Rapid determination of five arsenic species in polished rice using HPLC-ICP-MS

Transcription

1 Rapid determination of five arsenic species in polished rice using HPLC-ICP-MS Application note Food safety Authors Bing Yue China National Center For Food Safety Risk Assessment, Beijing, China Juane Song, Ke Xie, Zhao-jun Yun and Jian-qiu Mi Agilent Technologies, Beijing, China Introduction Arsenic (As) is a well-known toxic element that can be present in the environment and foodstuffs. Consequently it is a highly regulated element that is widely monitored. However, As speciation analysis rather than total As analysis is important because its biological toxicity is highly dependent on its chemical form. For example, if a seaweed sample is known to contain a high level of As, but mainly in the form of AsB, then there is no potential risk from its consumption as AsB is non-toxic. Among the five major species of arsenic,, MMA, As(III) DMA and AsB, only the two inorganic forms (As(III) and ) are toxic. They are also carcinogenic to humans, so need to be measured, especially in foodstuffs. Arsenic in rice is of special concern in countries where it is consumed daily. Rice plants absorb As from soil and water. Since rice requires a lot of water to grow compared to other plants, the potential for arsenic accumulation is increased. In this study, we present a rapid method for the determination of the toxic As species in rice using an Agilent 1260 HPLC system equipped with an Agilent ZORBAX SB-Aq column coupled to an Agilent 7900 ICP-MS.

2 Experimental Instrumentation An Agilent 1260 HPLC system comprising a quaternary pump, autosampler and vacuum degasser was coupled to an Agilent 7900 CP-MS. The column exit was simply connected to the nebulizer of the ICP-MS using PFA tubing. The Agilent ZORBAX SB-Aq (part number , 4.6 mm id x 250 mm, 5 μm) reversed phase column was maintained at room temperature throughout the analysis. The mobile phase was 20 mm citric acid and 5 mm sodium hexane sulfonate, adjusted to ph = 4.3 with sodium hydroxide. Two injection volumes, 5 μl or 100 μl, were evaluated. HPLC and ICP MS parameters are given in Table 1. The Agilent 7900 ICP-MS with the Octopole Reaction System (ORS 4 ) collision/reaction cell (CRC) provides sensitive and specific analysis of As in the presence of multiple interferences. In some regulatory guidance, food samples must be extracted using HCl to simulate gastric conditions. In this case, the formation of polyatomic ions based on ArCl and CaCl may cause spectral interferences on the sole isotope of As at m/z 75. However, operating the 7900 ICP-MS ORS 4 in helium mode effectively removes these matrix-based polyatomic interferences on As. Table 1. Operating conditions of the Agilent 1260 HPLC and 7900 ICP-MS ICP-MS parameters RF power Sampling depth Carrier gas Makeup gas Cell gas KED Isotopes monitored Dwell time HPLC parameters Mobile phase Column Flow rate Temperature Injection volume Run time 1550 W 8 mm 0.6 L/min 0.5 L/min He at 4 ml/min 3 V 75 As, 35 Cl 1.0 s/ 75 As; 0.1 s/ 35 Cl Isocratic, 20 mm C 6 H 8 O 7 and 5 mm C 6 H 13 NaO 3 S, adjusted to ph 4.3 with NaOH Agilent ZORBAX SB-Aq 1.2 ml/min Ambient 5 μl or 100 μl 4 min Chemicals and reagents Stock standards of, MMA, As(III), DMA and AsB were purchased from NIMC (National Institute of Metrology China, Beijing, China). High purity citric acid ( 99.5%), sodium hexane-sulfonate ( 98%) and sodium hydroxide ( 99.99%) were purchased from Sigma Aldrich Chemicals (UK). Ultra-pure grade HNO 3 (Sigma Aldrich, UK) was used for the sample preparation. Sample preparation The polished rice samples were obtained from different locations in the following provinces of China: Jiangsu, Fujian, Liaoning, Chongqing and Jiangxi. The polished rice samples were crushed and sieved through a 30 mesh strainer to make rice flour g of the rice flour sample was weighed into a polypropylene centrifuge tube (50 ml). 15 ml of 0.15 M HNO 3 was added and the sample was vortexed for 30 seconds. The tightly capped tube was left to stand overnight at room temperature; it was then placed in an oven at 90 C for 2.5 hours where it was agitated for 30 seconds, every 30 minutes. After the extraction had finished, the sample was cooled in a refrigerator at 4 C. The cooled rice flour suspension was centrifuged at 8000 rpm for 15 minutes and then the supernatant was passed through a 0.22 μm polyether sulphide (PES) filter. The filtrate was stored at 4 C and analyzed within 24 hours to minimize any species inter-conversion. Results and discussion Separation and quantification Calibration standard solutions containing the five As species at different concentrations (1 ppb, 5 ppb, 10 ppb, 50 ppb, 100 ppb and 200 ppb) were prepared and analyzed. The chromatogram for the 10 ppb calibration standard (5 μl injection) shows the five As species are well separated in 4 minutes (Figure 1). The results were used to create calibration curves for the As species, as shown in Figure 2. 2

