Insecticides and herbicides in soils 81
Miscellaneous
Katz and Strusz et al [143] have described gas and thin layer chromatographic methods for the determination of traces of Chlorobromuron (3,4 chromo-3-chlorophenyl,
1.1 methoxy, 1-mechylurea) and its metabolites. The samples were extracted in a
Soxhlet apparatus of ethyl acetate for 6h. Each extract was then acidified with anhydrous acetic acid and evaporated to dryness in a steam bath; 15 µl portions were
applied to a 0.1 mm layer of silica gel. The chromatogram was developed twice with
hexane-ethyl acetate (15:2). After drying, the chromatogram was developed with
chloroform-pyridine (10:1), dried again, and sprayed with Mitchell reagent (20 ml
of 2-phenoxyethanol added to the solution of 1.7 g silver nitrate in 5 ml of water, the
mixture is diluted with acetone to 200 ml, then one drop of 30% aqueous hydrogen peroxide is added), and the spots were located under UV radiation. For gas
chromatographic confirmation, standards were treated as above and corresponding
unsprayed sample areas were removed, extracted with 5 ml of ethyl acetate and evaporated to 1 ml; 5 µl aliquots were injected on to a glass column packed with 1.5%
of XE-60 on Gas-Chrom Q. The column was temperature programmed from 75 to
230
◦ C at 50
◦ per minute. Nitrogen was the carrier gas and detection was by flame
ionisation.
The method based on immunosorbents coupled on-line with liquid
chromatography-atmospheric pressure chemical ionization mass spectrometry
[144], has been applied to the determination of substituted urea type herbicides in soil.
3.6 IMIDAZOLINONE HERBICIDE
The imidazolinones are a relatively new class of herbicides used to control a wide
spectrum of broad-leafed weeds and grasses in a variety of agricultural commodities
[145]. These herbicides are very potent weed killers and are used in doses that are
substantially lower than those of conventional herbicides. The members of this class
of herbicides have similar structural features centred around the imidazolinone ring and
an attached aromatic system bearing a carboxylic acid moiety. Imidazolinones show
excellent activity against annual land perennial grasses and broad-leafed weeds when
applied either pre- or post-emergence. They function by inhibiting acetohydroxy acid
synthesis, the feedback enzyme in the biosynthesis of branched-chain essential acids
[145–147]. This enzyme is not present in animals. The imidazolinone ring of herbicides
is amphoteric and can behave as a weak base or a weak acid. The movement of the
acid imidazolinones in the soil can be strongly influenced by many soil properties, the
most important of which are pH, organic matter and clay content. Binding of the acid
imidazolinones increases as pH decreases. Basic herbicides protonate and are adsorbed
on negatively charged soil colloids. Acidic herbicide anions also become protonated as
pH decreases, reducing the repulsive forces present when the molecule is dissociated,
thus increasing molecular adsorption [148–152]. The typical low application rates
used for imidazolinone herbicides make their chemical analysis difficult.
Reddy and Locke et al isolated [153] the herbicide Imazaquin from soil by carbon
dioxide supercritical fluid chromatography [154]; corn root bioassay and electrospray
mass spectrometry have also been used to determine this herbicide. Heber et al [155]
determined the herbicides Imazethapyr and Imazapyr in a 0.1 M sodium carbonate
Miscellaneous
Katz and Strusz et al [143] have described gas and thin layer chromatographic methods for the determination of traces of Chlorobromuron (3,4 chromo-3-chlorophenyl,
1.1 methoxy, 1-mechylurea) and its metabolites. The samples were extracted in a
Soxhlet apparatus of ethyl acetate for 6h. Each extract was then acidified with anhydrous acetic acid and evaporated to dryness in a steam bath; 15 µl portions were
applied to a 0.1 mm layer of silica gel. The chromatogram was developed twice with
hexane-ethyl acetate (15:2). After drying, the chromatogram was developed with
chloroform-pyridine (10:1), dried again, and sprayed with Mitchell reagent (20 ml
of 2-phenoxyethanol added to the solution of 1.7 g silver nitrate in 5 ml of water, the
mixture is diluted with acetone to 200 ml, then one drop of 30% aqueous hydrogen peroxide is added), and the spots were located under UV radiation. For gas
chromatographic confirmation, standards were treated as above and corresponding
unsprayed sample areas were removed, extracted with 5 ml of ethyl acetate and evaporated to 1 ml; 5 µl aliquots were injected on to a glass column packed with 1.5%
of XE-60 on Gas-Chrom Q. The column was temperature programmed from 75 to
230
◦ C at 50
◦ per minute. Nitrogen was the carrier gas and detection was by flame
ionisation.
The method based on immunosorbents coupled on-line with liquid
chromatography-atmospheric pressure chemical ionization mass spectrometry
[144], has been applied to the determination of substituted urea type herbicides in soil.
3.6 IMIDAZOLINONE HERBICIDE
The imidazolinones are a relatively new class of herbicides used to control a wide
spectrum of broad-leafed weeds and grasses in a variety of agricultural commodities
[145]. These herbicides are very potent weed killers and are used in doses that are
substantially lower than those of conventional herbicides. The members of this class
of herbicides have similar structural features centred around the imidazolinone ring and
an attached aromatic system bearing a carboxylic acid moiety. Imidazolinones show
excellent activity against annual land perennial grasses and broad-leafed weeds when
applied either pre- or post-emergence. They function by inhibiting acetohydroxy acid
synthesis, the feedback enzyme in the biosynthesis of branched-chain essential acids
[145–147]. This enzyme is not present in animals. The imidazolinone ring of herbicides
is amphoteric and can behave as a weak base or a weak acid. The movement of the
acid imidazolinones in the soil can be strongly influenced by many soil properties, the
most important of which are pH, organic matter and clay content. Binding of the acid
imidazolinones increases as pH decreases. Basic herbicides protonate and are adsorbed
on negatively charged soil colloids. Acidic herbicide anions also become protonated as
pH decreases, reducing the repulsive forces present when the molecule is dissociated,
thus increasing molecular adsorption [148–152]. The typical low application rates
used for imidazolinone herbicides make their chemical analysis difficult.
Reddy and Locke et al isolated [153] the herbicide Imazaquin from soil by carbon
dioxide supercritical fluid chromatography [154]; corn root bioassay and electrospray
mass spectrometry have also been used to determine this herbicide. Heber et al [155]
determined the herbicides Imazethapyr and Imazapyr in a 0.1 M sodium carbonate