aoac for mercury
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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