Review on Current Analytical Methods with Chromatographic and Nonchromatographic Techniques for New Generation Insecticide Neonicotinoids

Neonicotinoid insecticides are a major group and the newest group among insecticides. They permeate the entire body of a plant and show excellent effects for the control of diseases and harmful insects. The history of neonicotinoid insecticides from development to their market release dates back to the late 1800s. Nicotine (Fig. 1), an alkaloid in tobacco leaves, is an early neonicotinoid insecticide that has been used as a natural insecticide, but it is extremely toxic to mammals (LD50 = 50–60 mg/kg; Tomizawa & Casida, 2005). Therefore, studies using nicotine as a model compound have been conducted actively to develop new pesticides with highly selective toxicity. In the 1970s, Shell developed nithiazine (Fig. 1), which showed strong insecticide activity (Soloway et al., 1978, 1979), although the compound was unstable in its application and remained commercially unavailable. Subsequently, the development of nithiazine derivatives was continued based on the relation between the chemical structure of nicotine compounds and insecticide activity. Eventually, Nihon Tokushu Noyaku Seizo (currently Bayer Crop Science) developed imidacloprid (Fig. 1) (Shiokawa et al., 1994). Subsequent to imidacloprid, acetamiprid (Nippon Soda Co. Ltd.), nitenpyram (Takeda Chemical Industries, currently Sumitomo Chemical Takeda Agro Co.), thiamethoxam (Ciba, currently Syngenta), thiacloprid (Bayer Crop Science), dinotefuran (Mitsui Chemicals Inc.) and clothianidin (Takeda Chemical Industries, currently Sumitomo Chemical Takeda Agro Co.) have been released on the market (Tomlin, 2003) with the subsequent new and recent development of imidaclothiz in China (Nantong Jiangshan Agrochemical and Chemical Co. Ltd.) (Fig. 1). Neonicotinoid insecticides express insecticide activity by acting on the nicotinic acetylcholine receptor, nAChR, which is present on the postsynaptic membrane of the insect nerve. Excellent insecticide effects are expressed on hemipteran pest species including aphids, whitefly, and planthoppers by this mechanism of action. Although nAChR is present in both insects and mammals, neonicotinoid insecticides that act on them are highly selectively toxic to insects because the recognition site of insect nAChR is lipid-soluble, whereas mammalian nAChR must be ionized to a high degree. It is considered that the

selectivity results from the fact that neonicotinoid insecticides are not, unlike nicotine, fully ionic compounds. Therefore, they are transferred easily in lipophilic insect body fluids to reach nAChR to express the action, although affinity to nAChR is low in mammals (Kagabu, 1996;Shiokawa et al., 1994;Tomizawa, 1994;Tomizawa & Casida, 2003. Neonicotinoid insecticides have come into worldwide use because, as described above, they have highly selective toxicity. Their toxicity to mammals, fish, and birds is low. They show a superior effect to control diseases and harmful insects that are resistant to insecticides including organophosphorus insecticides, carbamate insecticides, and synthetic pyrethroid insecticides. Recently however, it has been suggested that neonicotinoid insecticides are a possible cause of colony collapse disorder (CCD), i.e., sudden disappearance of bees that are pollinators of vegetables and fruits in modern agriculture (Decourtye & Devillers, 2010;El Hassani et al., 2008;Girolami et al., 2009;Iwasa et al., 2004;Mommaerts et al., 2010;Nauen et al., 2001). Studies have been undertaken to elucidate the relation between CCD and neonicotinoid insecticides, but the cause of CCD remains unclear. As described above, neonicotinoid insecticides have come into use as insecticides of the next generation to replace classical insecticides such as organophosphorus insecticides for the stable supply of various crops. However, the influence on the ecosystem related to useful insects such as bees has been noted. Further studies must be undertaken from various viewpoints such as food safety including crops, environment and ecological influence, i.e., for risk management and risk assessment. In this chapter, the author reports analytical methods that constitute the underlying technology that is indispensable for studies of risk management and risk assessment of neonicotinoid insecticides in the form of review to systemize cases reported to date, as well as to organize the trend, current situation, and future directions observed in the overview of respective analytical methods.

