Spectroscopic Methods for the Detection of Organophosphate Pesticides –A Preview

Organophosphate pesticides are the ester forms of phosphoric acid usually considered as secure for agriculture uses due to their relatively fast degradation rates. Organophosphorus pesticides have been extensively used in the area of agriculture to manage insect or pests of a number of economically important crops. Organophosphate pesticides are well-known as the inhibitor of acetylcholinesterase activity, not in insects only, but can also affect the nervous system of other organisms as well as humans. Organophosphorus pesticides are not restricted to anticholinesterase action, but comprise genotoxicity and teratogenicity including other environmental and ecological adverse impact. Such severe health and ecological consequences signify a requirement for a better understanding of the fate of organophosphates in the environment. By kept all these things in mind we have written a review on organophosphorus pesticides. In this review we have previewed the different methods of spectroscopic methods of detection including UV-visible, X-ray, Mass analysis, NMR, electrochemical analysis (sensor based) and FTIR. Among all these mass and electrochemical studies were flourished till date and considered as advanced techniques for the analysis of other pesticides also.

issues have aroused the public concern during last few years.By kept all these views in mind we are going to carve a review on OPs, divided into following parts.

Structural Properties of OPs
Organophosphate pesticides derived from phosphorus analog PH 3 having phosphorus as a core nuclei involved in oxidation states III and V. Basically organophosphate is the general name for esters of phosphoric acid.Hydrolyzed derivatives of phosphorus formal incorporation of additional oxygen atoms gives phosphinic acid (O=PH 2 OH) and phosphonic (phosphorus) acid [O=PH(OH) 2 ].Notably, these species may tautomerize between P(V) and P(III), that is, H 2 P(O)OH to HP(OH) 2 .Also a tetrahedral structure [O=PH(OH) 2 ] is more established than its isomer phosphonic acid, P(OH) 3 .This form can be stabilized by coordination with some metals .

Methods of Detection of OPs
The combination of FTIR, NMR and Mass data is often sufficient to determine completely the structure of an unknown molecule.There has been a longstanding interest, first among inorganic chemists, about how organophosphates bind to metals, and next by analytical chemists, about how an adequate detection device can be engineered.Ligand optimization and reporting media continue to be explored in OPs detection efforts that involve various spectroscopic techniques such as UV-Visible, FTIR, Mass and NMR spectroscopy.

UV-Visible Spectroscopy
In the field of pesticides UV-Vis.Spectroscopy play the vital role in the detection and interaction of metal ions with organic ligands i.e. pesticides, especially with transition metal ions.P=O can obscure absorbance.Agents themselves are not significant absorbers or emitters in the UV-Vis spectral region 17 unless they are specifically modified with a fluorescent coumarin-type leaving group. 180][21][22][23] The ZnFe 2 O 4 -TiO 2 composite photocatalyst is prepared by sol-gel method and used to degrade acephate successfully. 24Cu(II)-Glyphosate complex was studied in tea at pH 5 at absorbance 250 nm. 25 A UV-Visible based study to detection of monocrotophos at 490 nm included the effect of temperature and different regent viz.2, 4, dinitrophenylhydrazine and NaOH at different pH level. 26The photolysis of phorate has been studied as a thin film on a glass surface and in a solution of methanol water (60:40) by ultraviolet light (λ > 290 nm).The rate of disappearance of phorate in the solution show first order kinetics with a rate constant of 4.9 × 10 -5 s -1 . 27Metal complexes of Fe(III), Al(III), Co(II) and Zn(II) studied by using UV-Spectrophotometer, it observed that the complexes of methylphosphonic acid, formed the M-(CH 3 PO 3 )

