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: [email protected] (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: [email protected]
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 the six-way valve
was in LOAD position, and IO3 was delivered to the cell of the
coulometric ED by pump 1 and reduced to I2 at the electrode in
PECI cell filled with mobile phase 1, then the resulting I2 passed
through PCEI and had been collected in the polyether ether ketone ( peek) loop (Figure 1A). The process would take 30 s to get
ready for the next stage. At step 2, the six-way was immediately
switched to INJECT position and then pump 2 transferred the
Figure 1. Schematic diagram of PECI-HPLC-UV system: (A) step 1 and (B) step 2.
Determination of Iodate by HPLC-UV 281
mobile phase 2 to inject I2 from opposite end of the peek loop to
HPLC-UV system for subsequent analysis. I2 was isolated by the
column and detected by the UV detector. The high-voltage
switching six-way valve made the cell and electrode of PECI
link to atmosphere all the time. Therefore, the PECI on-line coupling with HPLC could avoid the high back pressure of the column (Figure 1B).
Operating conditions of the PECI system
To meet the requirement of determination, the high reduction
efficiency (RE) ofIO3 in PECI was considered as a key target.
Three operating conditions that can influence the RE of the
PECI system, including applied potential, pH value and salt concentration of mobile phase 1, were investigated in detail.
First, 10 mg mL21 of I2 and IO3 standard solutions were prepared by diluting their stock solutions with water. Then, IO3 was analyzed by the PECI-HPLC-UV system in the form of I2
under different conditions listed in Table II. Then, the RE value
was calculated by the following equation:
Amount of I produced
Reduction efficiency (RE) ¼
Amount of total IO3 100%
where the amount of I2 produced is calculated as moles of I2
produced by IO3 reduction and the amount of total IO3 is
the moles of IO3 in the standard solution.
Result
Operating conditions of the PECI system
Effect of pH
The effect of pH on the RE of IO3 is shown in Figure 2A. The
figure shows that the RE values of IO3 were very low within
pH 4.0 –7.5. When pH was adjusted from 4.0 to 2.0, the RE was
increased obviously and then kept unchanged when the pH further reduced from 2.0 to 1.0.
Effect of potential and salt concentration
Figure 2B shows the effect of salt concentration on RE of IO3 .
When the salt concentration was changed in the range of 50 –
200 mmol L21, the RE of IO3 increased from 49.12 to 70.00%.
High salt concentration is not necessary because it can increase
the viscosity of electrolysis solution and leading to the slow release of I2 from the electrode. High salt concentration may
also suppress molecule or ion diffusion from the working electrode. The effect of the applied potential on RE was also
Table II
The Operation Condition Screening of PECI System
Factor
Level
Other condition retained same
pH
Salt concentration
Applied potential
7.5, 5.0, 4.0, 3.0, 2.0 and 1.0
50, 120 and 200 mmol L21
2600, 2700, 2900, 21,100, 21,500 and 21,700 mV
120 mmol L21 of salt concentration; 21,500 mV of applied potential
pH 3.0; 21,500 mV of applied potential
120 mmol L21 of salt concentration; pH 3.0
Figure 2. The effect of pH (A), salt concentration (B) and applied potential (C) on reduction deficiency of IO3 (10.0 mg mL21).
282 Wang et al.
investigated, and results were showed in Figure 2C. The more
negative applied potential, the more intensive reduction takes
place at the electrode. Six levels were designed by us for the applied potential in the study such as 2600, 2700, 2800, 2110,
21,500 and 21,700 mV. The RE value with 21,500 and
21,700 mV was 52.52 and 67.44%, respectively.
Recovery and linearity
As shown in Table III, the recovery of the method was in the
range of 79.5 – 83.2% and the relative standard deviation (RSD)
was in the range of 2.0 – 4.3%. The curves (y ¼ 25.751x –
732.92) were linear in the range of 1.0 – 10.0 mg mL21 for IO3 with a correlation coefficient of 0.999 or greater. The detection
limit and quantity limit were 9 and 20 ng, respectively.
The IO3 in iodized salt was determined by PECI on-line coupling with high-performance liquid chromatograph (PECI-HPLC),
and results are given in Table III (8).
Discussion
An appropriate pH value can improve the reduction and oxidation at an electrode. In particular, acidic pH is beneficial to reduction and alkaline pH is beneficial to oxidation. According to
Faraday’s law of electrolysis, the mass of produced substance at
an electrode by an electrochemical reaction is proportional to
the number of mass of electrons transferred at that electrode.
When a molecule or ion undergoes oxidation or reduction in a
solution, it may undergo a three-step process: diffusion to the
electrode surface, oxidation or reduction and diffusion away
from the vicinity of the working electrode. For the first step,
Table III
Determination of Potassium Iodate in Salt Iodized (n ¼ 3)
IO3 transferred from the bulk solution to the electrode surface
was affected by the flow rate, diffusion coefficient and viscosity
of the mobile phase 1. In general, diffusion of ion or molecule in
liquid at room temperature is relatively slow, and either too
fast or too slow flow of mobile phase 1 is not appropriate for
diffusion. Therefore, 358C cell temperature and flow rate
0.2 mL min21 were chosen in the present study. In the second
step, electron transfer is primarily determined by applied potential of the electrode. As shown in Figure 3, the reduction of IO3 was very weak with applied potential ranging from 2600 to
21,100 mV. With the increasing in applied potential from
21,100 to 21,700 mV, the amount of I2 increased significantly.
To obtain high reduction of IO3 , 200 mmol L21 sodium dihydrogen phosphate ( pH 2.0), mobile phase 1, 21,700 mV applied
potential, 0.2 mL min21 flow rate and 378C cell temperature
were selected as operating conditions in this work. The UV response of iodide after reduction is almost 10 times more than
that of the initial form (Figure 3). On the other hand, as the iodate was detected through the formation of iodide and this ion
can be effectively isolated from chloride in the sample medium, a
low interference exists during the analysis. It can be concluded
that the method is specific and sensitive by using the PECI instrument under the selected conditions.
Conclusion
A novel PECI on-line coupled with HPLC was developed, and a
novel method based on PEC-HPLC-UV for amplifying the UV response of iodate (IO3 Þ was described. The proposed method
was simple and highly sensitive with good precision. The
PECI will be used widely in the field of HPLC analysis in the
future.
Acknowledgments
Sample
Added iodate (mg kg21)
Round iodate (mg kg21)
Recovery%
RSD%
1
2
3
1
10
50
0.795
8.04
41.54
79.5
80.4
83.1
4.3
2.0
2.8
We acknowledge Tijun Zhou of Shiseido China Co., Ltd for
his assistance in equipments repair. We also thank Peng
Tan and Shangyi Chen of Beijing Techmate Technology
Corporation Limited and Haiyan Li of Beijing Institute of
Pharmacology and Toxicology for helpful discussions and technical assistance.
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Figure 3. The chromatography diagram of IO3 (10.0 mg mL21), initial form (about RT
13.5 min) and after reduction (about RT 8.0 min).
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