Analytical methods based on chromatographic techniques
Chromatography is a powerful tool for the determination of pesticides that might remain in widely various matrices such as food including crops, living bodies, and the environment. It also is used as an important method for the analysis of neonicotinoid insecticides, which were introduced into the environment about twenty years ago as the successor of organophosphorus, carbamate, and synthetic pyrethroid insecticides. This section refers to sample pre-treatments including extraction and clean-up needed before chromatographic determination and gives an exhaustive summary of the trend related to the development of residue analysis of neonicotinoid insecticides by chromatography.

Sample pre-treatment procedures prior to chromatographic determination
To analyze minute amounts of residual pesticides in complex matrices accurately, sample pre-treatment procedures must be conducted before chromatographic determination. It is no exaggeration to say that the results of the procedures have a decisive influence on the reliability of the data measured. This also applies to the analysis of neonicotinoid insecticides. In pesticide residue analyses conducted in the mid-1990s, when imidacloprid became available on the market, gas chromatograph (GC) equipped with so-called elementselective detectors, for example, electron capture ionization detector (ECD) for detection of pesticides with halogen atoms such as organochlorine insecticides and synthetic pyrethroid insecticides or flame photometric detector (FPD) for detection of pesticides with phosphorus atoms or sulfur atoms such as organophosphorus insecticides was mainly used. On the other hand, high-performance liquid chromatograph (HPLC) equipped with UV detectors or diode array detectors (DAD) were used for detection of pesticides that were unstable to heat. It is important in all measurement methods to conduct sample pre-treatment procedures to obtain accurate measurement data. Table 1 shows that sample pre-treatment procedures roughly consist of (1) extraction of the target pesticide from the sample and (2) separation of the target pesticide from the extract and clean-up. For (1) extraction of neonicotinoid insecticides, shaking extraction with organic solvents such as acetone, acetonitrile, or methanol (Baskaran et al., 1997;de Erenchun et al., 1997;Mohan et al., 2010;Tokieda et al., 1997bTokieda et al., , 1998, blending extraction with a homogenizer (Agüera et al., 2004;Blasco et al., 2002aBlasco et al., , 2002bDi Muccio et al., 2006;Fernandez-Alba et al., 1996Ferrer et al., 2005;Hengel & Miller, 2008;Hernández et al., 2006;Ishii et al., 1994;Jansson et al., 2004;Kamel et al., 2010;Mateu-Sánchez et al., 2003;Obana et al., 2002Obana et al., , 2003Sannino et al., 2004;Ting et al., 2004;Tokieda et al., 1997aTokieda et al., , 1997bVenkateswarlu et al., 2007), and ultrasonic extraction (Bourgin et al., 2009;García et al., 2007;Ishii et al., 1994;Liu et al., 2005Liu et al., , 2010Mayer-Helm, 2009;Rancan et al., 2006aRancan et al., , 2006bZhang et al., 2010) are commonly used. In addition to these, Bourgin et al. (2009) extracted five insecticides including imidacloprid from seeds coated with acetonitrile and Xiao et al. (2011) extracted seven neonicotinoid insecticides from bovine tissues with water by accelerated solvent extraction (ASE) to give quantitative extraction efficiency. For extraction from liquid samples such as water, milk and wine, solid-phase extraction (SPE) packed with e.g. C 18 or diatomaceous earth (Baskaran et al., 1997;Economou et al., 2009;Ferrer & Thurman, 2007;Pirard et al., 2007;Seccia et al., 2005Seccia et al., , 2008Zhou et al., 2006) and liquid-liquid extraction (Galera et al., 1998;Vilchez et al., 1996Vilchez et al., , 2001 are used. Moreover, matrix solid-phase dispersion (MSPD) was first reported in 1989. A small amount of solid or semisolid sample was blended with adsorbents such as C 18 to be packed in a disposable plastic column. It was then extracted with a suitable organic solvent (Barker et al., 1989). The method has been applied to the extraction of pesticides, pharmaceuticals, and antibiotics since then (Barker, 2000a(Barker, , 