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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 (2012) 154–159
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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Simultaneous determination of iodide and iodate in povidone iodine solution by ion chromatography with homemade and exchange capacity controllable columns and column-switching technique Zhongping Huang, Zuoyi Zhu, Qamar Subhani, Wenwu Yan, Weiqiang Guo, Yan Zhu ∗ Department of Chemistry, Xixi Campus, Zhejiang University, Hangzhou 310028, China
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Article history: Received 11 April 2012 Received in revised form 7 June 2012 Accepted 14 June 2012 Available online 25 June 2012 Keywords: Ion chromatography Iodide Iodate Polystyrene-divinylbenzene Column-switching Povidone iodine solution
a b s t r a c t A simple ion chromatographic method for simultaneous detection of iodide and iodate in a single running was proposed, with columns packed with homemade functionalized polystyrene-divinylbenzene (PSDVB) resins and column-switching technique. Homemade resins were functionalized with controllable amounts of quaternary ammonium groups. The low-capacity anion-exchange column and high-capacity anion-exchange column were prepared, due to the resins having different exchange capacities. With this method, iodide and iodate in povidone iodine solution were detected simultaneously in a short time with iodide being eluted off first. A series of standard solutions consisting of target anions of various concentrations from 0.01 mg/L to 100 mg/L were analyzed. Each anion exhibited satisfactory linearity, with correlation coefficient r ≥ 0.9990. The detection limits (LODs) for iodide and iodate obtained by injecting 100 L of sample were 5.66 and 14.83 g/L (S/N = 3), respectively. A spiking study was performed with satisfactory recoveries between 101.2% and 100.6% for iodide and iodate. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ion chromatography (IC) is one of the most commonly applied techniques for the separation of various ionic compounds [1–3]. Since IC offers many possibilities of application, it is used in different branches of current analytical chemistry. However, a complicated sample will require a long analytical time because of a wide range of retention times of ions, and sometimes our targets are ions with only weak retention and strong retention. So there is no need to separate all the ions in the sample. The column-switching technique is one of the effective methods to solve the above problem. There are several column-switching techniques used in IC. Ion-exclusion chromatography columns and ion-exchange chromatography columns are commonly used as switching columns in ion chromatographic system. The former [4–8] is mainly utilized to eliminate the matrices of weakly ionized compounds such as organic acids and weak inorganic acids. The latter [9–11] is widely adopted for on-line sample clean-up of concentrated organic solvents and pre-concentration of target ions, as well as the elimination of high salinity [12]. Zhu et al. [13] proposed a novel method for fast determination of hexafluorophosphate and some inorganic anions by the column-switching
∗ Corresponding author. Tel.: +86 571 88273637; fax: +86 571 88273637. E-mail address: [email protected] (Y. Zhu). 0021-9673/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chroma.2012.06.059
technique. With the above technique, ions strongly retained are expected to be eluted off into the conductivity detector prior to ions weakly retained. The key point of this method is to find the proper exchange capacity columns, which is difficult and discommodious for commercial columns. The laboratory must keep a large reserve of columns to be chosen. So, it is necessary to propose a simple method for preparing resins and columns, which is suitable for laboratory preparation and the control of exchange capacity. As an essential micronutrient in seawater and sea products, iodine, existing as iodide and iodate, plays a special role in biological processes, including geochemically or biologically active processes, hazardous contaminant, etc. [14]. The determination of iodine in seawater has long been an essential task in marine chemistry. On the other hand, iodide is an essential component of the thyroid hormones that play a decisive role in human growth and metabolism [15], especially of the brain. Iodide deficiency in humans could cause several diseases or problems, such as goiter, stillbirth and miscarriage, neonatal and juvenile thyroid deficiency, dwarfism, mental defects, deaf-mutism, spastic weakness and paralysis [16]. Several methods have been developed for the estimation of iodine in seawater and iodized edible salt, such as spectrophotometry [17], spectrofluorimetry using cadmium sulfide quantum dots as fluorescence probes [18], electrochemical determination with modified electrode [19,20], gas chromatography combined with mass spectrometry after the derivation of iodine [21–23], transient isotachophoresis capillary zone electrophoresis with UV detection
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[24–26], and flow inject with chemiluminescence [27] or UV detection [28,29]. Additionally, the effective separation method, IC, has been used to determine iodine species, with amperometric detection [30,31], UV detection [32], inductively coupled plasma mass spectrometry (ICP-MS) [33,34], or with conductivity detection [35]. However, most of the methodologies described in the literature concerning iodine speciation are focused only on the iodide determination or iodate determination. It is necessary to oxidize iodide or reduce iodate for the determination of the total amount of iodine, resulting in a lack of methods permitting simple, rapid, and low-cost determination both of iodide and iodate in a single running. Iodate and iodide in seawater have been estimated simultaneously using anion-exchange chromatographic separation and ICP-MS detection (IC-ICP-MS) [33,34]. This is a useful method, the main limitation being equipment cost. Simultaneous determination of iodide and iodate could be carried out in a single running with the column-switching technique, without complex pretreatments. And only an IC system with two valves is needed, with low requirement of instruments. In the present study, in order to shorten the analytical time and accomplish a simultaneous detection of iodide and iodate in a single running, a simple IC technique was proposed, with homemade columns and column-switching technique. All the mentioned columns were packed with homemade functionalized polystyrene-divinylbenzene resins, due to the lack of proper commercial columns which could be used to separate iodide and iodate simultaneously. Homemade resins are based on polystyrene-divinylbenzene (PS-DVB) co-polymer functionalized with a amount of quaternary ammonium groups. The low-capacity anion-exchange resin and high-capacity anion-exchange resin have been prepared, respectively, due to the controllable exchange capacity. The column with low-capacity resin is able to retain iodide which is strong retention, and those anions weakly retained are almost not reserved. The column with high-capacity resin is able to separate anions with weak retention, such as fluoride and iodate. With the column of low-capacity and column-switching technique, those anions weakly retained are analyzed by the subsequent elution through the column of high-capacity, but iodide did not pass through the high-capacity column and was eluted off first. The simultaneous analysis of iodide and iodate in povidone iodine solution has been realized. The result shows that the system we adopted with homemade stationary phases and column-switching technique is a robust, convenient, and practical device for simultaneous detection of iodide and iodate.
