Simple and Sensitive Method for Chromium(VI) Determination in Children’s Toys

Everyday items including toys contain dangerous substances, which can pose a threat to children. This required national and EU-wide standards to impose tighter restrictions for hazardous substances in toys.

EU directive 2009/48/EC, also called the Toy Safety Directive, was established on July 20, 2013. It has stipulated stringent restrictions on the migration limits for certain toxic substances in toys, such as the carcinogenic chromium(VI).

The term migration limit applies to the highest quantity of a dangerous substance that a product is allowed to discharge. In the latest directive, this type of value has now substituted the limit values stipulated in terms of bioavailability that were characterized in the earlier version. The new directive compares a distinction between three kinds of materials utilized in toys:

• Sticky or liquid materials
• Pliable or dry, brittle, powder-like materials
• Scraped-off materials

All the above groups are subject to varied migration limits based on the accessibility of dangerous substances through swallowing, licking, sucking, or extended contact with skin. Table 1 shows the limit values applicable to hexavalent chromium as per the new EU directive.

Table 1. The three types of toy materials and their migration limits for chromium(VI) according to EU directive 2009/48/EC, plus some examples

Free Choice of Test Method

The test approaches related to the Toy Safety Directive, including preparation of samples and evaluation of results, are outlined by the European standard EN 71 Part 3. In sample preparation, a migration solution has to be produced by discharging toxic substances from the toy material under practical conditions - emulating exactly what occurs when the material is put in the mouth, swallowed, or comes into contact with skin. As long as the analysis method is validated, users are free to select their own preferred method.

Ion chromatography, combined with UV/VIS detection after column derivatization, has been shown to be suited for measuring chromium(VI) in scraped-off toy materials, pliable or brittle, dry, powder-like materials, according to the migration limits stipulated in Table 1.

It is possible to automate this technique to a large extent. This, along with the pre-concentration of chromium(VI) and elimination of matrix, guarantees highly accurate and reliable analysis. To better describe this technique, the resulting data highlights the process of investigating samples of colored pencils, inks, varnishes, watercolors, finger paint, and opaque white paint.

Figure 1. 940 Professional IC Vario with 944 Professional UV/VIS Detector Vario, 943 Professional Reactor Vario and the 858 Professional Sample Processor.

Sample Digestion Using Synthetic Gastric Juice

Hydrochloric acid is used to extract chromium(VI) from the toy material at body temperature. This form of sample digestion mimics the way gastric juice dissolves the toxic substance from the swallowed toy material.

Subsequently, the sample acquired through this technique is physically neutralized and diluted. Here, dilution has to be performed, because the sample contains a high ion concentration, which does not allow for pre-concentration of chromium(VI). The high ion concentration was caused by extraction of hydrochloric acid and successive neutralization.

Automation Improves Convenience and Safety

After automating all of the remaining steps, a sample was used to equilibrate the entire sample flow path (Figure 2). A defined sample volume is fed onto the pre-concentration column with a Dosino. This Dosino enables precise control of the injection volume, which ultimately forms the basis of consistent measurement, particularly when working with low analyte concentrations.

Figure 2. Diagram of the IC system for the determination of chromium(VI) in toy samples.

After physical extraction, dilution, and neutralization of the samples, the automated analysis process occurs as per the measurement setup shown in Figure 2. The use of Dosinos for dosing both the sample and post-column reagent instead of pumps provides several advantages, because it enables accurate control of the sample volume, dosing of post-column reagent, which is synchronized with the column flow, and allows automatic switching between the rinsing solution and the post-column reagent.

Matrix Elimination and Preconcentration for Reliable Trace Analysis

An anion exchanger is included in the pre-concentration column that binds chromium(VI). Both the organic matrix and the sample contain cations, and these are removed by washing the matrix and the sample with 50% acetone. Anions that are weakly binding to the exchanger, such as chloride, are also removed.

Matrix elimination helps to prevent intrusive peaks that impact the assessment of the chromium(VI) peak, and prolongs the working life of the analytical column. The rinsing solution contains acetone, which removes organic sample components and ions as well as pigments. Pigments can impede with VIS detection and they should be removed completely.

During the course of the procedure, the hexavalent chromium remains in the pre-concentration column. Due to the extended retention time of the hexavalent chromium, about 10 mL of rinsing solution can be utilized without eluting chromium(VI). This means that most of the matrix can be washed out.

Separation and Detection

The eluent releases chromium(VI) from the pre-concentration column and subsequently continues through the separation column and finally reaches the post-column reactor, where it reacts with the post-column reagent 1,5-diphenylcarbazide, as shown in Figure 3.

Figure 3. When combined with hexavalent chromium, 1,5-diphenylcarbazide forms complexes that can be detected using the UV/VIS detector thanks to their absorption in the VIS range.

1,5-diphenylcarbazide forms optically active complexes when mixed with chromium(VI). Thanks to their absorption in the VIS range, these complexes can be measured quantitatively with the UV/VIS detector. Subsequently, MagIC Net software is utilized to regulate the system and assess the quantified data. Figure 4 shows two determinations of a standard solution comprising 0.04 µg/L chromium(VI) in a matrix.

Figure 4. Two determinations of a standard solution containing 0.04 µg/L chromium(VI) in a matrix corresponding to that of a neutralized migration solution – this contains HCl, Na₂CO₃, NaHCO₃, and (NH₄)₂SO₄ (red and blue). The reference curve (black) shows the chromatogram of ultrapure water.

Conclusion

This article has described a simple and sensitive approach to measure chromium(VI) in toy migration solutions, prepared in line with the DIN EN 71 standard. VIS detection is done after post-column reaction with diphenylcarbazide. Hexavalent chromium in the single digit ppt range is precisely measured with this method, thus meeting the 10 ppt limit stipulated by the EU directive 2009/48/EC.

References

[1] EU Directive 2009/48/EC

[2] Metrohm Appplication Note AN-U-068

[3] DIN EN 71-3:2013-07: Safety of toys - Part 3: Migration of certain elements.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.