2000b. MSPD is also applied to the extraction of neonicotinoid insecticides from crop, honey, and fruit juice samples (Blasco et al., 2002a;Pous et al., 2001;Radišić et al., 2009;Totti et al., 2006). In any event, the first obstacle in the development of analytical methods for pesticide residues is to secure extraction efficiency from measurement samples. It is probably important to select extraction conditions that are suitable for the physicochemical features of the target pesticide and for the characteristics of the measurement sample. In the process of (2) clean-up, classical liquid-liquid partitioning and column chromatography have been used. However, because of concern over the health impact of the use of organic solvents in large quantities on analytical staff as well as the environmental load, the strong need exists for the reduction of organic solvents used in sample pretreatment procedures (Wan & Wong, 1996). SPE packed with widely various adsorbents such as silica gel, Florisil, C 18 , polymeric materials, graphitized carbon black (GCB), and ion exchange resin has come into common use these days, making a great contribution to reduction of the use of organic solvents (Fritz & Macka, 2000). In the 2000s, marked technical innovation in analytical instruments has taken place: LC-MS and LC-MS/MS have been brought into use for the determination of neonicotinoid insecticides. These methods have higher measurement sensitivity than classical HPLC-UV and HPLC-DAD. Moreover, in many cases, sample pre-treatment procedures have reportedly included only the extraction and dilution of the extract, with subsequent measurement, without even the need for additional clean-up (Table 1). The most common extraction procedure is the following: a measurement sample is homogenized; then it is dehydrated and extracted with ethyl acetate and anhydrous sodium sulfate with subsequent solvent evaporation and then LC-MS determination (Blasco et al., 2002a(Blasco et al., , 2002bFernández-Alba et al., 2000) or LC-MS/MS determination (Agüera et al., 2004;Jansson et al., 2004;Venkateswarlu et al., 2007). In 2003, Anastassiades et al. (2003 reported fast, simple and easy sample pre-treatment procedures by extraction with acetonitrile and dispersive SPE. The method that was presented is known as a quick, easy, cheap, effective, robust, and safe (QuEChERS) method, which is a breakthrough in which the sample pre-treatment is completed via acetonitrile extraction, dehydration, and salting-out with anhydrous magnesium sulfate and sodium chloride. Thereafter, dehydration and clean-up of the extract are done using dispersive SPE with anhydrous magnesium sulfate and primary secondary amine (PSA), which is a weak anion exchange adsorbent. Reportedly dispersive SPE using PSA is extremely effective for the removal of organic acids, polar dye components, and saccharides (Anastassiades et al., 2003). QuEChERS is very different from general sample pre-treatment procedures by organic solvent extraction, re-extraction with the organic solvent phase, and clean-up with SPE in that no process of concentration (evaporation of solvents) is needed. The concentration process, a procedure that must be conducted after processes such as extraction and clean-up, unexpectedly accounts for a large percentage of the sample pre-treatment time. Therefore, QuEChERS is a technology that contributes much to speeding up and simplification of sample pre-treatment procedures. In addition, the amounts of organic solvents used are extremely as small: about 10 mL of acetonitrile per sample. For those reasons, it can be concluded that the technology has met all of the requirements described above (Wan & Wong, 1996 With the presentation of QuEChERS as a turning point, many reports have described multiresidue analysis of various pesticides including neonicotinoid insecticides (Ferrer & Thurman, 2007;Frenich et al., 2008;Kamel, 2010;Kmellár et al., 2008Kmellár et al., , 2011Leandro et al., 2007;Lehotay et al., 2005;Payá et al., 2007;Romero-González et al., 2011). Additionally, reagents and adsorbents for QuEChERS are marketed as kits, implying that the method is extremely practical as multiresidue analysis for pesticide residues.