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) Model 9900-013 six-port valve fitted with a 100 L sampling loop, a Rheodyne (Cotati, CA, USA) P/N PR070108B ten-port valve and an EGC-KOH eluent generator. Suppression was achieved with a Dionex ASRS-4mm suppressor. The columns used in the study were prepared following the procedure shown in Section 2.3. 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 JY92- ultrasonic disrupter (Scientz Biotechnology Co. Ltd., Ningbo, China) was employed to emulsify organic compounds. The Ultimate 3000 UV detector (Dionex, USA) was used to determine dynamic adsorption. The content of nitrogen in the stationary phase
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was analyzed by utilizing an elemental analyzer (Flash EA1112, Thermofinnigan). The scanning electron microscopy (SEM) images were obtained using Hitachi S-4700 field emission scanning electron microscope (Hitachi, Japan). All columns were packed using a QP 6000 packing pump (Chuang Xin Tong Heng Science and Technology Co. Ltd., Beijing, China). 2.2. Reagents The used reagents were obtained from Huipu Chemical Reagent Co. Ltd. (Hangzhou, China), unless otherwise noted. Styrene (ST), divinylbenzene (DVB) (55+%, Zhengguang Chemical Plant, Hangzhou, China) and glycidol methacrylate (GMA) (95+%, Tokyo Kasei Kogyo Co. Ltd., Tokyo, Japan) were used after distillation under reduced pressure. Azobisisobutyronitrile (AIBN) and benzyl peroxide (BPO) were recrystallized before use. 1,4-Butanediol diglycidyl ether (BDDE, 60% in H2 O, v/v) and methylamine (MA, 40% in H2 O, v/v) were bought from Aladdin Chemical Co. Ltd. (Shanghai, China). Polyvinylpyrrolidone (PVP, K-30), dibutyl phthalate (DBP), polyvinyl alcohol (PVA, 1750 ± 50), sodium dodecylsulfonate (SDS) and toluene were all employed in the synthesis. A water purification system (Millipore, Milford, MA, USA) was used to further deionize distilled water for all eluents and sample mixtures. All reagents employed for the synthesis were of analytical reagent grade. The standard stock solutions containing 1000 mg/L of target ions were prepared by dissolving appropriate amount of salts that contained ions of interest in 100 mL deionized water. Sample solutions were produced by dissolving proper amount of povidone iodine solution in 100 mL deionized water, and filtered through a 0.22-m membrane filter and SPE-C18 column (Tianjin Fuji Science & Technology Co. Ltd., Tianjin, China) before injection. 2.3. Synthetic procedure of functionalized PS-DVB column 2.3.1. Synthesis of polymer seeds Monodisperse polystyrene (PS) seed particles were prepared according to the method that we reported before [36]. The procedure was as follows: 3 g PVP and 100 mL mixture of ethanol and water (95:5, v/v) were added to a 250 mL flask, and then a mixture of 0.7 g AIBN and 17.75 g ST was added dropwise. The mixture was stirred overnight at 70 ◦ C under dry nitrogen. After filtration and washing, the average particle size of the seeds obtained was about 1.8 m by SEM. 2.3.2. Preparation of PS-DVB beads A blend of 0.56 g polymer seeds and 15 mL SDS aqueous solution (0.2%, w/v) was placed into a 500 mL three neck flask. Then, an emulsified solution containing 4 g DBO, 30 mL SDS aqueous solution (0.2%, w/v) was added and stirred overnight. Whereafter, another emulsified mixture comprised of 30 g organic compounds and 250 mL PVA aqueous solution (1%, w/v) was prepared by an ultrasonic disrupter and poured into the flask for swelling. The above organic compounds consisted of ST, DVB, toluene, BPO and SDS. The amount of toluene equalled the sum of ST and DVB, while BPO was only 1.5% of the sum (w/w). The amount of SDS was 0.25% of the PVA aqueous solution (w/v). After 24 h, the temperature was increased to 70 ◦ C under nitrogen atmosphere and lasted for another 24 h. Subsequently, the resulting beads were washed successively with hot water and alcohol. Then the beads were extracted with toluene for 48 h. After washing and drying, the resulting particle size of the stationary phase was about 7 m. 2.3.3. Functionalization of exchange capacity controllable PS-DVB beads A blend of 4.0 g PS-DVB beads and 150 mL SDS ethanol solution (0.04%, w/v) was placed into a 250 mL four neck flask. An ethanol
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Fig. 1. Scheme of stationary phase synthesis. (A) Copolymerization of PS-DVB beads and GMA; (B) first reaction cycle with primary amine; (C) first reaction cycle with diepoxide monomer; (D) second reaction cycle with primary amine; (E) second reaction cycle with diepoxide monomer.
