Part 4 Multiple Pesticide Residue Analytical Methods
6.3 Column chromatography (not required for water samples)
Fill the chromatographic tube with approx. 30 ml hexane, push a glass wool plug through to the lower end, and trickle in 8 g Florisil. Allow air bubbles to escape, and drain the hexane to the top of the Florisil. Dissolve the residue derived from 6.2 in approx. 10 ml hexane, transfer the solution onto the column, and allow to percolate. Rinse the flask several times with 5-ml portions of hexane and transfer each rinsing onto the column before the preceding portion has soaked in. Continue to elute the column with hexane at a rate of 1.5 ml/min until a total of 120 ml eluate has been collected. Rotary-evaporate the eluate to dryness.
6.4 Gas-chromatographic determination
Dissolve the residue derived from 6.2 or 6.3 in acetone and make up to 10 ml (VEnd). Inject an aliquot of this solution (Vj) into the gas chromatograph.
Operating conditions
Gas chromatograph Varian 3700
Column 1 Glass, 2 mm i.d., 1.2 m long; packed with 5% OV-17 on Chromosorb G AW-DMCS, 60-80 mesh
Column temperature 3 min 200 °C, programmed to rise at 15°C/min from 200 to 250 °C, and 17 min 40 s after injection from 250 to 300 °C
Column 2 Glass, 2 mm i.d., 1.2 m long; packed with 5% OV-225 on Chromosorb G AW-DMCS, 60-80 mesh
Column temperature 3 min 150 °C, programmed to rise at 15°C/min from 150 to 180 °C, 8 min after injection from 180 to 200 °C, and 15 min 20 s after injection from 200 to 250 °C
Injection port temperature 300 °C
Detector Flame photometric detector equipped with dual flame system (Varian) and 394-nm sulphur filter (Tracor) Temperature 300 °C
346 Method S 24
Gas flow rates Nitrogen carrier, 30 ml/min Hydrogen, 140 ml/min
Air, flame 1, 80 ml/min; flame 2, 200 ml/min
Attenuation 8- 10 ~9
Recorder 1 mV; chart speed 5 mm/min Injection volume 5 ul
Column 1 Column 2 Retention times for derivatives
from
cyhexatin, azocyclotin 9 min 30 s 7 min fentin 11 min 12 min fenbutatin oxide 22 min 30 s 20 min
7 Evaluation
7.1 Method
Quantitation is performed by measuring the peak areas of the sample solutions and compar- ing them with the peak areas obtained for the corresponding derivative standard solutions.
Equal volumes of the sample solutions and the derivative standard solutions should be in- jected; additionally, the peaks of the solutions should exhibit comparable areas.
7.2 Recoveries and lowest determined concentration
The recoveries from untreated control samples of plant material, soil and water, fortified with cyhexatin, azocyclotin, fenbutatin oxide and fentin hydroxide at levels of 0.01 to 0.05 mg/kg, ranged from 80 to 100%. The routine limit of determination was 0.02 to 0.05 mg/kg for plant material and soil, and 0.01 mg/1 for tap water.
7.3 Calculation of residues
The residue R, expressed in mg/kg, of an identified organotin compound is calculated from the following equation:
R_ FA-VEnd-Wst where
G = sample weight (in g) or volume (in ml)
VEnd = terminal volume of sample solution from 6.4 (in ml)
Vj = portion of volume VEnd injected into gas chromatograph (in \i\) WSt = amount of compound injected with derivative standard solution (in ng) FA = peak area obtained from Vl (in mm2)
Fst = peak area obtained from WSt (in mm2)
Method S 24 347
- 200
- 150
16 8
Chromatograms 1 and 2. Untreated control sample of apples fortified with 0.05 mg/kg each of cyhexatin or azocyclotin (1), fentin hydroxide (2), and fenbutatin oxide (3); 50 mg aliquots injected. Upper chromatogram, GLC column 1; lower chromatogram, GLC column 2.
348 Method S 24
8 Important points
The method does not differentiate between azocyclotin and cyhexatin or between fentin acetate, chloride and hydroxide.
Metabolites of the general formulae R2SnX2 occur only at low levels in foodstuffs. They can also be determined by this method. Unter the conditions given in 6.4, they do not interfere with the determination of the parent compounds.
If the recoveries are less than expected, it is advisable to use more methyl magnesium chloride solution.
9 Reference
E. Mollhoff Methode zur gaschromatographischen Bestimmung des Akarizids Peropal und seiner Metaboliten in Pflanzen, Boden, Wasser und Kleintierfutter, Pflanzenschutz-Nachr. 30, 249-263 (1977).
