Ureas cleanup

Several cleanup methods have been developed for the determination of urea pesticides, involving different basic procedures, such as liquid-liquid partition (30-32,34,36,37), steam distillation (31), and liquid-solid chromatography (9,30,32,34,36,38,56-58). Different factors, e.g., water solubility, ionic and polarity properties, thermal stability, and the molecular weight of the compounds, determine the choice of the cleanup method. Moreover, micro-cleanup procedures and online enrichment techniques have been introduced for the automated determination of phenylureas (60). Table 6 summarizes the use of the different cleanup procedures in the determination of urea pesticides. 

Table 6 Cleanup Procedures for Sample Extracts Used in the Determination of Urea Pesticides...

Recently, EWOD actuation chips were developed into a multiplexed device that was used to simultaneously cleanup four samples. A sequence of seven actuation steps were performed for each sample (1) generation of sample droplets (0.02 p.L), (2) transport and drying of sample droplets, (3) generation of rinsing droplets, (4) transport of rinsing droplets to the sample sites for selective dissolution of urea, (5) transport and disposal of the rinsing droplets, (6) generation of MALDI matrix solution droplets, and (7) delivery of matrix droplets to the dried peptide spots. 

Solid-phase scavenger methods are employed with increasing frequency as a prehminary reaction cleanup step in combinatorial chemistry, and have recently become commercially available (Argonaut, Calbiochem-Novabiochem, Varian, Alltech). lilly researchers first reported on this approach, employing sohd supported electrophiles and nucleophiles for reaction purification in acylation and alkylation reactions. Yield and purity values reported were 90-95% and 50-99%, respectively, for a library generated by reductive amination. Parke-Davis researchers achieved the removal of known reaction product impurities by the application of custom synthesized polymer supported reagents, specifically polystyrene-divinylbenzene supported derivatives of methylisocyanate and tm(2-aminomethyl)amine for cleanup of by-products resulting from urea, thiourea,sulfonamide,amide, and pyrazole libraries. 

Soil column extraction (SCE) SCE consists on a specific solvent percolation to a column packed with the sampled material (soil, sediment) containing herbicides. About 100mmoll potassium phosphate buffer adjusted to pH 8 at ambient temperature readily extracts imidazolinones, diphenyl ethers, sulfonyl ureas, aryloxyphenoxypropionic acids, and triazolopyrimidines from soil samples. The same SCE procedure was applied to 0.2-0.4% carbon-containing sediments for isolation of sulfonamides and triazines with recoveries ranging from 63% to 99%. Further cleanup (e.g., on Carbograph cartridges) or concentration (SPME) may sometimes be necessary. 

Spectroscopic, luminescence, turbidimetric, and electrochemical methods of detection have been combined with SIA for the successful determination of amino acids, sugars, and trace elements in matrices such as meats, vegetables, breads, wines, juices, and milks. Many of these methodologies required sample pretreatment and whilst most performed this in an offline manner there have been some reports of online sample cleanup. Microwave assisted digestion was performed in-line for the determination of phosphorous, calcium, magnesium, and iron in slurried foodstuffs, wine, milk, and soft drinks whilst gaseous diffusion allowed interference removal for the determination of urea in milk. 

Eight carbamate pesticides were analyzed in bovine rumen. The method was based on extraction, partition, and an addititmal sweep condistillation step. This cleanup gave more re oducible results compared to cellulose-carbon column cleanup. The spots were visualized with either anisaldehyde or lV,2,6-trichloto-benzoquinoneimine after reversed-phase TLC (68). Carbamate and urea herbicides such as chlorpropham, metobromuron, and chlorbromuron were determined in medicinal plants by TLC. First they were hydrolyzed by alkali to 3-chloro-, 4-bromo-, and 3-chloro-4-bromoaniIines. These aniline derivatives were detected with 4-dimethylaminobenzaldehyde after TLC separation (Table 4) (69). Af-methyl carbamate pesticides can be separated by reversed-phase chromatography and analyzed by MS (70). 

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