Carbamates separation techniques

Carbamates, urea, and triazine type compounds can be analyzed on an HPLC. For carbamates postcolumn derivatization technique may be applied. Carbamates separated on a C—18 column are hydrolyzed with NaOH and the products amines are then derivatized with cr-phthalaldehyde and 2-mercaptoethanol to... 

This technique has been applied to the determination of chlorinated insecticides, carbamate insecticides and substituted urea type herbicides in soil and chloroaliphatic hydrocarbons in non-saline sediments. Separation is usually achieved on thin layers of silica gel or alumina. 

In simple experiments, particulate silica-supported CSPs having various cin-chonan carbamate selectors immobilized to the surface were employed in an enantioselective liquid-solid batch extraction process for the enantioselective enrichment of the weak binding enantiomer of amino acid derivatives in the liquid phase (methanol-0.1M ammonium acetate buffer pH 6) and the stronger binding enantiomer in the solid phase [64]. For example, when a CSP with the 6>-9-(tcrt-butylcarbamoyl)-6 -neopentoxy-cinchonidine selector was employed at an about 10-fold molar excess as related to the DNB-Leu selectand which was dissolved as a racemate in the liquid phase specified earlier, an enantiomeric excess of 89% could be measured in the supernatant after a single extraction step (i.e., a single equilibration step). This corresponds to an enantioselectivity factor of 17.7 (a-value in HPLC amounted to 31.7). Such a batch extraction method could serve as enrichment technique in hybrid processes such as in combination with, for example, crystallization. In the presented study, it was however used for screening of the enantiomer separation power of a series of CSPs. 

NBD-C1 reacts with the liberated amines of dithio- and thio-carbamates to produce fluorescent derivatives. The products are separated by TLC [177]. The method is similar to that described earlier for methylcarbamate insecticides (see Section 4.2.4.2.2). The carbamates are hydrolyzed in base and the liberated amines are treated as described for methylcarbamates or by the technique described for biogenic amines (Section 4.2.1.2.2). 

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. 

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. 

Tewari, S.N. Singh, R. "TLC technique for the separation and identification of carbamate pesticides in post-mortem material", J. Chromatog., 1979, 172, 528. 

In pioneering work by Ptereira et al., the derivatization of racemic alcohols with (+)-[19] was carried out to produce diastereomeric carbamates that could be resolved on stainless steel capillary GLC columns coated with Carbowax 20M or OV-225 stationary phase (166). The retention times, however, ranged between 28 and 90 min, rendering the technique somewhat inconvenient. Gal et al. used capillary GLC columns to separate... 

In contrast, diamine 104-NH2 can be stored in CH2C12. Neda et al. have shown that upon contact with C02 (from the air) or with C02 in ether, 104-NH2 reacts spontaneously to form the primary ammonium carbamate salt 106 (2 1 adduct) as a colorless crystalline solid. The neat salt is quite stable. Upon heating above 120 °C in vacuo, C02 is removed and pure 1,2-diamine 104-NH2 can be recovered and used directly thereafter. In this simple fashion, even traces of QCI can be removed from 104-NH2 without recourse to H PLC, distillation, or other traditional techniques of separation and purification (Scheme 12.36) [81]. 

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. ... 

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 dehydration of ammonium carbamate is appreciable only at temperatures above the melting point (about 150°C) and this reaction can only proceed if the combined partial pressure of ammonia and carbon dioxide exceeds the dissociation pressure of the ammonium carbamate (about 100 atmospheres at 160°C and about 300 atmospheres at 200°C). Thus commercial processes are operated in the liquid phase at 160—220°C and 180—350 atmospheres. Generally, a stoichiometric excess of ammonia is employed, molar ratios of up to 6 1 being used. The dehydration of ammonium carbamate to urea proceeds to about 50—65% in most processes. The reactor effluent therefore consists of urea, water, ammonium carbamate and the excess of ammonia. Various techniques are used for separating the components. In one process the effluent is let down in pressure and heated at about 155°C to decompose the carbamate into ammonia and carbon dioxide. The gases are removed and cooled. All the carbon dioxide present reacts with the stoichiometric amount of ammonia to re-form carbamate, which is then dissolved in a small quantity of water and returned to the reactor. The remaining ammonia is liquefled and recycled to the reactor. Fresh make-up ammonia and carbon dioxide are also introduced into the reactor. Removal of ammonium carbamate and ammonia from the reactor effluent leaves an aqueous solution of urea. The solution is partially evaporated and then urea is isolated by recrystallization. Ammonium carbamate is very corrosive and at one time it was necessary to use silver-lined equipment but now satisfactory alloy steel plant is available. Urea is a white crystalline solid, m.p. 133°C. 

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