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A single pump cycling-column-switching technique coupled with homemade high exchange capacity columns for the determination of iodate in iodized edible salt by ion chromatography with UV detection
Food Chemistry 139 (2013) 144–148
Contents lists available at SciVerse ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
A single pump cycling-column-switching technique coupled with homemade high exchange capacity columns for the determination of iodate in iodized edible salt by ion chromatography with UV detection Zhongping Huang, Qamar Subhani, Zuoyi Zhu, Weiqiang Guo, Yan Zhu ⇑ Department of Chemistry, Xixi Campus, Zhejiang University, Hangzhou 310028, China
a r t i c l e
i n f o
Article history: Received 28 July 2012 Received in revised form 12 December 2012 Accepted 27 January 2013 Available online 6 February 2013 Keywords: Iodate Iodized edible salt Cycling-column-switching Ion chromatography
a b s t r a c t A single pump cycling-column-switching technique has been developed for the iodate analysis in edible salt. Homemade high exchange capacity columns were adopted for the separation of iodate and chloride. Iodate could be retained and concentrated in a homemade concentrator column after eluents passing through the suppressor. With UV detection, iodate exhibited satisfactory linearity in the range of 0.1– 10.0 mg/L with a correlation coefficient of 0.9996. The detection limit (LOD) was 45.53 lg/L, based on the signal-to-noise ratio of 3 (S/N = 3) and a 100 lL injection volume. Relative standard deviations (RSDs) for retention time, peak area and peak height were all less than 2.1%. Recoveries of added iodate were in the range of 98.4–101.6% for the spiked samples. The quantitative determination of iodate in edible salt was accomplished by this column-switching technique, without any pretreatment and interference. The results on six samples were statistically compared with results determined by conventional titrimetric method. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Iodine is one of the essential trace elements for producing thyroid hormones in the human body and in some other biological species (Buchberger, 1988; Yao, Chen, & Wei, 1999). Excessive iodine intake can contribute to certain thyroid disorders in susceptible individuals, whereas a deficiency of iodine in the diet can cause several diseases or problems, which include spontaneous abortion, increased infant mortality, cretinism, and stunting of intellectual functioning (Delange, Bürgi, Chen, & Dunn, 2002; Eckhoff & Maage, 1997). For the control of goitre, salt for edible purpose is compulsorily iodized by coating KIO3 on salt crystals and the recommended concentration of iodate in the salt is about 40 ppm. Today, iodized salt is an important source of iodine and cooking with iodized salt is highly promoted in China and many Asian countries. Several methods have been developed for the estimation of iodine in iodized edible salt. Conventional iodometric titration (Cunniff, 1990, chap. 11) was applied easily both in the laboratory and in the field, based on the redox reaction between iodate and iodide to give I2 that can then be titrated with thiosulphate ion. However, titration demanded of high skill of operators and was a time-consuming process, especially in the processes of preparing and standardizing the titrant of thiosulfate solution. Additionally, ⇑ Corresponding author. Tel./fax: +86 571 88273637. E-mail address: [email protected] (Y. Zhu). 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.01.070
the titration method might give false estimate of iodate with oxidizing agents such as bleaching powder (Rebary, Paul, & Ghosh, 2010). Special chemical reagents and complicated reactions were involved in spectrophotometric detection of iodate in iodized edible salt (Borges, Peixoto, Feres, & Reis, 2010; Shabani, Ellis, & McKelvie, 2011; Xie & Zhao, 2004). Estimation of iodate in iodized edible salt by transient isotachophoresis-capillary zone electrophoresis has been reported recently, with carcinogenic chromate ion as an internal standard (Wang, Zhao, Shen, Tang, & Wang, 2009). Reduction of iodate was necessary in the HPLC detection of iodate in iodized edible salt (Xu, Li, Gu, & Paeng, 2004). Electrochemical determination with modified electrode has been accomplished with unsatisfactory relative standard deviation (Kosminsky & Bertotti, 1999). The use of ion chromatography (IC) is limited for direct determination of iodate in iodized edible salt. The high chloride concentration of the matrix does not permit a clean separation and conductimetric determination of iodate in edible salt. After sample pretreatment with on-guard silver cartridge for the removal of the large excess of chloride ion, iodate in edible salt was successfully determined by IC with conductivity detection (Kumar, Maiti, & Mathur, 2001). Unfortunately, a fresh Ag cartridge was required for each sample. In addition, it was necessary to reduce iodate to iodide in estimation of iodate in iodized salt by IC with integrated amperometric detection (Rebary et al., 2010) or UV/visible detection (Bichsel & Gunten, 1999; Weinberg & Yamada, 1998). Iodate in edible salt or seawater has been estimated using
Z. Huang et al. / Food Chemistry 139 (2013) 144–148