solution consisting of 0.8 g GMA and 0.2 g AIBN was added dropwise under nitrogen atmosphere, and the temperature was increased to 70 ◦ C for 24 h. Subsequently, the resulting beads were washed successively with hot water and alcohol. GMA with double bonds could copolymerize with the residual double bonds on the surface of PS-DVB beads and also between themselves, to form a superficial microsphere layer. Then the beads were quaternized by a multi-step synthesis [37] shown in Fig. 1. The basic condensation polymerization chemistry depends on MA and BDDE as monomers to build a branching polymer that has quaternary ammonium anion exchanger sites. 20 mL MA (4%, v/v) was added to the resin and allowed to react, later filtered and was rinsed with deionized water. Next 20 mL BDDE (10%, v/v) was added to the resin and allowed to react, later filtered and was rinsed with deionized water. In each step, the synthesis time was 30 min at a temperature of 60 ◦ C. Both the steps were repeated
2, 3, 5, 7, 9 and 11 times to make 2, 3, 5, 7, 9 and 11 layers on beads, respectively. The resulting beads were anion-exchanger resin with quaternary ammonium groups. 2.3.4. Column packing procedure All the columns (PEEK, 150 mm × 4.0 mm) were slurry packed with the stationary phases obtained above, by pressing with deionized water as packing solvent under a working pressure of 40 MPa. The volume of packing solvent passing through the column should be at least 500 mL. 2.4. Breakthrough curves The breakthrough curves of the anion-exchange resins were determined with a Dionex ICS 2100 system and an UV detector. An eluent containing 0.02 mol/L NO3 − was employed to determine the
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Fig. 2. Chromatographic instrument configuration for this column-switching system. (A) Separating anions with weak retention from strong retention (0–1.5 min); (B) analyzing the target anions and washing analytical column II.
adsorption capacities of all homemade columns, with the flow rate at 1.0 mL/min. The PEEK tubes in front of the analytical columns were filled with eluent before determination, to minimize void volume.
2.5. System operation procedure There were three steps involved for the simultaneous analysis of iodide and iodate in povidone iodine solution (Fig. 2): (1) filling the sample loop, (2) separating anions with weak retention from strong retention in analytical column I, (3) analyzing the target anions and washing analytical column II. First, the sample was loaded into the 100 L sample loop. To ensure that the loop contained a representative sample, at least four loop volumes were injected. Then, the six-port valve containing sample was switched to inject position and the ten-port valve was also switched to inject position to transport the sample into analytical column I (Fig. 2A). Anions with weak retention were washed down from analytical column I without almost any retention because of the low anionexchange capacity, but anions with strong retention were retained and washed down slowly. The result was that the anions with weak retention were all eluted into analytical column II but anions with strong retention were kept on analytical column I. After that, the two valves were switched to load positions, respectively, and the subsequent analysis started (Fig. 2B). With analytical column I and column-switching technique, those anions weakly retained were analyzed by the subsequent elution through analytical column II, but iodide did not pass through analytical column II and was eluted off first. It was difficult to perform for commercial columns, due to the particular exchange capacities of analytical column I and II.
3. Results and discussion 3.1. Characterizations of the stationary phase 3.1.1. Elemental analysis Nitrogen atoms in the quaternary ammonium groups were a main place where interaction between the stationary phase and analyte occurred. The content of nitrogen atoms in the stationary phase decided the anion-exchange capacity. Elemental analysis was performed for the PS-DVB beads modified with different numbers of bonded polymer layers: 2, 3, 5, 7, 9 and 11 layers. The percentages of nitrogen were as follows: 0.34%, 0.66%, 0.79%, 0.91%, 1.08% and 1.51% for 2, 3, 5, 7, 9 and 11 polymer layers in the stationary phases, respectively. As reported, the percentage of nitrogen in the stationary phases was increased with the number of bonded layer but not exponentially. That was because of the steric hindrance exponential growth. The anion-exchange capacity of the stationary phase was still controllable, though the percentage of nitrogen did not increase exponentially. 3.1.2. Chromatographic investigation on the PS-DVB-based stationary phase The ion chromatographic evaluation was performed using a test mixture of inorganic anions composed of fluoride, chloride, nitrite, bromide, nitrate and sulfate. Inorganic anions were separated with all synthesized stationary phases, which contained 3, 5 and 7 bonded layers, using a 15 mM hydroxide solution as the mobile phase. Representative chromatograms using the 3-, 5-, 7- and 9-layered stationary phase are shown in Fig. 3. Under these conditions, the stationary phase with nine bonded layers has too strong retention,
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Fig. 3. Separations of conventional inorganic anions on the: (A) 3-, (B) 5-, (C) 7-, (D) 9-layers bonded phases. Column dimensions: 4.0 mm I.D. by 150 mm; eluent: 15 mM KOH; flow rate: 1.0 mL/min; injection volume: 25 L; conductivity detector. Peaks: (1) fluoride, 2 mg/L; (2) chloride, 4 mg/L; (3) nitrite, 10 mg/L; (4) bromide, 10 mg/L; (5) nitrate, 10 mg/L; (6) sulfate, 10 mg/L.
separating fluoride and chloride in 35 min. And the stationary phase with three bonded layers has low retention and selectivity. Other stationary phases allowed separating all the inorganic anions in the test mixture with good resolution and asymmetry factors. The hydrophilicity of the resins is very good because of the quaternary ammonium functions surrounded by hydroxy groups. High resolution between fluoride and the water dip could be obtained. The retention order was typical for the elution of inorganic anions in IC. Higher retention was obtained with the same charge and higher radius, which was because of the different hydration enthalpies. The order of elution was independent of the number of bonded layers in the stationary phase. Symmetrical peaks of all tested anions were observed by using all synthesized stationary phases. As expected, the retention factor of all anions strongly depended on the number of bonded layers. The retention of anions increased from 3 to 9 bonded layers as the increase of the number of quaternary ammonium groups present in the structure of the bonded phase. Unfortunately, the increase of the ion exchange capacity of the column, generated by an increase of the number of exchange layers, might cause an impairment of the mass transfer kinetics, leading to peak broadening and loss of separation efficiency. It might be improved by adding some organic solvents into the eluent. 3.1.3. Breakthrough curves Anion exchange capacity could be obtained according to the breakthrough curve. An eluent containing 0.02 mol/L NO3 − was employed to determine the adsorption capacities of all the columns, with the flow rate at 1.0 mL/min and UV detector. The PEEK tubes in front of the columns were filled with eluent before determination, to minimize void volume. The anion exchange capacities of the homemade columns were as follows: 0.028 mmol/column, 0.078 mmol/column, 0.136 mmol/column, 0.196 mmol/column, 0.320 mmol/column and 0.420 mmol/column, for 2, 3, 5, 7, 9 and 11 polymer layers of the stationary phases, respectively. As expected, the anion exchange capacities increased from 2 to 11 bonded layers as the increase of nitrogen content in the stationary phases. Fluoride and iodate could only be separated on an 11-layered stationary phase with low concentration of eluent, because of their very similar retention. The exchange-capacity of the column with the 3-layer phase was a little high for the elution of iodide in the column-switching system. So analytical columns I and
Fig. 4. Chromatogram of three anions separated by the column-switching IC system. Injection volume: 100 L; flow rate: 1.0 mL/min; conductivity detector; eluent concentration: 3.5 mmol/L KOH generated by an EGC-KOH eluent generator; switching point: 1.5 min. Peaks: (1) iodide, 25 mg/L; (2) fluoride, 0.25 mg/L; (3) iodate, 25 mg/L.
