台灣地區稻米中砷物種與鎘濃度分析探討

Abstract

亞洲地區的飲食習慣是以稻米為主食，就目前所知，稻米中主要含有三價砷（As(III)）、五價砷（As(V)）、單甲基砷酸（MMA）和二甲基砷酸（DMA）這四種物種。一般而言，無機砷對人體的毒性較有機砷為高，因此經由稻米砷物種的量測可幫助在評估暴露及健康影響時，更為貼近實際情形。本研究針對台灣地區的稻米進行重金屬濃度分析，並開發砷物種萃取的方法，以進一步探討在稻米中的含量及其對人體的影響，並探討稻米煮熟後的砷濃度變化。並利用實驗所得之數據進行蒙地卡羅模擬，計算國人每日的鎘及無機砷攝取量。本研究依據台灣各地區鄉鎮的蓬萊米產量資料，從各縣市選出1-3個蓬萊米產量最高的鄉鎮，再依產量佔全國分佈比例，訂定採樣個數。總共收集118個稻米樣本，經由冷凍乾燥及磨粉後進行微波消化，最後利用感應耦合電漿質譜儀（ICP-MS）檢測其中的砷及鎘含量。在砷物種方法開發方面，嘗試利用不同種溶劑及萃取方法找出最適合本研究稻米樣本之研究方法。萃取回收率以超音波震盪加上微波輔助的效果最好，多數樣本及稻米標準品的萃取回收率可達到80 %以上。本研究稻米中金屬濃度分析檢測結果為：總砷含量為0.17±0.04 mg/kg（0.09-0.37 mg/kg）、三價砷含量為0.11±0.03 mg/kg（0.05-0.26 mg/kg）、五價砷含量為0.02±0.01 mg/kg（0.004-0.06 mg/kg）、DMA含量為0.02±0.01 mg/kg（0.01-0.06 mg/kg）、MMA含量為0.001±0.004 mg/kg（未檢出-0.03 mg/kg）。各地區稻米中砷含量並無明顯差異，但砷物種分析結果發現，台灣稻米含有三價砷的比例最高，DMA次之，MMA含量最少。在鎘含量分析方面，平均鎘含量為0.06±0.05 mg/kg（0.01- 0.40 mg/kg），宜蘭和南投地區平均稻米中鎘含量明顯較其他地區為高，分別為0.19±0.10 mg/kg及0.15±0.07 mg/kg。煮熟後的稻米中總砷及鎘含量皆明顯降低（10-36 %）。而熟飯中砷物種分析方面，觀察到三價砷的比例經由烹煮的過程後會降低，DMA的比例則會上升。將所測得之生米與熟飯中砷含量進行國人米製品攝食之鎘與無機砷暴露量評估，利用蒙地卡羅模擬，結果顯示99 %的國人皆並未超出WHO所建議的鎘或無機砷每日最大容許攝入量（MTDI）。實驗結果顯示台灣地區食用米中總砷及鎘含量在安全範圍內，而經由洗米煮飯的過程可降低一部分的總砷及鎘含量。經由人體攝食量暴露評估模擬，大部分國人經由食用米暴露於鎘與無機砷之攝取量尚在可接受範圍內。

Rice is the staple food in Asia. The major arsenic species in rice grain include arsenite (As(III)), arsenate (As(V)), monomethyarsonic acid (MMA), and dimethylarsinic acid (DMA). In general, inorganic arsenic species are more toxic than those of organic arsenic species. Therefore, it is necessary to analyze the arsenic species in rice in Taiwan to optimize the exposure assessment for the ingestion of rice. The objective of this study is to analyze arsenic and cadmium in white rice samples and cooked rice in Taiwan, and to determine the optimal extraction procedure for arsenic species in rice. With the obtained data, we used Monte Carlo simulation to calculate cadmium and inorganic arsenic daily intake value. First of all, based on the production of japonica crop, we chose one to three villages/towns with top crop production of japonica in every county in Taiwan and decided the sample number for each village/town, according to the ratio of rice crop production of the individual village/town to the whole Taiwan. In total, 118 white rice samples were collected, and then dried by freeze dryer and ground by a rice miller. Pretreated rice samples were acid digested by microwave. Total arsenic and cadmium concentrations in digested samples were determined by ICP-MS. For arsenic speciation, we first evaluated the efficacy of different solvent mixtures and techniques for the optimal extraction of arsenic species in rice samples. The most appropriate method was ultrasonic process followed by microwave extraction with recovery rate of above 80 % for most white rice samples. The average concentration for total arsenic was 0.17±0.04 μg/g (0.09-0.37 μg/g), for arsenite 0.11±0.03 μg/g (0.05-0.26 μg/g), for arsenate 0.02±0.01 μg/g (0.004-0.06 μg/g), for DMA 0.02±0.01 μg/g (0.01-0.06 μg/g), and for MMA 0.001±0.004 μg/g (ND-0.03 μg/g). There was no difference in the total arsenic concentration for rice samples of different counties in Taiwan. For arsenic speciation analysis, arsenite was the major species determined in rice sample and MMA was the least one. For the analysis of cadmium concentrations in rice samples, the average concentration of cadmium was 0.06±0.05 μg/g (0.007-0.40 μg/g). There were higher cadmium concentrations in Yilan and Nantou, with averages of 0.19±0.10 μg/g and 0.15±0.07 μg/g, respectively. The total arsenic concentration was significantly reduced in cooked rice, with higher percentage of reduction observed for raw rice samples with higher arsenic concentration. With respect to arsenic species in cooked rice samples, the proportion of arsenite rose while that of DMA decreased. We used Monte Carlo simulation to estimate cadmium and inorganic arsenic daily intake value. The results showed that 99% of people did not have cadmium or inorganic arsenic intake from rice exceeding the WHO suggested maximum tolerable daily intake value (MTDI). In conclusion, the concentrations of total arsenic and cadmium in rice samples in Taiwan are acceptable, which would be further reduced through washing and cooking process. Results of the Monte Carlo simulation indicated that, currently, the daily cadmium and inorganic arsenic intake from rice in Taiwan, is still within the acceptable range.