anion-exchange chromatographic separation and ICP-MS detection (IC-ICP-MS) (Chen, Megharaj, & Naidu, 2007; Zhang et al., 2010). This is a useful method, the main limitation being equipment cost. As an effective sample pretreatment method, column-switching technique has been widely used in ion chromatographic system due to its automated analysis. Zhong et al. (2011) proposed a single pump column-switching technique for the determination of trace anions in different concentrated organic matrices by IC. In this chromatographic system, only a single pump with a ten-port valve and six-port valve were involved, much simpler than other column-switching techniques. The later, two pumps with two valves or a pump with four valves were necessary. In addition, considering the eluent was neutralized to water in the suppressor, anions were supposed to be retained and concentrated in a concentrator column for the next analysis. To the best of our knowledge, there was no report on the direct determination of iodate in iodized edible salt by IC with conductivity or UV/visible detection without any pretreatment. In the present study, a single pump cycling-column-switching technique coupled with homemade high exchange capacity columns was first proposed, to analyze iodate directly in iodized edible salt by IC with UV detection. Homemade high exchange capacity columns were adopted, due to the lack of proper commercial IC columns permitting a clean separation of iodate with the high concentration chloride of the matrix. Iodate could be separated completely with the high concentration chloride on the homemade columns, due to the high exchange capacity. The homemade columns were based on polystyrene-divinylbenzene (PSDVB) resins, functionalized with a large amount of quaternary ammonium groups (Huang et al., 2012). With the single pump cycling-column-switching technique, the estimation of iodate in iodized edible salt has been realized, with no interference from cations or the large excess of chloride ion in matrices. 2. Experimental 2.1. Equipment A Dionex (Sunnyvale, CA, USA) ICS 2100 was employed for all the chromatographic separations, equipped with a dual-piston serial pump, a DS6 heated conductivity detector, a column heater, a Rheodyne (Cotati, CA, USA) P/N PR070108B ten-port valve fitted with a 100 lL sampling loop, a Rheodyne (Cotati, CA, USA) Model 9900-013 six-port valve and an EGC-KOH eluent generator. Suppression was achieved with a Dionex ASRS-4mm suppressor. A Dionex Ultimate 3000 UV detector was used to determine iodate. Polyether ether ketone (PEEK) tubes with the lengths as short as possible were used to connect all chromatographic hardware. The eluent flow rate was 1.0 mL/min. Data were collected with Chromeleon 6.80 chromatogram workstation (Dionex, USA). A PEEK column (150 mm 4.0 mm) was used as an analytical column and a PEEK column (35 mm 4.6 mm) was used as a concentrator column. Both the columns were packed with homemade anion exchange resins. The resins with particle size about 7 lm were prepared according to the method we reported before, functionalized with 11-layer quaternary ammonium groups (Huang et al., 2012). The pressure of the concentrator column placed behind of the suppressor should be less than 100 psi to protect the suppressor.
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The stock solution of 1000 mg/L iodate was prepared by dissolving 0.1224 g potassium iodate (Tianjin Chemical Reagent Co. Ltd., Tianjin, China) in water and diluting to the mark in a 100 mL volumetric flask. Standard solutions were prepared by dilution of the stock solution of 1000 mg/L iodate with 2% (w/v) NaCl (Huipu Chemical Reagent Co. Ltd., Hangzhou, China) water solution in a 100 mL volumetric flask. Iodized edible salt were obtained from the local market. Sample solutions were prepared by dissolving an accurate weight of 2 g to the nearest 0.1 mg of edible salt in water and making up to 100 mL. By this preparation, sample solutions would contain approximately 2% (w/v) NaCl. 2.3. System operation procedure There were four steps involved for the estimation of iodate in iodized edible salt (Fig. 1): (1) filling the sample loop, (2) eluting cations from the analytical column into the waste, (3) separating and concentrating iodate in the concentrator column, (4) the second separation of iodate on the analytical column and eluting off the large excess of chloride ion and other anions. First, the sample was loaded into the 100 lL sample loop on the ten-port valve. To ensure that the loop contained a representative sample, at least four loop volumes were injected. Then, the ten-port valve containing sample was switched to inject position while the six-port valve was kept on inject position (Fig. 1A). Cations were all eluted out from the analytical column into the waste. After that, the six-port valve was switched to load position. Iodate was separated on the analytical column for the first time and concentrated in the concentrator column (Fig. 1B). After the complete concentration of iodate, the six-port valve was switched back to inject position for the second separation of iodate (Fig. 1C). Because of the weak retention of iodate and the high exchange capacity of the analytical column, iodate was still eluted off earlier than chloride with good resolution. The large excess of chloride and other anions were all eluted out from the analytical column into the waste after the elution of iodate. A gradient elution was used to achieve a satisfactory resolution and short analysis time. 3. Results and discussion 3.1. Characteristics of homemade columns The analytical column, packed using a QP 6000 packing pump (Chuang Xin Tong Heng Science and technology Co. Ltd., Beijing, China), had a pressure less than 1500 psi when the eluent at 1.0 mL/min. While the concentrator column, packed by the drypacking method, had a pressure less than 100 psi when the eluent at 1.0 mL/min. An eluent containing 0.02 mol/L NO3 was employed to determine the exchange capacities, and the capacities were 0.420 mmol/column and 0.031 mmol/column respectively for the analytical column and concentrator column. Compared with the Dionex IonPac AS11-HC (4 mm 250 mm) analytical column, the capacity of the analytical column was much higher, making it possible to separate iodate with the high concentration chloride of the matrix. The concentrator column with the pressure less than 100 psi at 1.0 mL/min was quite suitable for this proposed system, because the suppressor was easy to leak if the back pressure more than 100 psi.
