AOAC Official Method 986
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375A4E4C8AED418BA7DAFDCFFF207581 |
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0.02 |
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1 |
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日期： |
2024-7-30 |
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10.6.15,AOAC Official Method 986.20,Ethylene Dibromide in Grains,and Grain Products,Gas Chromatographic Method,First Action 1986,Final Action 1987,A. Principle,Whole grains and intermediate grain-based products are extracted,by soaking in acetone–H2O; ready-to-eat products are extracted by,hexane co-distillation. Portions of extracts are dried and analyzed by,gas chromatography with electron capture detection.,B. Reagents,(a) Solvents.—2,2,4-Trimethylpentane (isooctane), acetone, and,hexane (all pesticide quality). Check for interferences by injecting,5 mL into GC system operated as described under Apparatus.,(b) Calcium chloride.—Analyzed reagent grade, anhydrous,8 mesh.,(c) Sodium sulfate.—Analyzed reagent grade, anhydrous, granular.,(d) Sulfuric acid (concentrated).—Analyzed reagent grade.,(e) Standard solutions.—(1) Stock solution.—Prepare in 50 mL,volumetric flask equipped with Teflon-lined screw cap. Add ca,40 mL isooctane to flask and weigh flask + isooctane to nearest,0.1 mg. Introduce 20 mL pure reference standard (EPA Reference,Standard P480) into isooctane and reweigh to determine weight of,EDB. Dilute to volume with isooctane and calculate concentration in,mg/mL. Store in freezer. (2) Working standard solution.—Prepare in,hexane by serial dilution of stock solution, (e)(1), to final concentration,of ca 4 pg/mL. Store in glass container with Teflon-lined screw,cap. Store in refrigerator or freezer when not in use.,C. Apparatus,(a) Volumetric flasks.—50 mL with Teflon-lined screw caps,(Thomas Scientific Co., No. 0319-037).,(b) Soaking vessels.—250 mL Erlenmeyers with Teflon-lined,screw caps (Thomas Scientific Co., No. 4903-K23) or 250 mL media,bottles with Teflon-lined screw caps.,(c) Teflon liners.—For Erlenmeyers (Thomas Scientific Co.,No. 2390-H82).,(d) Test tubes.—15 mL with Teflon-lined screw caps (Thomas,Scientific Co., No. 9212-K42).,(e) Centrifuge.—For use with test tubes.,(f) Distilling trap.—Barrett, 20 mL graduated, 24/40 (Thomas,Scientific Co., No. 7133-K44).,(g) Condenser.—Friedrich, 24/40 (Fisher Scientific Co.,No. 07-744-5).,(h) Heating mantle.—To fit 500 mL round-bottom flask.,(i) Variable electric transformer.—0–120 V (Powerstat,Model 1168, or equivalent.),( j ) Magnetic s t i r r e r.—Thermolyne Model (7200,Barnstead/Thermolyne Corp, 2555 Kerper Blvd, Dubuque, IA,52001, USA) or equivalent, with magnetic stirring bar, 1 ′ 5,16 in.,(25 ′ 0.79 cm),(k) Gas chromatograph.—Equipped with 1.8 m ′ 4 mm glass,column packed with 10% SP-1000 on 80–100 mesh Supelcoport,(Supelco, Inc., No. 1-1872) and constant current 63Ni electron capture,detector (Hewlett Packard 5730 or equivalent) operated under,following conditions: temperatures—injector 200°C, oven 115°C, detector,350°C,CH4–Ar (5+95) carrier gas 40 mL/min. Adjust attenuator,to give 1,2 FSD for 20 pg EDB. Retention time of EDB is ca 4 min. Condition,new GC column by holding at 60°C for 2 h with 40 mL/min carrier,gas flow. Slowly increase temperature to 200°C and hold overnight.,Cool to 115°C, let equilibrate, and checkEDBresponse. If proper sensitivity,is not achieved, longer conditioning at 200°C may be required.,D. Extraction and Cleanup,Store all laboratory samples in freezer until just before analysis.,(a) Whole grain and intermediate processed grain products.—,Weigh 50 g into appropriate soaking vessel. Add 150 mL acetone–,H2O (5 + 1) and seal with Teflon-lined screw cap. Swirl; let,whole grains soak 48 h at 20–25°C, with intermittent swirling. Follow,same procedure for intermediate products, except soak 16 h.,Using disposable pipet, transfer ca 10 mL supernate into 15 mL test,tube, add 1–2 g CaCl2, B(b), secure Teflon-lined screw cap, and shake,2 min. If all CaCl2,B(b), dissolves, add more and shake again. Let stand,30 min or centrifuge. Proceed with Determination.,(b) Ready-to-eat products.—Weigh 20 g into 500 mL,round-bottom flask. Add 150 mL H2O and stirring bar. While cooling,flask under stream of cold tap H2O or swirling in ice–H2O bath,slowly add 25 mL concentratedH2SO4. Stopper flask to prevent loss,of EDB. Pipet 10.0 mL hexane into flask immediately before,co-distillation. Connect round-bottom flask to Barrett trap and,Friedrich condenser. Place flask in heating mantle on top of stirrer,and turn on stirrer. Co-distil hexane and EDB into Barrett trap, using,heating mantle with transformer set at 75% full power. Continue distillation,until 1–2 mL H2O is collected. Remove flask from heating,mantle to prevent further H2O distillation. Drain lower H2O layer,and discard. Drain hexane into 15 mL test tube, add 2–3 g Na2SO4,B(c), secure Teflon-lined screw cap, and shake vigorously 1 min. Let,stand 3……
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