3 Table 2. Detection limits (ppb) for, MMA, As(III), DMA and AsB with 5μL and 100 μl injections Injection volume MMA As(III) DMA AsB (μl) Figure 1. Chromatogram of a 10 ppb As (conc. as As element) species mixture standard (5 μl injection) Figure 3 shows the chromatograph of a 50 ppt As species standard mixture (100 μl injection) demonstrating the ultra-trace analysis capability of the method. Figure 3. Chromatogram of 50 ppt As species mixture (100 μl injection) Certified Reference Material (CRM) analysis The method was applied to the analysis of two rice flour CRMs; NIST 1568a and NIST 1568b. Though only total As content is certified for NIST 1568a, (290 ± 30 ppb), the CRM has been well-characterized in the literature. The two CRMs were extracted following the aforementioned sample preparation method and analyzed for the five As species. Figure 2. Calibration graphs for, MMA, As(III),DMA and AsB showing excellent linearity. Detection limits Detection limits (DLs) for each arsenic species were calculated as the concentration equivalent to three times the peak-to-peak baseline noise (S/N) of the standard chromatogram. The results are summarized in Table 2. As shown, the DL of the five As species were between ppb and ppb with a 5 μl injection and ppb and ppb with a 100 μl injection. Tables 3 and 4 summarize the quantified results of the inorganic As species, MMA, DMA and the total As content in the two CRMs, the certified values or reported values in the literature. For this study, total As was calculated from the sum of inorganic As, MMA, DMA and AsB. As shown, the quantified inorganic As (ias) in NIST 1568b and total As in both CRMs are in good agreement with the certified values and within the standard deviation. The quantified results for each species are also in good agreement with reported values in the literature [1-4]. 3

4 Table 3. Quantification of As species in NIST 1568b Rice Flour a This study Certified Value MMA As(III) DMA ias b Total As 62.0 ± ± ± ± ± ± ± ± ±14 a value is mean ± SD of ten injections, b inorganic arsenic: sum of As(III) and Table 4. Comparison of quantified As species in NIST 1568a Rice Flour determined in this study compared to values from the literature a. The certified total As content of NIST 1568a is 290 ± 30 ppb. MMA As(III) DMA ias b Total As Ref. [1] 44±2 12±0.8 52±1 173± ±2 Ref. [2] 50.3± ± ± ± ±9.9 Ref. [3] 53.7± ± ± ± Ref. [4] 13.6± ± ± ±3.8 This 64±5 14±2 57±4 140± ±4 study a value is mean ± SD of ten injections, b inorganic arsenic: sum of As(III) and Spiked sample recovery Spiked sample recovery was checked for the five As species. A mixture of, MMA, As(III), DMA and AsB was spiked into three rice sample extracts at 1 ppb, 5 ppb and 10 ppb. Three replicates were prepared (spike 1, 2 and 3). As shown in Table 5, good recoveries ranging from 85% to 110% were achieved for all species, at all concentrations, in each of the spiked solutions. Precision Precision of the method was checked by repeatedly analyzing a rice sample, with ten separate injections. Table 6 shows the concentration results for all the As species, indicating the excellent reproducibility for the ten separate analyses. The %RSDs for all As species were less than 4%, and less than 2% for inorganic As. Table 6. Results of reproducibility of rice # 4 (n = 10) with 5 μl injection Sample MMA As(III) DMA AsB ias N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D Average % RSD 1.3% 0.9% 3.9% 0.5% Table 5. Results of spiked sample recovery with 5 μl injection Sample name Spike Conc. MMA As(III) DMA AsB Conc.(ppb) Rec.(%) Conc.(ppb) Rec.(%) Conc.(ppb) Rec.(%) Conc.(ppb) Rec.(%) Conc.(ppb) Rec.(%) N.D N.D. Spike % % % % % Spike 2 1 ppb % % % % % Spike % % % % % N.D N.D. Spike % % % % % Spike 2 5 ppb % % % % % Spike % % % % % N.D N.D. Spike % % % % % Spike 2 10 ppb % % % % % Spike % % % % % 4

5 Commercial rice sample analysis The As concentration was determined in the 20 different commercial rice samples and the results are summarized in Table 7. As shown,, As(III) and DMA were found in all 20 rice samples. The inorganic As and DMA content varied significantly among the rice samples, likely reflecting the As concentration in the growing environment of the original plant. Although the concentration of toxic inorganic As varied among the samples, all of the rice samples contained lower limit than the upper level of 0.2 mg/kg recommended by Codex Alimentarius Commission. Table 7. Quantitative results (μg/kg) for all five As species in 20 rice samples Sample MMA As(III) DMA AsB ias name N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D N.D Conclusions The separation of five arsenic species in polished rice was carried out using an Agilent 1260 Infinity LC coupled to an Agilent 7900 ICP-MS. The method is rapid - all five species including the toxicologically relevant inorganic forms As(III) and were determined in 4 minutes, with excellent sensitivity, accuracy and precision. Any potential interferences on As, such as ArCl +, were removed by the ORS 4 cell operating in helium mode. Inorganic arsenic and DMA species were detected in all of the twenty rice samples at various concentrations. However, none of the samples contained more than Codex Alimentarius Commission s recommended limit of 0.2 mg/kg. The validated methodology outlined here is suitable for the rapid determination of, MMA, As(III), DMA and AsB at trace levels in rice and other food samples. References 1. Narukawa T., Inagaki K., Kuroiwa T., Chiba K, The extraction and speciation of arsenic in rice flour by HPLC-ICP-MS [J], Talanta 77, 2008, Huang J H, Ilgen G, Blank value, adsorption, preconcentration, and preservation for arsenic speciation of the environmental water samples [J]. Analytica Chimica Acta, 2004, 512: Caruso J.A, B Hymer C, Heitkemper D.T., An evaluation of extraction techniques for arsenic species from freeze-dried apple sample [J]. Analyst, 2001, 126: Toni LIorente-Mirandes et al., A fully validated method for the determination of arsenic species in rice and infant cereal products, Pure Appl. Chem., 2012, vol. 84, No. 2,

6 Agilent shall not be liable for errors contained herein or for incidental or consequential damages in connection with the furnishing, performance or use of this material. Information, descriptions, and specifications in this publication are subject to change without notice. Agilent Technologies, Inc Published July 15, 2015 Publication number: EN