Pesticides postcolumn derivatization

Direct determination of urea pesticides by high-performance liquid chromatography has been widely reported in the literature (10,32-36,127-130). Ultraviolet detection has often been used (32,33,35,36,60,127) with usually acceptable sensitivity, although this detector is nonspecific and the sensibility is, in general, low. To overcome this problem, several techniques have been assayed, such as precolumn enrichment (60), postcolumn derivatization (34,10), and the use of other detection techniques such as the electrochemical (129), photoconductivity (128,130), and fluorescence detectors (9,10,34). Table 9 summarizes representative papers using these techniques in HPLC analysis. 

Carbamates and substituted ureas are a numerous group of pesticides widely used to control weeds, pests, and diseases in fruit trees, vegetables, and cereals. Carbamate residues in foods are commonly extracted with water-miscible solvents and determined by using a liquid chromatograph equipped with a sensitive detector, frequently a UV detector. In addition, to obtain adequate detection selectivity, the postcolumn fluorimetric labeling technique is used for methyl carbamates. Substituted ureas are normally extracted from foods with organic solvents, and they can be determined directly by HPLC-UV or after postcolumn derivatization by fluorescence determination of their derivatives. 

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). 

Liquid chromatography is suitable for analysis of polar, low-volatile, and thermally labile pesticides, such as phenylureas and carbamates. In spite of the high sensitivity of postcolumn derivatization and fluorescence detection, or the robustness of UV detection, MS offers the advantages of high sensitivity and selectivity. 

Chiron, S. and Barcelo, D., Determination of pesticides in drinking water by online solid-phase disk extraction followed by various liquid-chromatographic systems,/ Chromatogr., 645, 125-134, 1993. Sundaram, K. M. S. and Curry, J., High-performance liquid-chromatographic methods for the analysis of aminocarb, mexacarbate, and some of their At-methylcarhamate metabolites by postcolumn derivatization with fluorescence detection, J. Chromatogr. A, 672, 117-124, 1994. 

Vassilakis, I., Tsipi, D., and Scoullos, M., Determination of a variety of chemical classes of pesticides in surface and ground waters by off-line solid phase extraction, gas chromatography with electron-capture and nitrogen-phosphorus detection, and high-performance liquid chromatography with postcolumn derivatization and fluorescence detection, J. Chromatogr. A, 823, 49-58, 1998. 

Online postcolumn derivatization for LC-fluori-metric analysis of nonfluorescent or weakly fluorescent pesticides has mainly involved fluorogenic labeling of N-methyl carbamates and metabolites. The labeling reaction is a two-step process consisting of alkaline hydrolysis and posterior reaction of the hydrolysis product with o-phthalaldehyde and 2-mercaptoethanol to yield the highly fluorescent l-hydroxyethylthio-2-methylisoindole (EPA method 531.1). The detection limits for surface waters are between 20 and 30ngl . ... 

UV or FL detection of the target compounds is sometimes not achievable due to the lack of suitable chromophoric/fluorophoric molecular sites. This could be overcome by derivatization. Two postcolumn derivatization modes are noticeable for pesticide detection. One relates to the fluorescent detection of carbamates based on their reaction with ortho-phtalaldehyde (OPA), and the other deals with the photoirradiation of benzoylurea insecticides (namely diflubenzuron, triflumuron, hexaflumuron, lufenuron. 

Carbamate pesticides can be monitored in die UV at 220 nm, by postcolumn derivatization with o-phdialaldehyde and fluorescence detection, or by thomospray MS [198]. Following US EPA Methods 531.1 and 531.5 for drinking water (C g column with A = 330 nm, ex 465 nm, em and a 30-min 0/100 — 0/100 methanol/ water gradient), baseline separation of 12 V-methyl and W-methyloxime carbamates and selected oxidation products was achieved with detection limits of <0.6 ppb [199]. 

The UV detector has been the most widely used in the LC determination of pesticides [59,63,69,71,77,106,107,109,111,122,147,148]. However, at present, the diode array detector is usually preferred to obtain the UV spectrum for each individual compound and confirm e presence of target analytes [35 8,45,48,50,71,74,76,108,149]. Carbamate pesticides are usually determined with fluorescence detector, following LC-postcolumn derivatization of methylamine (formed in the previous hydrolysis of pesticide) with OPA reagent [68,74]. Recently, a postcolumn detection system was used for the... 

Some parameters affect the conversion of nonflu-orescent compounds into fluorescent ones, e.g., UV irradiation time and the nature of the solvent used (see Table 1). For example, pesticides such as fenvalerate, diflubenzuron, and deltamethrin are efficiently converted into fluorescent products (high signal with short irradiation time) in protic solvents, while for fenitrothion and chlorpyriphos polar apro-tic solvents are the choice. Comparing the analytical performances of the photoconversion method with other derivatization approaches, it can be considered a rapid, sensitive, and precise method for quantifying several classes of pesticides in stationary media and as postcolumn detection mode in HPLC and in FIA. 

Traditional detectors and postcolumn derivatiz-ation Traditionally, LC served mostly for analysis of single or small groups of non-GC-amenable pesticide residues for which derivatization for GC was unsuitable or complicated. Besides a lower number of possible theoretical plates, the main reason for its inferior position as compared to GC consists in the... 

Newsome et al. [974] studied the chromatography of 11 carbamate pesticides (e.g., aldicaib, aldicarb sulfoxide and sulfone, oxamyl, cathofiiran, 3-hydroxy- and 3-ketocaiboiuran) on a Cg column. A postcolumn o-phthalaldehyde derivatization technique (A = 336 run, ex 440 nm, em) was compared with an atmospheric pressure ionization MS detector. A 20-min 12/78 -> 70/30 acetonitrile/water gradient generated good resolution and peak shapes. Detection limits forall compounds were tabulated and were 0.1 ng for fluorescence and 1 ng for APCl/MS. 

Derivatives are used in LC primarily to improve the response of an analyte to a specific detector or, less frequently, to increase the stability of the analyte in a particular separation system or the chromatographic separation of a mixture yielding overlapping peaks [133,137]. Most derivatization procedures introduce chromophores or fluorescent groups into functionalized molecules of pesticides. The reagent most widely used to obtain fluorescent derivatives is o-phthaldehyde (OPA), which is employed in a postcolumn reaction with the pesticides [68,74]. 

---