FTIR Spectroscopy
IR spectroscopy is indispensible for many systems mentioned herein; direct access to monitoring the phosphoryl stretch is very useful.Electron-poor ions such as Fe(III) and Cu(II) favor stronger coordination and give [P=O] stretching frequencies concomitantly lower by 30-100 cm -1 .Some other functionality, such as perchlorate Cl-O stretches, can obscure P=O absorbance.In OPs from literature following main stretching and bending frequencies observed in different solvents. 29,30OPs having the thio as well as amino salts, for the thio group ν(S-H) and ν(C-S) observed at 2550 and 730 cm -1 .Ammonium salts are divided into following parts as per their IR frequencies; these are primary, secondary and tertiary ammonium salts.In all these salts ν(C-N) observed between the region 1380 -1250 cm -1 and ν(N-H) between the 1550 -1630 cm -1 .For the ammonium salt a strong band of ν(N-H) at 1430cm -1 also reported in literature.In the FTIR study of copper complexes, for the yellow complex; [Cu(Ph 3 P=O) 2 Cl 2 ] the ν(P=O) observed at 1142 cm -1 .For the dark red complex; [Cu(Ph 3 P=O) 2 Br 2 ] the ν(P=O) observed at 1145,1169 cm -1 .For free Ph 3 P=O, ν(P=O) is 1195 cm -1 .These compounds were found to be tetrahedral.The cation moiety Cu(II)-(Me 3 PO) 4 was square planar.Arsenic analogs were yellowbrown ν(P=O) at 840 cm -1 and olive green ν(P=O) at 842 cm -1 .Perchlorate species were also obtained, but the C=O stretching bands in the IR spectrum obscured the P=O stretches.In MgI 2 complex with diphenylphosphinates; it was found that the PO-C bond, not the P-OC bond, was cleaved. 15In the interaction of Cu(II) with DIMP the IR frequency of ν(P-O) and ν(P-O…H) observed at 1016 and 1206 cm -1 . 31Metal complexes of Fe(III), Al(III), Co(II) and Zn(II) studied by using FTIR, it observed that the complexes of methylphosphonic acid, formed the M-(CH 3 PO 3 ) 3 •3H 2 O, and aminomethylphosphonic acid, formed the M-OH(NH 3 CH 2 PO 3 ) 2 •H 2 O. Also complexes of N-phosphonomethylglycine, were prepared and the formula was M-OH(−−OOC-CH 2 - FTIR study indicating that the complexes form by the chelation of P=O and N-H bonds. 28In the FTIR study of interaction of OPs herbicide glyphosate with Fe(III) in aqueous solution at pH 4, suggests that coordination of Fe(III), or more likely Fe(OH) 2+ species, occurs through the phosphonic group, glyphosate shows no evidence of coordinating the metal through the carboxylate anion or the amino group; however, significant changes are observed in the range for the phosphonic group vibrations as expected for metal-phosphonate coordination. 32

NMR Spectroscopy
Nuclear magnetic resonance (NMR) is a spectroscopic method that is even more important to the structure elucidation than other spectroscopic techniques.Many nuclei may be studied by NMR techniques viz. 13C, 1 H, 31 P, 15 N, and 19 F. Any atomic nucleus that possesses either odd mass, odd atomic number split in the radiofrequency region.NMR gives information about the number of magnetically distinct atoms of the type being studied. 28NMR spectroscopy supports many areas of chemistry and science.NMR spectra have found great utility in monitoring reactions and characterizing new compounds and also in detection of OPs and their fragments. 33,34There are numerous nuclei that can be brought to bear in OPs studies, especially the 31 P nucleus.Analytes that contain characteristic 13 C, 1 H, and 31 P, as well as 15 N and 19 F, NMR signals can be probed. 35Other NMR active nuclei and related experiments also are found in the literature.The types of experiments have involved HSQC NMR, as well as magic angle spinning. 15Next, shift reagents can change the phosphorus δ value. 15One early study involved mixtures of (Me 2 N) 3 P=O, DMMP, or (MeO) 3 P=O, in the presence of Be 2+ and Al 3+ ionic centers and a series of phosphates was treated with Co 2+ and Fe 3+ . 36The species(MeO) 3 P=O, (EtO) 3 P=O, and (MeO) 2 P=O(Me) were all found to give shifts upon binding; also P-H decoupling was observed.With this technique, changes in P-S bonding can be monitored. 37Variable-temperature 31 P NMR spectroscopy was used in studying resin-based systems with two DMMP adsorption sites, the macro-reticular region and the quaternary ammonium hydroxide ion-exchange sites. 38It was found that DMMP may migrate from one site to another.There is also a report of lanthanide-induced shifts from authors who have also been active in the organophosphonate sensor area. 39An NMR spectroscopic assay was also developed to conveniently determine the purity of live agents of OPs derivatives. 31P NMR spectroscopy can also be used in studies that involve enzymes that degrade agents.Studies that successfully determine discrete cleavage events have used 31 P. 40 Nuclei other than 13 C, 1 H, and 31 P NMR also occasionally hold prominence.The 27 Al, 113 Cd, and 199 Hg nuclei have been utilized in terms of monitoring the adducts and mineralization. 15A 1 H-NMR study of phorate deals to photodegredation of phorate, indicated the P-S bond destruction.

X-ray Diffraction
X-ray diffraction is among the unswerving techniques to studies and elucidates the intricacies of metal ligand structure and probable binding possibilities that help open up a casement for future sensing possibilities.In study of [R 3 P=O-M n+ ] moieties (R = alkyl, aryl) and (M = any metal), the mean P=O bond length in [ P=O-Mn + ] interactions is 1.48 Å and the mean M-O bond length is 2.33 Å. 15 The importance of these structures is that they resemble a deprotonated acid fragment bound through three atoms to one metal center.(CH 2 ) 2 could be thought of as holding the place of [-P=OMe(OR)-].This motif is pragmatic with respect to a degraded sample in which the (-OR) has been hydrolyzed off.Metal complexes of Fe(III), Al(III), Co(II) and Zn(II) studied by using XRD-Spectrophotometer, it observed that the complexes of methylphosphonic acid formed the M-(CH 3 PO 3 ) 3 •3H 2 O, and aminomethylphosphonic acid, formed the M-OH(NH 3 CH 2 PO 3 ) 2 •H 2 O. Also complexes of N-phosphonomethylglycine were prepared and the formula was M-OH(-OOC-CH 2 -NH 2 + -CH 2 -PO 3 2-)•2.25H 2 O is proposed.X -ray study representing that metal ions concerned in octahedral structure. 28

