New technique for the determination of picloram and other herbicides containing carboxylic acid and ester groups

Abstract

THE NEED for simple and sensitive methods for residue analysis of herbicides and other pesticides is apparent. Gas chromatography is often employed, but compounds containing an acid group must be esterified before injection into the chromatograph. Diazomethane or BF3-methanol is usually employed in the esterification process (1-4). The esterification of picloram (4-amino-3,5,6-trichloropicolinic acid) in soil, water, or forage samples has several disadvantages including the handling of toxic reagents, cumbersome extraction procedures and, most importantly, the predominance of interfering peaks, causing a lack of usable sensitivity.This paper describes an improved analytical technique based on the on-column decarboxylation of picloram, and the determination of the decarboxylated product by electroncapture gas chromatography. EXPERIMENTAL Apparatus. A Barber-Coleman gas chromatograph Model 5630 equipped with an electron capture detector and a 6-foot spiral glass ( 5 mm i.d.) column was modified to include a flow-through high temperature pyrolysis unit. The pyrolysis unit was constructed of vycor glass tubing (5 mm i.d.) packed with vycor chips (2-4 mm squares) and was connected ahead of the chromatographic column. The first 6 inches of the column were packed with 2 to 4 mm vycor chips, the next 3.5 feet with 3 Z SE-30 on 60-to 80-mesh Chromosorb W and the last 2 feet with 10% DC-200 on 60-to 80-mesh Gaschrom Q. The injector, pyrolysis, column, and detector temperatures were 290, 385, 165, and 240 "C, respectively. The carrier gas was nitrogen at an inlet pressure of 28 psi and a flow rate of 110 ml per minute. Further details on the construction of the pyrolysis unit have been described (5).Procedure. The appropriate amounts of picloram were dissolved in diethyl ether which had been saturated with water at 25 "C. Two microliters of this solution were injected into the pyrolysis chamber with an ice-cold syringe. The retention time and the concentration of the decarboxylated product were compared with those of a sample prepared in a preparative hot-tube reactor under conditions identical to those employed in gas chromatography. The decarboxylated product was found to be 4-amino-2,3,5-trichloropyridine; it had a melting point (149-149.5 "C), a mixed melting point undepressed, and NMR spectrum identical to those of a sample obtained by an unambiguous synthesis (6).

Gas chromatography

“…Picioram and other herbicides containing carboxylic acid and esf,er groups may be determined by on-column decarboxylation over Vycor chips a t 385 "C and measurement of the decarboxylated product (423). Reaction-QC on Ca(OH)., was used for the separation and identification of carbamate and urea herbicides (874).…”

“…Langer and coworkers (578) recommended a reaction GC column as a source of unstable reagents and demonstrated this use by conversion of trioxanetetrahydrofuran to formaldehyde and dicyclopentadiene to cyclopentadiene. Picloram and other herbicides containing carboxylic acid and ester groups may be determined by on-column decarboxylation over Vycor chips at 385 °C and measurement of the decarboxy lated product (423). Reaction-GC on Ca(OH)2 was used for the separation and identification of carbamate and urea herbicides (874)• The pesticide, V-methylcarbamate at levels as low as 10~9 gram was determined (663) by reaction GC both by on-column thermal degradation to methyl isocyanate and by on-column transesterification with methanol and sodium hydroxide, quantitatively yielding methyl bofuran and its major metabolite, 3-hydroxylcarbofuran, in lettuce were also evaluated by this procedure (962).…”

Pesticide residues

“…Riva and Carisano (331) reported on the direct GLC determination of carbaryl. A 1-m glass column packed with 0.5% SE-30 on 100-200 mesh silanized Gas-Chrom P support at 168 °C was used. The carbaryl was quantitated with a KCl-treated thermionic detector.…”

“…Hall and co workers (168) described a new technique for the determination of picloram and other herbicides, carboxylic acid, and ester groups. The procedure is based on the on-column decarboxylation of picloram and the determination of the decarboxylated product by electron-capture GLC.…”