2.2. Reagents 3.2. Selection of chromatographic parameter All reagents were of analytical reagent grade and a water purification system (Millipore, Milford, MA, USA) was used to further deionize distilled water for all eluents and sample mixtures.
In the procedure of anions analysis, flow velocity plays an important role, which influences the total pressure of the
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Z. Huang et al. / Food Chemistry 139 (2013) 144–148
Fig. 1. Chromatographic instrument configuration for the column-switching IC system. (A) Eluting cations from the analytical column into the waste (0–2.8 min); (B) separating and concentrating iodate in the concentrator column (2.8–5.3 min); (C) the second separation of iodate on the analytical column and eluting off the large excess of chloride ion and other anions. Injection volume: 100 lL; flow rate: 1.0 mL/min; UV detector at 213 nm; eluent concentration: 30 mmol/L (0–2.8 min), 20 mmol/L (2.8– 12 min) and 30 mmol/L (12–25 min) generated by an EGC-KOH eluent generator.
Z. Huang et al. / Food Chemistry 139 (2013) 144–148
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Table 1 Gradient elution and operation procedure of the cycling-column-switching technique. Time (min)
CKOH (mmol/L)
Ten-port valve
Six-port valve
0 – 2.8 2.8 – 5.3 5.3 – 12 12 – 20 20 – 25
30 20 20 30 30
Inject Inject Inject Inject Load
Inject Load Inject Inject Inject
chromatographic system, the valve-switching time and the analytical time. The flow rate of this ion chromatographic system was set at 1.0 mL/min with the total pressure less than 3000 psi and no suppressor leakage, for only two columns involved in the system. An EGC-KOH eluent generator was utilized to generate high-purity and contaminant free potassium hydroxide on-line successively using only deionized water, which produced a low background, a low detection limit as well as an excellent reproducibility. Most importantly, the eluent of KOH could be neutralized to water in the suppressor, which having rather low eluting power for the anions retained in the concentrator column. A gradient elution was used to achieve a satisfactory resolution and short analysis time, Table 1. A Dionex ASRS-4mm suppressor was used to turn KOH into water. Its current was set at 75 mA. The eluent from the outlet of the suppressor was collected and tested. The result showed that the pH value approached 7 and the conductivity was less than 2 lS. It could be used as the rinsing reagent of the concentrator column.
Fig. 3. Peak area of iodate under different column-switching times. Concentration of standard solution: 5 mg/L; injection volume: 100 lL; flow rate: 1.0 mL/min; UV detector at 213 nm; eluent concentration: 30 mmol/L (0–2.8 min), 20 mmol/L (2.8– 12 min) and 30 mmol/L (12–25 min) generated by an EGC-KOH eluent generator.
3.3. Optimization of column-switching conditions In this system, the column-switching time should be adjusted when the flow rate and concentration of the eluent were fixed. The first switching of the six-port valve (To load position, Fig. 1B) should be set at the point when the cations (Peak 1, Fig. 2) were all eluted out from the analytical column but iodate (Peak 2, Fig. 2) was not. It was set at 2.8 min according to Fig. 2. The concentrating time of iodate was also need to be optimized. Different column-switching times were compared from 4.7 to 5.5 min. According to Fig. 3, as the concentrating time was prolonged, the peak area of iodate increased first then did not change. Therefore, the second switching of the six-port valve (to inject position, Fig. 1C) was set at 5.3 min, with the total concentrated time of
Fig. 4. Analysis of rock salt and sea salt by the column-switching IC system. Injection volume: 100 lL; flow rate: 1.0 mL/min; UV detector at 213 nm; eluent concentration: 30 mmol/L (0–2.8 min), 20 mmol/L (2.8–12 min) and 30 mmol/L (12–25 min) generated by an EGC-KOH eluent generator.
2.5 min, then iodate was eluted through the analytical column for the second separation. In this chromatographic system, iodate was eluted through the analytical column and detector twice, and there were two peaks of iodate on the chromatograms. Due to the weak retention of iodate and the high exchange capacity of the analytical column, iodate was still eluted off earlier than chloride with good resolution. The analysis of next sample should not be started until chloride was eluted from the analytical column. It was worth to mention that the peak (Peak 2, Fig. 2) was not Gaussian distribution, for the impairment of the mass transfer kinetics caused by the co-eluting of cations. However, after concentration and the second separation, a symmetrical peak (Peak 3, Fig. 2) was obtained. Therefore, the separation with only gradient elution was not satisfied for the estimation of iodate in iodized edible salt. Fig. 2. Chromatogram of iodate (5 mg/L) in standard solution separated by the column-switching IC system. Injection volume: 100 lL; flow rate: 1.0 mL/min; UV detector at 213 nm; eluent concentration: 30 mmol/L (0–2.8 min), 20 mmol/L (2.8– 12 min) and 30 mmol/L (12–25 min) generated by an EGC-KOH eluent generator. Peak: (1) cations; (2) iodate; (3) iodate.
