9.2.22 AOAC Official Method 971.21 Mercury in Food Flameless Atomic Absorption Spectrophotometric Method First Action 1971 Final Action 1976 PDF

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9.2.22 (c) Magnesium perchlorate.—Drying agent placed in filter flask AOAC Official Method 971.21 (Figure 971.21). Replace as needed. [Caution: Mg(ClO4)2 is explo- Mercury in Food sive when in contact with organic substances.] Flameless Atomic Absorption Spectrophotometric Method ( d ) M e r c u r y...

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9.2.22 AOAC Official Method 971.21 Mercury in Food Flameless Atomic Absorption Spectrophotometric Method First A... Martua Saragih

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9.2.22 AOAC Official Method 971.21 Mercury in Food Flameless Atomic Absorption Spectrophotometric Method First Action 1971 Final Action 1976

(Rinse all glassware before use with HNO3 [1 + 9].) A. Apparatus

(a) Atomic absorption spectrophotometer.—(Instrumentation Laboratory, Inc., 113 Hartwell Ave, Lexington, MA 02173, USA, Model 153 [or successors], or equivalent.) Equipped with Hg hollow cathode lamp and gas flow-through cell (Figure 971.21), 25 (id) × 115 mm with quartz windows cemented in place. Operating conditions: wavelength 253.7 nm, slit width 160 µm, lamp current 3 ma, and sensitivity scale 2.5. (b) Diaphragm pump.—(Neptune Dyna-Pump, or equivalent.) Coat diaphragm and internal parts of pump with acrylic-type plastic spray. Use 16 gage Teflon tubing for all connections. (c) Water condenser.—12–18 (id) × 400 mm borosilicate, 24/40 standard taper joint, modified to hold 6 mm Raschig rings. Fill condenser with Raschig rings to height of 100 mm; then place 20 mm layer of 4 mm diameter glass beads on top of rings. (d) Gas inlet adapter.—24/40 standard taper, e.g., Kontes Glass Co. No. 181000. (e) Digestion flask.—250 mL boiling flask with 24/40 standard taper joint. B. Reagents

(a) Reducing solution.—Mix 50 mL H2SO4 with ca 300 mL H2O. Cool to room temperature and dissolve 15 g NaCl, 15 g hydroxylamine sulfate, and 25 g SnCl2 in solution. Dilute to 500 mL. (b) Diluting solution.—To 1 L volumetric flask containing 300–500 mL H2O, add 58 mL HNO3 and 67 mL H2SO4. Dilute to volume with H2O.

(c) Magnesium perchlorate.—Drying agent placed in filter flask (Figure 971.21). Replace as needed. [Caution: Mg(ClO4)2 is explosive when in contact with organic substances.] ( d ) M e r c u r y s t a n d a r d s o l u t i o n s . — ( 1 ) S t o c k s o l ution.—1000 µg/mL. Dissolve 0.1354 g HgCl2 in 100.0 mL H2O. (2) Working solution.—1 µg/mL. Dilute 1 mL stock solution to 1 L with 0.5M H2SO4. Prepare fresh daily. C. Determination

Weigh 5.0 g test portion into digestion flask; add 25 mL 9M H2SO4, 20 mL 7M HNO3, 1 mL 2% sodium molybdate solution, and 5–6 boiling chips. Connect condenser (with H2O circulating through it) and apply gentle heat ca 1 h. Remove heat and let stand 15 min. Add 20 mL HNO3–HClO4 (1 +1) through condenser. Turn off H2O circulating through condenser and boil vigorously until white fumes appear in flask. Continue heating 10 min. Cool. Cautiously add 10 mL H2O through condenser while swirling liquid in flask. Again boil solution 10 min. Remove heat and wash condenser with three 15 mL portions H2O. Cool solution to room temperature. Completely transfer digested sample with H2O to 100 mL volumetric flask and dilute to volume with H2O. Transfer 25.0 mL aliquot from each test portion to another digestion flask. Adjust volume to ca 100 mL with diluting solution, B(b). Adjust output of pump to ca 2 L air/min by regulating speed of pump with variable transformer. Connect apparatus as in Figure 971.21, except for gas inlet adapter. With pump working and spectrophotometer zeroed, add 20 mL reducing solution to diluted aliquot. Immediately connect gas inlet adapter and aerate ca 3 min. (Adjust aeration time to obtain maximum A.) Record A, disconnect pressure on “out” side of pump, and open vent on filter flask to flush system. Prepare reagent blank and standard curve by adding 0, 0.2, 0.4, 0.6, 0.8, and 1.0 µg Hg to series of digestion flasks. To each flask add 100 mL diluting solution. Finally, add reducing solution and aerate standards as for test portion. Plot standard curve from least squares linear regression of A against µg Hg. (See “Definitions of Terms and Explanatory Notes,” item [25], or use calculator which performs linear regression.) De-

Figure 971.21—Apparatus for flameless atomic absorption analysis.

© 2000 AOAC INTERNATIONAL

termine µg Hg in aliquot from curve. If µg Hg determined falls outside range of calibration, repeat determination with smaller aliquot of test solution to bring µg Hg into region of standard curve. From size of aliquot used, determine total µg Hg in original test portion. Concentration Hg (µg/kg) =

µg Hg g test portion

Reference: JAOAC 54, 202(1971). CAS-7439-97-6 (mercury)

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