Determination of neonicotinoid insecticides based on chromatographic methods
Chromatographic determination of neonicotinoid insecticides are classifiable into two groups that use HPLC or GC (Table 1). However, because neonicotinoid insecticides are generally degraded by heat, additional processes such as derivatization are necessary for GC determination, making sample pre-treatment procedures more complicated than HPLC determination. Vilchez et al. (1996) and Navalón et al. (1997) used a hydrolyzed compound of imidacloprid [1-(6-chloro-3-pyridylmethyl)imidazolidin-2-one] formed by heat treatment of a measurement sample under alkaline conditions for GC-MS determination of imidacloprid in soil, water, and crops. MacDonald and Meyer (1998) extracted imidacloprid from white pine with water-containing methanol under acidic conditions with diluted sulfuric acid, cleaned up with C 18 SPE and a self-prepared Florisil minicolumn, heptafluorobutyryl-derivatized, and then subjected to GC-MS determination. As presented above, it is presumed that utilization of HPLC rather than GC is more advantageous for the determination of neonicotinoid insecticides from the viewpoints of speedup and simplification of sample pre-treatment procedures. It also is readily apparent that most cases reported to date used HPLC for determination (Table 1). In determination by HPLC, both HPLC-UV and HPLC-DAD contributed greatly to analyses of neonicotinoid insecticides until the 2000s, when MS and MS/MS began to become popularly used. Since the report by Ishii et al. (1994), HPLC-UV and HPLC-DAD have been applied to the determination of neonicotinoid insecticides in various matrices (Table 1). In addition to UV and DAD, methods were developed in which imidacloprid and thiamethoxam were separated using a column and then converted into electrochemically active compounds by ultraviolet irradiation to be detected by an electrochemical detector (ECD) (Rancan et al., 2006a(Rancan et al., , 2006b, or converted into fluorescent substances to be detected using a fluorescence detector (FLD) (García et al., 2007). ECD and FLD are generally more sensitive than UV or DAD. They are applied to residue analysis in the bodies of bees, where determination at low concentrations must be done. In fact, HPLC-UV and DAD have less measurement sensitivity and selectivity than either LC-MS or MS/MS, which are the most widely used methods today. Therefore, thorough clean-up is indispensable for the pre-treatment procedures of samples consisting of complicated matrices such as crops. Liquid-liquid partition has been used since the initial phase of the market release of imidacloprid (Fernandez-Alba et al., 1996;García et al., 2007;Ishii et al., 1994;Liu et al., 2005;Singh et al., 2004). However, that technique presents problems: e.g. a large amount of organic solvent is used; and an emulsion is formed at the liquid-liquid interface depending on the extraction sample that is used. For that reason, it has been increasingly replaced by clean-up mainly by SPE (Mohan et al., 2010;Obana et al., 2002;Ting et al., 2004;.  used re-extraction with diatomaceous earth SPE and clean-up with GCB/NH 2 SPE in the development of simultaneous analysis by HPLC-DAD of seven neonicotinoid insecticides released on the market. However, the recovery of nitenpyram was not satisfactory (not more than 40%). According to their discussion, matrix components in the sample affected nitenpyram in some way, leading to the factor of reduced recovery.
In the 2000s, a dramatic increase occurred in the number of cases reported on the residue pesticide analysis by HPLC equipped with quadrupole MS, ion-trap MS, tandem MS (MS/MS), and time-of-flight MS (TOF-MS). Utilization of MS enabled not only the detection of trace pesticide residues in various matrices with high accuracy but also the elucidation of their respective chemical structures. Obana et al. (2003) reported a method of extracting five neonicotinoid insecticides from 12 crop samples using methanol, clean-up with GCB SPE, and determination by LC-MS. They suggested that the analytical method that was developed was effective as a regular monitoring method. In determination methods including neonicotinoid insecticides by LC-  . It is exactly the benefit of LC-MS/MS, with which highly sensitive determination is possible. Ultra-performance liquid chromatography (UPLC) was developed only a few years ago in which the mobile phase can be flown at a high pressure (about 15,000 psi) using a short column of about 50 mm packed with C 18 of particle size of not more than 2 m. Its application to pesticide residue analysis has been examined because the utilization of UPLC enables not only the achievement of highly sensitive determination, but also improved high throughput attributable to reduced measurement time as well as substantially smaller amounts of organic solvents (mobile phase) used than in conventional HPLC (Frenich et al., 2008;Kamel et al., 2010;Leandro et al., 2007;Liu et al., 2010;Romero-González et al., 2011;Zhang et al., 2010). Liu et al. (2010) used UPLC-MS/MS to construct simultaneous analyses of seven neonicotinoid insecticides in crops and livestock products, obtaining good recovery except for the lower recovery of nitenpyram (not more than 70%) in some of samples including potatoes and cabbages. Romero-González et al. (2011) constructed highly sensitive ultra-rapid analysis of more than 90 pesticides including acetamiprid, imidacloprid, thiacloprid, and thiamethoxam as well as mycotoxins by the combination of sample pretreatment by QuEChERS and UPLC-MS/MS. As described above, the trend in the development of residue analysis of various pesticides including neonicotinoid insecticides by LC-MS and LC-MS/MS can be summarized via the overview of several cases. LC-MS and LC-MS/MS are suitable for highly sensitive determination of only slightly volatile and heat-unstable pesticides. They enable quantification only with extremely simple and rapid sample pre-treatment compared to those of HPLC-UV, HPLC-DAD, and GC with an element-selective detector, which requires complicated sample pre-treatment procedures. Particularly MS/MS can be characterized as an effective method for the structural analysis of the target pesticide and its confirmation because much chemical information can be acquired by obtaining product ions from the precursor ion. Moreover, little interference occurs by matrix components because ions can be selected at will, thereby enabling highly sensitive determination. Recently, newly marketed insecticides tend to be included in the subjects for HPLC determination. Therefore, increasing need is expected for LC-MS and LC-MS/MS in the future. Although many advantages of LC-MS and LC-MS/MS are described above, the matrix effect must also be noted, which is a problem in the chromatographic determination of pesticide residues that might be present in various samples.
