Residue analysis experiments of carbamate insecticides

Summary

Urethane is generally non-corrosive and its storage stability is excellent. Its greatest advantage is that it remains in the plant for only a short time, and most carbamates are degraded into the appropriate metabolites, which usually have the same or greater activity as the parent compound, within a short time after application.

Source: Food Safety Testing Technology (Chemical Industry Press)

Operation method

Residue analysis of carbamate insecticides

Principle

Carbamate insecticides usually have the following general formula. See FIGS. 2-4. wherein the hydroxyl compound corresponding to the ester group, R1OH, tends to be weakly acidic, R2 is a methyl group, and R3 is either hydrogen or a group that can be easily broken by chemical or biological means. For the urethane as a whole, the structural variations are mainly in the ester group, and the hydroxyl compound corresponding to the ester group is generally required to be weakly acidic, such as an enol, a phenol, a hydroxamic acid, and so on; another variable part of the structure is the substituent on the nitrogen atom, wherein the hydrogen on the nitrogen atom can be replaced by one or two methyl groups or by an acyl group. According to the variation of the structure, the carbamates can be divided into four types: N,N-dimethyl carbamate, N-methyl carbamate aryl ester, N-methylamine, formic acid oxime ester, N-acyl (or hydroxythio)-N-methyl carbamate. Most carbamate insecticides are soluble in a variety of organic solvents, but less soluble in water, with the exception of a few methylcarbamate oxime esters such as aldicarb and methomyl.

Materials and Instruments

Vegetables Fruits
Acetone NaCl solution Dichloromethane Anhydrous sodium sulfate Methanol
Stoppered conical flasks Filter flasks Gel columns Dispensing funnels Fraction collectors