10 Author
Bayer AG, Agrochemicals Sector, Research and Development, Institute for Product Informa- tion and Residue Analysis, Monheim Agrochemicals Centre, Leverkusen, Bayerwerk, E. Mollhoff
Methyl Carbamate Insecticides s 25
Apples, beans (green), carrots, cherries, head cabbage, Gas-chromatographic kohlrabi, leeks, lettuce, radishes, strawberries, tomatoes determination (German version published 1989)
1 Introduction
The method permits the identification and quantitative determination of 14 methyl carbamate insecticides and four metabolites, including ten aromatic and four aliphatic N-methyl carb- amates as well as one N,N-dimethyl carbamate (see Table 1). The compounds are determined, without derivatization, under conditions by which decomposition of the carbamates is prevented.
Table 1. Relative retention times (RRT) of methyl carbamates, relative to lauronitrile (I; retention time 4 min 25 s) and to octadecanonitrile (II; retention time 13 min 46 s).
Methyl carbamate RRT I RRT II 0.41 0.45 0.48 0.48 0.52 0.57 0.57 0.64 0.65 0.73 0.75 0.76 0.77 0.81 0.82 0.86 0.91 0.99 1.00
2 Outline of method
The methyl carbamate residues are extracted from plant material with ethyl acetate; an aliquot of the extract is evaporated to dryness. The residue is extracted with water and the solution passed through a RP-18 disposable cartridge using an acetonitrile-water mixture as eluant. The
Lauronitrile Aldicarb Butocarboxim Methomyl Propoxur Thiofanox Promecarb Bendiocarb Carbofuran Aminocarb Primicarb Ethiofencarb 3-Keto-carbofuran Desmethyl-pirimicarb Dioxacarb
Carbaryl
Mercaptodimethur (Methiocarb) 3-Hydroxy-carbofuran
Desmethylformamido-pirimicarb Octadecanonitrile
1.00 1.28 1.39 1.51 1.51 1.62 1.77 1.77 1.99 2.04 2.27 2.35 2.37 2.39 2.51 2.56 2.67 2.83 3.08
350 Method S 25
eluate is shaken with dichloromethane, and the dichloromethane phase is evaporated. The methyl carbamates are determined by capillary gas chromatography with splitless injection using a thermionic detector.
3 Apparatus
Homogenizer, e.g. Ultra-Turrax (Janke & Kunkel)
Laboratory centrifuge, e.g. Varifuge type 4120 (Heraeus-Christ), with 150-ml stainless steel tubes
Pear-shaped flasks, 100-ml and 50-ml, with ground joints and glass stoppers Rotary vacuum evaporator, 30-40 °C bath temperature
Glass syringe, 10-ml, with Luer-lock fitting Separatory funnel, 100-ml, with PTFE stopcock
Test tubes, 10-ml, with screw cap and PTFE gasket, e.g. SVL45 (Corning No. 611-52) Gas chromatograph equipped with thermionic nitrogen-specific detector
Microsyringe, 10-ul
Important: All glassware must be rinsed with acetone immediately before use.
4 Reagents
Acetone, chemically pure, dist.
Acetonitrile, reagent grade (Merck No. 800015), dist.
Cyclohexane, chemically pure, distilled from a sodium suspension Dichloromethane, chemically pure, dist.
Ethyl acetate (Riedel-de Haen No. 27227), dist.
Methanol, chemically pure, distilled from potassium hydroxide Water, deionized and dist.
Eluting mixture: acetonitrile + water 1:1 v/v
Internal standard solution: 1 ng/ml lauronitrile (Merck-Schuchardt No. 805345) and 2 octadecanonitrile (Aldrich No. 12,258-0) in cyclohexane
Methyl carbamate standard solutions: 1-2 ng/ml of each compound in cyclohexane (diluted from a stock solution in ethyl acetate)
RP-18 disposable cartridge: Sep-Pak Cartridge C18 (Millipore No. 51910) Sodium hydrogen carbonate, p.a.
Air, synthetic, re-purified Hydrogen, re-purified Nitrogen, re-purified
Method S 25 351
5 Sampling and sample preparation
The analytical sample is taken and prepared as described on pp. 17 ff, Vol. 1.
6 Procedure
6.1 Extraction
Weigh 25 g of the chopped analytical sample (G) into a centrifuge tube and (only in the case of fruit) add 0.5 g sodium hydrogen carbonate. Add 80 ml ethyl acetate and homogenize for 2 min. Rinse the rod of the homogenizer twice with 10-ml portions of ethyl acetate, add the rinsings to the centrifuge tube, and centrifuge the homogenate for 10 min at 5000 r.p.m. Pipet a 40-ml portion (VR1) from the clear supernatant solution (VEx =100 ml) into a 100-ml pear- shaped flask and rotary-evaporate to near dryness. Remove the last traces of solvent with a gentle stream of air or nitrogen.