Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 200–205
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Comparison of rapidly synergistic cloud point extraction and ultrasound-assisted cloud point extraction for trace selenium coupled with spectrophotometric determination Xiaodong Wen ⇑, Yanyan Zhang, Chunyan Li, Xiang Fang, Xiaocan Zhang College of Pharmacy and Chemistry, Dali University, Dali, Yunnan 671000, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A comparative study of RS-CPE and
A comparative study of RS-CPE and UA-CPE was carried out. RS-CPE and UA-CPE were firstly coupled with UV–vis for Se detection.
UA-CPE was carried out. RS-CPE and UA-CPE were firstly coupled with UV–vis for Se detection. Conventional UV–vis accomplishes ultra-trace detection via the coupling. Be useful in less developed area without more advanced analytical instruments.
a r t i c l e
i n f o
Article history: Received 4 November 2013 Received in revised form 10 December 2013 Accepted 15 December 2013 Available online 21 December 2013 Keywords: Rapidly synergistic cloud point extraction Ultrasound-assisted cloud point extraction Preconcentration Spectrophotometric determination Selenium
a b s t r a c t In this work, rapidly synergistic cloud point extraction (RS-CPE) and ultrasound-assisted cloud point extraction (UA-CPE) were firstly compared and coupled with spectrophotometer for selenium preconcentration and detection. The established RS-CPE pretreatment was simple, rapid and high effective. The extraction time was only 1 min without heating process. Under the effect of ultrasound, UA-CPE accomplished extraction efficiently although the extraction procedure was relatively time-consuming. In this study, RS-CPE and UA-CPE were firstly applied for selenium preconcentration and coupled with conventional spectrophotometer. Their applications were expanded and the analytical performance of spectrophotometric determination for selenium was considerably improved. The influence factors relevant to RS-CPE and UA-CPE were studied in detail. Under the optimal conditions, the limits of detection (LODs) for selenium were respectively 0.2 lg L 1 of RS-CPE and 0.3 lg L 1 of UA-CPE with sensitivity enhancement factors (EFs) of 124 and 103. The developed methods were applied to the determination of trace selenium in real water samples with satisfactory analytical results. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Trace and ultra trace analysis of metal elements is one of important tasks and academic investigation in analytical chemistry. With ⇑ Corresponding author. Tel.: +86 872 2257 393. E-mail address: [email protected] (X. Wen). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.074
the development of analytical instruments, ultra trace analysis has become easier for advanced apparatus such as electrothermal atomic absorption spectrometry (ET-AAS) [1,2], inductively coupled plasma optical emission spectrometry (ICP-OES) [3,4] and inductively coupled plasma mass spectrometry (ICP-MS) [5,6]. Compared with those sensitive but expensive and energy consuming instruments, spectrophotometer is very cost-effective and
X. Wen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 200–205
widely used even in some less developed area. The choke point of spectrophotometric determination is relatively poor sensitivity. If solve this problem, the instrument could be more useful. Through coupling of some advanced preconcentration techniques with traditional spectrophotometer, trace or ultra-trace metal ions could be successfully detected [7,8]. In this work, rapidly synergistic cloud point extraction (RS-CPE) and ultrasound-assisted cloud point extraction (UA-CPE) were firstly coupled with conventional spectrophotometer to pre-concentrate and determine trace selenium. As one of advanced and miniaturized extraction techniques, cloud point extraction (CPE) or micelle-mediated extraction has been well-established and applied in many fields of chemistry, especially in analytical chemistry. Recently, Yazdi [9] reviewed the surfactant-based extraction methods, which introduced relatively detailed researches in this field. Besides that, CPE has been well-reported in the literatures for coupling with a variety of instrumental methods in recent years, including flame atomic absorption spectrometry (FAAS) [10], thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) [11], [12], GF-AAS [13], W-coil ET-AAS [14], ICP-OES [15,16], ICP-MS [17], gas chromatography (GC) [18], high-performance liquid chromatography (HPLC) [19], capillary electrophoresis (CE) [20], etc. On the other hand, the processes and patterns of CPE have been deeply investigated and developed. Traditional CPE depends on heating in water bath to induce cloud point phenomenon and accomplish extraction process, which is time- and labor-consuming. Recently, many researches devoted to improve traditional CPE [21–24]. Rapidly synergistic cloud point extraction (RS-CPE) has been developed by our group and considerably simplified and accelerated traditional CPE procedure [25–27]. As mentioned in our previous work, octanol worked as cloud point revulsant and synergic reagent, which