II were packed with 2-layered and 11-layered stationary phases respectively, considering the functions of both the columns in the separation system (Fig. 2). 3.2. Selection of chromatographic parameter An EGC-KOH eluent generator was utilized to generate highpurity and contaminant free potassium hydroxide on-line. In order to wash out the anions with weak retention from analytical column I rapidly and separate from anions with strong retention, the concentration of KOH should be not too low. However, considering the resolution of the anions eluted into analytical column II, the concentration of KOH should be not too high. An eluent containing 3.5 mmol/L KOH was employed to analyze iodide, fluoride and iodate in povidone iodine solution, successively 30 mmol/L KOH to wash analytical column II with the flow rate at 1.0 mL/min. Common anions were all eluted off from the analytical column II directly under high concentration of eluent, after the separation of fluoride and iodate. In this system, the column-switching time should be adjusted when the flow rate and concentration were fixed. It should be set at the point when the anions with weak retention were all eluted out from analytical column I but the anions with strong retention were not. Here the switching-time was set at 1.5 min to ensure the separation of iodide, fluoride and iodate. The concentration of KOH was increased to 30 mmol/L after iodate eluted out from analytical column II at 30 min. Fig. 4 was the chromatogram of standard solution containing 25 mg/L iodide, 0.25 mg/L fluoride and 25 mg/L iodate. With the good resolution between fluoride and iodate, there was no interference for the determination of iodate. 3.3. Validation and application To illustrate an application of the developed method, a povidone iodine solution sample was selected for the determination of iodide, fluoride and iodate. After filtration through a 0.22-m membrane filter and SPE-C18 column to eliminate the sample matrix, the samples were subjected to ion chromatographic analysis. Under the conditions in Fig. 4, a series of standard solutions consisted of target anions of various concentrations from 0.01 mg/L to 100 mg/L were analyzed. Each anion exhibited satisfactory linearity with correlation coefficient r ≥ 0.9990. The detection limits (LODs),
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Table 1 Calibration parameters (six data points) for the three analytes in standard solutions. Analyte
Linear range (mg/L)
Correlation, r
LOD (S/N = 3) (g/L)
Iodide Fluoride Iodate
1–100 0.01–1 1–100
0.9996 0.9998 0.9999
5.66 0.96 14.83
RSD (n = 6) % Retention time
Peak area
Peak height
0.38 0.44 0.21
1.21 1.43 2.33
0.97 3.05 1.36
polystyrene-divinylbenzene columns could work stably. Due to the low sensitivity of the conductivity detector, simultaneous determination of iodide and iodate in seawater could not be accomplished because of their low concentrations. It will be feasible for the determination of lower concentrations of iodide and iodate in seawater or edible salt, when combined with ICP-MS instead of the conductivity detector. Acknowledgements This research was financially supported by National Natural Science Foundation of China (No. 20775070, J0830413, 20911140271), Zhejiang Provincial Natural Science Foundation of China (No. R4080124, J20091495, Y4110532). References
Fig. 5. Analysis of povidone iodine solution by the column-switching IC system. Injection volume: 100 L; flow rate: 1.0 mL/min; conductivity detector; eluent concentration: 3.5 mmol/L KOH (0–30 min) and 30 mmol/L KOH (after 30 min) generated by an EGC-KOH eluent generator; switching point: 1.5 min. Peaks: (1) iodide, 16.73 mg/L; (2) fluoride, 0.0361 mg/L; (3) iodate, 12.73 mg/L. Table 2 Method performance of anions in povidone iodine solution (n = 5). Analyte
Found (mg/L)
Added (mg/L)
Recovery (%)
Iodide Fluoride Iodate
16.73 ± 0.06 0.0361 ± 0.0008 12.73 ± 0.07
17 0.03 13
101.2 91.7 100.6
based on the signal-to-noise ratio of 3 (S/N = 3), were calculated based on a 100 L injection volume. The coefficients of determination (r), the detection limit (LOD) of each anion and the linear ranges were all shown in Table 1. Then the RSDs of the six replicate analyses were calculated, which were ≤0.44% based on retention time and ≤2.33% based on peak area. On peak height, the max. of RSD was 3.05% while the min. was 0.97%. A representative chromatogram of anions in povidone iodine solution is shown in Fig. 5. The protocol used the simpler external standard calibration technique, established recovery data, and was applicable for anion concentrations in the typical range of interest for povidone iodine solution. The concentrations of anions in 100fold diluted povidone iodine solution were found to be 16.73 mg/L iodide, 0.0361 mg/L fluoride and 12.73 mg/L iodate, respectively. The recovery of iodide, fluoride and iodate in povidone iodine solution was 101.2%, 91.7% and 100.6%, respectively (Table 2). 4. Conclusion A simple IC technique for simultaneous detection of iodide and iodate in a single running was proposed, with homemade functionalized polystyrene-divinylbenzene columns and columnswitching technique. Determination of iodide and iodate in povidone iodine solution was performed using the proposed IC system. During system analysis, the homemade functionalized
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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Simultaneous determination of iodide and iodate in povidone iodine solution by ion chromatography with homemade and exchange capacity controllable columns and column-switching technique Zhongping Huang, Zuoyi Zhu, Qamar Subhani, Wenwu Yan, Weiqiang Guo, Yan Zhu ∗ Department of Chemistry, Xixi Campus, Zhejiang University, Hangzhou 310028, China
a r t i c l e
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Article history: Received 11 April 2012 Received in revised form 7 June 2012 Accepted 14 June 2012 Available online 25 June 2012 Keywords: Ion chromatography Iodide Iodate Polystyrene-divinylbenzene Column-switching Povidone iodine solution
