Carbamates, HPLC analysis

This material is enantiomerically pure, as confirmed by HPLC analysis of the 3,5-dinitrophenyl carbamate using a chiral stationary phase. The carbamate derivative of DAI B... 

Tables 8 and 9 summarize the separation conditions used in HPLC analysis of carbamate and urea pesticides.

Carbamate pesticides can be determined using different detectors in GC or HPLC analysis. A characteristic feature of a carbamate molecule is the nitrogen atom, which can form the bases for quantitation and some carbamates also contain chlorine, sulfur, or other heteroatoms in the molecule. This allows the use of various detection techniques for their determination (139,140), such as electrical conductivity (165), alkali flame (141) photometry, and mass spectrometry (44,166). 

For HPLC analysis [172], an aliquot portion of the benzene layer is injected into a system consisting of 0.5% BOP 03,/3 -oxydipropionitrile) coated on Zipax and hexane-ethanol (19 1) as the mobile phase, or of Corasil as the stationary phase and hexane-acetone (49 1) as the mobile phase. A typical HPLC chromatogram of some separated DNS-carbamate derivatives is shown in Fig. 4.64. 

The reaction performed on cyclohexene and ( )- ,2-diphenylethylene yielded a mixture of dia-stereomeric hydroxy carbamates 1, whose ratio was determined by H- and 13C-NMR spectroscopy, by using Eu(fod)3, and by HPLC analysis (Table 8). Furthermore, when an optically active auxiliary was used, the carbamates were cleaved with iodotrimethylsilanc and the enantiomeric excesses were determined by measuring the optical activity of the hydroxy amines. 

Combination of a nonracemic isocyanate and a l,3-disul)sliluted distannoxane has provided a new method for determination of the optical purity of chiral alcohols (Scheme 12.174) [316]. When a chiral alcohol was reacted with commercially available (l )-l-(l-naphthyl)efhyl isocyanate in the presence of 1,3-disubstituted distannoxane, formation of the desired carbamates occurred rapidly, with acid-labile functional groups such as ester, THP and /Miydroxyketone remaining intact. Subsequent HPLC analysis of the resulting carbamate revealed a pair of well-separated peaks of diastereomers derived from both enantiomeric alcohols. 

Figure 7.11. HPLC analysis of carbamates according to U.S. EPA Method 531.1 using postcolumn reaction and fluorescence detection. Reprinted with permission from reference 25.

In both tests the polymer sample was reacted with CO2 at low pressure (—100 bar) and 80°C, followed by flow-through of CO2 at 300 bar and SO C. A hazy liquid was collected, and HPLC analysis indicated that essentially all of the cyclics were removed in the first fraction. Ethane was then introduced and the system was heated to 150°C, well above the reversion temperature of 100°C found with the model compound. Ethane at 15 bar was slowly flushed through the system for — 1 h to promote the reversal of the carbamic acid to amine. The pressure was then raised to 300 bar and a sample was collected. The remainder of the polymer was extracted with ethane at a higher pressure of 400 bar. Table 9.19 gives the results of the first test, and table 9.20 gives the results of the second test. 

An hplc assay was developed suitable for the analysis of enantiomers of ketoprofen (KT), a 2-arylpropionic acid nonsteroidal antiinflammatory dmg (NSAID), in plasma and urine (59). Following the addition of racemic fenprofen as internal standard (IS), plasma containing the KT enantiomers and IS was extracted by Hquid-Hquid extraction at an acidic pH. After evaporation of the organic layer, the dmg and IS were reconstituted in the mobile phase and injected onto the hplc column. The enantiomers were separated at ambient temperature on a commercially available 250 x 4.6 mm amylose carbamate-packed chiral column (chiral AD) with hexane—isopropyl alcohol—trifluoroacetic acid (80 19.9 0.1) as the mobile phase pumped at 1.0 mL/min. The enantiomers of KT were quantified by uv detection with the wavelength set at 254 nm. The assay allows direct quantitation of KT enantiomers in clinical studies in human plasma and urine after adrninistration of therapeutic doses. 

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." " ... 

Reversed-phase HPLC followed by post-column derivatization and subsequent fluorescence detection is the most common technique for quantitative determination of oxime carbamate insecticides in biological and environmental samples. However, for fast, sensitive, and specific analysis of biological and environmental samples, detection by MS and MS/MS is preferred over fluorescence detection. Thus, descriptions and recommendations for establishing and optimizing HPLC fluorescence, HPLC/ MS, and HPLC/MS/MS analyses are discussed first. This is followed by specific rationales for methods and descriptions of the recommended residue methods that are applicable to most oxime carbamates in plant, animal tissue, soil, and water matrices. 

The recommended technique for the determination of oxime carbamates and their metabolites by HPLC/MS and HPLC/MS/MS is positive ESI. Electrospray is a soft ionization technique and is suitable for thermally labile compounds. Ions are produced in the liquid phase at quasi-ambient temperature and atmospheric pressure, thus leaving the fragile pesticides intact. For oxime carbamates, the molecular adducts that can be monitored during HPLC/MS analysis with electrospray in positive mode are [M- -H]+, [M- -Na]+, or [M- -NH4]+, depending on the nature of mobile phase used. ... 

For HPLC/MS/MS analysis, a triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and speciflcity in the quantitative determination of oxime carbamates and their metabolites. This allows... 

The European drinking water guidelines set a maximum admissible concentration of 0.10 pgL for individual pesticides and their related compounds in drinking water. The recommended methods for oxime carbamates and their metabolites in groundwater and surface water are suitable for detection limits below 0.10 pgL. The first method is the HPLC/lluorescence multiresidue method of de Kok et al The second method is an HPLC/MS/MS method of DuPont Crop Protection which is specific for oxamyl and may be applicable for the analysis of other oxime carbamates in heavily polluted waters. 

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. 

A Cis disk has been used to remove chlorotriazine, atrazine, metabolites, organophosphorus compounds, phenylurea, and carbamate pesticides from seawater prior to analysis by HPLC [392],... 

High-performance LC is the favored technique for the determination of carbamates, since many of them lack the thermal stability necessary for gas chromatographic analysis. Most HPLC methods for methyl and phenyl carbamate pesticides have employed reversed-phase chromatography with C18 or C8 columns and aqueous mobile phases (47,50,105,106). Two different solvent sys-... 

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). 

G Blaicher, W Pfannhauser, H Woidich. Problems encountered with the routine applcation of HPLC to the analysis of carbamate pesticides. Chromatographia 13 438-446, 1980. 

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