versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.53 n.1 Concepción mar. 2008
J. Chil. Chem. Soc, 53, N° 1 (2008)
SEPARATION AND DETERMINATION OF SIX METAL CATIONS BY CAPILLARY ZONE ELECTROPHORESIS
JINZHANG GAO*, XIANGLI SUN, WUYANG, HAIFENG FAN, CHONGYANG LI AND XUEFENG MAO
(Chemistry & Chemical Engineering College, Northwest Normal University, Lanzhou 730070, P. R. China), e-mail: firstname.lastname@example.org
A rapid, simple and reliable method was devised for separation and determination of six metal cations, based on the capillary zone electrophoresis. The results showed that the complete separation of K+,Cu2+, Zn2+, Mn2+, Pb2+and Cd2+canbe achieved within 5 min. Anew electrolyte system containing 15 mmol-L"1 imidazole as a background absorbance reference and 8 mmol-L"1 malonic acid, 2 mmol-L-1 18-crown-6 ether as the complexing agents, 10%methanol as anorganic modifier, by using acetic acid to adjust the pH=4.40 was developed. The applied voltage was 20 kV at 25 °C. Under the optimum conditions, 6 ions were separated and determined with the correlation coefficient of 0.9984-0.9993. The detection limits (S/N=3) from 0.05 (K+)to 0.75 (Zn^mg-L1. The repeatability of migration times was less than 0.60% and of peak area ranged from 3.4 to 5.6% (n=6). The results were compared with AAS for analyzing the real samples of waste-water and snow water.
Keywords: Electrolyte composition; metal cations; capillary zone electrophoresis
As a separation technique the capillary electrophoresis (CE) is extremely useful in organic analysis and has been used successfully in many areas. In marked contrast to the extensive studies of organic compounds, very few works of inorganic ions have been reported. The first paper concerning the indirection detection of inorganic ions by using CE was reported by Hjerten  in 1967. Since then, the application to the separation and determination of inorganic substances has developed rapidly and many papers have been published [2-3], in which the capillary zone electrophoresis (CZE) is one of the widely used capillary electrophoresis techniques, and some of critical reviews have been summarized [4-7]. The use of capillary zone electrophoresis for the analysis of cations in water samples has been studied [8-12]. In recent years, although many instrumental analysis have been used in environmental water samples, such as ion chromatography (IC) , atomic absorption spectroscopy (AAS) , as well as inductively coupled plasma combined with mass spectrometry (ICP-MS) , the capillary zone electrophoresis holds the promise of even better separations and lower sample consumption . Simultaneous determination of copper, cobalt and cadmium, manganese and lead were found to be influence by the derivative spectrophotometry . For the purpose, in this paper a new electrolyte system contained 15 mmol-L"1 imidazole as background absorbance reference, 8 mmol-L"1 malonic acid and 2 mmol-L"1 18-crown-6 ether as the complexing agents, 10% methanol as an organic modifier, pH adjusted by acetic acid (50%, V/V) to 4.40 was developed. The satisfactory results indicated that the proposed method could be used in water analysis.
Separation was performed on a P/ACE 5510 apparatus (Beckman Instruments, USA) equipped with a UV detector and wavelength filters (190, 200, 214, 254 and 280 run). A fused silica capillary (total length 47 cm, 75 um.i d; Yongnian Fiber Optic Factory, China) was used. The distance from the point of injection to the detection window was 40 cm. Gold software System was used for data acquisition. A mode 211 pH-meter (HANNA, Italy) was used. Electromigration injection was carried out by using a voltage of 5 KV for 5 S. The separation temperature of 25 °C and the applied separation voltage of 20 kV were used, respectively.
2.2. Reagents and solutions
All reagents were of analytical grade and used as received from Tianjin Chemical Reagent Factory, China. Imidazole and 18-crown-6 ether were obtained from Sigma (Sigma Chemical Co., St. Louis, MO, USA). All solutions and electrolytes were prepared with ultra-pure water from a Milli-Q system (Millipore, Germany). Stock standard solutions of various metal cations (lg-L [) were obtained by dissolving their corresponding inorganic salts (nitrates, sulfates or chlorides), and stored in refrigerator prior to use.
Before the use, the capillary was rinsed with O.lmol-L"1 NaOH and ultra-pure water for 5 min, followed by the used carrier electrolyte for 10 min. The capillary was rinsed for 2 min with carrier electrolyte between runs. All electrolytes and samples were filtered through a 0.45 urn membrane filter and degassed by ultrasonication prior to analysis.
RESULTS AND DISCUSSION
In general, most inorganic anions and cations have no strong absorption in UV-visible spectral region. For improving the sensitivity, complex reactions are commonly used, that is, adding some complexing agent (such as a-hydroxyisobutyric acid (HIBA), citrate, EDTA and acetic acid) into the electrolytic solution. If so, some factors in operation, e.g., the composition and concentration of buffer, pH adjustment as well as voltage and temperature, should be optimized in advance.
