Measurement of Inorganic Arsenic Species in Rice after Nitric Acid Extraction by HPLC-ICPMS: Verification Using XANES

Abstract

The measurement of As species in rice is normally accomplished by extraction followed by HPLC-ICPMS analysis. This method, however, has not been comprehensively validated by comparing these speciation results with XANES, which does not require sample extraction, due to the challenge of conducting XANES analysis at very low As concentrations. In this study As speciation data using nitric acid extraction/HPLC-ICPMS and XANES are compared to verify the efficacy of using 2% v/v nitric acid extraction and HPLC-ICPMS to measure inorganic As, DMA, and MA in reference rice materials and common rice varieties obtainable in Australia. Total As and As species (As(III), As(V), DMA, and MA) concentrations measured in 8 reference materials were in agreement with published values. XANES analysis was performed on 5 samples having total As concentrations ranging from 0.198 to 0.335 μg g(-1). XANES results gave similar proportions of total As(III), As(V), and DMA to HPLC-ICPMS. XANES was able to distinguish two forms of As(III): As(III) and As(III)GSH. Total As concentrations in rice samples varied from 0.006 to 0.45 μg g(-1) As (n = 47) with a mean ± std of 0.127 ± 0.112 μg g(-1) As with most As present as inorganic species (63 ± 26%). DMA was found in nearly all the rice samples with the majority of samples containing concentrations below 0.05 μg g(-1) As while MA concentrations were negligible (<0.003 μg g(-1) As). Six rice varieties produced in Australia, China, and Spain all had elevated DMA concentrations (0.170-0.399 μg g(-1) As) that were correlated with total As concentrations (r(2) = 0.7518). In conclusion, comparison of As speciation by HPLC-ICPMS and XANES showed that similar As species were detected indicating the appropriateness of using 2% v/v nitric acid for extraction of rice prior to speciation. Common rice varieties obtainable in Australia generally have low As concentrations with most As present as inorganic As.

Imaging element distribution and speciation in plant cells

“…19,No. 3 analysis and in situ bulk XANES [67] (Figure 3A). The samples had low As concentrations ranging from 0.20 to 0.34 mg/g.…”

“…This study provided evidence of good agreement between the two techniques and demonstrated the high sensitivity of XANES offered by modern synchrotron beamlines. In rice grains, As is present as As(III), As(III)-thiol complexes, As(V), and DMA [43,67].…”

“…Probing chemical speciation of elements in plant tissues. (A) Synchrotron X-ray absorption near edge structure (XANES) arsenic K-edge spectra of arsenic standards and five rice grain samples [67]. Abbreviations: As(III), arsenite; As(III)-GSH, arsenite-glutathione complex; As(V), arsenate; DMA, dimethylarsinic acid.…”

Arsenic translocation in rice cultivation and its implication for human health

“…The most common use of XAS for As speciation is the analysis of X-ray absorption near-edge structure (XANES), which reveals the most probable chemical environment around As by matching the spectral characteristics, including whiteline energy, to the standard compounds. This method has provided good results for the speciation of As compounds in rice (Maher et al, 2013). Studies comparing both methods demonstrate that compounds such as As III -S have not been found by the traditional method, but have been found by XAS; this has been attributed to its oxidation during extraction with nitric acid.…”

Inorganic Arsenic Determination in Food: A Review of Analytical Proposals and Quality Assessment Over the Last Six Years

“…The iAs content varied with the region of Italy the samples came from. Maher et al 188 extract arsenic species using 2% HNO 3 before measurement by the coupled technique. Both cation and anion exchange columns are used for separation.…”