It has been pointed out that, as in GC-ECD and GC-FPD determination, quantitative determination is not possible because of the matrix effect when the sample clean-up is insufficient. Lee and Wylie (1991) have reported interesting observations by which the susceptibility (to the matrix effect) of several GC detectors is examined for individual crop samples. The matrix effect is a phenomenon that is also observed in LC-MS and LC-MS/MS determination; it was shown earlier that when the target pesticide is eluted together with matrix components in the sample, ion suppression or ion enhancement occurs during the ionization process, engendering error in the determination result (Niessen et al., 2006). The matrix effect can be avoided using a matrix-matched standard method or using isotope dilution with isotopically labeled internal standards (Niessen et al., 2006), in addition to thorough sample clean-up. In the matrix-matched standard method, the matrix effect is evaluated through the comparison of the responses of the standard solution and the target pesticide prepared in the measurement sample solution free of the target pesticide; this method is commonly used as a means to correct the matrix effect (Di Muccio et al., 2006;Economou et al., 2009;Ferrer et al., 2005;Ferrer & Thurman, 2007;Fidente et al., 2005;Frenich et al., 2008;Hernández et al., 2006;Kamel, 2010;Kamel et al., 2010;Kanrar et al., 2010;Kmellár et al., 2008;Leanro et al., 2007;Mayer-Helm, 2009;Payá et al., 2007;Pirard et al., 2007;Radišić et al., 2009;Romero-González et al., 2011;Totti et al., 2006;Venkateswarlu et al., 2007;Xiao et al., 2011;Xie et al., 2011). Xie et al. (2011) reported observation of ion enhancement in dinotefuran, imidacloprid, and thiacloprid, as well as ion suppression in acetamiprid, clothianidin, and thiamethoxam by the matrix effect, and that acetamiprid and thiamethoxam among these were significantly affected. However, because the recovery was improved by correction using the matrixmatched standard method, they emphasized the effectiveness of the method in the avoidance of the matrix effect. In any event, the matrix effect should be regarded as a common problem of pesticide residue analysis by HPLC and GC; it goes without saying that it is most important to evaluate the matrix effect that might be derived from the subject sample when the development of a new analytical method is attempted or when an established, existing analytical method is applied.

Analytical methods based on nonchromatographic techniques
Section 2 summarized the trends up to now in the development of the determination of neonicotinoid insecticides by chromatography. This section refers to analytical methods based on nonchromatographic techniques. The list in Table 2 shows that major nonchromatographic methods include flow injection analysis (FIA), direct MS analysis, and enzyme-linked immunosorbent assay (ELISA), which is an immunochemical determination using an antigen-antibody reaction with high specificity or selectivity. This section summarizes the trend and the current situation of the development of analytical methods for neonicotinoid insecticides using these.