a b s t r a c t A simple ion chromatographic method for simultaneous detection of iodide and iodate in a single running was proposed, with columns packed with homemade functionalized polystyrene-divinylbenzene (PSDVB) resins and column-switching technique. Homemade resins were functionalized with controllable amounts of quaternary ammonium groups. The low-capacity anion-exchange column and high-capacity anion-exchange column were prepared, due to the resins having different exchange capacities. With this method, iodide and iodate in povidone iodine solution were detected simultaneously in a short time with iodide being eluted off first. A series of standard solutions consisting of target anions of various concentrations from 0.01 mg/L to 100 mg/L were analyzed. Each anion exhibited satisfactory linearity, with correlation coefficient r ≥ 0.9990. The detection limits (LODs) for iodide and iodate obtained by injecting 100 L of sample were 5.66 and 14.83 g/L (S/N = 3), respectively. A spiking study was performed with satisfactory recoveries between 101.2% and 100.6% for iodide and iodate. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ion chromatography (IC) is one of the most commonly applied techniques for the separation of various ionic compounds [1–3]. Since IC offers many possibilities of application, it is used in different branches of current analytical chemistry. However, a complicated sample will require a long analytical time because of a wide range of retention times of ions, and sometimes our targets are ions with only weak retention and strong retention. So there is no need to separate all the ions in the sample. The column-switching technique is one of the effective methods to solve the above problem. There are several column-switching techniques used in IC. Ion-exclusion chromatography columns and ion-exchange chromatography columns are commonly used as switching columns in ion chromatographic system. The former [4–8] is mainly utilized to eliminate the matrices of weakly ionized compounds such as organic acids and weak inorganic acids. The latter [9–11] is widely adopted for on-line sample clean-up of concentrated organic solvents and pre-concentration of target ions, as well as the elimination of high salinity [12]. Zhu et al. [13] proposed a novel method for fast determination of hexafluorophosphate and some inorganic anions by the column-switching
∗ Corresponding author. Tel.: +86 571 88273637; fax: +86 571 88273637. E-mail address: [email protected] (Y. Zhu). 0021-9673/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chroma.2012.06.059
technique. With the above technique, ions strongly retained are expected to be eluted off into the conductivity detector prior to ions weakly retained. The key point of this method is to find the proper exchange capacity columns, which is difficult and discommodious for commercial columns. The laboratory must keep a large reserve of columns to be chosen. So, it is necessary to propose a simple method for preparing resins and columns, which is suitable for laboratory preparation and the control of exchange capacity. As an essential micronutrient in seawater and sea products, iodine, existing as iodide and iodate, plays a special role in biological processes, including geochemically or biologically active processes, hazardous contaminant, etc. [14]. The determination of iodine in seawater has long been an essential task in marine chemistry. On the other hand, iodide is an essential component of the thyroid hormones that play a decisive role in human growth and metabolism [15], especially of the brain. Iodide deficiency in humans could cause several diseases or problems, such as goiter, stillbirth and miscarriage, neonatal and juvenile thyroid deficiency, dwarfism, mental defects, deaf-mutism, spastic weakness and paralysis [16]. Several methods have been developed for the estimation of iodine in seawater and iodized edible salt, such as spectrophotometry [17], spectrofluorimetry using cadmium sulfide quantum dots as fluorescence probes [18], electrochemical determination with modified electrode [19,20], gas chromatography combined with mass spectrometry after the derivation of iodine [21–23], transient isotachophoresis capillary zone electrophoresis with UV detection
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[24–26], and flow inject with chemiluminescence [27] or UV detection [28,29]. Additionally, the effective separation method, IC, has been used to determine iodine species, with amperometric detection [30,31], UV detection [32], inductively coupled plasma mass spectrometry (ICP-MS) [33,34], or with conductivity detection [35]. However, most of the methodologies described in the literature concerning iodine speciation are focused only on the iodide determination or iodate determination. It is necessary to oxidize iodide or reduce iodate for the determination of the total amount of iodine, resulting in a lack of methods permitting simple, rapid, and low-cost determination both of iodide and iodate in a single running. Iodate and iodide in seawater have been estimated simultaneously using anion-exchange chromatographic separation and ICP-MS detection (IC-ICP-MS) [33,34]. This is a useful method, the main limitation being equipment cost. Simultaneous determination of iodide and iodate could be carried out in a single running with the column-switching technique, without complex pretreatments. And only an IC system with two valves is needed, with low requirement of instruments. In the present study, in order to shorten the analytical time and accomplish a simultaneous detection of iodide and iodate in a single running, a simple IC technique was proposed, with homemade columns and column-switching technique. All the mentioned columns were packed with homemade functionalized polystyrene-divinylbenzene resins, due to the lack of proper commercial columns which could be used to separate iodide and iodate simultaneously. Homemade resins are based on polystyrene-divinylbenzene (PS-DVB) co-polymer functionalized with a amount of quaternary ammonium groups. The low-capacity anion-exchange resin and high-capacity anion-exchange resin have been prepared, respectively, due to the controllable exchange capacity. The column with low-capacity resin is able to retain iodide which is strong retention, and those anions weakly retained are almost not reserved. The column with high-capacity resin is able to separate anions with weak retention, such as fluoride and iodate. With the column of low-capacity and column-switching technique, those anions weakly retained are analyzed by the subsequent elution through the column of high-capacity, but iodide did not pass through the high-capacity column and was eluted off first. The simultaneous analysis of iodide and iodate in povidone iodine solution has been realized. The result shows that the system we adopted with homemade stationary phases and column-switching technique is a robust, convenient, and practical device for simultaneous detection of iodide and iodate.