3.1. Optimization of the running buffer
The running buffer composition and concentration affect basically the sensitivity and selectivity. Padarauskas et al.  pointed out that the separation efficiency was proportional to running buffer concentration. The larger running buffer concentration, the higher separation efficiency. In this study, we use imidazole in buffer as the background absorption at 214 nm to examine the operation conditions. The influence of imidazole concentration was studied from 5 to 18 mmol-L"1 while the background electrolyte (BGE) was at pH=4.40. Too low concentration may broaden the cation peak to cause difficult in separation. With increasing the concentrations of electrolyte, a better resolution was observed. However, when the concentration of imidazole was over 15 mmol-L"1, the large Joule heat would produce to make the baseline excursion. Thereby the suitable concentration of 15 mmol-L"1 was chosen.
Generally, most of single cations are difficult to separate directly by CE, as their migration velocities are very close to each other. The use of some weak complexing agents, such as HIBA, malic, glycolic, tartaric and malonic, can improve the separation efficiency. In this study, malonic acid of 8 mmol-L"1 was used for separating 6 cations, just showing in Figure 1.
Fig 1 shows that K+ and Mn2+ cannot be complete baseline separation by using malonic acid only. Some papers reported [16,19,20] that the addition of crown ether (18-crown-6 ether) could improve the selectivity. For improving the separation between K+ and Mn2+ ions we investigated the effect of 18-crown-6 ether on the separation showing in Figure 2. Results indicated that adding 2 mmol-L"1 of 18-crown-6 ether in the electrolyte makes a good baseline separation for 6 cations within 5 min.
In the pre-experimental, the addition of methanol was found to improve the property of electrolyte solution. In this study, 10% of methanol was suitable. More or less than the value would decrease the separation efficiency.
3.2. pH of the running buffer
The separation of metal cations should carry out in acidic media, due to the precipitation of heavy metal ions being formed in basic solutions . Moreover, the electroosmotic flow (EOF) can be affected by the pH of running buffer. In the experimental, the effect of pH on the sensitivity was studied in the range of 3.80-4.80. If the pH were less than 3.80, the separation between Pb2+ and Cd2+would be difficult; more than pH 4.40, the migration time would be prolonged and the noise dramatically increased. As shown in Figure 3, the optimum condition was chosen at pH = 4.40.
3.3. Choice of voltage and temperature
Generally, both applied voltage and temperature affect migration time of ions in electrolyte. With increasing the applied voltage the migration time of ions shortens; the higher temperature could also cause the migration time to be short. That is to say, under higher voltage and temperature the speed of ions would be faster. However, high voltage can cause the Joule-heat formed to raise temperature, and higher temperature gives a bad repeatability. A suitable way (20 kV at 25 °C) in the study was adopted. As shown in Figure 4, under the chosen conditions, a complete baseline separation for 6 ions (K+, Cu2+, Zn2+, Mn2+, Pb2+and Cd2+ with 10 mg-L"1) was done within 5 min.
3.4. Quantitative analysis
Under the proposed conditions, a mixture containing 6 metal ions was detected. The linear range, detection limit and correlation coefficient are listed in Table 1. To ensure the corrected data mentioned above, each point was detected in triplicate. The correlation coefficients were limited from 0.9984 to 0.9993. Limit of detection (LOD) was defined as the analyte concentrations corresponding to a signal equal to three times the background noise (S/N=3), which were found to be from 0.05 to 0.75 mg-L"1.
The repeatability of the proposed method was evaluated by a standard solution containing 6 ions with 5 mg-L1; which was determined repeatedly for 8 times. Table 2 indicated that the relative standard deviation (RSD) of peak area was ranged from 3.4% to 5.6%.
Under the optimum conditions, some of real samples, such as wastewater and snow water, were examine by the proposed method. The data obtained by CZE are in good agreement with those detected by atomic absorption spectrometry (AAS). Moreover, the recovery test was also carried out with the satisfactory results. All results are given in Figure 5, Table 3 and Table 4.
Although capillary zone electrophoresis is used to determining the organic compounds, the application to inorganic ions is quite successful, too. In this paper, we studied the separation of 6 ions by CZE. The background electrolyte consists of 15 mmol-L"1 imidazole, 8 mmol-L1 malonic acid, 2 mmol-L"1 18-crown-6 ether, and 10% methanol. The pH of solution was 4.40 at 25 °C and applied voltage 20 kV. Under conditions, 6 metal-ions in real samples were separated and detected successfully within 5 min. The proposed method can be used in environmental analysis.
This work was supported in part by the Natural Science Foundation of Gansu Province (3ZS041-A25-028), the Invention Project of Science & Technology (KJCXGC-01, NWNU), and the Key Lab of Polymer Materials of Gansu Province, China.
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(Received: 18May 2007 - Accepted: 11 January 2008)