3.4. Validation and application After the optimization of the column-switching conditions, a linear calibration curve for the standard iodate solutions in the
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Z. Huang et al. / Food Chemistry 139 (2013) 144–148 Table 2 Method performance of iodate in iodized edible salt solutions (n = 5). Sample
Sea salt I Sea salt II Sea salt III Rock salt I Rock salt II Rock salt III
Results by proposed method
Iodometry (mg/kg)
Found (mg/kg)
Added (mg/kg)
Recovered
%
31.14 ± 0.08 30.52 ± 0.05 33.25 ± 0.07 34.09 ± 0.04 34.27 ± 0.06 38.37 ± 0.05
25 25 25 25 25 25
24.78 ± 0.05 24.60 ± 0.05 24.87 ± 0.04 25.33 ± 0.07 24.95 ± 0.07 25.41 ± 0.04
99.1 98.4 99.5 101.3 99.8 101.6
range of 0.1–10.0 mg/L was obtained with UV detection at 213 nm, for its better sensitivity to iodate than conductivity detection. A representative chromatogram of iodate in standard solution was shown in Fig. 2. Iodate exhibited satisfactory linearity with a correlation coefficient r = 0.9996. The detection limit (LOD) was 45.53 lg/L, based on the signal-to-noise ratio of 3 (S/N = 3) and a 100 lL injection volume. Relative standard deviations (RSDs) for retention time, peak area and peak height were all less than 2.1%. Six samples of edible salt marketed by six different manufacturers were analyzed. Fig. 4 shows the chromatograms of rock salt and sea salt. The large excess of chloride in the sample matrix has no interference for the determination of iodate. The iodate peak has been confirmed from the enhancement of the intensity of the iodate peak in the chromatogram by spiking the sample with a known amount of iodate. Recoveries of added iodate were in the range of 98.4–101.6% for these spiked samples. The external standard method was applied for quantitative determination of iodate in iodized edible samples. Table 2 gives the results of the ion chromatographic analysis of iodate in six commercial samples. To evaluate the proposed method, the samples were also analyzed with the conventional iodometric titration (Cunniff, 1990, chap. 11). At the 95% confidence level, the differences between the results obtained by the proposed method and the conventional method were statistically not significant, in Table 2. The concentrations of iodate measured by this method are also in good agreement with those claimed by the manufacturer. 4. Conclusion A single pump cycling-column-switching technique for the direct determination of iodate in iodized edible salt by IC with UV detection was developed, coupled with homemade high exchange capacity columns. Iodate was retained and concentrated in the concentrator column, and eluted through the analytical column and detector twice. The main advantages of the proposed method are the lack of special chemical reagents and complicating reactions required, low cost, good stability and without toxic and hazardous substances or any pretreatment. The estimation of iodate in iodized edible salt has been realized, with no interference from cations or the large excess of chloride ion in matrices. There are no significant differences between the results obtained by the proposed method and the conventional titrimetric method, and the values of iodate concentrations determined in the six commercial samples are in close agreement with the expected values as claimed by the manufacturer. Acknowledgements This research was financially supported by National Natural Science Foundation of China (Nos. 20775070, J0830413,
31.25 ± 0.23 31.02 ± 0.20 33.56 ± 0.19 34.21 ± 0.35 33.98 ± 0.27 38.08 ± 0.25
20911140271), Zhejiang Provincial Natural Science Foundation of China (Nos. R4080124, J20091495, Y4110532). References Bichsel, Y., & Gunten, U. V. (1999). Determination of iodide and iodate by ion chromatography with postcolumn reaction and UV/Visible detection. Analytical Chemistry, 71, 34–38. Borges, S. S., Peixoto, J. D. S., Feres, M. A., & Reis, B. F. (2010). Downscaling a multicommuted flow injection analysis system for the photometric determination of iodate in table salt. Analytica Chimica Acta, 668, 3–7. Buchberger, W. (1988). Determination of iodide and bromide by ion chromatography with post-column reaction detection. Journal of Chromatography A, 439, 129–135. Chen, Z. L., Megharaj, M., & Naidu, R. (2007). Speciation of iodate and iodide in seawater by non-suppressed ion chromatography with inductively coupled plasma mass spectrometry. Talanta, 72, 1842–1846. Cunniff, P. (1990). Official methods of analysis (16th ed.). Arlington: Association of Official Analytical Chemists, pp. 31. Delange, F., Bürgi, H., Chen, Z. P., & Dunn, J. T. (2002). World status of monitoring of iodine deficiency disorders control programs. Thyroid, 12(10), 915–924. Eckhoff, K. M., & Maage, A. (1997). Iodine content in fish and other food products from East Africa analyzed by ICP-MS. Journal of Food Composition and Analysis, 10, 270–282. Huang, Z. P., Zhu, Z. Y., Subhani, Q., Yan, W. W., Guo, W. Q., & Zhu, Y. (2012). Simultaneous determination of iodide and iodate in povidone iodine solution by ion chromatography with homemade and exchange capacity controllable columns and column-switching technique. Journal of Chromatography A, 1251, 154–159. Kosminsky, L., & Bertotti, M. (1999). Determination