AOAC Official Method 992
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4A8A64523E8B4667B5FD46D73F09CF57 |
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0.02 |
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2 |
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日期： |
2024-7-30 |
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37.1.64,AOAC Official Method 992.09,Sugar-Beet-Derived Syrups,in Frozen Concentrated Orange Juice,d18OMeasurements in Water,Stable Isotope Ratio Mass Spectrometric Method,First Action 1992,Final Action 1997,(Applicable to classification of frozen concentrated orange juice,with d18O value <+8.9‰ as diluted with groundwater-prepared,product.),Results of the interlaboratory study supporting acceptance of the,method:,100% and 90% orange juice samples, mean d18O value = 12.24‰:,sr = 0.88; sR = 1.81; RSDr = 7.2%; RSDR = 14.8%,75% and 60% orange juice samples, mean d18O value = 6.73‰:,sr = 0.30; sR = 1.76; RSDr = 4.5%; RSDR = 26.1%,A. Principle,Test sample is equilibrated with CO2 to achieve isotopic equilibrium,between oxygen in test sample water and oxygen in CO2.,After equilibration, CO2 is removed and purified, and 18O/16O is,measured by isotope ratio mass spectrometer. Differences in d18O,values for pure orange juice concentrate (63–67° Brix; mean,d18OSMOW = +14.28‰;SMOWis standard mean ocean water) and,ground water used in preparation of sugar beet syrups (usual,range of d18OSMOW = -5 to -10‰) provide a measure of beet syrup,in orange juice concentrate.,B. Apparatus,(a) Equilibration system.—60 mL plastic syringes, 2.5 cm,22 gauge needles, 1.8 cm diameter steel balls, 40 cm diameter wheel,to which 12 syringes can be attached and rotated at 10–12 rpm by,electric motor.,(b) Purification system.—Vacuum-tight glass manifold including,liquidN2 traps, sample collection bottle, and high vacuum source,(Figure 992.09).,(c) Ratio mass spectrometer.—Instrument designed or modified,for isotope ratio measurement and capable of accuracy of 0.01% of,abundance at mass 46.,C. Reagents,(a) Pressurized carbon dioxide.—100%.,(b) Reference water standard.—Standard mean ocean water,(SMOW).,(c) Carbonate standards.—(1) NIST RM 8544 NBS 19 (limestone),(2) NIST RM 8543 NBS 18 (carbonite), or equivalent.,(d) Phosphoric acid.—100%.,D. Preparation of Test Sample,Place steel ball in barrel of syringe, pour 30 mL test sample into,barrel. Insert plunger and expel as much air as possible from syringe.,Attach 2.5 cm 22 gauge needle. Use rubber septum attached,to pressurized reservoir of pure CO2 to add 20 cc of CO2. Remove,syringe from septum and seal needle by piercing a No. 000 rubber,stopper. Attach syringe to wheel and rotate in vertical plance at,10–12 rpm for 32 h. Maintain air (test sample) temperature at,25 ± 0.5°C. Insert needle into septum on preparation line (see Figure,992.09). Transfer CO2 from syringe into preparation line;,then move CO2 into evacuated portion of preparation line up to,first stopcock and cool finger with liquid nitrogen. Pump away,any air through multitrap 1, also cooled in liquid nitrogen. Water,vapor is retained in multitraps 1 and 2, which are cooled with dry,ice–acetone slurry. For each test sample set, reference water standard,(SMOW) should be analyzed 2 times. Analyze a standard before,and after test samples.,E. Determination,Calibrate reference gas of mass spectrometer using at least 2 carbonate,standards. React carbonate standards with 100% H3PO4 at,25 ± 0.1°C. Make any necessary corrections due to instrumental error,such as zero enrichment, peak tailing, or gas mixing. Correct for,17O contribution to mass 45.,F. Calculations,Calculate d13C and d18O from:,dHE(‰) = [(HE/LETest Sample)/(HE/LEStandard) – 1] ′ 1000,where HE and LE are heavy and light isotopic species of element E,respectively. A frozen orange juice concentrate with d18O value of,+15‰would have an 18O/16Oratio15‰greater than 18O/16Oratio in,SMOW, and be “heavy” in 18O relative to the standard.,Convert analytical data obtained relative to reference gas to PDB,scale using the equation:,d(x - PDB) = d(x - B) + d(B - PDB)+ [(d(x - B))(d(B - PDB))10–3],where (x - B) and (x - PDB) refer to analysis of test sample (x) relative,to standard (B) and to PDB, and (B - PDB) is analysis of standard,(B) relative to PDB, all d values expressed in ‰ (standard B,defined as carbon dioxide source used, derived from standard limestone,graphite, or crude oil).,Convert d18OPDB values to d18OSMOW by adding -0.26‰.,. 2000 AOAC INTERNATIONAL,Figure 992.09—Schematic diagram of vacuum line for,collection and purification of CO2: (A) sample bottle for,collection of CO2; (B) multitrap 2; (C) multitrap 1; (D),cold finger; (E) sample inlet, illustrated in detail; (F),3-way stopcock; (G) capillary glass tubing; (H) silicone,rubber septum; (I) equilibrated CO2 in gaseous phase;,(J) water sample.,Correct sample values by average deviation from expected value,found for water standards by adding or subtracting appropriate ‰,value. A d18O value……
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