Species analysis with the LC-ICPMS Detecting Toxins: Arsenic in Your Rice?

Heavy metal contamination in food and water poses a threat to humans and animals. The toxicity of elements such as chromium, mercury and arsenic varies depending on their oxidation state. Food analysis is a very effective method for positively identifying the oxidation state.

Heavy metal contamination in food and drinking water is an ever-present risk, and constant monitoring is needed to keep it at bay. While it is true that a number of different metals are involved in vital processes inside the human body, excessive consumption of the same elements can be harmful to health or even cause certain types of cancer. To protect public health, Commission Regulation (EC) No 1881/2006 sets maximum levels for certain heavy metals in foodstuffs. Regulation (EC) No 2015/1006 of 25 June 2015 changed the maximum levels for arsenic in rice [2] (also see the LAB Info box). Since 2018, all EU member states are urged to set new maximum levels for inorganic arsenic in rice.

The danger posed by toxic arsenic in foodstuffs

History, the cinema and television constantly remind us that arsenic is a poison. Evidence shows that the ingestion of inorganic arsenic in particular seriously increases the risk of lung, skin and badder cancer [2]. Contamination can be present right in the drinking water. Also, plants can absorb and store arsenic from water and from the soil. In today’s globalized world, food containing arsenic can be distributed to additional population centers, even in areas where until recently arsenic contamination was not a problem. Rice in particular has the capacity to absorb more arsenic than other types of grain [3]. Levels as high as 20 - 900 µg/kg have been detected in the past [4].

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Oxidation numbers and types of bonding

The different oxidation states and types of bonding determine the toxicity of an element. Inorganic arsenic and organic arsenic compounds can be present in rice. Inorganic arsenic is more toxic than organic arsenic compounds. In addition, a distinction can be made between the far more toxic arsenic (III) and arsenic (V). Consequently, highly accurate qualitative and quantitative determination of the element’s state is crucial.

Dissecting chemical properties with the LC-ICPMS

Distinguishing between the oxidation states and chemical state is called species analysis. Coupling selective HPLC with highly sensitive ICPMS equipment creates a reliable and highly impressive analysis technique. Through the use of chromatography, complex samples are separated out into their individual constituents. The arsenic content is then determined using ICPMS. The next step is to generate a chromatogram to detect arsenic which has a specific mass-to-charge ratio of 75. This article presents a method for accurate, highly-sensitive food analysis of organic and inorganic arsenic in rice using the in-line combination of a Shimadzu ICPMS-2030 and a Shimadzu HPLC (Prominence, inert LC system). From a single software user interface, highly accurate, user-friendly investigations can be carried out on extremely complex samples with the LC-ICPMS system (Fig.2).

Sample preparation and measurement conditions

Two certified rice samples were analyzed: white rice (certified standard substance: NMIJ CRM 7503-a) and brown rice (certified standard substance: NMIJ CRM 7532-a). For each analysis, 0.5 g of the sample was added to 2 ml of 0.15 mol nitric acid and heated at 100 °C for six hours. Distilled water was subsequently added to make 10 g of the solution which was then placed in a centrifuge at 3,000 rpm for 20 minutes. The supernatant was filtered using a micro-membrane filter (pore size 0.45 μm) and diluted twice with a mobile phase.

An inert Prominence HPLC system, which was coupled in-line with a Shimadzu ICPMS-2030, was used to measure the rice samples. Using a standard addition kit, an internal standard can be added to the samples and the calibration standards. A calibration curve is used to quantify the inorganic arsenic species (As(III) and As(V)) and dimethylarsenic acid (DMAA).

Table 2 lists the settings for the HPLC and the ICPMS. The measurement results for the certified rice samples of white and brown rice respectively are shown in Table 1.

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