of iodate in salt samples with amperometric detection at a molybdenum oxide modified electrode. Electroanalysis, 11(9), 623–626. Kumar, S. D., Maiti, B., & Mathur, P. K. (2001). Determination of iodate and sulphate in iodized common salt by ion chromatography with conductivity detection. Talanta, 53, 701–705. Rebary, B., Paul, P., & Ghosh, P. K. (2010). Determination of iodide and iodate in edible salt by ion chromatography with integrated amperometric detection. Food Chemistry, 123, 529–534. Shabani, A. M. H., Ellis, P. S., & McKelvie, I. D. (2011). Spectrophotometric determination of iodate in iodised salt by flow injection analysis. Food Chemistry, 129, 704–707. Wang, T. L., Zhao, S. Z., Shen, C. H., Tang, J., & Wang, D. (2009). Determination of iodate in table salt by transient isotachophoresis–capillary zone electrophoresis. Food Chemistry, 112, 215–220. Weinberg, H. S., & Yamada, H. (1998). Post-ion-chromatography derivatization for the determination of oxyhalides at sub-PPB levels in drinking water. Analytical Chemistry, 70, 1–6. Xie, Z. H., & Zhao, J. C. (2004). Reverse flow injection spectrophotometric determination of iodate and iodide in table salt. Talanta, 63, 339–343. Xu, X. R., Li, H. B., Gu, J. D., & Paeng, K. J. (2004). Determination of iodate in iodized salt by reversed-phase high-performance liquid chromatography with UV detection. Chromatographia, 60, 721–723. Yao, S. Z., Chen, P., & Wei, W. Z. (1999). A quartz crystal microbalance method for the determination of iodine in foodstuffs. Food Chemistry, 67(3), 311–316. Zhang, W. N., Liu, X. Q., Jia, X. Y., Han, Y., Liu, X. L., Xie, X. J., et al. (2010). Fast speciation of iodide and iodate in edible salt and human urine by short column IC coupled with inductively coupled plasma MS. Chromatographia, 72, 1009–1012. Zhong, Y. Y., Zhou, W. F., Zhu, H. B., Zeng, X. L., Ye, M. L., Zhang, P. M., et al. (2011). A single pump column-switching technique coupled with polystyrenedivinylbenzene–carbon nanotubes column for the determination of trace anions in different concentrated organic matrices by ion chromatography. Analytica Chimica Acta, 686, 1–8.
Contents lists available at SciVerse ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
A single pump cycling-column-switching technique coupled with homemade high exchange capacity columns for the determination of iodate in iodized edible salt by ion chromatography with UV detection Zhongping Huang, Qamar Subhani, Zuoyi Zhu, Weiqiang Guo, Yan Zhu ⇑ Department of Chemistry, Xixi Campus, Zhejiang University, Hangzhou 310028, China
a r t i c l e
i n f o
Article history: Received 28 July 2012 Received in revised form 12 December 2012 Accepted 27 January 2013 Available online 6 February 2013 Keywords: Iodate Iodized edible salt Cycling-column-switching Ion chromatography
a b s t r a c t A single pump cycling-column-switching technique has been developed for the iodate analysis in edible salt. Homemade high exchange capacity columns were adopted for the separation of iodate and chloride. Iodate could be retained and concentrated in a homemade concentrator column after eluents passing through the suppressor. With UV detection, iodate exhibited satisfactory linearity in the range of 0.1– 10.0 mg/L with a correlation coefficient of 0.9996. The detection limit (LOD) was 45.53 lg/L, based on the signal-to-noise ratio of 3 (S/N = 3) and a 100 lL injection volume. Relative standard deviations (RSDs) for retention time, peak area and peak height were all less than 2.1%. Recoveries of added iodate were in the range of 98.4–101.6% for the spiked samples. The quantitative determination of iodate in edible salt was accomplished by this column-switching technique, without any pretreatment and interference. The results on six samples were statistically compared with results determined by conventional titrimetric method. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Iodine is one of the essential trace elements for producing thyroid hormones in the human body and in some other biological species (Buchberger, 1988; Yao, Chen, & Wei, 1999). Excessive iodine intake can contribute to certain thyroid disorders in susceptible individuals, whereas a deficiency of iodine in the diet can cause several diseases or problems, which include spontaneous abortion, increased infant mortality, cretinism, and stunting of intellectual functioning (Delange, Bürgi, Chen, & Dunn, 2002; Eckhoff & Maage, 1997). For the control of goitre, salt for edible purpose is compulsorily iodized by coating KIO3 on salt crystals and the recommended concentration of iodate in the salt is about 40 ppm. Today, iodized salt is an important source of iodine and cooking with iodized salt is highly promoted in China and many Asian countries. Several methods have been developed for the estimation of iodine in iodized edible salt. Conventional iodometric titration (Cunniff, 1990, chap. 11) was applied easily both in the laboratory and in the field, based on the redox reaction between iodate and iodide to give I2 that can then be titrated with thiosulphate ion. However, titration demanded of high skill of operators and was a time-consuming process, especially in the processes of preparing and standardizing the titrant of thiosulfate solution. Additionally, ⇑ Corresponding author. Tel./fax: +86 571 88273637. E-mail address: [email protected] (Y. Zhu). 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.01.070
