Due to excessive use in agriculture, organophosphorus (OP) pesticides are one of the most widely occurring pollutants in the environment. They inhibit acetylcholinesterase (AChE) activity and cause serious nerve problems in animals and humans, particularly affecting the nerve-muscle junction. Sensitive OP detection and monitoring of degradation are the two challenges of analytical chemistry. Broadly, two detection methods have evolved: Gas chromatography (GC) hooked to a suitable detection system as the compounds are volatile in nature, and more recently, the immobilized biocatalyst systems, detecting by AChE inhibition chemically linked to spectrophotometric or electrical conductance changes. Traditionally GC-mass spectroscopy (GC-MS) and its improvised version, ion trap GC-MS/MS have been used as highly sensitive detection methods for OP. Due to the non-availability of proper standards, the oxidizing products of native OP’s, particularly the oxon compounds remained undetected. A more recent approach is based on triple quadrupole mass spectroscopy as the detection device for GC (GC-QqQ-MS). This method helps detect traces of a large number of OP’s and also the derivatives in fruits and vegetables before being marketed. This is based on a principle that once the pesticides are separated by GC they are ionized both positively and negatively under ion trap and then fragmented in QqQ MS in MS/MS mode. The structure of OP and products can be deduced from the fragmentation pattern in the spectrum. It is possible to quantify the OP’s so that biosafety levels can be marked. Although chlorination is practiced for water purification, the process leads to oxidation of thiono (P=S) form of OP to active oxon forms (P=O), which are more harmful. Simultaneous detection of parental and oxon forms of OP’s was possible by GC-MS if the water samples were passed through solid-phase extraction prior to GC-MS. In SPE-GC-MS special DB-5 column was used to separate the parental and oxon forms of the OP.
Electronic sensory devices have been developed later to screen OP in a large number of environmental samples. In one of the methods, electric eel AChE-mediated variation in current, due to OP-caused inhibition of the enzyme, was recorded using a tetracyanocyanoquinodimethane (TCNQ)-modified graphite electrode as against Ag/AgCl reference electrode. A sol-gel was prepared to immobilize AChE in the SiO2 network, resulting in tetrameric mesh capturing the enzyme. This enzyme biosensor had very high thermal and photochemical stability in aqueous and organic phases. Acetylthiocholine was used as a substrate that got catalytically converted to thiocholine, and this reacted chemically with a redox modulator, TCNQ (reduced) to dithiocholine resulting in the formation of oxidized TCNQ and charge (electrons). The reaction is as follows:
CH3(C=O)S(CH2)2N+(CH3)3 + H2O → CH3COOH + HS(CH2)2N+(CH3)3 2HS(CH2)2N+(CH3)3 + 2TCNQ (red) → [S(CH2)2N+(CH3)3]2 + 2TCNQ (ox) + 2H+ + 2e-
A constant potential difference between TCNQ-coated graphite and Ag/AgCl electrodes gives current proportionate to catalytic activity. The presence of OP’s would quantitatively inhibit the enzyme and thereby the current flow. The advantage is that both parental and oxon forms of OP’s can be detected in this method. A relatively new approach utilizes photothermal detection of OP’s using thermal lens spectrophotometry in conjunction with a bio-analytical flow injection analysis system. This method is particularly suitable for monitoring the photolytic degradation products of OP compounds mediated through reactive oxygen radicals. This method is developed by immobilizing the eel AChE to the controlled pore glass and then packed in a column and azinphos-methyl is used as standard OP. The carrier mobile phase contains acetylthiocholine iodide substrate and 5’,5’-dithiobis (2-nitrobenzoic acid). The catalysis generates 5-to-2-nitrobenzoate absorbing at 410 nm. Since this irradiance also generates heat conductance, thermal lens spectrophotometry is employed as a superior detection system. The presence of OP’s (parental and oxons) would reduce the catalysis rate and decrease thermal conductance.
These chemistry-based application methods have revolutionized the pesticide detection system than the conventional bioassays. Further innovations in new chemical techniques or adaptation of existing techniques would be highly desirous to detect new generation pesticides in the environment.
Sources
Badea, M., M. Romanca, C. Draghici, J.-L. Marty, C. V. V. C. O. Marques, D. R. Mendes, O. P. Amarante Jr. and G. S. Nunes. “Multidisciplinary Collaboration for Environmental Protection using Biosensors. Detection of Organophosphate Insecticides in Aqueous Medium.” Journal of Brazilian Chemical Society 17.4 (2006): 807-811.
Franko, Mladen, Mohamed Sarakha, Anja Cibej, Ales Boskin, Mojca Bavcon and Polonca Trebse. “Photodegradation of pesticides and application of bioanalytical methods for their detection.” Pure and Applied Chemistry 77.10 (2005): 1727–1736.
Frenich, Antonia Garrido, Manuel J. Gonzalez-Rodrýguez, Francisco J. Arrebola, and Jose L. Martýnez Vidal. “Potentiality of Gas Chromatography-Triple Quadrupole Mass Spectrometry in Vanguard and Rearguard Methods of Pesticide Residues in Vegetables.” Analytical Chemistry 77 (2005): 4640-4648.
Tahara, Maiko, Reiji Kubota, Hiroyuki Nakazawa, Hiroshi Tokunaga and Tetsuji Nishimura. “Analysis of Active Oxon forms of nine Organophosphorous Pesticides in Water Samples using Gas Chromatography with Mass Spectrometric detection.” Journal of Health Science 52.3 (2006) 313-319.