Determination of Trace Elements in Rice Products Using Flame and Graphite Furnace Atomic Absorption Spectrometry

Rice is the second most prevalent cereal crop in the world, with an annual global production of approximately 600 million tons. It is the staple food of most Asian countries, with a daily consumption per person of between 200 and 400 g. Rice can be contaminated by toxic heavy metals present in water or soil. Large amounts of these elements are responsible for acute or chronic poisoning, resulting in damaged or reduced mental and central nervous function; lower energy levels; and damage to blood composition, lungs, kidneys, liver, and other vital organs. Long-term exposure may result in slowly progressing physical, muscular, and neurological degenerative processes.1 Monitoring the presence of heavy metals in rice and its products is therefore of particular importance on an essential, nutritional, and toxicological level. 

A number of serious incidents of rice contamination have occurred in recent years, including the one that took place in the Toyama Prefecture, Japan. During the early twentieth century, cadmium was released by mining companies into the Jinzu River in Toyama. The river water was used to irrigate rice fields, and cadmium was subsequently absorbed by the growing rice. Consumption of the contaminated rice resulted in the mass cadmium poisoning of hundreds of people in the region, who suffered softening of bones, anemia, and kidney failure. In response to these kinds of disasters, global regulatory bodies have introduced stringent legislation to specify the permissible levels of heavy metals in foodstuffs.

Current legislation

In the U.S., the safety of food products for human consumption is preserved under the Federal Food, Drug and Cosmetics Act (FFDCA) enforced by the U.S. FDA. The Act forbids the introduction or delivery for introduction into interstate commerce of any food that is adulterated or misbranded. Within this framework, tolerances for poisonous ingredients in food are specified.2

Commission Regulation (EC) No. 1881/20063 sets maximum levels for the heavy metals cadmium, lead, and mercury in certain foodstuffs. The rule requires that contaminants be kept at levels that are toxicologically acceptable in order to protect public health. Maximum levels are set at a strict level, which is reasonably achievable by following good agricultural, fishery, and manufacturing practices, and taking into account the risk related to the consumption of the food. For rice, the regulation specifies an upper limit of 0.2 mg/kg wet weight of cadmium and lead.

The EU Codex Alimentarius 198-19954 standard for rice requires that rice be free from heavy metals in amounts that may represent a hazard to human health. This standard applies to husked rice, milled rice, and parboiled rice, all intended for direct human consumption, either presented in packaged form or sold loose from the package directly to consumers. It does not apply to other products derived from rice or to glutinous rice.

The Food Hygiene Law of the People’s Republic of China5 has been introduced to ensure food hygiene, prevent food contamination, safeguard the health of people, and improve their physical fitness. According to the law, food shall be nontoxic and harmless; conform to proper nutritive requirements; and have appropriate sensory properties such as color, fragrance, and taste. China has also set an upper limit of 0.2 mg/kg of cadmium in rice.6

Such legislation has produced a requirement to monitor rice and other foodstuffs for trace content of heavy metals. Flame and furnace atomic absorption (AA) spectrometry is the technique of choice for this application, providing precise analyses at parts per million (ppm) and parts per billion (ppb) levels.

Atomic absorption spectrometry

AA spectrometry is ideal for high-throughput food safety laboratories requiring fast and regular flame and graphite furnace analyses of heavy metals. The latest technological advancements have seen the introduction of AA spectrometers designed to accommodate both types of analysis. In these instruments, changeover from flame to furnace mode is entirely software controlled, facilitating automatic operation, even with the most difficult samples. Overall, increased performance, flexibility, and simplicity are achieved, even in cases of challenging detection limits.

Analysis of multiple nutritional and toxic elements in food products is accurately and rapidly determined with optimized methods. Simple sample preparation and method optimization provide a rapid and effective solution for precise and reliable analysis at minor and trace levels. Flame atomic absorption can be used as a fast screening tool for the analysis of nutritional elements such as copper, manganese, and zinc, while graphite furnace atomic absorption can be used for the accurate determination of toxic elements such as cadmium and lead. In addition, AA spectrometry provides an efficient screening tool compared to more complex and expensive techniques, such as inductively coupled plasma-optical emission spectrometry (ICPOES) or mass spectrometry (ICP-MS).

A study was performed to evaluate the performance of AA spectrometry for the analysis of copper, zinc, manganese, cadmium, and lead in rice products.

Experimental

Reagents and standards

Trace analysis-grade nitric acid was used for both flame and graphite furnace analysis. For flame analysis, copper, manganese, and zinc master standards of 1000 mg/L were used to prepare 0-, 0.5-, 1-, 2-, and 5-ppm substandards. The multielement standards were diluted to volume with 1% nitric acid.

Cadmium and lead master standards of 1000 mg/L were used for graphite furnace analysis. The cadmium and lead standards were prepared at 5 ppb and 10 ppb, respectively, using the master standards, and were diluted to volume with 1% nitric acid. Ammonium nitrate was also used as a furnace matrix modifier in a quantity of 20 µg in 10-µL injection for cadmium and 50 µg in 10-µL injection for lead.

Sample preparation

Three samples were analyzed: a rice flour Certified Reference Material (CRM, IRMM-804) and two samples of retail products purchased from a local supermarket—rice flour and whole white rice. Samples were dried for 12 hr at 85 °C to a constant weight. They were then cooled, and portions of approximately 0.25 g were accurately weighed and transferred to microwave digestion vessels. A volume of 4 mL nitric acid was added to each of the vessels, which were left uncovered for 1 hr.

Another 4 mL nitric acid was added to each vessel, and the vessels were sealed and samples digested in a high-pressure closed microwave digestion system by ramping over 20 min to 170 °C. Samples were left to cool before increasing their volume to 250 mL with deionized water for cadmium analysis. Duplicate samples were prepared for the analysis of copper, lead, manganese, and zinc, with the digests made up to 50 mL using deionized water.

A spiked sample of the supermarket rice flour was prepared to assess the sample preparation and subsequent analysis of cadmium and lead by graphite furnace AA. Spikes were added to obtain a final concentration of approximately 75% of the CRM rice flour standard.

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