the titration method might give false estimate of iodate with oxidizing agents such as bleaching powder (Rebary, Paul, & Ghosh, 2010). Special chemical reagents and complicated reactions were involved in spectrophotometric detection of iodate in iodized edible salt (Borges, Peixoto, Feres, & Reis, 2010; Shabani, Ellis, & McKelvie, 2011; Xie & Zhao, 2004). Estimation of iodate in iodized edible salt by transient isotachophoresis-capillary zone electrophoresis has been reported recently, with carcinogenic chromate ion as an internal standard (Wang, Zhao, Shen, Tang, & Wang, 2009). Reduction of iodate was necessary in the HPLC detection of iodate in iodized edible salt (Xu, Li, Gu, & Paeng, 2004). Electrochemical determination with modified electrode has been accomplished with unsatisfactory relative standard deviation (Kosminsky & Bertotti, 1999). The use of ion chromatography (IC) is limited for direct determination of iodate in iodized edible salt. The high chloride concentration of the matrix does not permit a clean separation and conductimetric determination of iodate in edible salt. After sample pretreatment with on-guard silver cartridge for the removal of the large excess of chloride ion, iodate in edible salt was successfully determined by IC with conductivity detection (Kumar, Maiti, & Mathur, 2001). Unfortunately, a fresh Ag cartridge was required for each sample. In addition, it was necessary to reduce iodate to iodide in estimation of iodate in iodized salt by IC with integrated amperometric detection (Rebary et al., 2010) or UV/visible detection (Bichsel & Gunten, 1999; Weinberg & Yamada, 1998). Iodate in edible salt or seawater has been estimated using
Z. Huang et al. / Food Chemistry 139 (2013) 144–148
anion-exchange chromatographic separation and ICP-MS detection (IC-ICP-MS) (Chen, Megharaj, & Naidu, 2007; Zhang et al., 2010). This is a useful method, the main limitation being equipment cost. As an effective sample pretreatment method, column-switching technique has been widely used in ion chromatographic system due to its automated analysis. Zhong et al. (2011) proposed a single pump column-switching technique for the determination of trace anions in different concentrated organic matrices by IC. In this chromatographic system, only a single pump with a ten-port valve and six-port valve were involved, much simpler than other column-switching techniques. The later, two pumps with two valves or a pump with four valves were necessary. In addition, considering the eluent was neutralized to water in the suppressor, anions were supposed to be retained and concentrated in a concentrator column for the next analysis. To the best of our knowledge, there was no report on the direct determination of iodate in iodized edible salt by IC with conductivity or UV/visible detection without any pretreatment. In the present study, a single pump cycling-column-switching technique coupled with homemade high exchange capacity columns was first proposed, to analyze iodate directly in iodized edible salt by IC with UV detection. Homemade high exchange capacity columns were adopted, due to the lack of proper commercial IC columns permitting a clean separation of iodate with the high concentration chloride of the matrix. Iodate could be separated completely with the high concentration chloride on the homemade columns, due to the high exchange capacity. The homemade columns were based on polystyrene-divinylbenzene (PSDVB) resins, functionalized with a large amount of quaternary ammonium groups (Huang et al., 2012). With the single pump cycling-column-switching technique, the estimation of iodate in iodized edible salt has been realized, with no interference from cations or the large excess of chloride ion in matrices. 2. Experimental 2.1. Equipment A Dionex (Sunnyvale, CA, USA) ICS 2100 was employed for all the chromatographic separations, equipped with a dual-piston serial pump, a DS6 heated conductivity detector, a column heater, a Rheodyne (Cotati, CA, USA) P/N PR070108B ten-port valve fitted with a 100 lL sampling loop, a Rheodyne (Cotati, CA, USA) Model 9900-013 six-port valve and an EGC-KOH eluent generator. Suppression was achieved with a Dionex ASRS-4mm suppressor. A Dionex Ultimate 3000 UV detector was used to determine iodate. Polyether ether ketone (PEEK) tubes with the lengths as short as possible were used to connect all chromatographic hardware. The eluent flow rate was 1.0 mL/min. Data were collected with Chromeleon 6.80 chromatogram workstation (Dionex, USA). A PEEK column (150 mm 4.0 mm) was used as an analytical column and a PEEK column (35 mm 4.6 mm) was used as a concentrator column. Both the columns were packed with homemade anion exchange resins. The resins with particle size about 7 lm were prepared according to the method we reported before, functionalized with 11-layer quaternary ammonium groups (Huang et al., 2012). The pressure of the concentrator column placed behind of the suppressor should be less than 100 psi to protect the suppressor.