Move

I. Overview of determination methods At present, the methods applied to the detection of carbamate pesticide residues on fruits and vegetables in China are divided into two categories: chemical and biological methods. The chemical method mostly adopts the traditional liquid-liquid extraction and column chromatographic clean-up pretreatment technology, and is determined by GC or HPLC. Because of the general lack of thermal stability of carbamate pesticides, GC will cause decomposition, but also lack of sensitive selective detector, therefore, in addition to the direct determination of compounds, more methods are derived from pesticides for the derivatives suitable for detection by electron capture detector, and then quantitative determination; high performance liquid chromatography commonly used detector is the ultraviolet detector, but not very sensitive; fluorescence detector because of its high sensitivity. The fluorescence detector has been used for the detection of carbamate pesticide residues because of its high sensitivity. Biological methods, including enzyme immunoassay and rapid enzyme method, are simple, fast and easy to operate, but the detection of fruits and vegetables and pesticides are limited. Liquid chromatography analysis using reversed-phase chromatography separation of carbamate pesticides, due to the absence of characteristic spectral properties of such pesticides, the general use of post-column derivatization method with fluorescence for detection. The general principle is a two-step process: the first step is to decompose the carbamate into free primary amines under strong alkaline conditions; the second step is to use o-phthalaldehyde to react with primary amines to produce fluorescent compounds for detection. Recently, it has been reported that the derivatization reaction can also be carried out by one-step catalysis, i.e., using MgO as catalyst, the carbamate will be degraded, and the degradation product will then be synthesized with o-phthalaldehyde to produce fluorescent compounds. 1. Pre-treatment is generally solvent plus shaking or Soxhlet extraction. Multi-residue analysis, the most commonly used extraction reagents are acetone, dichloromethane, petroleum ether, acetonitrile, ethyl acetate, methanol, etc.; the most commonly used purification step is after the separation by column chromatography or solid-phase extraction separation. The solvents commonly used for liquid-liquid partitioning are dichloromethane, petroleum ether, hexane and acetonitrile, and the separation columns commonly used are Florisilicate, Celite, silica gel, alumina, and C18 and aminopropyl bonded SPE columns. The recovery of strongly polar carbamate pesticides and their metabolites (e.g., aldicarb sulfoxide) using liquid-liquid partitioning was low, and solid-phase extraction is a better purification method. In addition, there are multi-residue analytical methods using gel chromatography for purification after extraction, which have been more commonly used in recent years. According to AndredeKok and Maurice Hemstra, carbamate insecticides are extracted with a mixed organic solvent acetone-dichloromethane-petroleum ether (1 + 1 + 1) for samples of vegetables and fruits with high aqueous content; for samples with low aqueous content, e.g., cereals, they are extracted with (1 + 1) acetone-dichloromethane extract and then cleaned up with a solid-phase extraction column in the aminobonded phase. The extract was then cleaned up with a solid phase extraction column using an amino-bonded phase. The extract was then cleaned up by an amino-bonded solid phase extraction column and eluted with a dichloromethane-methanol (99 + 1) eluent. After concentration, drying and dissolution, it was analyzed by liquid chromatography. It has been reported that they are now using the GilsonASPEC automated solid phase extraction system for sample cleanup to improve the accuracy and speed of analysis. In recent years, there are new techniques applied to the pretreatment of carbamate pesticides, such as matrix solid-phase dispersion and SPME and GPC systems to clean up the sample technology. 2. Determination Gas chromatography: The determination of residues of carbamate pesticides (NMCs) is mainly by gas chromatography (GC) and high performance liquid chromatography (HPLC). Due to the poor thermal stability of many carbamate pesticides, the application of gas chromatography is somewhat limited. For the more stable carbamate pesticides can also be determined by GC, such as carbofuran, methomyl, aldicarb, methomyl, nematicarb, dithiopyr, butylcarb, isopropylcarb, residual carbofuran and so on. It can also be determined by GC after derivatization, e.g., methiocarb is determined after derivatization with BSTFA silanization. Residue analysis can be done by ECD and NPD. Yang Dajin et al. used capillary GC and nitrogen-phosphorus detector to determine six carbamate pesticides in rice and vegetables, with good accuracy and precision, and the lowest limit of detection was 2-15 ug/g. In the detection of carbamate pesticides in a variety of crops, the method of capillary cold head injection coupled with mass spectrometry has been widely used, which is due to the short column, and the analytes reaching the mass spectrometer can be analyzed with the cold head injection. This method has been widely used in the detection of carbamate pesticides by capillary cold capillary injection and mass spectrometry. Song Ming et al. made a study on the stability of eight carbamates, and concluded that the chromatograms of oxime carbamates methomyl and aldicarb had only one peak, and no molecular ion peaks were seen in the mass spectrometry, indicating that these two compounds have been decomposed. Phenylcarbamate carbofuran, carbofuran, carbofuran, carbofuran, carbofuran and carbaryl had two peaks in the total ion flow diagram, respectively, for the original compounds and decomposition products, in which the retention time of the original compounds was longer, and the retention time of the decomposition products was shorter, and the difference between them was that the decomposition products did not have the molecular peaks of the original compounds. Through the analysis of mass spectra, it can be seen that the decomposition product of phenyl carbamate is its corresponding phenol, i.e., it is formed by a CONHCH3 in the molecule. The mass spectra of both the original