Carbamates postcolumn derivatization

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... 

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. 

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... 

Since BAs occurring in food do not exhibit satisfactory absorbance or fluorescence in the visible or ultraviolet range, chemical derivatization, either pre- (35-37) or postcolumn (38), is usually used for their detection in HPLC. The most frequently employed reagents for precolumn derivatization are fluorescamine, aminoquinolyl-lV-hydroxysuccinimidyl carbamate (AQC) (39, 40), 9-fluorenylmethyl chloroformate (FMOC) (41-43), 4-dimethylaminoazobenzene-4 -sul-fonyl chloride (dabsylchloride, DBS) (44), N-acetylcysteine (NAC) (45,46), and 5-dimethyl-amino-1-naphthalene-1-sulfonyl chloride (dansylchloride, DNS) (47,48), phthalaldehyde (PA), and orf/to-phthaldialdehyde (OPA) (49-51), together with thiols such as 3-mercaptopropionic acid (MPA) (37) and 2-mercaptoethanol (ME) (35,49). 

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. 

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