145
The stock solution of 1000 mg/L iodate was prepared by dissolving 0.1224 g potassium iodate (Tianjin Chemical Reagent Co. Ltd., Tianjin, China) in water and diluting to the mark in a 100 mL volumetric flask. Standard solutions were prepared by dilution of the stock solution of 1000 mg/L iodate with 2% (w/v) NaCl (Huipu Chemical Reagent Co. Ltd., Hangzhou, China) water solution in a 100 mL volumetric flask. Iodized edible salt were obtained from the local market. Sample solutions were prepared by dissolving an accurate weight of 2 g to the nearest 0.1 mg of edible salt in water and making up to 100 mL. By this preparation, sample solutions would contain approximately 2% (w/v) NaCl. 2.3. System operation procedure There were four steps involved for the estimation of iodate in iodized edible salt (Fig. 1): (1) filling the sample loop, (2) eluting cations from the analytical column into the waste, (3) separating and concentrating iodate in the concentrator column, (4) the second separation of iodate on the analytical column and eluting off the large excess of chloride ion and other anions. First, the sample was loaded into the 100 lL sample loop on the ten-port valve. To ensure that the loop contained a representative sample, at least four loop volumes were injected. Then, the ten-port valve containing sample was switched to inject position while the six-port valve was kept on inject position (Fig. 1A). Cations were all eluted out from the analytical column into the waste. After that, the six-port valve was switched to load position. Iodate was separated on the analytical column for the first time and concentrated in the concentrator column (Fig. 1B). After the complete concentration of iodate, the six-port valve was switched back to inject position for the second separation of iodate (Fig. 1C). Because of the weak retention of iodate and the high exchange capacity of the analytical column, iodate was still eluted off earlier than chloride with good resolution. The large excess of chloride and other anions were all eluted out from the analytical column into the waste after the elution of iodate. A gradient elution was used to achieve a satisfactory resolution and short analysis time. 3. Results and discussion 3.1. Characteristics of homemade columns The analytical column, packed using a QP 6000 packing pump (Chuang Xin Tong Heng Science and technology Co. Ltd., Beijing, China), had a pressure less than 1500 psi when the eluent at 1.0 mL/min. While the concentrator column, packed by the drypacking method, had a pressure less than 100 psi when the eluent at 1.0 mL/min. An eluent containing 0.02 mol/L NO3 was employed to determine the exchange capacities, and the capacities were 0.420 mmol/column and 0.031 mmol/column respectively for the analytical column and concentrator column. Compared with the Dionex IonPac AS11-HC (4 mm 250 mm) analytical column, the capacity of the analytical column was much higher, making it possible to separate iodate with the high concentration chloride of the matrix. The concentrator column with the pressure less than 100 psi at 1.0 mL/min was quite suitable for this proposed system, because the suppressor was easy to leak if the back pressure more than 100 psi.
2.2. Reagents 3.2. Selection of chromatographic parameter All reagents were of analytical reagent grade and a water purification system (Millipore, Milford, MA, USA) was used to further deionize distilled water for all eluents and sample mixtures.
In the procedure of anions analysis, flow velocity plays an important role, which influences the total pressure of the
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Fig. 1. Chromatographic instrument configuration for the column-switching IC system. (A) Eluting cations from the analytical column into the waste (0–2.8 min); (B) separating and concentrating iodate in the concentrator column (2.8–5.3 min); (C) the second separation of iodate on the analytical column and eluting off the large excess of chloride ion and other anions. Injection volume: 100 lL; flow rate: 1.0 mL/min; UV detector at 213 nm; eluent concentration: 30 mmol/L (0–2.8 min), 20 mmol/L (2.8– 12 min) and 30 mmol/L (12–25 min) generated by an EGC-KOH eluent generator.
Z. Huang et al. / Food Chemistry 139 (2013) 144–148
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Table 1 Gradient elution and operation procedure of the cycling-column-switching technique. Time (min)
CKOH (mmol/L)
Ten-port valve
Six-port valve
0 – 2.8 2.8 – 5.3 5.3 – 12 12 – 20 20 – 25
30 20 20 30 30
Inject Inject Inject Inject Load
Inject Load Inject Inject Inject
chromatographic system, the valve-switching time and the analytical time. The flow rate of this ion chromatographic system was set at 1.0 mL/min with the total pressure less than 3000 psi and no suppressor leakage, for only two columns involved in the system. An EGC-KOH eluent generator was utilized to generate high-purity and contaminant free potassium hydroxide on-line successively using only deionized water, which produced a low background, a low detection limit as well as an excellent reproducibility. Most importantly, the eluent of KOH could be neutralized to water in the suppressor, which having rather low eluting power for the anions retained in the concentrator column. A gradient elution was used to achieve a satisfactory resolution and short analysis time, Table 1. A Dionex ASRS-4mm suppressor was used to turn KOH into water. Its current was set at 75 mA. The eluent from the outlet of the suppressor was collected and tested. The result showed that the pH value approached 7 and the conductivity was less than 2 lS. It could be used as the rinsing reagent of the concentrator column.
Fig. 3. Peak area of iodate under different column-switching times. Concentration of standard solution: 5 mg/L; injection volume: 100 lL; flow rate: 1.0 mL/min; UV detector at 213 nm; eluent concentration: 30 mmol/L (0–2.8 min), 20 mmol/L (2.8– 12 min) and 30 mmol/L (12–25 min) generated by an EGC-KOH eluent generator.
