AOAC Official Method 968
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6554F6F073F4490EA5DDE299410D2543 |
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0.02 |
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2 |
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日期： |
2024-7-30 |
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10.6.04,AOAC Official Method 968.25?,Biphenyl Pesticide Residues,in Citrus Fruits,Thin-Layer Chromatographic–Spectrophotometric Method,First Action 1968,Final Action 1969,Surplus 1994,A. Principle,Biphenyl is extracted from blended peel or pulp by steam–liquid–,liquid extraction. Extract is subjected to TLC and biphenyl zone,is completely scraped from developed plate. Biphenyl is eluted from,adsorbent with alcohol for spectrophotometric determination.,B. Reagents,(a) Silica gel.—GF-254 (Brinkmann Instruments, Inc.,No. 7730).,(b) Biphenyl standard solutions.—(1) Stock solution.—Ca,0.5 mg/mL. Dissolve ca 50 mg accurately weighed biphenyl in,n-heptane and dilute to 100 mL with n-heptane. (2) Limit solution.—,Ca 0.01 mg/mL. Dilute 5 mL stock solution, (b)(1), to,250 mL with n-heptane.,Use stock standard solution, (b)(1), for spectrophotometric,quantitation after TLC step. Limit standard solution (b)(2) aids in locating,biphenyl zone and in estimating small amounts.,C. Apparatus,(a) Applicator.—For depositing thin layer on glass plates.,(b) Glass plates.—20 ′ 20 cm (8 ′ 8 in) or 5 ′ 20 cm ( 2 ′ 8 in); of,uniform thickness.,(c) Plastic board.—22 ′ 113 cm, with retaining edges 1.8 cm,wide along short and long sides.,(d) Developing jars or tanks.—Use equipment, 970.52G(a) (see,10.1.01), for 20 ′ 20 cm (8 ′ 8 in) glass plates and glass cylinders for,small plates. Cylinders can be covered with plastic caps.,(e) Spotting pipet.—100 mL.,(f) Tank liner.—Whatman 3MM paper cut to fit tank.,(g) Moisture test apparatus.—Similar to lighter-than-H2O volatile,oil trap, 962.17A(a), Figure 962.17 (see 43.1.14), with cold finger,condenser (Lurex Scientific, No. JM-8590, or equivalent).,D. Preparation of TLC Plates,Mix 40 g silica gel, B(a), with 80 mL H2O, shaking vigorously,few s, and finally swirling ca 30 s to eliminate air bubbles. Spread,slurry 0.3 mm thick over 5 plates. Let plates air dry in place ca 1 h.,Put plates in drying rack and place in 100°C oven 2 min. Remove,plates and store in desiccator over silica gel, B(a), or CaCl2 until,used. Plates may be stored up to 30 days.,E. Preparation of Test Sample,Sort out and discard rotten units. Completely peel 36 whole fruits,(include all white material under peel in peel portion). Weigh peelings,and peeled fruit, and calculate weight ratio of peelings to peeled,fruit.,(a) Peel.—Grind combined peel in food grinder. Blend 200 g,ground peel with 400 g H2O at high speed 5 min (or in five 1 min increments,if blender becomes very warm), using high-speed blender.,(Larger batches may be blended with large blender as long as,peel–H2O ratio is same.),(b) Peeled fruit.—Cut peeled fruit into small pieces and blend at,high speed 5 min (or in five 1 min increments if blender becomes,very warm).,F. Extraction,Accurately weigh ca 300 g recently blended peel slurry or ca 100 g,recently blended peeled fruit, and transfer to 1 L round-bottom standard,taper 29/42 flask with enough H2O to yield total volume of ca,500 mL; add few boiling chips (6 mesh granular SiC is convenient).,Connect extraction unit of moisture test apparatus to flask and fill,side arm withH2Oto overflowing. Place ca 3mL n-heptane on top of,H2O layer and insert cold finger cooled with very rapid flow of cold,H2O. Gradually heat flask with mantle (controlled by variable transformer),until even boiling is obtained, then intensely enough to,maintain vigorous boiling. Continue extraction 3 h from time mixture,starts boiling. (Wrap exposed portion of flask and connector,arm between flask and extraction unit with Al foil.) Initial carry-over,of froth does not interfere. After 3 h, discontinue heat and drain entire,contents of extractor into 125 mL separator. Discard lower layer,and drain heptane extract through 2.5 cm column of granular anhydrous,Na2SO4 (8–10 mm id column) into 10 mL volumetric flask.,Rinse separator with 1mL n-heptane and add rinse to column. Rinse,cold finger and extraction unit with five 2 mL portions alcohol, collecting,successive rinses in separator. Add 5 mL n-heptane to separator,and shake vigorously few s; add 50–75 mL H2O and shake,moderately few s. Let layers separate (lower layer may remain,slightly cloudy) and discard lower layer. Pass heptane layer through,same Na2SO4 column into volumetric flask. Rinse separator and column,with enough n-heptane to dilute to volume.,G. Thin-Layer Chromatography,Presaturate tank containing liner with n-heptane 31 h before use.,Establish imaginary spotting line 3 cm from bottom edge of plate.,For each intended spot, use tip of 100 mL pipet to scratch mark in adsorbent,layer just size of pipet tip. (Space spots evenly with maximum,of 7 spo……
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