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9.2.22AOAC 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 withHg hollow

cathode lamp and gas flow-through cell (Figure  971.21), 25 (id)  ×

115 mm with quartz windows cemented in place. Operating condi-

tions: 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 internalparts of pump with acrylic-type plastic

spray. Use 16 gage Teflon tubing for all connections.

(c)  Water condenser .—12–18 (id) ×400 mm borosilicate, 24/40standard taper joint, modified to hold 6 mm Raschig rings. Fill con-

denser 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)   Reducingsolution.—Mix50mLH2SO4 withca 300 mLH2O.

Cool to room temperature and dissolve 15 g NaCl, 15 g

hydroxylamine sulfate,and 25 g SnCl2 in solution. Dilute to 500mL.

(b)   Diluting solution.—To 1 L volumetric flask containing300–500 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 explo-

sive 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 u -

tion.—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 H2

SO4

. Prepare fresh daily.

C. Determination 

Weigh 5.0 g test portion into digestion flask; add 25 mL 9M

H2SO4,20mL7MHNO3, 1 mL 2% sodiummolybdatesolution, and

5–6boilingchips.Connectcondenser (with H2O circulatingthrough

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 swirl-

ing 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 volumetricflask and dilute to volume with

H2O. Transfer 25.0 mL aliquot from each test portion to another diges-

tion 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 Fig-

ure   971.21, except for gas inlet adapter. With pump working and

spectrophotometer zeroed, add 20 mL reducing solution to diluted

aliquot. Immediately connect gasinletadapter andaerateca 3 min.(Ad-

 just aeration time to obtainmaximum 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 1.0µg Hgto seriesof digestion flasks. Toeachflaskadd

100 mL diluting solution. Finally, add reducing solution and aerate

standards as for test portion.

Plot standard curve from least squares linear regression of   Aagainst µg Hg. (See “Definitions of Terms and Explanatory Notes,”

item [25], or use calculator which performs linear regression.) De-

 © 2000 AOAC INTERNATIONAL

Figure 971.21—Apparatus for flameless atomic absorption analysis.

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termine µg Hg in aliquot from curve. If µg Hg determined falls out-

side 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)

 © 2000 AOAC INTERNATIONAL