AOAC Official Method 994
|
ID： |
B217830E0F414D04BB00F84866F424AE |
|
文件大小(MB)： |
0.03 |
|
页数： |
4 |
|
文件格式： |
|
|
日期： |
2024-7-30 |
|
购买： |
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）：
41.1.35A,AOAC Official Method 994.15,Total cis- and trans-Octadecenoic Isomers,and General Fatty Acid Composition,in Hydrogenated Vegetable Oils and Animal Fats,Capillary Gas Chromatographic–Infrared,Spectrophotometric Method,First Action 1994,Final Action 1998,(Applicable to partially hydrogenated vegetable oils and terrestrial,animal fats containing >5% trans fatty acids. Method is not applicable,to hydrogenated marine oils and partially hydrogenated fish oils,which contain large levels of cis- and trans-isomers of C16, C18, C20,and C22 chain lengths.),See Table 994.15 for the results of the interlaboratory study supporting,acceptance of the method.,A. Principle,Total trans isomer content consists of trans fatty acids,[trans-octadecenoate (18:1t); mono-trans-octadecadienoate (18:2ct,or tc, described as 18:2t); trans,trans-octadecadienoate (18:2tt); and,mono-trans-octadecatrienoate (18:3 cct, ctc, and tcc, described as,18:3t)], which occur in hydrogenated vegetable oils and terrestrial,animal fats. Total trans content is determined by infrared,spectrophotometry (IR) using methyl elaidate as external standard.,Various isomers of 18:2tt, 18:2t, and 18:3t are resolved; their weight,percentages are determined by gas chromatography. Based on IR determination,weight percentage of 18:1t is calculated from equation.,Difference between total methyl octadecenoate (18:1, as sum of all,18:1 peaks in GC) and calculated 18:1t gives weight percentage of,cis-octadecenoate (18:1c).,B. Apparatus,(a) Gas chromatograph (GC).—With flame ionization detector,capillary column injection system (split ratio 1:100). Operating conditions:,injection port 225°C; detector 250°C. Temperature program:,initial 150°C, program rate 1.0°C/min, final 200°C, final hold,20 min. (Note: Operator may change operating conditions to obtain,optimum separation of isomeric fatty acid methyl esters.) Helium or,hydrogen carrier gas (399.99% purity) with oxygen scrubber in line.,(b) GCcolumn.—100m′ 0.25mmfused silica capillary column,coated with SP-2560 (available from Supelco, Inc., Bellefonte, PA,16823, USA) or other suitable capillary column coated with,cyano–alkylpolysiloxane (e.g., SP-2340, CP SIL-88) that provides,same elution pattern as in Figure 994.15.,(c) GCsyringe.—Maximumvolume 10 mL, graduated to 0.1 mL.,(d) Infrared spectrophotometer (IR).—Double-beam IR or Fourier,Transform IR (FTIR); capable of quantitative measurements at,1050–900 cm–1 with scale readable to 1 cm–1; holding fixed thickness,cells 0.1–1.0 mm with NaCl or KBr windows. All instruments,must be checked for wavelength accuracy and photometric scale accuracy,according to manufacturer’s instructions. Chart paper must,be linear in either wavelength or wave number and calibrated in either,transmission, T, or absorbance, A.,C. Reagents,GC reference standards.—Mixture of cis- and trans-isomers of,known composition; available from Nu Chek Prep, Inc., Elysian,MN 56028, or Supelco, Inc., Bellefonte, PA 16823, USA.,D. Preparation of Methyl Esters,Melt solid fats or free fatty acids at temperature ￡10°C above,melting point and mix. If cloudy, filter through filter paper. If diluted,sample is cloudy due to H2O, add small portion of anhydrous,Na2SO4 to melted sample, mix, and let settle before taking portion,for methylation. Using ca 400–500 mg fat, prepare methyl esters as,in 969.33 (see 41.1.28).,E. GC Analysis of Fatty Acid Composition,(a) GC performance specifications.—Inject 1–2 mL methyl esters,from GC reference standards, C, into GC. Select GC conditions,to obtain resolution of methyl esters at least equivalent to that in Figure,994.15.,(b) GC determination.—Inject 1–2 mL methyl esters (in hexane,or heptane solution) from test sample into GC. Compare retention,times of test sample with those of GC reference standards (see Figure,994.15).,F. IR Determination of Total trans Content,Perform as in 994.14 (see 41.1.36A).,G. Calculations,Calculate weight percentages of fatty acid methyl esters, WX, assuming,unity response factor for each component:,WX, % = PX/PT ′ 100,where PX = GC area counts of specific methyl ester peak; PT = total,area counts of all fatty acid methyl ester peaks in chromatogram.,Calculate weight percentage of 18:1t isomers, W18:1t:,W18:1t, % = Wtrans – (1.74 ′ W18:2tt) – 0.84 (W18:2t + W18:3t),where Wtrans = total trans content determined by IR; W18:2tt = total,weight percentage of all 18:2tt isomer peaks in GC; W18:2t = total,weight percentage of all 18:2t isomer peaks in GC; W18:3t = total,weight percentage of all 18:3t isomer peaks in GC; 1.74 and 0.84 =,correction factors for trans, trans fatty acids and mono-trans fatty,acids, respe……
……