AOAC Official Method 993
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15764E6485DA4DB7BA0F8B30B0A3F471 |
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0.02 |
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1 |
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日期： |
2024-7-30 |
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文本摘录（文本识别可能有误，但文件阅览显示及打印正常，pdf文件可进行文字搜索定位）：
2.4.02,AOAC Official Method 993.13,Nitrogen (Total) in Fertilizers,Combustion Method,First Action 1993,Final Action 1996,(Applicable to determination of 1–67% total nitrogen content in liquid,and solid fertilizer materials.),Results of Interlaboratory Study:,Liquid and solid fertilizers, 1.04–33.99% N:,sr = 0.11–0.26; sR = 0.14–0.39; RSDr = 0.77–10.99%;,RSDR = 1.15–13.80%,A. Principle,Nitrogen is released from fertilizer material through combustion,at high temperature under high-purity oxygen. Nitrogen is quantitatively,measured by thermal conductivity detection and reported as,weight/weight percent nitrogen in sample.,B. Apparatus,Combustion instrument.—Capable of measuring nitrogen by,combustion utilizing high-purity oxygen. Nitrogenous compounds,liberated must be fully converted to N2 gas. Other combustion byproducts,must be isolated either through gas chromatography or chemical,scrubbing. Containing combustion chamber capable of maintaining,minimum temperature of 950°C for liberation of nitrogen from sample,when high-purity (99.99%) oxygen is introduced into chamber, either,as aliquot or as carrier gas. Containing thermal conductivity detector capable,of detecting N2 gas liberated from combusted test sample. Containing,microprocessor capable of calibrating instrumentation with,standard reference material; subtracting nitrogen impurities in instrument,system and test sample encapsulation process; tracking analyses time; retention,of test sample, standard, and blank data; converting detector response,into w/w percent N2 in test sample; and controlling some, if not,all, instrument operating parameters.,C. Standard Reference Materials,A certified reference material of percent nitrogen content giving,detector response in same analytical range as fertilizer laboratory,sample to be analyzed is necessary for accurate analytical results.,Uric acid should be obtained from National Institute of Standards,and Technology (NIST) for comparison and measurement of reference,materials obtained from other sources. Standard operating procedure,should include weekly crosschecks of these standard,materials. Use NIST Standard Reference Material (SRM) to calibrate,instrument and to analyze for nitrogen content of other reference,materials using instrument parameters as in E. Other primary,standard materials for instrument calibration may be used if nitrogen,content is verified by NIST SRM. Except for uric acid (which liberates,HCN when heated), dry reference materials 2 h at 105°C before,use. Store reference materials in desiccator. Suggested reference,materials:,(a) Uric acid (33.23% N).—Clinical grade, 99.7% purity (SRM,No. 913, NIST, Gaithersburg, MD).,(b) Ammonium sulfate (21.20% N).—99.999% purity (Cat. No.,20450-1, Aldrich Chemical Co., Milwaukee, WI, is suitable).,(c) Ammonium nitrate (35.00% N).—99.999% purity (Cat. No.,25606-4, Aldrich Chemical Co., is suitable).,(d) Ammonium chloride (26.18% N).—99.99% purity (Cat. No.,32637-2, Aldrich Chemical Co., is suitable).,D. Performance Requirements,System must be capable of meeting or exceeding the following,minimum performance specifications:,(a) Analytical system must be able to measure nitrogen in fertilizer,materials containing 1–67% N.,(b) Accuracy of system must be demonstrated by performing,10 successive determinations of ammonium sulfate standard and,10 successive determinations of ammonium nitrate standard. Mean,of 10 determinations for reference materials must be within,±0.20 units of respective theoretical values. Standard deviations,must be ￡0.20% N for ammonium nitrate and ￡0.10% N for ammonium,sulfate standards.,(c) Grind fertilizer materials to suitable fineness to give relative,standard deviation (RSD) ￡1.0% for 10 successive nitrogen determinations.,Moisture content of solid fertilizer test sample must be same,before and after grinding to ensure correct analytical result.,E. Instrument Setup,Follow manufacturer’s recommendations for safe operation of instrument;,refer to Material Safety Data Sheet (MSDS) for safe handling,of chemicals. Ventilate exhaust gases to appropriate fume,removal system. For solid (but not liquid) reference materials or fertilizer,materials, add at least 4′ test portion/weight of powdered (use,mortar and pestle) sucrose to test sample container if instrument utilizes,only oxygen as carrier gas in combustion chamber.,Adjust combustion furnace temperature to 950°C or higher according,to manufacturer’s recommendations. Calibrate instrument according,to manufacturer’s instructions using NIST uric acid. Adjust,oxygen profile and other instrument parameters to achieve maximum,combustion of ammonium nitrate……
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