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Method validation for determination of some rare-earth elements in rice using ICP-MS

In the present work, an inductively coupled plasma mass spectrometer operating in single quadrupole was performed for determination of some rare-earth elements including La, Sm, Eu, Tl, and U in rice. The samples were prepared by microwave digestion with nitric acid. The fortifications of rice samples at 3 levels were studied in order to check the accuracy of the proposed method. The experiment was performed under the internal standard method with the coefficient of determination higher than 0.9990. The average recoveries of all elements were higher than 92% with the standard deviation less than 4.5.


Introduction
Rare earth elements (REEs) are members of Group IIIA in the Periodic Table and have similar physical and chemical properties [1]. The REEs are frequently subdivided into three groups: the light (lanthanum to europium), middle (samarium to holmium) and heavy REEs (gadolinium to lutetium). Information about the contents of rare-earth elements (REEs) in several kinds of samples is very important in geochemical, biochemical and environmental studies. In addition, nowadays REEs have been widely used in agriculture as a trace element fertilizer in all over the world. Therefore, the need for accurate trace analysis of REEs in environmental samples has increased. Because the content of REEs in environmental samples such as rice is generally very low, it is very difficult to determine them accurately by conventional analytical methods.
Owing to very low REE concentrations and the complex matrix, sophisticated analytical techniques have been developed for the determination of trace REEs in environmental, biological, and food samples, such as neutron activation analysis (NAA) [2][3][4], X-ray fluorescence (XRF) [5], isotopic dilution mass spectrometry (IDMS) [6], inductively coupled plasma atomic emission spectrometry (ICP-AES) [7−9], and ICP-MS [10]. Among these techniques, XRF suffers poor detection limits for REEs, which limit its application for REE detection in real samples. NAA is sensitive but requires access to a nuclear reactor, so it is not suitable for routine REE analysis. Although the IDMS assay allows fast and multi-elemental determination of REEs, the detection limit for REEs is still 2−3 orders of magnitude poorer than with ICP-MS.
In this study, we investigated the method validation for the determination some rare-earth elements including Lanthanum (La), Samarium (Sm), Europium (Eu), Thallium (Tl), and Uranium (U) in rice using inductively coupled plasma mass spectrometry with microwave digestion. The measurement was performed by internal standard method.

Sample collection and preparation
Thai rice samples were dried in an hot-air oven at 60 °C for 48 hour and kept in desiccator prior to analysis. The powdered rice samples (0.8 g) were weighed into 100 mL TFM vessels. After samples were spiked with the rare-earth standards at three concentration levels (0.1, 1, and 10 mg/kg), 10 mL of 60% HNO3 was then mixed in 100 mL closed TFM vessel. The closed vessels were placed for digestion in a Mars TM 6 Xpress microwave digestion system (CEM, NC, USA). The following digestion program was created at 15-min ramp and 30-min hold at 950 W. After cooling down to room temperature, all digested solutions were filtered through a 0.45 µm syringe filter. The sample solution was heated to dryness and reconstituted in 10 mL of 2% (V/V) HNO3 prior to analysis by inductively coupled plasma mass spectrometer (ICP-MS).

ICP-MS Analysis
All measurements were determined using an ICP-MS (Agilent 7900, Agilent Scientific Technology Ltd., USA). ICP-MS analyses were carried out with an internal standard calibration approach. Each solution was measured in triplicate. The operating conditions of ICP-MS instrument are shown in table 1.

Results and discussion
The

Conclusion
The proposed method gave linear calibrations in the range of 0.025-1.0 μg/L with correlation coefficients greater than 0.9997. Limits of detection (LODs) and limits of quantitation (LOQs) were 0.007-0.010 μg/L and 0.016-0.020 μg/L, respectively. Good precision was achieved with relative standard deviation lower than 6%. High accuracy, with the average recoveries higher than 92% was obtained. The proposed ICP-MS with microwave digestion has shown to be of high potential for the analysis of some rare-earth elements including Lanthanum (La), Samarium (Sm), Europium (Eu), Thallium (Tl), and Uranium (U) in rice.