FIA or MS analysis for direct determination of neonicotinoid insecticides
FIA is a method by which a predetermined amount of a sample solution is injected into carrier solution that flows continuously in a tube, and the target substance is detected or quantified using a detector, as might be done after a chemical reaction. Instruments used in www.intechopen.com FIA are generally inexpensive. Furthermore, the method is known to be capable of rapid, easy, and highly sensitive detection of trace substances (Lara et al., 2010;Llorent-Martínez et al., 2011). Reports on the determination of neonicotinoid insecticides by FIA have targeted imidacloprid to date. In all such studies, methods are constructed by which a measurement sample is irradiated with ultraviolet light for conversion into a fluorescent substance [1-(6chloro-3-pyridylmethyl)-2-(hydroxyimino)-3,4-didehydroimidalozolidene] which is detected using a spectrofluorometer (Flores et al., 2007;Vílchez et al., 2001). Alternatively, nitrite is detached from imidacloprid to be reduced into nitric oxide by iodide, which is detected by chemiluminescence detection with ozone (Lagalante & Greenbacker, 2007). All FIA methods show measurement sensitivity that is equal to or better than the detection limit of HPLC or GC. For the determination of liquid samples such as water, direct injection is possible with filtration only (Flores et al., 2007;Lagalante & Greenbacker, 2007). Actually, FIA is regarded as making a great contribution to fast, simple, and easy determination of pesticide residues, especially in liquid samples. García-Reyes et al. (2009) reported rapid in situ qualitative and quantitative analysis of 16 pesticides including nitenpyram and thiacloprid by desorption electrospray ionization MS (DESI-MS) and MS/MS (DESI-MS/MS). In their determination, crop samples were pretreated according to QuEChERS and the resulting sample solution was applied on the PTFE surface, while the skin of fruit and vegetable samples were fixed on glass slides, and electrospray was applied directly to the sample to ionize target pesticides in the sample. The measurement sensitivity was extremely high: on the order of g/kg, and the measurement accuracy was comparable to LC-MS. Although such an analytical method remains under development at present, it can be anticipated as a new, highly sensitive, and rapid screening method.

ELISA analysis for neonicotinoid insecticides as a rapid and simple preliminary screening method
Since Yalow and Berson applied radioimmunoassay (RIA) to the determination of insulin in the 1950s (Yalow & Berson, 1959, 1960, RIA has been widely used mainly in the field of clinical laboratory tests. Although immunoassay for pesticide determination was not reported in two decades. Subsequently, antibodies selective to DDT and malathion were developed (Centeno et al., 1970). In addition, RIA for parathion determination was developed (Ercegovich et al., 1981). After Engvall and Perlmann (1972) proposed enzyme immunoassay (EIA, ELISA) using enzyme-labeled antigens instead of radioisotope-labeled antigens, a marked increase existed in the development of ELISA for various pesticides including organophosphorus and synthetic pyrethroid insecticides (Hennion et al., 1998;Meulenberg et al., 1995;Nunes et al., 1998;Shan et al., 2002).
The immunogenicity of small molecules such as pesticides themselves is extremely low. Therefore, it is necessary for the development of antigens to these compounds to design and synthesize hapten molecules that imitate the chemical structure of the target substances. It is known that the measurement sensitivity and selectivity of the resulting antibody is strongly dependent on the chemical structure of the designed hapten molecule (Shan et al., 2002;Szurdoki et al., 1995). However, the importance of hapten design is not explained in this chapter. ELISA, based on an antigen-antibody reaction, is a method used to detect residual pesticides, etc., in various samples consisting of complex matrices such as food samples including crops. Therefore, fast analysis can be achieved because significant laborsaving is possible in complicated sample pre-treatment procedures before chromatographic determination. Moreover, ELISA is regarded as an economical, straightforward, and easy analytical method because only small amounts of organic solvents are used, instruments requiring expertise are not needed, and multisample treatment is possible using 96-well microplates (Ellis, 1996). In contrast, ELISA is disadvantageous compared to chromatographic determination in that it is limited to the determination of a single pesticide, it is incapable of identification, and it might produce a false positive result when it cross-reacts to a compound with similar chemical structure because it is a selective analytical method. As shown in Table 3, ELISA for the determination of neonicotinoid insecticides