Determination of Chromium in Cosmetics—Flame Atomic Absorption Spectrophotometry

1 Scope of Application

This method is applicable for the determination of chromium in cosmetics, with a minimum detection limit of 0.1 mg/kg.

2 Principle

After sample digestion, chromium exists in ionic form within the test solution. When chromium ions in the solution are atomized, their ground-state atoms absorb resonance lines emitted by a chromium hollow cathode lamp. The absorption intensity is directly proportional to the chromium content in the sample. Quantification is achieved by comparing the measured absorption line intensity with a standard series.

3 Reagents

3.1 DDTC Solution: Dissolve 2 g of sodium diethyldithiocarbamate (DDTC, [(C₂H₅)₂NCS₂Na·3H₂O]) in water to make 100 mL.

3.2 10% Ammonium Persulfate Solution.

3.3 1+1 ammonia solution.

3.4 Sodium acetate buffer solution: Mix 59 mL of 1 mol/L acetic acid with 141 mL of 1 mol/L sodium acetate, adjust pH to 5.0.

3.5 Methylisobutylcopper (MIBK).

3.6 Chromium standard stock solution: See Diphenylcarbazide spectrophotometric method (3.1).

3.7 Chromium standard working solution: See Diphenylcarbazide spectrophotometric method (3.2).

4 Instruments

4.1 AA-1800C atomic absorption spectrophotometer and accessories.

4.2 Separating funnel: 125 ml separating funnel.

5.Analytical Procedure

5.1 Sample Preparation

5.1.1 Wet Digestion Method: Same as the diphenylcarbazide spectrophotometric method (5.1.1).

5.1.2 Wet-Dry Digestion Method: Same as the diphenylcarbazide spectrophotometric method (5.1.2).

5.2 Determination

Place the test solution treated according to 5.1.1 or 5.1.2 into a 100 mL beaker. Add 5 mL of 10% ammonium persulfate solution, then adjust the pH to 3.0–4.0 using 1+1 ammonia solution. Cover with a watch glass and heat in a boiling water bath for 15 min. After cooling, transfer to a 125 mL separatory funnel. Rinse the beaker three times with 15 mL of pure water each time; combine the rinsing solutions into the separatory funnel. Separately, prepare chromium standard working solutions (10 μg/ml) at concentrations of 0.0, 0.50, 1.00, 5.00, 10.00, and 20.00 ml.

Add 60 ml of pure water to the separatory funnel. Add 5 ml of sodium acetate buffer to each standard and sample solution, shake well, then add 5 ml of DDTC solution and and 10 mL MIBK. Shake for 3 minutes, then let stand in the dark until layers separate. Discard the aqueous layer. Transfer the MIBK solution to a 10 mL stoppered colorimetric tube. If turbid, centrifuge at 2000 rpm for 2–3 minutes until clear before proceeding with the assay. Proceed as follows using atomic absorption spectrophotometry. Measure absorbance at 357.9 nm. Plot a concentration-absorbance standard curve and calculate the sample content.

6 Calculation

C = (A - B) / m

Where:

C – Chromium concentration in the cosmetic, μg/g;

A – Chromium content in the sample, μg, obtained from the standard curve;

B – Chromium content in the reagent blank, μg, obtained from the standard curve;

M – Sample mass, g.