hartmann 1959), would have increased susceptibility to the effects of anticholinesterase agents such as methyl parathion. Repeated measurements of plasma cholinesterase activity (in the absence of organophosphate exposure) can be used to identify individuals with genetically determined low plasma cholinesterase. 

A recent method to screen the urine for alkyl phosphates as an indicator of exposure to organophosphate insecticides shows that the method can be used to determine environmental exposure to a specific organophosphate pesticide. The method was found to be sensitive, identifying low levels of exposure to insecticides in the environment by quantitation of urinary phosphates (Davies and Peterson 1997). The test is limited in that it is only useful for assessing recent exposure, due to the short half-life of the organophosphate pesticides. 

A recent method, still in development, for determining total 4-nitrophenol in the urine of persons exposed to methyl parathion is based on solid phase microextraction (SPME) and GC/MS previously, the method has been used in the analysis of food and environmental samples (Guidotti et al. 1999). The method uses a solid phase microextraction fiber, is inserted into the urine sample that has been hydrolyzed with HCl at 50° C prior to mixing with distilled water and NaCl and then stirred (1,000 rpm). The fiber is left in the liquid for 30 minutes until a partitioning equilibrium is achieved, and then placed into the GC injector port to desorb. The method shows promise for use in determining exposures at low doses, as it is very sensitive. There is a need for additional development of this method, as the measurement of acetylcholinesterase, the enzyme inhibited by exposure to organophosphates such as methyl parathion, is not an effective indicator of low-dose exposures. 

In the OPMBS, all compounds were determined that occurred in the tolerance expression for organophosphate pesticides being supported by the registrants in each of the 13 commodities. Thus, compounds to be determined were those that might occur as residues of organophosphate pesticides whose continued use was supported by members of the OPMBS Task Force. 

OPMBS data were intended to support a valid estimate of the dietary exposure of populations and sub-populations to organophosphate residues in fresh fmits and vegetables. The results of the study were presented to the EPA in a report, with appropriate summaries. All of the study results, i.e., residue levels of each compound determined in each sample of each commodity, were also provided to the EPA in a database. EPA has recently notified the task force that the OPMBS study on the frequency and magnitude of organophosphate residues in fruits and vegetables is acceptable. The EPA is expected to utilize the data in a new assessment of potential dietary risk from organophosphate residues. 

Analytical Methods for Determining Organophosphate Ester Hydraulic Fluids in Biological Samples... 

Organophosphate Ester Hydraulic Fluids. The biomarkers of effects after exposure to organophosphate ester hydraulic fluids are well established in cases of delayed neuropathy (clinical signs of peripheral neuropathy). Further study would be helpful to determine whether certain effects (such as diarrhea after oral exposure) are due to direct action of the toxic agent on the target organ or to inhibition of acetylcholinesterase at the acetylcholine nerve receptor site on the organ. 

This property of organophosphate esters may be of environmental importance since phosphoric acid diesters are much more soluble and very little is known concerning the environmental toxicity of these compounds. The available data do not provide sufficient descriptions of the experimental methods to determine if the rates are reliable (Barnard et al. 1961 Ciba-Geigy 1984e, 1986 Howard and Deo 1979 Mayer et al. 1981 Wolfe 1980). The majority of reports provide only a minimum of information and exclude important facts such as the duration of the experiments and the concentration of buffers. Despite the lack of experimental detail, published rate constants for base-catalyzed hydrolysis appear to be reasonably consistent and suggest that the hydrolytic half-life of triphenyl phosphate will vary from... 

Enzymes can be used not only for the determination of substrates but also for the analysis of enzyme inhibitors. In this type of sensors the response of the detectable species will decrease in the presence of the analyte. The inhibitor may affect the vmax or KM values. Competitive inhibitors, which bind to the same active site than the substrate, will increase the KM value, reflected by a change on the slope of the Lineweaver-Burke plot but will not change vmax. Non-competitive inhibitors, i.e. those that bind to another site of the protein, do not affect KM but produce a decrease in vmax. For instance, the acetylcholinesterase enzyme is inhibited by carbamate and organophosphate pesticides and has been widely used for the development of optical fiber sensors for these compounds based on different chemical transduction schemes (hydrolysis of a colored substrate, pH changes). 

P. Mulchandani, W. Chen, and A. Mulchandani, Flow injection amperometric enzyme biosensor for direct determination of organophosphate nerve agents. Environ. Sci. Technol. 35, 2562-2565 (2001). 

A. Mulchandani, I. Kaneva, and W. Chen, Biosensor for direct determination of organophosphate nerve agents using recombinant Escherichia coli with surface-expressed organophosphorus hydrolase. 2. Fiber-optic microbial biosensor. Anal. Chem. 70, 5042-5046 (1998). 

K.A. Law and S.PJ. Higson, Sonochemically fabricated acetylcholinesterase micro-electrode arrays within a flow injection analyser for the determination of organophosphate pesticides. Biosens. Bioelectron. 20, 1914-1924 (2005). 

S. Jin, Z. Xu, J. Chen, X. Liang, Y. Wu, and X. Qian, Determination of organophosphate and carbamate pesticides based on enzyme inhibition using a pH-sensitive fluorescence probe. Anal. Chim. Acta 523, 117-123 (2004). 

R.P. Deo, J. Wang, I. Block, A. Mulchandani, K.A. Joshi, M. Trojaowicz, F. Scholz, W. Chen, and Y. Lin, Determination of organophosphate pesticides at a carbon nanotube/organophosphorus hydrolase electrochemical biosensor. Anal. Chim. Acta 530, 185—189 (2005). 

Lores et al. [382] discussed the determination of the organophosphate insecticide fenthion in seawater. The method comprises a solvent extraction followed by silica gel clean-up procedure prior to determination by gas chromatography with thermionic detection. Detection levels of 0.01 pg/1 were achieved. 

McLean, S., M. Sameshina, T. Katayama, J. Iwata, C.E. Olney, and K.L. Simpson. 1984. A rapid method to determine microsomal metabolism of organophosphate pesticides. Bull. Japan. Soc. Sci. Fish. 50 1419-1423. 

Mulchandani, A. Chen, W. Mulchandani, P. Wang, J. Rogers, K. R., TITEL, Biosensors for direct determination of organophosphate pesticides, Biosens. Bioelectron. 2001, 16, 225 230... 

Garcia M, Rodriguez I, Cela R (2007) Optimisation of a matrix solid-phase dispersion method for the determination of organophosphate compounds in dust samples. Anal Chim Acta 590 17-25... 

---