was first developed in 2000 using polyclonal antibody (PoAb) specific to imidacloprid (Li & Li, 2000). It was followed by a report by Lee et al. (2001) in which the measurement sensitivity was improved to be approximately twice that obtained using another hapten. Watanabe et al. (2001) and Kim et al. (2004) developed monoclonal antibody (MoAb) specific to imidacloprid, and constructed ELISA that is 5-fold to 20-fold more sensitive than that obtained using PoAb. In addition to imidacloprid, ELISA using MoAb specific to acetamiprid (Watanabe et al., 2001) and thiamethoxam (Kim et al., 2003(Kim et al., , 2006 were also developed. As described above, ELISA uses highly specific antigen-antibody reaction. Theoretically, it responds sensitively only to the trace pesticide in matrices. In fact, however, ELISA is susceptible to the matrix effect described in Section 2.2, and it is important to evaluate the matrix effect for individual measurement samples (Jourdan et al., 1996;Nunes et al., 1998;Skerritt & Rani, 1996). While the matrix effect can be avoided or reduced by SPE, etc., such measures would eliminate advantages of ELISA shown above. Table 3 shows that the easiest method to avoid the matrix effect is dilution of the sample extract (mainly methanol extract) with water or phosphate buffer. The recovery from various measurement samples were generally good in all reports (Byrne et al., 2005;Eisenback et al., 2009;Kim et al., 2006;Ma et al., 2009;Watanabe et al., 2001;Watanabe et al., 2004aWatanabe et al., , 2004bWatanabe et al., , 2006Watanabe et al., , 2011Xu et al., 2010). The measurement sensitivity of ELISA is apt to be affected by the concentration of extraction solvent (mainly methanol) coexisting in the sample solution during measurement (Nunes et al., 1998). The sample extraction is diluted not only to avoid the matrix effect effectively but also to reduce the influence of the organic solvent.
Recently, kit-based ELISA for neonicotinoid insecticides was developed and marketed by Horiba Ltd. (Kyoto, Japan) and Envirologix Inc. (Portland, ME). Kit-based ELISA package reagents needed for determination (96-well microplate pre-coated with antibody, washing solution, substrate solution, and stopping solution, etc.) can be used easily for monitoring tests of a specific neonicotinoid insecticide. However, it is important in the use of kit-based ELISA to remember that the matrix effect should be evaluated in advance (Byrne et al., 2005;Watanabe et al., 2004aWatanabe et al., , 2004bWatanabe et al., , 2006Watanabe et al., , 2011. In any event, when ELISA is applied to a sample, it might be affected by the matrix effect. Therefore, it is important to examine in advance if the matrix effect is present, and to examine methods to avoid it. At least, ELISA can be applied sufficiently to routine analysis, especially as a screening method, by solving this problem.

Overview of analytical methods for pesticide residues including neonicotinoid insecticides
Sections 2 and 3 summarize trends in the determination methods for neonicotinoid insecticides and the problems discovered during the development of analytical methods such as the matrix effect. At present, highly accurate and sensitive multiresidue analysis by G C o r H P L C e q u i p p e d w i t h M S o r M S / M S a s a d e t e c t o r i s t h e m a j o r t r e n d i n t h e determination not only of neonicotinoid insecticides but also of pesticides in general. It goes without saying that pesticide residue analysis is an indispensable basic technology that is useful to secure the safety of food, including crops, as well as in various research fields such as environment and ecological impact evaluation. However, it is questionable because it is entirely dependent only on GC-MS, LC-MS, and LC-MS/MS for pesticide residue analysis. Particularly when pesticide residues are analyzed to secure food safety, two situations are assumed: (1) testing of foods on the market, and (2) testing of crops before shipment. In the former situation, the history of pesticides that have been used is often unknown. Therefore multiresidue analysis using MS or MS/MS is suitable, in which as many pesticides as possible can be tested and unknown ingredients can be identified. On the other hand, in the latter situation, the subject pesticide can be selected based on the history of use. Therefore there might be cases in which it is more sensible to choose a rapid, simple, and easy method such as ELISA described in Section 3 over multiresidue analysis which has better than necessary performance. In any event, among the numerous analytical methods, it is necessary to make a wise choice for a suitable method via accurate comprehension of the analytical objectives. One-year routine application of a new method based on liquid chromatographytandem mass spectrometry to the analysis of 16 multiclass pesticides in vegetable samples.