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Chocolate’s Dark Side

Tue, 10/05/2010 - 7:11am
Hazel Dickson, Applications Chemist, Thermo Fisher Scientific, Cambridge, UK

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Studies show that excessive cadmium consumption can cause various health issues in humans, including nausea, gastrointestinal pain and softening of bones. In addition, one of the main risks of cadmium consumption is the accumulation of heavy metal within the kidneys which can lead to kidney damage, eventually causing renal failure. The International Agency for Research on Cancer has classified cadmium as a human carcinogen and has generated data that links cadmium to an increased risk of lung, bladder and breast cancer.

The following provides an in-depth account of the reagents, standards, sample preparation and furnace parameters required to analyze trace levels of cadmium in chocolate using graphite furnace atomic absorption (AA) spectrometry.

Chocolate trouble
Cocoa, milk and fats are the main ingredients found in chocolate and each of these is a potential source of cadmium. In 2003, the European Commission requested that the Scientific Panel on Contaminants in the Food Chain (CONTAM) assess the negative impact cadmium in foodstuffs can have on human health. After evaluating approximately 140,000 data on the existence of cadmium in various foodstuffs across 20 Member States, CONTAM found some of the highest cadmium concentrations to be in chocolate. The negative impact of cadmium on human health is becoming an increasing concern, and stringent regulations are being enforced by regulatory authorities worldwide to minimize human exposure to cadmium and safeguard human health.

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Table 1. Furnace parameters for the analysis of cadmium in chocolate.
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Currently the provisional tolerable weekly intake (PTWI) of cadmium is set at 7 µg/kg body weight. This is documented in the Codex Alimentarius Commission’s General Standard for Contaminants and Toxins in Foods. This translates to 70 µg/day for a person who weighs 70 kg. However, the standard states that cadmium intake should be limited as much as possible, as it offers no nutritional benefit. The standard also demands that regular monitoring takes place to assess the level of cadmium found in foodstuffs.
The Food and Agriculture Organization/World Health Organization (FAO/WHO) Joint Expert Committee on Food Additives has also defined the PTWI for chemicals such as cadmium at 7 µg/kg body weight by assessing the amount of the chemical that can be ingested weekly over a lifetime without noticeable health risk. The U.S. Food and Drug Administration (FDA) states that specified food additives may contain cadmium at maximum levels that range from 0.05 to 0.13 ppm.

California regulates cadmium in foodstuffs through the Safe Drinking Water and Toxic Enforcement Act enforced in 1986, more popularly referred to as Proposition 65. Typical maximum levels of cadmium in foodstuffs, including chocolate, are currently between 0.05 and 0.2 mg/kg wet weight.

In the U.S., cadmium levels in the food supply range from 2 to 40 ppb and the daily adult intake of cadmium is estimated to be approximately 30 µg. The determination of cadmium levels in chocolate is therefore an important issue. It is essential that chocolate samples are analyzed accurately to ensure consumer safety. Atomic absorption (AA) spectrometry is a simple solution for the accurate and rapid analysis of trace elements, such as cadmium, in foodstuffs, ensuring compliance with global legislation. A simple sample preparation procedure combined with a fully optimized analysis method results in accurate detection well below current recommended limits for the concentration of cadmium in foodstuffs.

This powerful technology allows quick and easy optimization for precise flame and furnace analyses of cadmium in foodstuffs at parts per million and parts per billion levels. It is necessary to analyze cadmium at trace levels in the case of chocolate. For this type of analysis, graphite furnace AA spectrometry is the most suitable 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.

Experimental
A Thermo Scientific iCE 3500 AA spectrometer was used to demonstrate the use of AA spectrometry for the effective analysis of cadmium in chocolate. The system was operated using the wizard-driven Thermo Scientific SOLAAR software to ensure quick and easy optimization and method development.

All standards and reagents used for this application were purchased from Fisher Scientific, including trace metal grade nitric acid (69%), hydrogen peroxide (>30% w/v) and ammonium nitrate. The sub-standards were prepared using a 1000 mg/L cadmium master standard.

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

Standard and reagent preparation
For spiking of samples prior to digestion, a 1-mg/L cadmium sub-standard was prepared in deionized water. Following this, the 1-mg/L sub-standard was used to prepare a 10-µg/L sub-standard for calibration. The 10-µg/L sub-standard consisted of 7% nitric acid and 1% hydrogen peroxide to matrix match to the digested samples. Blank and diluent were also prepared at 7% nitric acid and 1% hydrogen peroxide. To enable deposition of 20 µg of ammonium nitrate in a 10-µL aliquot, a matrix modifier was prepared at 2 g/L.

Table 1 shows the furnace temperature parameters. The SOLAAR software features an “optimize furnace parameters” wizard, which was used to determine the most suitable temperatures for ashing and atomizing of the digested chocolate samples. GFTV was used to observe the deposition of the sample into the cuvette and to optimize the position of the injection capillary. The 10 µg/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 10 µg/L.

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

Results
During the analysis of cadmium in chocolate, a segmented fit curve was used for generation of the calibration. To evaluate the recovery of cadmium, spiked samples were prepared. This was done by adding 0.5-m aliquots of the 1-mg/L cadmium standard to samples of chocolate. After this, the proposed method (0.5 mL of 1 mg/L cadmium results in addition of 5 µg/L to the final sample) was used for preparing and analyzing chocolate samples.

Experimental results show that cadmium was detected in small amounts in all three chocolate samples, with the maximum calculated at 0.04 mg/kg. All samples fell below typical current legislation for the recommended maximum levels of cadmium in foodstuffs. For an average adult of 70 kg weight to exceed the PTWI, approximately 12 kg/week of the dark chocolate analyzed in this experiment would need to be consumed. However, this would not be recommended for a balanced and healthy diet.
Spiked recoveries were performed on the three analyzed chocolate samples and were all found to be satisfactory.

These results demonstrate that graphite furnace AA spectrometry is an ideal tool for simple, easy and accurate analysis of trace levels of cadmium in chocolate.

For further information please visit: Thermo Fisher Scientific

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