A simple, economical, and sensitive capillary electrophoresis (CE) method integrated with capacitively coupled contactless conductivity detection was developed for the determination of metal ions such as K +, Na +, Mg 2+, Sr 2+, Ca 2+ in drinking water. 18-Crown-6 ether and Hexadecyltrimethylammonium Bromide (CTAB) were employed as complexing reagents. The effects of electrolyte additives, citric acid buffer solution, and other separation conditions of CE were comprehensively investigated and carefully optimized. The best results were obtained in a running buffer solution composed of citric acid (12 mM), 18-crown-6 ether (0.2 mM), and CTAB (0.015 mM) at pH 3.5. Under these conditions, a complete separation of five metal ions was successfully achieved in less than 12 min. The limits of detection for the optimal procedure were determined to be in the range of 0.02 - 0.2 mg·L -1. The repeatability with respect to migration times and peak areas, expressed as relative standard deviations, was better than 2.3% and 5.1%, respectively. Evaluation of the efficiency of the methodology indicated that it was reliable for the determination of metal ions in six different brands of drinking water samples.
Most of the alkali and alkaline earth metal ions play an important role in numerous processes in the human body, such as volume and osmotic regulation, myocardial rhythm, blood coagulation, and neuromuscular excitability. The detection of any deviation from the normal concentration ranges of these species is very useful in the diagnosis of metabolic disorders and abnormalities [
The traditional methods for the analysis of metal ions in environmental samples include spectrophotometry and atomic absorption spectroscopy, capable of detecting only a single element at a time. Therefore, these methods are time-consuming and laborious [
Till date, there are few studies describing the CE determination of inorganic ions in formulations of drinking water. Some of these studies have described the determination of K+, Na+, Mg2+, and Ca2+ ions in real samples [
All chemicals used were of analytical-grade. Acetic acid, L-histidine (His), MES, 18- crown-6 ether, potassium chloride (KCl), sodium chloride (NaCl), magnesium chloride (MgCl2), strontium chloride (SrCl2), and calcium chloride (CaCl2) were purchased from Sinopharm Chemical Reagent Co., Ltd. Organic additives, 18-crown-6 ether and CTAB were obtained from Aldrich (Milwaukee, WI, USA).
KCl, NaCl, MgCl2, SrCl2, and CaCl2 were dissolved in deionized water (Milli-Q water purification system, Millipore, Milford, MA) to prepare the stock solution with concentration of 100 mg∙L−1. For the analyses of inorganic ions, a 10 mM stock solution of 18-crown-6 ether and 1 mM stock solution of CTAB were prepared and added to the running buffer in order to allow the complete separation of five metal ions. Fresh running buffer solutions were prepared daily. All the solutions were stored in a refrigerator at 4˚C until their use. Prior to the experiments, all the solutions were filtered through 0.22 μm polypropylene Acrodisc syringe filter (Xinya Purification Instrument Factory, Shanghai, China) and sonicated for 5 min to remove bubbles.
Drinking water samples were obtained from local supermarket in Shanghai, China. Prior to the analysis, the water samples were filtered through polypropylene Acrodisc syringe filter (nominal pore size 0.22 μm). All the samples were kept in a refrigerator until the experiments were performed and were diluted to specified concentration in our testing range with the buffer solution prior to analyses.
The CE-C4D system used was similar to a previously reported system [
All the quantitative determinations of metal ions in drinking water samples were made by the standard addition method (n = 3), where the peak area obtained for each analyte was used for quantitative interpretation and calculations. All data treatment was performed by using Microcal Origin 7.0 (Microcal Software, Northampton, MA, USA).
In C4D, the response arises from the difference in conductivity between analytes and BGE co-ions. In order to achieve a high signal-to-noise ratio (S/N), a large difference between the conductance of the analytes and electrolyte is needed [
replacement of MES/His by Cit, capable of complexing with Mg2+ and Sr2+, and Ca2+ at low pH. In systems involving contactless conductivity detection, parameters such as concentration of the electrolyte solution, pH of the BGE, and concentration of the complexing agents must be taken into account; therefore, in this study, these parameters were investigated in detail.
Next parameter investigated was the concentration of Cit in the BGE. The effect of the concentration of Cit on the migration time was studied in the range 8 - 15 mM.
In order to improve the resolution of analytes, a small amount of the cationic surfactant CTAB was added to the BGE. Adsorption of CTAB micelles onto the capillary surface resulted in the formation of a layer with positive charge causing a reduction or even a complete reversal of the electroosmotic flow.
tween Ca2+ and Na+ improves. Nonetheless, the baseline becomes unstable and resolution of the five metal ions becomes worse due to higher concentration of CTAB (0.02 mM). Thus, CTAB concentration of 0.015 mM seemed to be a suitable compromise and was selected as the optimized concentration.
Under the tested experimental conditions discussed above, a phenomenon was observed that Sr2+ and Mg2+ could not be separated completely by using only Cit buffer and CTAB. The separation of some comigrating cations can be fine-tuned by the addition of 18-crown-6 ether to the BGE. 18-Crown-6 ether is known to form inclusion complexes with several inorganic cations, such as K+ and Sr2+. The complex formation depends on the sizes of both the cation and the crown ether cavity.
