Determination of Iodate by HPLC-UV after On-Line Electrochemical Reduction to Iodide

Journal of Chromatographic Science 2015;53:280– 284 doi:10.1093/chromsci/bmu053 Advance Access publication July 6, 2014 Article Determination of Iodate by HPLC-UV after On-Line Electrochemical Reduction to Iodide Tao Wang1†, Weimei Lin2†, Xueliang Dai2†, Lijun Gao1, Bing Wang2 and Dongqin Quan1* 1 Beijing Institute of Pharmacology and Toxicology, Beijing 100850, People’s Republic of China, and 2Beijing Techmate Technology Corporation Limited, Beijing 100070, People’s Republic of China *Author to whom correspondence should be addressed. Email: (D. Q.) † These authors contributed equally to this work. Received 6 April 2013; revised 1 May 2014 In this study, a novel on-line pre-column electrochemical instrument (PECI) coupled with high-performance liquid chromatography (HPLC) was developed, and a novel method based on PEC–HPLC-UV for amplifying the ultraviolet (UV) response of iodate (IO3 2 ) was studied. Iodate undergoes reduction in the PECI, and the resulting I2 was injected to an HPLC system and detected by a UV detector. For IO3 2 analysis, conditions that can influence the reduction efficiency, including applied potential, pH value and salt concentration, were investigated in detail. In an appropriate condition, the UV response of iodate after passing through PECI was almost 10 times more than that of the initial form with good precision (relative standard deviation 2.0 –4.3%). The detection limit and quantity limit were 9 and 20 ng, respectively. It can be concluded that the proposed method is simple and highly sensitive. reversed-phase or ion chromatography mode column and direct UV detection has been developed for the separation and quantification of periodate, iodate and iodide (8, 9). It is also aimed to develop a new method based on improving the UV absorption of iodate. The purpose of this approach was to develop a novel instrument including pre-column electrochemical instrument (PECI) and coupling it with HPLC-UV to determine IO3  in iodized salt. In this study, IO3  undergoes reduction in PECI and produces iodide (I2 ): IO3  þ 6e  þ 6Hþ ! I þ 3H2 O. Then, the resulting I2 was retained and isolated by a column and detected by an UV detector. The UV response of I2 is nearly 10 times more than that of IO3  . Therefore, the proposed method is simple and highly sensitive. Experimental Introduction Iodine deficiency is the greatest single cause of preventable brain damage and mental retardation (1, 2). Remarkable success has been achieved by common use of iodized salt in China since 1994. However, occasional adverse effects occurred. The principal effect is iodine-induced hyperthyroidism (3). Therefore, the China National Standard decided that iodized salt must contain: no less than 25 mg kg21, and no more than 50 mg kg21 of iodine. At the very beginning, salt was “ iodized” by the addition of potassium iodide (KI); nowadays, the most common form of iodine in iodized salt is potassium iodate (KIO3). Many methods based on different principles have been proposed for determination of iodate (IO3  Þ, including spectrophotometry (4, 5), ion chromatography (6) and high-performance liquid chromatography (HPLC) (7). In recent years, ion chromatography has been used to determine iodide in seawater, urine and other natural samples. At the very beginning, ion chromatography equipped with an ultraviolet (UV) detector was developed in which the electrochemical detector (ED) was used to detect the iodide. However, some challenges still exit, in particular the instability of ED. On the other hand, formation of large amount of matrix ions (chloride, sulfate and other organic ions) impedes the determination of the target analysts by the way of saturating the active sites of ion-exchange column; and high price also hinders from spreading the ion chromatography in Chinese laboratories. HPLC with UV detector becomes a more significant method among all the methods of IO3  analysis and it is more commonly used in conventional analysis laboratories. An HPLC system with a Apparatus The HPLC-UV system consisted of a 3001 high-pressure pump equipped with a 3010 degasser, a 3002 UV-visible detector, a 3006 autosampler and a 3004 column oven (Shiseido, Tokyo, Japan). A TSK-GEL-NH2-100 column (Tosoh, Tokyo, Japan) was used for analysis. The PECI system comprised an HTEC500 high-pressure pump equipped with a dependant degasser, a PEC-500 ED (Eicom, Kyoto, Japan), which was used as the reduction reactor, and a 3011 high-voltage switching six-way valve (Shiseido), which overcomes the high back pressure of the column and protect the electrode and cell in the PECI. The operating conditions for PECI-HPLC-UV are given in Table I. The samples were introduced by the autosampler (Shiseido), transferred by a mobile phase into the reduction reactor and then accommodated in the loop before detecting by the HPLC-UV system. Standard solution and reagents Reverse osmosis-Milli Q water (18 MV) (Millipore Corp., Bedford, USA) was used for all solutions and dilutions. The iodide and iodate stock solutions were 1.0 mg mL21, which were prepared by dissolving 0.1103 g of potassium iodide (Sigma, USA) and 0.1024 g of potassium iodate (Sigma, Milwaukee, USA) in 100 mL of water, respectively. The stock solutions were stored under dark condition at 48C. The working standard solutions were prepared by suitable dilution of the stock solutions with water. # The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: Iodized salt was prepared by adding potassium iodate to sodium chloride (China National Pharmaceutical Group, Beijing, China). Acetonitrile (ACN) was purchased from Fisher Scientific (HPLC grade, Fair lawn, NJ, USA). Analytical grade sodium dihydrogen phosphate and phosphoric acid were bought from China Table I The Operating Conditions for PECI-HPLC-UV PECI system Mobile phase 1 Flow rate Applied potential Peek loop volume Switch internal time Cell temperature HPLC-UV system Mobile phase 2 Stationary phase Flow rate Column temperature Detected wavelength Sodium dihydrogen phosphate buffer solution (pH 7.5 –2.0, salt concentration 50 –200 mmol L21) 0.2 mL min21 2600 to 21,700 mV 200 mL 30 s 358C Acetonitrile –50 mmol L21 sodium dihydrogen phosphate buffer solution (pH 3.0) (50 : 50, v/v) TSK-GEL-NH2 0.5 mL min21 358C 215 nm National Pharmaceutical Group (Beijing, China). All mobile phases were degassed prior to use either by vacuum or by ultrasonic wave. Procedure A schematic diagram of PECI-HPLC-UV system was illustrated in Figure 1. A PECI, consisting of a pump, a sampler, a high-voltage switching six-way valve and a coulometric ED, was used for online coupling with the HPLC-UV system. As shown in Figure 1, the analyzed chemicals were sent to the HPLC-UV system and detected by the UV detector after passing through the PECI system undergoing a reduction or oxidation. The designed analytical programs included two steps. Initially, when (...truncated)