Carbamates analytical techniques

Historically the earliest Ni-containing enzyme to be described was urease from jack bean meal, which was crystallized by James Sumner in 19261. However, analytical techniques did not allow urease to be recognized as a Ni-containing enzyme until 50 years later. Urease catalyses the hydrolysis of urea to ammonia and carbamate, which spontaneously hydrolyses to give carbonic acid and a second molecule of ammonia. 

In the past, TLC and spectroscopy were the firsts analytical techniques used for the determination of carbamate pesticides, with their inherent difficulties (particularly their limited sensitivity). With the inception of GC and LC, TLC applications were restricted to sample cleanup, metabolic studies, and, perhaps most important, confirmatory analyses in clinical and medical-legal cases requiring positive, unambiguous identification, and quantitative determination.At present, TLC continues to offer attractive features, such as, parallel sample processing for a high sample throughput, accessibility of the sample for postchromatographic evaluation free of time constraints, detection in the presence of the stationary phase, which is somewhat independent of mobile phase and normally used only once. Table 24.5 shows selected examples of the use of TLC for the determination of carbamates. 

The most common analytical technique to analyze carbamate pesticide residues in fruits,... 

The most confirmatory analytical techniques, however, are the GC/MS, LC/MS or liquid chromatography-electrospray mass spectrometry (LC/ESl/MS/MS). Carbamates can also be measured by GC/NPD under N-mode. 

Lord and Pawliszyn" developed a related technique called in-tube SPME in which analytes partition into a polymer coated on the inside of a fused-silica capillary. In automated SPME/HPLC the sample is injected directly into the SPME tube and the analyte is selectively eluted with either the mobile phase or a desorption solution of choice. A mixture of six phenylurea pesticides and eight carbamate pesticides was analyzed using this technique. Lee etal. utilized a novel technique of diazomethane gas-phase methylation post-SPE for the determination of acidic herbicides in water, and Nilsson et al. used SPME post-derivatization to extract benzyl ester herbicides. The successful analysis of volatile analytes indicates a potential for the analysis of fumigant pesticides such as formaldehyde, methyl bromide and phosphine. 

The increased use of IV-methyl carbamate insecticides in agriculture demands the development of selective and sensitive analytical procedures to determine trace level residues of these compounds in crops and other food products. HPLC is the technique most widely used to circumvent heat sensitivity of these pesticides. However, HPLC with UV detection lacks the selectivity and sensitivity needed for their analysis. In the late 1970s and early 1980s, HPLC using post-column hydrolysis and derivatization was developed and refined with fluorescence detection to overcome these problems. The technique relies on the post-column hydrolysis of the carbamate moiety to methylamine with subsequent derivatization to a fluorescent isoindole product. This technique is currently the most widely used HPLC method for the determination of carbamates in water" and in fruits and vegetables." " ... 

APCI can help to reduce matrix effects when analyzing for carbamate insecticides. Eor example, when analyzing for methiocarb in citrus products, the apparent recoveries were in the region of 50% with ESI. However, on changing to APCI, the apparent recoveries were increased to 110%. This is an example where APCI can be an alternative API method if matrix effects are a problem with ESI. It is important to note that the analyte must show sufficient sensitivity to both API techniques. 

Currently, HPLC/fiuorescence is still the most common technique for the determination of residues of oxime carbamates. With the introduction of ESI and APCI MS interfaces, HPLC/MS analysis for oxime carbamates in various sample matrices has become widespread. However, for a rapid, sensitive, and specific analysis of biological and environmental samples, HPLC/MS/MS is preferred to HPLC/MS and HPLC/fiuorescence. With time, improved and affordable triple-quadrupole mass spectrometers will be available in more analytical laboratories. With stricter regulatory requirements, e.g., highly specific and conclusive methods with lower LOQ, HPLC/MS/MS will be a method of choice for oxime carbamates and their metabolites. 

Benzimidazole carbamates have generally been determined by HPLC (20,124-126), although other methods have been reported based on enzyme immunoassay (147) or in gas chromatography (124). The commercial availability of high-performance thin-layer chromatography (HPTLC) plates with layers of very different polarities has improved the sensitivity, selectivity, and analytical precision of TLC, which has led to a reconsideration of the use of this technique for qualitative and quantitative analysis (54). 

OP compounds and carbamate are widely used as insecticides, pesticides, and warfare agents [20,21], Detection of pesticides is usually carried out by multiresidue methods (MRMs) of analysis, which are able to detect simultaneously more than one residue and have been developed mainly based on chromatographic techniques. Two groups of MRMs are used (i) multiclass MRMs that involve coverage of residues of various classes of pesticides, and (ii) selective MRMs, which concern multiple residues of chemically related pesticides (e.g., IV-methyl carbamate pesticides (NMCs), carboxylic acids, phenols, etc.). As foods are usually complex matrices all of the pre-analytical steps (matrix modification, extraction, and clean-up) are often necessary. 