Furthermore, effect of the injection time ranging from 2 to 10 s was investigated. Variation of the injection time from 2 - 10 s at 17 kV led to an increase in the peak heights of these five cations. However, simultaneously the band broadening of signals and low separation efficiency were also observed. When the injection time exceeded 6 s, the problem related to peak broadening in conjunction with peak distortion became more obvious (results not shown). Thus, the abovementioned discussion indicated that 6 s in-
jection time could be selected as the optimal condition.
Above parameters related to BGEs showed that simultaneous determination of five metal ions in the real samples was possible. The electrolyte solution consisted of 12 mM Cit, 0.015 mM CTAB, and 0.2 mM 18-crown-6 ether at pH 3.5 (monitored by a PHS-3C Acidometer). The optimal injection time and separation voltage were 6 s and 17 kV, respectively. The separation was achieved in less than 12 min. The total capillary length used was 75 cm and the effective capillary length was 65 cm. The resulting electropherograms of the simultaneous determination of five metal ions are depicted in
In order to evaluate the developed method for the quantitative purpose, linearity figures and limits of detection (LOD) were determined. The linearity of the method was determined by constructing a calibration curve with different concentrations of the five metal ions. The linear regression coefficients were always higher than 0.9950. Repeatability of the relative area and migration time, expressed as relative standard deviations (RSD), was better than 5.1% and 2.3% (n = 8), respectively, indicating a high degree of precision. All the validation data are listed in
The detection limits (the concentrations providing peak heights which are 3 times as tall as the baseline noise) for K+, Na+, Mg2+, Sr2+, Ca2+ ions were determined to be below 0.2 mg∙L−1 for all but one of the ions for this experimental system, which is well below the requirements for the application envisaged.
The methodology based on the CE- C4D was applied to the analysis of five metal ions in six different drinking matrices. The quantitative results (obtained using a calibration
Compound | Linear regressiona | Linearity (mg∙L−1) | Correlation (R2) | LODb (mg∙L−1) | RSD% (time) | RSD% (area) |
---|---|---|---|---|---|---|
K+ | y = 10.387x + 1.146 | 0.05 - 20 | 0.9973 | 0.02 | 1.3 | 2.7 |
Ca2+ | y = 3.747x + 0.151 | 0.25 - 20 | 0.9968 | 0.1 | 2.1 | 3.6 |
Na+ | y = 4.837x + 0.766 | 0.1 - 15 | 0.9984 | 0.05 | 1.7 | 4.8 |
Sr2+ | y = 1.671x + 0.058 | 0.4 - 15 | 0.9955 | 0.2 | 2.3 | 5.1 |
Mg2+ | y = 5.076x + 0.612 | 0.1 - 20 | 0.9972 | 0.05 | 1.4 | 4.2 |
aLinear regression based on peak area (mV s) vs. concentration (mg∙L−1). bEstimated on the basis of S/N = 3.
Sample | Cation concentration (mg∙L−1)b | ||||
---|---|---|---|---|---|
K+ | Ca2+ | Na+ | Sr2+ | Mg2+ | |
1 | 8.2 | 34.8 | 5.7 | 0.5 | 36.3 |
2 | ND | 69.2 | 3.8 | 0.8 | 10.9 |
3 | 3.1 | 2.7 | 4.5 | 0.6 | ND |
4 | 2.9 | 54.2 | 43.9 | 0.6 | 9.4 |
5 | 4.3 | 21.9 | 15.5 | ND | 8.2 |
6 | 0.61 | 51.7 | 2.1 | ND | 5.4 |
aDetermination was performed in triplicate and experimental conditions were similar to those mentioned in Figures 3-5. bRSD (n = 3): 3% - 10%.
curve method) are listed in
The perfect separation of metal ions K+, Na+, Mg2+, Sr2+, and Ca2+ in drinking water samples could be achieved by CE-C4D using a Cit buffer solution containing CTAB and 18-crown-6 ether as complexing reagents. The proposed method did not require any sample retreatment, except dilution with running buffer, and thus did not significantly change the composition of the original sample. It was easy to optimize the composition of the running buffer by using a conductivity detector. Under the optimal conditions, the developed method exhibited a very good quantitative performance in terms of accuracy and precision with an analysis time of less than 12 min for all the ions.
This work was financially supported by the national program on development of scientific instruments and equipments (2011YQ150072), the National Natural Science Foundation of China (No. 21575042) and the talent development program of Minhang District.
Chen, W.J., Gao, F., Zhang, Y., Zhang, Y., Li, Y., Zhang, Y.T., Wang, Q.J. and He, P.G. (2016) Sensitive Determination of Metal Ions in Drinking Water by Capillary Electrophoresis Coupled with Contactless Conductivity Detection Using 18-Crown-6 Ether and Hexadecyltrimethylammonium Bromide as Complexing Reagents. American Journal of Analytical Chemistry, 7, 737-747. http://dx.doi.org/10.4236/ajac.2016.711066