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) Model 9900-013 six-port valve fitted with a 100 L sampling loop, a Rheodyne (Cotati, CA, USA) P/N PR070108B ten-port valve and an EGC-KOH eluent generator. Suppression was achieved with a Dionex ASRS-4mm suppressor. The columns used in the study were prepared following the procedure shown in Section 2.3. 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 JY92- ultrasonic disrupter (Scientz Biotechnology Co. Ltd., Ningbo, China) was employed to emulsify organic compounds. The Ultimate 3000 UV detector (Dionex, USA) was used to determine dynamic adsorption. The content of nitrogen in the stationary phase
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was analyzed by utilizing an elemental analyzer (Flash EA1112, Thermofinnigan). The scanning electron microscopy (SEM) images were obtained using Hitachi S-4700 field emission scanning electron microscope (Hitachi, Japan). All columns were packed using a QP 6000 packing pump (Chuang Xin Tong Heng Science and Technology Co. Ltd., Beijing, China). 2.2. Reagents The used reagents were obtained from Huipu Chemical Reagent Co. Ltd. (Hangzhou, China), unless otherwise noted. Styrene (ST), divinylbenzene (DVB) (55+%, Zhengguang Chemical Plant, Hangzhou, China) and glycidol methacrylate (GMA) (95+%, Tokyo Kasei Kogyo Co. Ltd., Tokyo, Japan) were used after distillation under reduced pressure. Azobisisobutyronitrile (AIBN) and benzyl peroxide (BPO) were recrystallized before use. 1,4-Butanediol diglycidyl ether (BDDE, 60% in H2 O, v/v) and methylamine (MA, 40% in H2 O, v/v) were bought from Aladdin Chemical Co. Ltd. (Shanghai, China). Polyvinylpyrrolidone (PVP, K-30), dibutyl phthalate (DBP), polyvinyl alcohol (PVA, 1750 ± 50), sodium dodecylsulfonate (SDS) and toluene were all employed in the synthesis. A water purification system (Millipore, Milford, MA, USA) was used to further deionize distilled water for all eluents and sample mixtures. All reagents employed for the synthesis were of analytical reagent grade. The standard stock solutions containing 1000 mg/L of target ions were prepared by dissolving appropriate amount of salts that contained ions of interest in 100 mL deionized water. Sample solutions were produced by dissolving proper amount of povidone iodine solution in 100 mL deionized water, and filtered through a 0.22-m membrane filter and SPE-C18 column (Tianjin Fuji Science & Technology Co. Ltd., Tianjin, China) before injection. 2.3. Synthetic procedure of functionalized PS-DVB column 2.3.1. Synthesis of polymer seeds Monodisperse polystyrene (PS) seed particles were prepared according to the method that we reported before [36]. The procedure was as follows: 3 g PVP and 100 mL mixture of ethanol and water (95:5, v/v) were added to a 250 mL flask, and then a mixture of 0.7 g AIBN and 17.75 g ST was added dropwise. The mixture was stirred overnight at 70 ◦ C under dry nitrogen. After filtration and washing, the average particle size of the seeds obtained was about 1.8 m by SEM. 2.3.2. Preparation of PS-DVB beads A blend of 0.56 g polymer seeds and 15 mL SDS aqueous solution (0.2%, w/v) was placed into a 500 mL three neck flask. Then, an emulsified solution containing 4 g DBO, 30 mL SDS aqueous solution (0.2%, w/v) was added and stirred overnight. Whereafter, another emulsified mixture comprised of 30 g organic compounds and 250 mL PVA aqueous solution (1%, w/v) was prepared by an ultrasonic disrupter and poured into the flask for swelling. The above organic compounds consisted of ST, DVB, toluene, BPO and SDS. The amount of toluene equalled the sum of ST and DVB, while BPO was only 1.5% of the sum (w/w). The amount of SDS was 0.25% of the PVA aqueous solution (w/v). After 24 h, the temperature was increased to 70 ◦ C under nitrogen atmosphere and lasted for another 24 h. Subsequently, the resulting beads were washed successively with hot water and alcohol. Then the beads were extracted with toluene for 48 h. After washing and drying, the resulting particle size of the stationary phase was about 7 m. 2.3.3. Functionalization of exchange capacity controllable PS-DVB beads A blend of 4.0 g PS-DVB beads and 150 mL SDS ethanol solution (0.04%, w/v) was placed into a 250 mL four neck flask. An ethanol
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Fig. 1. Scheme of stationary phase synthesis. (A) Copolymerization of PS-DVB beads and GMA; (B) first reaction cycle with primary amine; (C) first reaction cycle with diepoxide monomer; (D) second reaction cycle with primary amine; (E) second reaction cycle with diepoxide monomer.