Carbamate pesticides are best analyzed by HPLC using postcolumn deriva-tization technique. Some common carbamate pesticides are listed in Table 2.19.1. Compounds are separated on a C-18 analytical column and then hydrolyzed with 0.05 N sodium hydroxide. Hydrolysis converts the carbamates to their methyl amines which are then reacted with o-phthalaldehyde and 2-mercaptoethanol to form highly fluorescent derivatives. The derivatives are detected by a fluorescence detector. o-Phthaladehyde reaction solution is prepared by mixing a 10-mL aliquot of 1% o-phalaldehyde solution in methanol to 10 mL of acetonitrile containing 100 pL of 2-mercaptoethanol and then diluting to 1 L with 0.05 N sodium borate solution. 

Air analysis for carbamate pesticides may be performed by sampling air over 1 pm PTFE membrane. The analytes collected over the membrane are extracted with methylene chloride, exchanged into methanol, and analyzed by HPLC using postcolumn derivatization technique as described above. Certain pesticides may be analyzed too by the colorimetric method (see Part 3 under individual compounds). 

Although an excellent detector for PAEis, the fluorometer is not widely used in environmental analysis, as the number of environmental pollutants with fluorescent spectra is limited. The sensitivity and selectivity of the fluorometer are also used in the A-methyl carbamate pesticide analysis (EPA Method 8318). These compounds do not have the capacity to fluoresce however, when appropriately derivatized (chemically altered), they can be detected fluorome-trically. The process of derivatization takes place after analytes have been separated in the column and before they enter the detector. This technique, called post column derivatization, expands the range of applications for the otherwise limited use of the fluorometer. 

The HPLC/MS technique used in EPA Method 8321 is best suited for analysis of thermally unstable compounds that are hard to analyze with conventional GC methods, such as organophosphorus pesticides, chlorinated herbicides, and carbamates. In this technique, the detection with mass spectrometry provides the ultimate selectivity. The sensitivity for each individual compound depends on the interferences in a given environmental matrix and on the chemical nature of the analyte. 

A Note on Post-Column Reaction Techniques A post-column reaction unit is an online derivatization system that supplies reagents to the column eluent into a heated chamber to convert the analytes into more chromophoric forms for higher sensitivity detection. Some common applications of post-column reaction systems are amino acid analysis using ninhydrin (with visible detection), and carbamate pesticide analysis using o-phthaldehyde (with fluorescence detection).]... 

A method for analysis of polar pesticides in wine by the use of automated in-tube SPME coupled with LC/ESI-MS was proposed (Wu et al., 2002). In-tube SPME is a microextraction and preconcentration technique that can be coupled on-line with high-performance liquid chromatography (HPLC), suitable for the analysis of less volatile and/ or thermally labile compounds. This technique uses a coated open tubular capillary as an SPME device and automated extraction. Using a polypyrrole coating, six phenylurea pesticides (diuron, fluometuron, linuron, monuron, neburon, siduron) and six carbamates (barban, car-baryl, chlorpropham, methiocarb, promecarb, propham) were analyzed in wine. Structures of compounds are reported in Fig. 9.4. Due to the high extraction efficiency of the fiber toward polar compounds, benzene compounds, and anionic species, LODs ranging between 0.01 and 1.2pg/L were achieved, even if the sample ethanol content affects the recoveries of analytes. 

This chapter deals with the properties of carbamate pesticides, many of which influence the design and choice of analytical methods for their determination. It reviews the chromatographic techniques used to quantify carbamate pesticides in environmental samples, with special emphasis on GC and LC. Other chromatographic techniques, viz., capillary electrophoresis (CE), thin-layer chromatography (TLC), supercritical fluid chromatography (SFC), and sample preparation procedures, are also discussed. 

SPME was hrst used by Pawliszyn et al. in 1990. It is a two-step process conductive to the simultaneous extraction and preconcentration of analytes form sample matrices. In the first step, a fused-silica fiber coated with a polymeric stationary phase is exposed to the sample matrix where the analyte partitions between the matrix and the polymeric stationary phase. In the second step, the fiber/analyte is transferred to the analytical instrument for desorption, separation, and quantification. SPME has a number of advantages over traditional extraction techniques for pesticides. In fact, it is fast, simple, solvent-free, and easily automated for both GC and HPLC instruments. It exhibits good linearity and sensitivity. Thus, carbamate and organophosphorus pesticides in golf course samples were successfully extracted by SPME and analyzed by HPLC by Jinno et al. ... 

Accurate, sensitive analytical methods are required for reliable environmental control analyses. GC is currently the most flexible and sensitive method for residue analysis. One alternative technique in growing use for the determination of carbamates is LC (or high-performance liquid... 

Carbamate Pesticides Environmental Samples Extraction Technique cleanup Technique Reagent/Other Pretreatment Detection Chromatographic Column Analytical Figures of Merit Ref. 

Many analytes that are not amenable to GC (e.g., thermolabile compounds) can be separated by SFC. Separations via SFC are often more efficient and faster than traditional LC analyses. Wider coverage of supercritical fluid methods can be found in the literature on SFC. " This technique has also been employed for the determination of carbamate pesticides in environmental samples (see Table 24.5). ... 

The analysis of pesticide residues and metabolites in foods can be a difficult problem because of the complexity of the matrix and the trace levels at which the analytes must be detected. Capillary SFC has been shown to be a very effective technique for the analysis of trace levels of pesticides, especially when a selective detector such as the NPD is used (18). Figure 7 shows an example of the determination of selective carbamate pesticides in parsley. The sample was prepared by extraction of 1 gram of... 

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