Insecticides and herbicides in soils 77
Immunoassay-and chemiluninescence-based methods have also been used [102,
103] to determine methyl-2-bendimadazole and Aldicarb and Paraoxon in soil.
Leistra et al [104] have studied the rate of leaching of Methomyl (S-methylcarbamoxy thioacetimidate or S-methyl-N-(methyl carbamoyl)oxy thioacetimidate)
from greenhouse soils into watercourses. Its adsorption on and leaching from soil were
studied. Adsorption on three typical greenhouse soils was weak to moderate, and the
half-life ranged from three to 14 days. These data were used in mathematical models to predict the pesticide’s behaviour; only very small amounts would be leached,
depending on the rate of transformation in the soil and the amount of irrigation
water used.
N-methylcarbamate and N,N’-dimethylcarbamates have been determined in soil
samples by hydrolyses with sodium bicarbonate and the resulting amines and diamines
reacted with 4-chloro-7-nitrobenzo-2,1,3-oxadiazole in isobutyl methyl ketone solution to produce fluorescent derivates [105]. These derivatives were separated by thin
layer chromatography on silica gel G or alumina with tetrahydrofuran-chloroform
(1:49) as solvent. The fluorescence is then measured in situ (excitation at 436 nm,
emission at 528 and 537 nm).
3.5 SUBSTITUTED UREA HERBICIDES
Supercritical Fluid Extraction
McNally et al have applied supercritical fluid extraction chromatography to the
determination of Diuron and Linuron in soil [106]. Schlaeppi et al [107] have
described an automated magnetic particle-based chemiluminescent immunoassay for
the determination of Trisulfuron in soil [108].
The presence of free anilines or other metabolites in soils and plants has been
reported [109–114]. Some work has suggested that they are very strongly bound
to soil-components. The presence in soils of metabolites of Linuron that possess
the urea structure have been reported [160, 161]. These are produced mainly by
microbiological degradation. The dimethyl derivative is considered to be inactive
whereas the monomethyl metabolite has a phytotoxicity approaching that of the parent
herbicide [161].
McNally and Wheeler et al [115] used supercritical fluid extraction coupled
to supercritical fluid chromatography to determine sulfonylurea herbicides in soil.
Klafterback et al [116, 117] used supercritical fluid extraction with methanol-modified
carbon dioxide followed by high-performance liquid chromatography with UV detection to determine sulfonylurea herbicides obtained on a C 18 solid-phase extraction
disc. Alternatively the determination was carried out by gas chromatography of the
dimethyl derivatives of the sulfonylurea herbicides, employing an electron capture or
a NP detector on the gas chromatograph.
Gas chromatography
Gas chromatography of phenylurea herbicides is difficult because of their ease of
decomposition. Procedures have been reported in which careful control of conditions
allows these compounds to be chromtographed intact [118–121]. Alternatively, the
Immunoassay-and chemiluninescence-based methods have also been used [102,
103] to determine methyl-2-bendimadazole and Aldicarb and Paraoxon in soil.
Leistra et al [104] have studied the rate of leaching of Methomyl (S-methylcarbamoxy thioacetimidate or S-methyl-N-(methyl carbamoyl)oxy thioacetimidate)
from greenhouse soils into watercourses. Its adsorption on and leaching from soil were
studied. Adsorption on three typical greenhouse soils was weak to moderate, and the
half-life ranged from three to 14 days. These data were used in mathematical models to predict the pesticide’s behaviour; only very small amounts would be leached,
depending on the rate of transformation in the soil and the amount of irrigation
water used.
N-methylcarbamate and N,N’-dimethylcarbamates have been determined in soil
samples by hydrolyses with sodium bicarbonate and the resulting amines and diamines
reacted with 4-chloro-7-nitrobenzo-2,1,3-oxadiazole in isobutyl methyl ketone solution to produce fluorescent derivates [105]. These derivatives were separated by thin
layer chromatography on silica gel G or alumina with tetrahydrofuran-chloroform
(1:49) as solvent. The fluorescence is then measured in situ (excitation at 436 nm,
emission at 528 and 537 nm).
3.5 SUBSTITUTED UREA HERBICIDES
Supercritical Fluid Extraction
McNally et al have applied supercritical fluid extraction chromatography to the
determination of Diuron and Linuron in soil [106]. Schlaeppi et al [107] have
described an automated magnetic particle-based chemiluminescent immunoassay for
the determination of Trisulfuron in soil [108].
The presence of free anilines or other metabolites in soils and plants has been
reported [109–114]. Some work has suggested that they are very strongly bound
to soil-components. The presence in soils of metabolites of Linuron that possess
the urea structure have been reported [160, 161]. These are produced mainly by
microbiological degradation. The dimethyl derivative is considered to be inactive
whereas the monomethyl metabolite has a phytotoxicity approaching that of the parent
herbicide [161].
McNally and Wheeler et al [115] used supercritical fluid extraction coupled
to supercritical fluid chromatography to determine sulfonylurea herbicides in soil.
Klafterback et al [116, 117] used supercritical fluid extraction with methanol-modified
carbon dioxide followed by high-performance liquid chromatography with UV detection to determine sulfonylurea herbicides obtained on a C 18 solid-phase extraction
disc. Alternatively the determination was carried out by gas chromatography of the
dimethyl derivatives of the sulfonylurea herbicides, employing an electron capture or
a NP detector on the gas chromatograph.
Gas chromatography
Gas chromatography of phenylurea herbicides is difficult because of their ease of
decomposition. Procedures have been reported in which careful control of conditions
allows these compounds to be chromtographed intact [118–121]. Alternatively, the