Elsevier

Food Chemistry

Volume 213, 15 December 2016, Pages 609-615
Food Chemistry

Simultaneous determination of arsenic and mercury species in rice by ion-pairing reversed phase chromatography with inductively coupled plasma mass spectrometry

https://doi.org/10.1016/j.foodchem.2016.07.003Get rights and content

Highlights

  • First technique for simultaneous speciation analysis of As and Hg in rice flour by HPLC-ICP-MS was developed.

  • Optimization of the chromatographic method was proposed.

  • Methods for the simultaneous extraction of As and Hg species in rice were performed.

  • Good precision and recoveries of the method were obtained in analysis of rice flour samples.

Abstract

An analytical method using reversed phase chromatography–inductively coupled plasma mass spectrometry for arsenic and mercury speciation analysis was described. The effect of ion-pairing reagent on simultaneous separation of four arsenic (arsenite, arsenate, monomethlyarsonate and dimethylarsinate) and three mercury species (inorganic mercury (HgII), methylmecury and ethylmercury) was investigated. Parameters including concentrations and pH of the mobile phase were optimized. The separation and re-equilibration time was attained within 20 min. Meanwhile, a sequential extraction method for arsenic and mercury in rice was tested. Subsequently, 1% HNO3 microwave-assisted extraction was chosen. Calibration curves based on peak area measurements were linear with correlation coefficient greater than 0.9958 for each species in the range studied. The detection limits of the species were in the range of 0.84–2.41 μg/L for arsenic and 0.01–0.04 μg/L for mercury, respectively. The proposed method was then successfully applied for the simultaneous determination of arsenic and mercury species in rice flour standard material and two kinds of rice from local markets.

Introduction

Arsenic (As) and mercury (Hg) are well known as toxic elements because of their potential risks to human health. Exposure to As or Hg has been linked to an increased risk of many physiological disorders and various types of cancer (Mir et al., 2007). With the rapid development of modern industry, As and Hg are often present as a result of human activities, such as mining/processing of ore and wood treatment (Afton, Kubachka, Catron, & Caruso, 2008). Since As and Hg can be taken up and accumulated by crops, the safety and quality of agricultural products are subject to serious threats from contaminated sources.

Unfortunately, rice, a staple crop for Chinese people, is very efficient in As and Hg accumulation (Ma et al., 2014, Ren et al., 2014, Rothenberg et al., 2011, Sommella et al., 2013). Along with drinking water and seafood, the consumption of rice has become the major contributor to As and Hg, thereby causing potential health risks (Sun, Williams, & Zhu, 2009). Studies have shown a positive correlation between health risks and the total As and Hg concentrations of rice (Fang et al., 2014, Qian et al., 2010). The toxicities of As and Hg depend on their chemical speciation. Generally, inorganic As is more toxic than organic As, and the levels of toxicity of As compounds are as follows: arsenite (AsIII) > arsenate (AsV) > monomethlyarsonate (MMA) > dimethylarsinate (DMA) (Gurkan et al., 2015, Moreda-Piñeiro et al., 2011). Interestingly, some organic As species such as arsenobetaine and arsenosugars have been proved to be nontoxic (Gómez-Ariza et al., 2004, Nam et al., 2010). Evidence has shown that the major Hg species generally found in biological samples are either methylmecury (MeHg) or inorganic mercury (HgII) (Doker and Bosgelmez, 2015, Li et al., 2007). The levels of toxicity of Hg compounds are as follows: MeHg > ethylmercury (EtHg) > HgII. Therefore, measuring the total As and Hg concentration alone is not enough to assess the hazards of As and Hg.

In recent years, a number of analytical techniques have been widely employed for the speciation analysis of As or Hg, including gas chromatography (GC) or high performance liquid chromatography (HPLC) coupled with element specific techniques such as atomic absorption spectroscopy (AAS), atomic emission spectrometry (AES), atomic fluorescence spectroscopy (AFS), and inductively coupled plasma mass spectrometry (ICP-MS) (Do et al., 2001, Pasias. et al., 2013, Pelcova et al., 2015, Zmozinski et al., 2015). HPLC techniques are better suited for the separation of As or Hg due to the relatively wide compatibility of mobile phase composition and the easiness of sample preparation (Lin et al., 2008, Liu et al., 2013). ICP-MS also has the advantages of high sensitivity, wide linearity, low detection limit, and multielemental analysis (Iserte, Roig-Navarro, & Hernández, 2004). Therefore, coupling of ICP-MS with HPLC is the most widely used technique for the individual speciation of As or Hg that has been employed successfully on a number of different matrices (Khan et al., 2015, Moreno et al., 2013, Raber et al., 2012, Souza et al., 2013). C18 reversed-phase chromatography with the ion-pairing reagent tetrabutylammonium hydroxide (TBAH) has been employed in the separation of four selenium and four As species within 18 min by HPLC coupled with ICP-MS (Afton et al., 2008). Currently, however, a single method for the simultaneous separation of common As and Hg species in one chromatographic run has not yet been reported. Gómez-Ariza et al. (2004) established a method for the simultaneous determination of Hg and As species in natural freshwater by HPLC (hydrides generation and cold vapor) coupled with a home modified AFS and a standard AFS system. However the use of modified AFS makes this method somewhat complex, and it cannot be widely used for other matrices. To the best of our knowledge, no information is available regarding the simultaneous speciation analysis of As and Hg in rice using HPLC–ICP-MS.

The main objective of this work was to develop a method for the simultaneous speciation analysis of As and Hg species using HPLC–ICP-MS. Seven common environmentally and biologically observed As and Hg species standards were baseline separated on a C18 chromatography column via ion-pairing reversed phase chromatography. To illustrate the potential applicability of the proposed method, we successfully optimized the chromatographic method applied to the certified reference materials (CRMs) NIST 1568a rice flour, GBW 10043 rice flour, and two kinds of rice flour from local markets.

Section snippets

Reagents and materials

All the solutions were prepared with doubly deionized water (DDW) (18  cm−1, Millipore-Q American). The following commercial products were used: Nitric acid (65%) was purchased from Merck, methanol (HPLC grade) was obtained from Kermel, ammonium dihydrogen phosphate (GR) was purchased from Simopharm Chemical Reagent Co., Ltd, l-cysteine (Reagent Grade) from Solarbio Science & Technology Co., Ltd, and both ammonium acetate (GR) and tetrabutylammonium hydroxide (TBAH) 40% in water

Optimization of chromatographic separation conditions

Previous studies showed that pH 5.5–6.5 was optimal for As species and successfully separation of As species was achieved at pH 6.0 on a C18 column with ion-pairing reagent (Afton et al., 2008). Therefore, according to previous publications (Afton et al., 2008, Li et al., 2007), a preliminary study was attempted to select the mobile phase at pH 6.0 using Agilent Eclipse plus C18 column without using any ion-pairing reagents (mobile A, 15 mmol/L NH4H2PO4 for As; mobile phase B, 3% (v/v) methanol,

Conclusions

In this study, an analytical technique for simultaneous speciation of As and Hg in rice flour by HPLC-ICP-MS was established, and 1% HNO3 microwave-assisted extraction was chosen as the simultaneous extraction for As and Hg species in rice flour. All As and Hg species could be separated within 20 min (including 10 min re-equilibration time) by using TBAH as the ion-pairing reagent on a C18 column. The approach is simple and sensitive and can be used for the simultaneous speciation analysis of As

Conflict of Interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (31471680), the China Special Fund for Grain-scientific Research in the Public Interest (201313007), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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