successfully decreased the cloud point temperature (CPT) of surfactant to realize the room temperature CPE without heating. The process of RS-CPE was very rapid at room temperature without heating and the extraction time was only about 1 min. Ultrasound-assisted cloud point extraction (UA-CPE) is one of improved CPE methods [28]. In this method, ultrasonic wave was used to assist CPE process and the reactions and clouding phenomena were accelerated. Ultrasound can accelerate the interactive rate between the surfactant and aqueous phase so that the target analytes could be well extracted into the surfactant-rich phase. In this work, RS-CPE and UA-CPE were compared and coupled with conventional spectrophotometer to determine trace selenium. The purpose of this work was to compare the analytical performance of the improved two CPE methods. The effect of ultrasound and revulsant/synergic reagent was investigated and compared in this study. To the best of our knowledge, this was the first comparative study for the analytical performance of the two CPE methods. As advanced CPE methods, RS-CPE and UA-CPE have not been widely applied and especially for UA-CPE most of the reported target analytes were organic compounds [29]. In this work, trace selenium was extracted and detected as a target element. Water is the potential source that metal elements enter human bodies, thus the determination of selenium in real water samples could afford some information of significant importance. Without any doubt, the developed couplings could be expanded to the determination of many other metals. The main parameters influencing extraction and determination were investigated in detail. The analytical performance of the two CPE methods was compared in this work. The characteristics and performance parameters of the developed methods RS-CPE and UA-CPE were described below.
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Experimental Apparatus UV–vis Spectrophotometer Model TU-1901 (Beijing Puxi General Instrument Co., Ltd., Beijing, China) was used for the investigation and determination. An ultrasonic cleaner with temperature control Model SB5200DT (Ningbo Xinzhi Biotechnology Co., Ltd., Ningbo, China) was used for ultrasonic extraction for UA-CPE. A centrifuge Model TDL-5-A (Shanghai Anting Scientific Instrument Factory, Shanghai, China) was used for phase separation experiments in 40 mL conical tubes. The pH values were measured by a pH-meter Model pHS-25 (Shanghai Hongyi Instrument Co., Ltd., Shanghai, China). A laboratory pure water system Model DZG-303A (Chengdu Tangshi Kangning Science and Technology Development Co., Ltd., Chengdu, China) was used to prepare ultra pure water. Reagents Selenium standard solution (1000 mg L 1) was purchased from National Standard Material Center (Beijing, China). Working standard solution was obtained daily by stepwise dilution from standard stock solution in ultra pure water. Non-ionic surfactant Triton X-114 (TX-114) (Sigma–Aldrich) was used as extractant. Octanol (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) worked as cloud point revulsant/synergic reagent in RS-CPE. Dithizone (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was used as the chelating reagent for selenium. Other chemical reagents including hydrochloric acid, methanol, ethanol, acetone, ethyl acetate and tetrahydrofuran (THF) were all of analytical grade, as well as the reagents mentioned above. Ultra pure water was used throughout to decrease blank. Procedures of RS-CPE and UA-CPE The experimental procedure of RS-CPE has been well elaborated in our previous work [25]. For each sample, 40 mL analytical solution containing selenium and proper amount of dithizone were mixed in the solution. After adjusting the pH value, the proper amounts of TX114 and octanol were added into the conical tube. After manual shaking the tube, the solution became turbid and micelle occurred by the effect of octanol. The extraction was accomplished rapidly during manual shaking process for 1 min. The complex of Se-dithizone was captured by the micelle and dispersed octanol. For UA-CPE, the procedure was similar with traditional CPE and relatively time-consuming. For each sample, 40 mL analytical solution containing selenium and proper amount of dithizone were mixed in the solution. After adjusting the pH value, the optimal volume of TX-114 was added into the conical tube. Ultrasound was applied to assist and accelerate cloud point extraction under the equilibration temperature of 50 °C for 20 min, which was accomplished in an ultrasonic cleaner with temperature control. After centrifugation at 3000 rpm for 5 min, the turbid solution became pellucid and the micelle rich phase presented deeply color. The water phase was moved carefully and then the proper type (methanol) and volume (3 mL) of solvent was used to dilute the concentrated sample, considering the sampling demand of spectrophotometer. After that, the resultant sample solution was transported to spectrophotometer for determination. Sample collection and preparation Real water samples were all gotten from local place. Lake water was gotten from the famous Erhai Lake (Dali, China).