solution consisting of 0.8 g GMA and 0.2 g AIBN was added dropwise under nitrogen atmosphere, and the temperature was increased to 70 ◦ C for 24 h. Subsequently, the resulting beads were washed successively with hot water and alcohol. GMA with double bonds could copolymerize with the residual double bonds on the surface of PS-DVB beads and also between themselves, to form a superficial microsphere layer. Then the beads were quaternized by a multi-step synthesis [37] shown in Fig. 1. The basic condensation polymerization chemistry depends on MA and BDDE as monomers to build a branching polymer that has quaternary ammonium anion exchanger sites. 20 mL MA (4%, v/v) was added to the resin and allowed to react, later filtered and was rinsed with deionized water. Next 20 mL BDDE (10%, v/v) was added to the resin and allowed to react, later filtered and was rinsed with deionized water. In each step, the synthesis time was 30 min at a temperature of 60 ◦ C. Both the steps were repeated
2, 3, 5, 7, 9 and 11 times to make 2, 3, 5, 7, 9 and 11 layers on beads, respectively. The resulting beads were anion-exchanger resin with quaternary ammonium groups. 2.3.4. Column packing procedure All the columns (PEEK, 150 mm × 4.0 mm) were slurry packed with the stationary phases obtained above, by pressing with deionized water as packing solvent under a working pressure of 40 MPa. The volume of packing solvent passing through the column should be at least 500 mL. 2.4. Breakthrough curves The breakthrough curves of the anion-exchange resins were determined with a Dionex ICS 2100 system and an UV detector. An eluent containing 0.02 mol/L NO3 − was employed to determine the
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Fig. 2. Chromatographic instrument configuration for this column-switching system. (A) Separating anions with weak retention from strong retention (0–1.5 min); (B) analyzing the target anions and washing analytical column II.
adsorption capacities of all homemade columns, with the flow rate at 1.0 mL/min. The PEEK tubes in front of the analytical columns were filled with eluent before determination, to minimize void volume.
2.5. System operation procedure There were three steps involved for the simultaneous analysis of iodide and iodate in povidone iodine solution (Fig. 2): (1) filling the sample loop, (2) separating anions with weak retention from strong retention in analytical column I, (3) analyzing the target anions and washing analytical column II. First, the sample was loaded into the 100 L sample loop. To ensure that the loop contained a representative sample, at least four loop volumes were injected. Then, the six-port valve containing sample was switched to inject position and the ten-port valve was also switched to inject position to transport the sample into analytical column I (Fig. 2A). Anions with weak retention were washed down from analytical column I without almost any retention because of the low anionexchange capacity, but anions with strong retention were retained and washed down slowly. The result was that the anions with weak retention were all eluted into analytical column II but anions with strong retention were kept on analytical column I. After that, the two valves were switched to load positions, respectively, and the subsequent analysis started (Fig. 2B). With analytical column I and column-switching technique, those anions weakly retained were analyzed by the subsequent elution through analytical column II, but iodide did not pass through analytical column II and was eluted off first. It was difficult to perform for commercial columns, due to the particular exchange capacities of analytical column I and II.
3. Results and discussion 3.1. Characterizations of the stationary phase 3.1.1. Elemental analysis Nitrogen atoms in the quaternary ammonium groups were a main place where interaction between the stationary phase and analyte occurred. The content of nitrogen atoms in the stationary phase decided the anion-exchange capacity. Elemental analysis was performed for the PS-DVB beads modified with different numbers of bonded polymer layers: 2, 3, 5, 7, 9 and 11 layers. The percentages of nitrogen were as follows: 0.34%, 0.66%, 0.79%, 0.91%, 1.08% and 1.51% for 2, 3, 5, 7, 9 and 11 polymer layers in the stationary phases, respectively. As reported, the percentage of nitrogen in the stationary phases was increased with the number of bonded layer but not exponentially. That was because of the steric hindrance exponential growth. The anion-exchange capacity of the stationary phase was still controllable, though the percentage of nitrogen did not increase exponentially. 3.1.2. Chromatographic investigation on the PS-DVB-based stationary phase The ion chromatographic evaluation was performed using a test mixture of inorganic anions composed of fluoride, chloride, nitrite, bromide, nitrate and sulfate. Inorganic anions were separated with all synthesized stationary phases, which contained 3, 5 and 7 bonded layers, using a 15 mM hydroxide solution as the mobile phase. Representative chromatograms using the 3-, 5-, 7- and 9-layered stationary phase are shown in Fig. 3. Under these conditions, the stationary phase with nine bonded layers has too strong retention,
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Fig. 3. Separations of conventional inorganic anions on the: (A) 3-, (B) 5-, (C) 7-, (D) 9-layers bonded phases. Column dimensions: 4.0 mm I.D. by 150 mm; eluent: 15 mM KOH; flow rate: 1.0 mL/min; injection volume: 25 L; conductivity detector. Peaks: (1) fluoride, 2 mg/L; (2) chloride, 4 mg/L; (3) nitrite, 10 mg/L; (4) bromide, 10 mg/L; (5) nitrate, 10 mg/L; (6) sulfate, 10 mg/L.
separating fluoride and chloride in 35 min. And the stationary phase with three bonded layers has low retention and selectivity. Other stationary phases allowed separating all the inorganic anions in the test mixture with good resolution and asymmetry factors. The hydrophilicity of the resins is very good because of the quaternary ammonium functions surrounded by hydroxy groups. High resolution between fluoride and the water dip could be obtained. The retention order was typical for the elution of inorganic anions in IC. Higher retention was obtained with the same charge and higher radius, which was because of the different hydration enthalpies. The order of elution was independent of the number of bonded layers in the stationary phase. Symmetrical peaks of all tested anions were observed by using all synthesized stationary phases. As expected, the retention factor of all anions strongly depended on the number of bonded layers. The retention of anions increased from 3 to 9 bonded layers as the increase of the number of quaternary ammonium groups present in the structure of the bonded phase. Unfortunately, the increase of the ion exchange capacity of the column, generated by an increase of the number of exchange layers, might cause an impairment of the mass transfer kinetics, leading to peak broadening and loss of separation efficiency. It might be improved by adding some organic solvents into the eluent. 3.1.3. Breakthrough curves Anion exchange capacity could be obtained according to the breakthrough curve. An eluent containing 0.02 mol/L NO3 − was employed to determine the adsorption capacities of all the columns, with the flow rate at 1.0 mL/min and UV detector. The PEEK tubes in front of the columns were filled with eluent before determination, to minimize void volume. The anion exchange capacities of the homemade columns were as follows: 0.028 mmol/column, 0.078 mmol/column, 0.136 mmol/column, 0.196 mmol/column, 0.320 mmol/column and 0.420 mmol/column, for 2, 3, 5, 7, 9 and 11 polymer layers of the stationary phases, respectively. As expected, the anion exchange capacities increased from 2 to 11 bonded layers as the increase of nitrogen content in the stationary phases. Fluoride and iodate could only be separated on an 11-layered stationary phase with low concentration of eluent, because of their very similar retention. The exchange-capacity of the column with the 3-layer phase was a little high for the elution of iodide in the column-switching system. So analytical columns I and
Fig. 4. Chromatogram of three anions separated by the column-switching IC system. Injection volume: 100 L; flow rate: 1.0 mL/min; conductivity detector; eluent concentration: 3.5 mmol/L KOH generated by an EGC-KOH eluent generator; switching point: 1.5 min. Peaks: (1) iodide, 25 mg/L; (2) fluoride, 0.25 mg/L; (3) iodate, 25 mg/L.
