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9.2.08AOAC Official Method 973.34

Cadmium in Food

Atomic Absorption Spectrophotometric Method

First Action 1973

Final Action 1974

A. Princ ip le

Material is digested with HNO3, H2SO4, and H2O2. All reactive

metals are extracted from solution, after adjustment to ca pH 9, with

dithizoneCHCl3. Cd is removed by stripping CHCl3solution with

dilute HCl and de termined by AA spectrophotometry at 228.8 nm.

B. Reagents and Appara tu s

(Thoroughly wash all new glassware and glassware which has

contained high Cd concentra tion with 8M HNO3, and rinse with

H2O. Cover beakers with watch glasses during all op erations.)

(a) Nitric acid.Low in Pb and Cd (GFS Chemical, Inc.,

No. 63).

(b) Hy droge n pe rox id e .50% (Fisher Scien tific Co.,

No. H-341).

(c) Citric acid.Monohydrate, fine crystal.

(d) Thymol blue indicator.See945.58B(h) (see9.2.07).(e ) D i t h i z o n e s o l u t i o n s . ( 1 ) C o n c e n t r a t e d

solution.1 mg/mL. Prepare 200 mL in CHCl3. (2) Dilute

solution.0.2 mg/mL. Dilute con centrated so lution 1 + 4 with

CHCl3. Prepare fresh daily.

(f) C a d m i u m s t a n d a r d s o l u t i o n s . ( 1 ) S t o c k

solution.1.0 mg/mL. Dissolve 1.000 g Cd (Fisher Scientific

certified 99.9% pure, or equivalent) in 165 mL HCl in 1 L volumetric

f lask. Di lute to volume wi th H 2O. (2 ) In termediate

solution.10 g/mL. Dilute 10 mL stock solution with 2N HCl to

1 L. Pre pare just be fore use. (3) Working solutions.Dilute 0, 1, 5, 10,

and 20 mL intermediate solution to 100 mL with 2N HCl (0, 0.1, 0.5,

1.0, and 2.0g Cd/mL, respectively).

(g ) A t o mi c a bso rp t i on spec t rophot ome t e r .Wi t h

hollow-cathode Cd lamp and 10 cm burner head for airC2H2flame;

wave length 228.8 nm, range 02.0g/mL.

C. Digest ion

Weigh 50.0 g test portion into 1.5 L beaker. Add several boiling

chips or beads, and cover. Carefully add 25 mL HNO3, cover, and

warm gently with flame to initiate reaction. (Meker-type burners are

preferred throughout for their versatil ity and speed.) When reac tion

subsides, add 25 mL HNO3, warm again, and continue until 100 mL

HNO3has been added. (Alternatively, add 100 mL HNO3all at once,

with caution, and let stand at room tempera ture over night.) Heat

until most NO fumes have evolved; con trol ex cessive froth ing by

cooling or quench ing with H2O from wash bottle. Only some

cellulose and fatty materials, if any, remain undissolved.

To remove any fat visible in hot so lution, proceed as fol lows: Coolbeaker in ice, and decant clear, aque ous solution from coagulated oils

and solids through glass wool pad into 1 L beaker. Add 100 mL H2O to

1.5 L beaker with fat, heat, swirl vigorously to rinse fat, chill, and filter

as before. Wash fun nel and glass wool pad with ca 20 mL H2O.

Add 20 mL H2SO4to test solution, dilute to ca 300 mL with H2O,

and evaporate over flame until charring be gins. When char ring

becomes exten sive, cau tiously add 50% H2O2, 1 mL at time. Let

re action subside be fore adding next por tion of oxidant, and never

add >1 mL at a time. Continue ad ditions of 2O2until solution is

colorless. Heat vigorously to SO3 fumes, adding more H2O2 asre quired to re move char. Heat vig orously to expel excess H2O2. Cool

colorless digest to room temperature.

Prepare re agent blank of 100 mL HNO3, 20 mL H2SO4, and same

amounts of H2O as added to test portion. Cau tiously add same

amounts 50% H2O2, as above, and remove all HNO3from blank.

Carry blank through same oper ations as test por tion.

D. Extract ion

Add 2 g citric acid to cooled digest and cau tiously dilute to ca

25 mL with H2O. Add 1 mL thymol blue indicator and adjust to ca

pH 8.8 by slowly adding NH4OH while cool ing in ice bath, un til

solution changes from yellowish green to greenish blue. Transfer

quantitatively to 250 mL separator, using H2O, and di lute to ca

150 mL.Cool solution, and extract with two 5 mL portions concentrated

dithizone solution, shak ing 12 min each time. Continue extraction

with 5 mL portions dilute dithizone solution until last 5 mL portion

dithizone extract shows no change in color. Combine dithizone

extracts in 125 mL separa tor; wash with 50 mL H2O, and trans fer

solvent to another 125 mL sepa rator. Extract H2O wash with 5 mL

CHCl3and add this to dithizone ex tracts. Add 50 mL 0.2M HCl to

combined dithizone ex tracts, shake vigorously 1 min, and let layers

separate; discard dithizone layer. Wash aqueous solution with 5 mL

CHCl3and discard CHCl3. Quantitatively transfer aqueous solution

to 400 mL beaker, add boil ing chips, and evaporate carefully to

dryness. Carefully rinse down sides of beaker with 1020 mL H2O

and again evaporate to dryness.

E. Determ inat ion

Set instrument to previously established optimum conditions, using

airC2H2oxidizing flame and 228.8 nm res onant wave length. Dis solve

dry residue in 5.0 mL 2M HCl and determineAof test and stan dard

solutions against 2M HCl as blank. Flush burner with H2O between

readings. Use scale expansion controls to obtain 410expansion, as

convenient. De termine Cd from curve ofAagainstg Cd/mL:

Concentration (mg Cd/kg) =g Cd

mL

mL 2M HCl

g test portion

For concentration >2.0g Cd/mL, di lute solution with 2M HCl.

Reference: JAOAC56,876(1973).

CAS-7440-43-9 (cadmium)

2005 AOAC INTERNATIONAL

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