AOAC Methods in Codex STAN 234 (Preliminary Methods Review)

J. ASSOC. OFF. ANAL. CHEM . (VOL. 65, NO. 4, 1982)

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GAJAN ET AL..

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suits of the C(illaborative study (15) are pre– med in Part II of this paper, which also appears this issue. All glassware and plasticware are soaked in · (v/v) HNO3 for a minimum of 24 h and n~ed with distilled, deionized water. Because b1,ratory contamination control is very im· rtant, all possible precautions must be taken .ivoid contamination of samples, reagents, and uipment. In order to obtain low quantitation l' units, the electrolyte and ashing aid solutions may require cleanup by either controlled po· tenlial electrolysis (16, 17) or ion exchange h1omatography (18-20). n ' W1 Preparation of Sample 1\ n, A5.0-10.0 g homogeni1.ed sample is weighed , . d- mt(l a 150-250 mL quartz beaker ashing vessel. o !\'ycor or Pyrex may be used.) The sample is n mixed with 5.0 mL K2SO 4 ashing solution. (The s ~,hi1,g solution is prepared by dissolving 50 g K~S0 4 in 400 mL water containing 10 mL HNO3 l d"l · t 500 L · h ) W an, 1uhng o m wit water. ater may ~" ,111ded, if necessary, to ensure good mixing. (~nncd foods may require a special homogeni– z,1tion procedure (21 , 22). The ashing vessel is covered with a quartz or Pyrex watch glass, and rnt' sample is dried in an oven at 110-120°C and then ashed in a furnace at 500-550°( for a min– imum of 4 h. Caution: Do not heat otier S00 °C if J1ii11g l'yrex and avoid excessive overslzooting of tcm- lf"-' m/ure. The sample is removed from the fur- n.1.:c and allowed to cool. Ii the ashing vessel contains a nonwhite ash, the sides of the ashing vessel are washed down 'with water, 2.0 mL HNOJ is added, and a glass r1,J is used to break up any solid particles. (A brownish-red color in the ash, possibly Fe2O3, is ac.:1'ptable and does not require the HNO 3 lnwment.) The sample is dried on a hot plate or under an infrared lamp and then is placed in afurnace at 500°C for 30 min. The sample is allowed to cool and, if necessary, the HNO3 treatment is repeated with 1.0 mL increments of H~0 3 until a white carbon-free ash is ob- tained. The carbon-free ash in the ashing vessel is di,~olved with 1.0 mL HNO 3 and approximately IOmL water. (Low heating on the hot plate may ~~ used to aid dissolution.) The solution is transferred to a 50 mL volumetric flask and di– luh:d to volume with water. Any precipitate is ~llC1wed to settle. At least 3 control reagent blanks, including any additional water or HNO~ used for sampl(' ashing, arc carried through the entire m~•thod. p er

Determination by DPASV A 5.0 mL aliquot of the sample solution is transferred to a polarographic cell, 5.0 mL of electrolyte solution (.1.7M in acetic acid, 1.25M in sodium acetate trihydrate, and 0.0lM in tar– taric acid) is added and the cadmium and lead in this cell S()!ution are determined by DPASV in the usual manner (10). An EG&G Princeton Applied Research Corp. (Princeton, NJ 08540) Model 174A coupled with a Model 315 elec– troanalytical controller and a Model 303 static mercury drop electrode, or equivalent instru· mentation, is used for the DPASV determination. The total amounts of cadmium and lead in thE' cell solution are quantitated by using the method of standard additions (23). Three additions to base level are performed. The amount of each analyte in the cell is calculated from the Jint-ar regression coefficients (24) of the standard ad– dition data. The concentration (ppm) of each analyte in thC' sample is calculated from the following equa· tion: ppm (µg/g) = [(ll - C)/A] X (50/W) where A = mL sample solution taken for analy– sis, B = µg analyte in sample cell solution, C = average µg analyte in reagent blank cell solu– tions, and W = total g sample. Detcrmfoation by LSASV A Model 2014 anodic stripping voltammeter {Environmental Sciences Associates, Inc., Bed– ford, MA 01730), or equivalent, equipped with 8 CMGEs is used for the LSASV determination. A mixture of 2.0 ml. sample solution and 3.0 mL electrolyte solution (l.7M in acetic acid, 1.25M in sodium acetate trihydrate, and O.0IM in tar– taric acid) is analyzed according to manufoctur· l~r's instructions. The peak current is measured and converted to micrograms by using a conversion factor. This factor is obtained for each CMGE by first detcrmining the microamperc response for a reagent blank. A standard addition to the cell solution is made and the microampere response is measured. The conversion factor is the amount (nanograms or micrograms) of the ad– dition divided by the difference between the 2 microampere responses. This factor should be verified periodically. The concentration (ppm) of each analytc iJi th<.• sampfo is calculated from the equation.

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