Electrochemical
Almost all current instrumental techniques that explicitly determine the presence of pesticides are generally exclusive and non-portable, such as FTIR, mass chromatography and NMR spectrophotometer etc.But electrochemical methods are simple, versatile, in terms of controlling and altering the behavior of redox materials.Transition metal ions here have vulnerable properties.Such equipment is not too bulky and has a portable device, principally in regards to recent lab-on-a-chip research efforts.Reviews in this area mostly deal with biosensing.Organophosphonate electrochemistry with sarin, (EtO) 2 (EtSCH 2 -CH 2 S)P=O, parathion, and malathion.There were reports of a mercury surface with which polaragram data was recorded.Chemical groups point outward and waters are replaced stepwise in a Cu 2+ -complex by the surface-active compounds R 3 P=O.2][43] Also, some studies involve the use of phthalocyanines.Thus, this section involves structurally positioned redox active metal ions that do not directly bind with OPs donor atoms.Recently, electronic tongue array, consisting of an eight working electrodes (Au, Pt, Ir, Rh, Cu, Co, Ni, and Ag) was used to detect nerve agent stimulants DCP and DECP in aqueous environments. 44,15

Mass Spectrometric
The fundamental principles of mass spectroscopy (MS) to determined the mass-tocharge ratio of the molecule by ionizing it by using different procedures.The number of ions with a particular mass-to-charge ratio is plotted as a function of that ratio.The types of MS techniques include MALDI-TOF, GC-MS, ESI, SPAMS, and desorption electrospray.MALDI-TOF and MS-TOF was presented as an effective way of determining widespread emergency events. 40A detection by MS of species that include (RO) 2 P=O(R 1 ), (RO)P=O(R 1 )F, (RO)P=O(R 1 )(SR 2 ), and (R 2 N)P=O(OR 1 )(CN). 44MS also provides support for some surface-based studies.A study include the development and inter-laboratory verification of LC-MS libraries for organic chemicals of environmental concern, includes the 129 pesticides in which the monocrotophos and phorate having the m/z at 193, 98 and 75.In the study it was observed that more than 90% data was accepted in both modes. 15The determination of OPs in human blood and water using solid-phase microextraction and gas chromatography with mass (GC-MS) spectrometric detection performed. 45,46A Multiresidue detection of pesticide in fishery products was conducted by using the tandem solidphase extraction technique.Study includes more than 50 OPs.[47] The interesting disulfide derivative [bis (diisopropylaminoethyl) disulfide] was determined from a soil sample (detected at 1 µg per 1.0 g of soil). 15Secondary ionization (IM-TOF-MS) was used in detecting pinacolyl methylphosphonate, diethyl phosphoramidate, and 2-(butylamino) ethanethiol. 15Some reports involve the mention of pesticides: malathion was studied with GC-FID. 47S-MS was used to support microsynthesis of various O,Odialkyl-N,N-dialkylphosphoramidates to generate a library of mass spectra. 48Additionally the methyl esters of N,N-dialkylaminoethane-2sulfonic acids, R 2 NCH 2 CH 2 S(O) 2 OCH 3 , were analyzed by GC-EI mass spectroscopy. 49Rapid determination of pesticide residues in Chinese materia medica using QuEChERS sample preparation followed by gas chromatographymass spectrometry. 50In a minipig model, plasma was used in studying (iPrO(P=O)Me-(OH)) and cyclohexyl-O(P=O)Me(OH).1][52][53] Complexes of Ni, Co, and Fe, were evaluated for the OPs sensing, but Cu 2+ gave the best response indicating that the ligand had to accommodate enough vacancy.Thus, at this early point it was concluded that "copper complexes may adsorb and desorb phosphorus esters in air." 15

CONCLUSION
Among the all methods of detections mass and electrochemical based methods are flourished in recent years but there is slow development in FTIR and UV based sensing and detection of OPs which is highlighting a gap in study.Even OPs have the short life time of decomposition but they persist and leach out in soil and environment which is the matter of huge concern, so there is need to develop the new moieties which minimize all these risks and hazards.For the future safeguard, there is need to development of antidotes for intoxication with neurotoxic is one of the most important task, not only because their potential use as chemical warfare defense agents, but also for the treatment of intoxication with organophosphorus pesticides, which are very intensively used in agriculture.There is the need to develop quicker, cheapest, portable methods for agent and pesticides sensing..