compound and the decomposition product yielded [M-58]+ ions and further decomposition. In addition, the decomposition of such carbamate standard solution with the time of placing, the total ion flow diagram of the newly prepared standard solution, the original compound's peak is obviously higher than that of the decomposition product, and after the solution has been placed for a period of time, the original compound's peak gradually weakened, while the decomposition product's peak gradually strengthened. High performance liquid chromatography (HPLC): High performance liquid chromatography (HPLC) with ultraviolet (UV) detector for the detection of two-component or single-component carbamate pesticide residues has also been reported. The detection wavelength commonly used for multi-residue analysis of NMCs in complex matrices is 255 nm, while in the analysis of residues of furadan and its metabolites, the detection wavelength of 280 nm is used. the sensitivity of such methods cannot meet the limit requirements of all NMCs and their metabolites. In recent years, the research on the detection of carbamate pesticide residues by high performance liquid chromatography (HPLC) has been more active at home and abroad. However, most of them are HPLC with post-column derivatization-fluorescence detector detection.In 1977, M0ye et al. firstly used post-column derivatization HPLC-fluorescence detection to determine the residues of NMCs. In the past 20 years, post-column hydrolysis and derivatization followed by fluorescence detection of NMCs in complex matrices has become more common. It requires the hydrolysis of NMCs at 90 ℃ to produce methylamine, which reacts with o-phthalaldehyde (OPA) in a base solution in the presence of 2-mercaptoethanol (2-ME) to produce an intensely fluorescent substance. This method has been used to determine 25 NMCs in 26 samples with recoveries ranging from 70% to 100%. This post-column derivatization system requires two reaction tubes and two reaction reagent pumps to be installed after the column, but can satisfy the method sensitivity requirements for all NMCs detection limits nowadays. Although HPLC-UV and HPLC-fluorescence detection have been widely used in the analysis of carbamate pesticide residues, the confirmation of some compounds is potentially problematic, especially in the analysis of complex samples such as soils and crops, where the separation of peaks is difficult, and therefore the use of HPLC in-line with MS is undoubtedly a good solution. Different interfaces such as particle beam and thermal spray have been used for the analysis of carbamate pesticides by HPLC-MS. The thermal spray interface can analyze small molecules with certain volatility, and nitrogen-containing compounds can produce strong thermal spray signals, which is more conducive to the analysis and detection of carbamate pesticides. The lowest detection limit for the determination of furadan in potato samples was up to 2.5 ng/g using HPLC-MS. R. T. Krause's HPLC method with post-column derivatization-fluorescence detector resulted in a low limit of detection of O.Ol mg/kg with a coefficient of variation of 4.7%. A post-column derivatization-high performance liquid chromatography-electrochemical detector method for the determination of carbamate pesticide residues in cereals and grains was also reported, with a low limit of detection (LOD) in the range of O.O5-O.l mg/kg and a coefficient of variation (CV) of 2.8 %. M. Sher Ali used a high performance liquid chromatography (HPLC) method with post-column derivatization-fluorescence detector for the determination of carbamate pesticide residues in liver with the use of a GPC (Gel Perfiltration) system for sample clean-up. De Kou and Hiemstra used post-column derivatization-fluorescence detector-high performance liquid chromatography (HPLC) for the determination of carbamate pesticides in fruits and vegetables with solid phase extraction. Xinming Jiang detected carbamate pesticides in soil by post-column derivatization-fluorescence detector-high performance liquid chromatography (HPLC), and the samples were determined directly without purification. Immunoassay for carbamate pesticides is also developing rapidly and mostly using cloned antibody technology, e.g., Lebotay et al. applied a commercial kit produced using enzyme immunoassay technology to determine furadan and aldicarb sulfone in meat and liver samples. Detection and analysis of carbamate pesticide residues in different media has always been a subject of interest, and a variety of detection techniques have been developed so far. Currently, various methods have their own advantages, but at the same time there are also different degrees of shortcomings, the application should be based on the actual conditions and research needs to choose the appropriate detection technology. Typical analytical methods Take the GPC cleanup-post-column derivatization HPLC method as an example to analyze the residue of carbamate in food. The sample was minced, 20 g was taken into a 500 mL stainless steel centrifuge tube, 10 mL of water and 40 mL of acetone were added, homogenized for 2 min, and the sample was centrifuged at 6000 g for 15 min. The acetone layer was removed from the sample and placed into a 300 mL aubergine flask. Add 40 mL of acetone into the residue, in order to make the acetone and residue well mixed, the residue was crushed with a spatula and mixed, centrifuged at 6000 g for 15 min, and the acetone layer was combined in a 300 mL aubergine flask. The acetone layer was combined in a 300 mL aubergine flask. The liquid was concentrated in an evaporator until the liquid volume became 1/2 of the original liquid volume. In a 300 mL separatory funnel, add 50 mL of saturated NaCl solution, 2 × 40 mL dichloromethane extraction, dichloromethane layer in a 200 mL beaker, add anhydrous sodium sulfate dehydration, filtered through a glass filter, filtrate evaporator concentrated to dryness, add 10 mL of dichloromethane - cyclohexane (1 +1), precipitation of insoluble turbid components of the 7-8 mL (3000 r / min, 5 min, clarification). (3000 r/ min, 5 min, clarified). 5 mL was purified by CFG. The purified solution was concentrated to 10 mL, and 1 mL was purified by C18 column. Elute with 5 mL of methanol and concentrate the eluate to 1 mL. Positive results can be confirmed by HPLC-MS.

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