3.3. Optimization of column-switching conditions In this system, the column-switching time should be adjusted when the flow rate and concentration of the eluent were fixed. The first switching of the six-port valve (To load position, Fig. 1B) should be set at the point when the cations (Peak 1, Fig. 2) were all eluted out from the analytical column but iodate (Peak 2, Fig. 2) was not. It was set at 2.8 min according to Fig. 2. The concentrating time of iodate was also need to be optimized. Different column-switching times were compared from 4.7 to 5.5 min. According to Fig. 3, as the concentrating time was prolonged, the peak area of iodate increased first then did not change. Therefore, the second switching of the six-port valve (to inject position, Fig. 1C) was set at 5.3 min, with the total concentrated time of
Fig. 4. Analysis of rock salt and sea salt by the column-switching IC system. Injection volume: 100 lL; flow rate: 1.0 mL/min; UV detector at 213 nm; eluent concentration: 30 mmol/L (0–2.8 min), 20 mmol/L (2.8–12 min) and 30 mmol/L (12–25 min) generated by an EGC-KOH eluent generator.
2.5 min, then iodate was eluted through the analytical column for the second separation. In this chromatographic system, iodate was eluted through the analytical column and detector twice, and there were two peaks of iodate on the chromatograms. Due to the weak retention of iodate and the high exchange capacity of the analytical column, iodate was still eluted off earlier than chloride with good resolution. The analysis of next sample should not be started until chloride was eluted from the analytical column. It was worth to mention that the peak (Peak 2, Fig. 2) was not Gaussian distribution, for the impairment of the mass transfer kinetics caused by the co-eluting of cations. However, after concentration and the second separation, a symmetrical peak (Peak 3, Fig. 2) was obtained. Therefore, the separation with only gradient elution was not satisfied for the estimation of iodate in iodized edible salt. Fig. 2. Chromatogram of iodate (5 mg/L) in standard solution separated by the column-switching IC system. Injection volume: 100 lL; flow rate: 1.0 mL/min; UV detector at 213 nm; eluent concentration: 30 mmol/L (0–2.8 min), 20 mmol/L (2.8– 12 min) and 30 mmol/L (12–25 min) generated by an EGC-KOH eluent generator. Peak: (1) cations; (2) iodate; (3) iodate.
3.4. Validation and application After the optimization of the column-switching conditions, a linear calibration curve for the standard iodate solutions in the
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Z. Huang et al. / Food Chemistry 139 (2013) 144–148 Table 2 Method performance of iodate in iodized edible salt solutions (n = 5). Sample
Sea salt I Sea salt II Sea salt III Rock salt I Rock salt II Rock salt III
Results by proposed method
Iodometry (mg/kg)
Found (mg/kg)
Added (mg/kg)
Recovered
%
31.14 ± 0.08 30.52 ± 0.05 33.25 ± 0.07 34.09 ± 0.04 34.27 ± 0.06 38.37 ± 0.05
25 25 25 25 25 25
24.78 ± 0.05 24.60 ± 0.05 24.87 ± 0.04 25.33 ± 0.07 24.95 ± 0.07 25.41 ± 0.04
99.1 98.4 99.5 101.3 99.8 101.6
range of 0.1–10.0 mg/L was obtained with UV detection at 213 nm, for its better sensitivity to iodate than conductivity detection. A representative chromatogram of iodate in standard solution was shown in Fig. 2. Iodate exhibited satisfactory linearity with a correlation coefficient r = 0.9996. The detection limit (LOD) was 45.53 lg/L, based on the signal-to-noise ratio of 3 (S/N = 3) and a 100 lL injection volume. Relative standard deviations (RSDs) for retention time, peak area and peak height were all less than 2.1%. Six samples of edible salt marketed by six different manufacturers were analyzed. Fig. 4 shows the chromatograms of rock salt and sea salt. The large excess of chloride in the sample matrix has no interference for the determination of iodate. The iodate peak has been confirmed from the enhancement of the intensity of the iodate peak in the chromatogram by spiking the sample with a known amount of iodate. Recoveries of added iodate were in the range of 98.4–101.6% for these spiked samples. The external standard method was applied for quantitative determination of iodate in iodized edible samples. Table 2 gives the results of the ion chromatographic analysis of iodate in six commercial samples. To evaluate the proposed method, the samples were also analyzed with the conventional iodometric titration (Cunniff, 1990, chap. 11). At the 95% confidence level, the differences between the results obtained by the proposed method and the conventional method were statistically not significant, in Table 2. The concentrations of iodate measured by this method are also in good agreement with those claimed by the manufacturer. 4. Conclusion A single pump cycling-column-switching technique for the direct determination of iodate in iodized edible salt by IC with UV detection was developed, coupled with homemade high exchange capacity columns. Iodate was retained and concentrated in the concentrator column, and eluted through the analytical column and detector twice. The main advantages of the proposed method are the lack of special chemical reagents and complicating reactions required, low cost, good stability and without toxic and hazardous substances or any pretreatment. The estimation of iodate in iodized edible salt has been realized, with no interference from cations or the large excess of chloride ion in matrices. There are no significant differences between the results obtained by the proposed method and the conventional titrimetric method, and the values of iodate concentrations determined in the six commercial samples are in close agreement with the expected values as claimed by the manufacturer. Acknowledgements This research was financially supported by National Natural Science Foundation of China (Nos. 20775070, J0830413,
31.25 ± 0.23 31.02 ± 0.20 33.56 ± 0.19 34.21 ± 0.35 33.98 ± 0.27 38.08 ± 0.25
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