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River water was collected from Xier River (Dali, China). Tap water was collected from our lab. Bottled mineral water and purified water were purchased from local market. All the environmental water samples were filtered through a 0.25 lm micropore membrane prior to use. The resultant samples were then subjected to the established RS-CPE and UA-CPE and subsequently analyzed by spectrophotometer. Results and discussion Study on the absorption spectra of complex In this work, conventional spectrophotometer was coupled with the advanced RS-CPE and UA-CPE preconcentration methods. The maximum wavelength of absorption for the resultant samples was the basis of the quantification analysis. In the preliminary experiment, the resultant samples were conveyed to the spectrophotometer to measure the absorption curves, according to the experimental procedures described above. For direct detection of selenium, the complex of Se-dithizone was determined in the range of wavelength from 400 nm to 600 nm. During the experiment, the concentration of 1000 lg L 1 selenium was used for wavelength scan without preconcentration. For RS-CPE and UACPE, 50 lg L 1 and 100 lg L 1 of selenium were respectively used to be pre-concentrated and detected (without optimization for both methods). From the results in Fig. 1, the preliminary enhancement of sensitivity for both methods was obvious and the extraction efficiency of RS-CPE for selenium maybe higher than that of UA-CPE. The results indicated that the maximum absorption wavelength after RS-CPE and UA-CPE was 420 nm, and the wavelength without extraction was 522 nm (moving to the short wavelength after extraction). During the determination, the blank absorbance of all reagents was corrected. Investigation and comparison of RS-CPE and UA-CPE parameters Effect of equilibration temperature and ultrasound time For UA-CPE, the procedure was similar with traditional CPE, which was accomplished by the effect of heating in water bath. In this method, ultrasonic wave was used to assist CPE process and the reactions and clouding phenomena were accelerated. Ultrasound can accelerate the interactive rate between the surfactant
Fig. 1. Absorption spectra for the determination of Se with/without extraction. RSCPE conditions: Se, 50 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.05% v/v; octanol volume, 1.0 mL; pH, 1.4; sample volume, 40 mL; dilution solvent, ethanol. UA-CPE conditions: Se, 100 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.10% v/ v; ultrasound time, 15 min; equilibration temperature, 50 °C; pH, 1.0; sample volume, 40 mL; dilution solvent, ethanol. Without extraction conditions: Se, 1000 lg L 1; dithizone, 1.25 10 5 mol L 1; pH, 1.0.
