Determinación de herbicidas derivados de la urea en aguas y vegetales

by Berrada, Houda

Abstract (Summary)

Substituted ureas are an important group of pesticides that are used as herbicides (phenylureas and sulfonylureas) and insecticides (benzoylureas). Phenylurea herbicides (PUHs) are largely used in field applications for pre- and post-emergence weed control in a variety of crops. The main groups of phenylurea herbicides are the N-phenyl-N,N-dialkylureas, N-phenyl-N-methoxyureas and compounds containing a heterocyclic group. More recently a group of sulfonylurea herbicides (SUHs) has been developed. Multiresidue and single residue methods generally consist of the same basic steps, but multiresidue methods allow the determination of a large number of pesticides in a single analysis, reducing thus time and cost of analysis. The Maximum Admissible Concentration (MAC) of pesticides in drinking water, defined by European Community Directive as 0.1 mg L-1 for individual pesticides and 0.5 mg L-1 for the sum of pesticides posed certain demands to the analytical methods for pesticide residue determination. Gas chromatographic conditions for determining eight phenylurea æchlortoluron, diuron, fluometuron, isoproturon, linuron, metabenzthiazuron, metobromuron and monuronæ and one sulfonylurea æchlorsulfuronæ herbicides were assessed. Degradation products of herbicides formed in the injector were used for identification. Most phenylureas formed their respective carbamic acid methyl esters, metabenzthiazuron formed an aminobenzothiazol and chlorsulfuron formed an aminotriazine plus a phenylsulfonamide. On-column injections of standards at different temperatures (from -30ºC to 400ºC) using columns with four stationary phases (BPX70, BPX35, BP10 and BPX5) were evaluated to identify the best chromatographic behaviour. The best suited column was BP10-30m. The effect of other factors such as injection mode, injector design, sample volume and the solvent used were also investigated. Instrument detection limits (IDLs) ranged from 0.05 ng for chlorsulfuron to 3 ng for monuron with the NPD and, from 0.01 ng for chlorsulfuron to 5 ng for metabenzthiazuron with the ECD. The RSDs (n=4) were lower than 4% at the 12-25 ng level. The method was applied to analyse surface waters extracted with C18 Empore disks with recoveries higher than 85%. Down to 0.1 mg·L-1 of each herbicide could be determined. Chlortoluron was found (11.4 mg·L-1) in a water sample and its presence was confirmed by gas chromatography-mass spectrometry. Degradation products of chlorsulfuron, chlortoluron, diuron, fluometuron, isoproturon, linuron, metabenzthiazuron, metobromuron, and monuron formed in the gas chromatographic injector were used for identification of their respective herbicide. The spectra of the derived compounds were obtained with a quadrupole mass spectrometric detector working in scan mode (20-450 amu). The solvent used for phenylurea herbicide injection (ethanol, methanol, dichloromethane, and acetonitrile) had influence on the generated compounds. When methanol and ethanol were used as solvents, the major products formed from phenylureas were carbamic acid esters. When acetonitrile or dichloromethane were used, the main derivatives were phenylisocyanates. However, chlorsulfuron and metabenzthiazuron generated a triazine plus a phenylsulfonamide and a benzothiazolamine respectively regardless of the assayed solvent. Linuron and diuron showed the same behaviour and gave degradation products with the same mass spectra. The thermal reactions occurred instantaneously in the injector block favoured by the high selected temperature (300ºC). The derived compounds from urea herbicides can be determined using a BP10-30m column and a Selected Ion Registering (SIR) program based on two to three m/z ions as a way to sensitively detect the presence of urea herbicides in environmental extracts. With standards in methanol the instrumental detection limits ranged from 0.1 pg for chlorsulfuron (detected as 2-chlorobenzensulfonamide) to 1 pg for monuron and metobromuron (both detected as their analogous carbamic acid methyl esters). The RSD were below 9 % at the 5 ng·L-1 level. The response was linear (r > 0.9986) within the 5 ng·L-1 to 25 mg·L-1 range. The unequivocal identification of some phenylurea herbicides was not always possible because some herbicides having analogous structures such as diuron and linuron gave the same derivative. Under the proposed conditions matrix effects were low enough to permit calibration with samples proceeding from ecological (non-pesticide treated) crops. Twelve commercial samples of each carrots, onions and potatoes were analyzed and only three samples of potatoes contained residues of linuron at levels below MRL.

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Bibliographical Information:

Advisor:Moltó Cortés, Juan Carlos; Font, Guillermina

School:Universitat de València

School Location:Spain

Source Type:Master's Thesis

Keywords:medicina preventiva i salut pública bromatologia toxicologia legal


Date of Publication:01/31/2002

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