Analysis of cadmium in chocolate by atomic absorption spectrometry

As a known human carcinogen, cadmium is a heavy metal used in a variety of applications such as steel plating, as a pigment in plastics and glasses and in the production of batteries. These industrial activities are the main route through which cadmium is released into the environment where it accumulates in water and soil and subsequently in plants, animals and fish through uptake and ingestion. One of the main routes of human exposure to cadmium is therefore through the intake of contaminated foodstuffs.

The main ingredients in chocolate are cocoa, milk and fats, each of which is a potential source of cadmium. The Scientific Panel on Contaminants in the Food Chain (CONTAM) was asked by the European Commission to assess the risks to human health related to the presence of cadmium in foodstuffs. From 2003 to 2007 CONTAM evaluated approximately 140 000 data on cadmium occurrence in various food commodities across 20 member states.

Chocolate was among the foodstuffs in which the highest cadmium concentrations were detected. Chocolate consumers and manufacturers around the globe are becoming increasingly concerned about the human health hazards caused by cadmium in chocolate. In response, global legislative bodies enforce strict regulations to minimise exposure to cadmium.

The Codex Alimentarius Commission, in its General Standard for Contaminants and Toxins in Foods, specifies the provisional tolerable weekly intake (PTWI) of cadmium at 7µg/kg body weight. The recommendation is to limit cadmium intake as it offers no nutritional benefit. The standard mandates the degree of cadmium contamination of food shall be assessed by regular monitoring.

The European Commission has introduced the EC 1881/2006 regulation, setting maximum levels for certain contaminants in foodstuffs. The regulation endorses the PTWI of 7µg/kg body weight and recommends the presence of cadmium in food must be reduced as much as possible. EC 1881/2006 specifies typical maximum levels of cadmium in foodstuffs between 0.05 - 0.2 mg/kg wet weight.

The determination of cadmium levels in chocolate is an important issue. Atomic absorption (AA) spectrometry provides a simple and uncomplicated solution for the precise and rapid analysis of trace elements in foodstuffs, ensuring optimal product quality in compliance with global legislation.

A straightforward sample preparation procedure combined with a fully optimised analysis method results in accurate detection well below current recommended limits for the concentration of cadmium in foodstuffs. This technology allows optimisation for precise flame and furnace analyses of cadmium in foodstuffs at parts per million and parts per billion levels. In the case of chocolate, it is necessary to analyse cadmium at trace levels. For this type of analysis, graphite furnace AA spectrometry is the most appropriate method. Analysis by graphite furnace AA spectrometry becomes even easier when using Graphite Furnace TeleVision (GFTV), which allows viewing of the sample inside the cuvette.

A Thermo Scientific iCE 3500 AA spectrometer was used to demonstrate AA spectrometry for the analysis of cadmium in chocolate. The system was operated using the wizard-driven Thermo Scientific SOLAAR software. All standards and reagents used for this application were purchased from Fisher Scientific. These included trace metal grade nitric acid (69 per cent), hydrogen peroxide (>30 per cent w/v) and ammonium nitrate. A 1000mg/l cadmium master standard was used to prepare the sub-standards.

Approximately 0.3g pieces of popular global brands of milk and dark variety chocolate were accurately weighed and transferred to microwave digestion vessels. After that, 7ml of nitric acid and 1ml of hydrogen peroxide were added and left to stand for five minutes before the vessels were sealed and samples digested in a high pressure microwave digestion system by ramping to 200°C for 10 minutes. Samples were maintained at this temperature for 20 minutes before being allowed to cool. The contents of the vessels were then quantitatively transferred to 100ml volumetric flasks with deionised water and made up to a final volume of 100ml.

A 1mg/l cadmium sub-standard was prepared in deionised water for spiking of samples prior to digestion. The 1mg/l sub-standard was then used to prepare a 10µg/l sub-standard for calibration. The 10µg/l sub-standard was made up in seven percent nitric acid and one percent hydrogen peroxide to matrix match to the digested samples. Blank and diluent were also prepared at seven per cent nitric acid and one percent hydrogen peroxide. A matrix modifier was prepared at 2g/l to allow deposition of 20µg of ammonium nitrate in a 10µl aliquot.


The furnace temperature parameters are shown in Table 1. The 'optimise furnace parameters' wizard of the SOLAAR software was used to determine the most suitable temperatures for ashing and atomising of the digested chocolate samples. GFTV was used to optimise the position of the injection capillary and to observe the deposition of the sample into the cuvette. The 10µg/l cadmium solution was used as the master standard for the method. The autosampler was programmed to automatically generate calibration standards at 2, 4, 6, 8 and 10µg/l.


All samples, blanks and standards were injected at a constant fixed volume of 10 µL, alongside an additional aliquot of 10µL of matrix modifier into an electrographite cuvette. Cadmium was analysed at 228.8nm and Zeeman background correction was used throughout.

A segmented fit curve was used for generation of the calibration for the analysis of cadmium in chocolate. Spiked samples were prepared to evaluate the recovery of cadmium. This was done by adding 0.5ml aliquots of the 1mg/l cadmium standard to samples of chocolate and then preparing and analysing using the proposed method (0.5ml of 1mg/l cadmium results in addition of 5µg/l to the final sample).

Cadmium was detected in small amounts in all three chocolate samples, with the maximum calculated at 0.04mg/kg (Table 2). However, all samples fell below typical current legislation for the recommended maximum levels of cadmium in foodstuffs.

For an average adult of 70kg weight, about 12kg per week of the dark chocolate analysed in this experiment would need to be consumed to exceed the PTWI. Nevertheless, this would not be advisable for a balanced and healthy diet!

Spiked recoveries were performed on the three analysed samples and were all found to be satisfactory. In addition, the method detection limit and characteristic concentration were calculated using the 'check instrument performance' wizard of the SOLAAR software. The method detection limit was found to be 0.029µg/l and the characteristic concentration 0.060µg/l.

- Hazel Dickson is an Applications Chemist with Thermo Fisher Scientific, Cambridge, UK.

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