II were packed with 2-layered and 11-layered stationary phases respectively, considering the functions of both the columns in the separation system (Fig. 2). 3.2. Selection of chromatographic parameter An EGC-KOH eluent generator was utilized to generate highpurity and contaminant free potassium hydroxide on-line. In order to wash out the anions with weak retention from analytical column I rapidly and separate from anions with strong retention, the concentration of KOH should be not too low. However, considering the resolution of the anions eluted into analytical column II, the concentration of KOH should be not too high. An eluent containing 3.5 mmol/L KOH was employed to analyze iodide, fluoride and iodate in povidone iodine solution, successively 30 mmol/L KOH to wash analytical column II with the flow rate at 1.0 mL/min. Common anions were all eluted off from the analytical column II directly under high concentration of eluent, after the separation of fluoride and iodate. In this system, the column-switching time should be adjusted when the flow rate and concentration were fixed. It should be set at the point when the anions with weak retention were all eluted out from analytical column I but the anions with strong retention were not. Here the switching-time was set at 1.5 min to ensure the separation of iodide, fluoride and iodate. The concentration of KOH was increased to 30 mmol/L after iodate eluted out from analytical column II at 30 min. Fig. 4 was the chromatogram of standard solution containing 25 mg/L iodide, 0.25 mg/L fluoride and 25 mg/L iodate. With the good resolution between fluoride and iodate, there was no interference for the determination of iodate. 3.3. Validation and application To illustrate an application of the developed method, a povidone iodine solution sample was selected for the determination of iodide, fluoride and iodate. After filtration through a 0.22-m membrane filter and SPE-C18 column to eliminate the sample matrix, the samples were subjected to ion chromatographic analysis. Under the conditions in Fig. 4, a series of standard solutions consisted of target anions of various concentrations from 0.01 mg/L to 100 mg/L were analyzed. Each anion exhibited satisfactory linearity with correlation coefficient r ≥ 0.9990. The detection limits (LODs),
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Table 1 Calibration parameters (six data points) for the three analytes in standard solutions. Analyte
Linear range (mg/L)
Correlation, r
LOD (S/N = 3) (g/L)
Iodide Fluoride Iodate
1–100 0.01–1 1–100
0.9996 0.9998 0.9999
5.66 0.96 14.83
RSD (n = 6) % Retention time
Peak area
Peak height
0.38 0.44 0.21
1.21 1.43 2.33
0.97 3.05 1.36
polystyrene-divinylbenzene columns could work stably. Due to the low sensitivity of the conductivity detector, simultaneous determination of iodide and iodate in seawater could not be accomplished because of their low concentrations. It will be feasible for the determination of lower concentrations of iodide and iodate in seawater or edible salt, when combined with ICP-MS instead of the conductivity detector. Acknowledgements This research was financially supported by National Natural Science Foundation of China (No. 20775070, J0830413, 20911140271), Zhejiang Provincial Natural Science Foundation of China (No. R4080124, J20091495, Y4110532). References
Fig. 5. Analysis of povidone iodine solution by the column-switching IC system. Injection volume: 100 L; flow rate: 1.0 mL/min; conductivity detector; eluent concentration: 3.5 mmol/L KOH (0–30 min) and 30 mmol/L KOH (after 30 min) generated by an EGC-KOH eluent generator; switching point: 1.5 min. Peaks: (1) iodide, 16.73 mg/L; (2) fluoride, 0.0361 mg/L; (3) iodate, 12.73 mg/L. Table 2 Method performance of anions in povidone iodine solution (n = 5). Analyte
Found (mg/L)
Added (mg/L)
Recovery (%)
Iodide Fluoride Iodate
16.73 ± 0.06 0.0361 ± 0.0008 12.73 ± 0.07
17 0.03 13
101.2 91.7 100.6
based on the signal-to-noise ratio of 3 (S/N = 3), were calculated based on a 100 L injection volume. The coefficients of determination (r), the detection limit (LOD) of each anion and the linear ranges were all shown in Table 1. Then the RSDs of the six replicate analyses were calculated, which were ≤0.44% based on retention time and ≤2.33% based on peak area. On peak height, the max. of RSD was 3.05% while the min. was 0.97%. A representative chromatogram of anions in povidone iodine solution is shown in Fig. 5. The protocol used the simpler external standard calibration technique, established recovery data, and was applicable for anion concentrations in the typical range of interest for povidone iodine solution. The concentrations of anions in 100fold diluted povidone iodine solution were found to be 16.73 mg/L iodide, 0.0361 mg/L fluoride and 12.73 mg/L iodate, respectively. The recovery of iodide, fluoride and iodate in povidone iodine solution was 101.2%, 91.7% and 100.6%, respectively (Table 2). 4. Conclusion A simple IC technique for simultaneous detection of iodide and iodate in a single running was proposed, with homemade functionalized polystyrene-divinylbenzene columns and columnswitching technique. Determination of iodide and iodate in povidone iodine solution was performed using the proposed IC system. During system analysis, the homemade functionalized
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