Fig. 2. Effect of equilibration temperature for UA-CPE. UA-CPE conditions: Se, 100 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.10% v/v; ultrasound time, 20 min; pH, 1.0; sample volume, 40 mL; dilution solvent, ethanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
and aqueous phase so that the target analytes could be well extracted into the surfactant-rich phase. Besides ultrasound, the lowest possible equilibration temperature is very desirable in the CPE process. When the temperature was lower than cloud-point temperature, no cloud-point phenomenon occurred. So, it was important to investigate the two key factors for UA-CPE. The results in Fig. 2 indicated that 50 °C was the optimal equilibration temperature. As shown in Fig. 3, ultrasound time was optimized from 2 min to 30 min. In the range of 2–20 min, the signals increased dramatically, which demonstrated the maximum extraction efficiency was obtained at ultrasound time (incubation time) of 20 min. Volume of octanol As cloud point revulsant and synergic reagent in RS-CPE, octanol efficiently lowered the cloud point temperature (CPT) of TX-114 below room temperature and assisted the subsequent extraction process. The developed RS-CPE procedure could be accomplished rapidly without heating or adding any salts. In this section, the volume of octanol was investigated from 0.2 mL to 1.5 mL. As shown in Fig. 4 , 0.2 mL of octanol was not enough to accomplish room temperature extraction effectively and the absorbance increased with the increasing of octanol volume. It was
Fig. 3. Effect of ultrasound time for UA-CPE. UA-CPE conditions: Se, 100 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.10% v/v; equilibration temperature, 50 °C; pH, 1.0; sample volume, 40 mL; dilution solvent, ethanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
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Concentration of chelating reagent As a principle reagent in RS-CPE and UA-CPE, dithizone was used to form selenium chelate and the concentration of dithizone was carefully studied in the range from 5.0 10 6 to 2.0 10 5 mol L 1, as shown in Fig. 6. It was found that the absorbance for both methods increased rapidly with the increasing of dithizone concentrations from 5.0 10 6 to 1.25 10 5 mol L 1 and at higher concentrations, the absorbance remarkably decreased. The excessive dithizone could be co-extracted into the micelle rich phase, thus decreased the extraction efficiency of target analytes. Therefore, 1.25 10 5 mol L 1 of dithizone concentration was chosen as the optimal condition of RS-CPE and UA-CPE for further experiments. In this section, the same concentration of selenium (50 lg L 1) was used in both CPE methods. The results well demonstrated that the extraction efficiency of RS-CPE for selenium was higher than that of UA-CPE. Fig. 4. Optimization of octanol volume. RS-CPE conditions: Se, 50 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.05% v/v; pH, 1.4; sample volume, 40 mL; dilution solvent, ethanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
Effect of pH For the extraction of metal ions, the complexing and extraction efficiencies are closely related to the pH of the solution system. In
found that 1.2 mL was the optimal volume, and the signals were decreased when octanol volume beyond 1.2 mL. The extraction efficiency basically kept constant with the increasing of octanol volume. The reason of decreasing of absorbance may be the increased viscosity of resultant samples. Effect of Triton X-114 concentration In this work, the extraction was accomplished in different patterns. For RS-CPE, the micelle-rich phase was induced under the effect of revulsant and synergic reagent octanol. For UA-CPE, the micelle of surfactant was occurred by the effect of heating and ultrasound in water bath. For the two methods, careful attention should be paid to the concentration of TX-114, which remarkably affected the extraction efficiency of the developed methods. As shown in Fig. 5, for RS-CPE it was found that the absorbance of selenium increased with the TX-114 concentration increasing from 0.0125% to 0.05% v/v. When TX-114 was excessive in the system, the absorbance rapidly decreased. The excessive surfactant could affect the efficiency of cloud point extraction. For UA-CPE, the trend was similar and 0.05% v/v of TX-114 was the optimal condition. After optimization, the best concentration of TX-114 in both systems was found to be 0.05% v/v.
Fig. 5. Optimization of Triton X-114 concentration. RS-CPE conditions: Se, 50 lg L 1; dithizone, 1.25 10 5 mol L 1; octanol volume, 1.2 mL; pH, 1.4; UACPE conditions: Se, 100 lg L 1; dithizone, 1.25 10 5 mol L 1; ultrasound time, 20 min; equilibration temperature, 50 °C; pH, 1.0; Sample volume, 40 mL; dilution solvent, ethanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
Fig. 6. Optimization of dithizone concentration. RS-CPE conditions: Se, 50 lg L 1; TX-114, 0.05% v/v; octanol volume, 1.2 mL; pH, 1.4; UA-CPE conditions: Se, 50 lg L 1; TX-114, 0.05% v/v; ultrasound time, 20 min; equilibration temperature, 50 °C; pH, 1.0; Sample volume, 40 mL; dilution solvent, ethanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
Fig. 7. Effect of pH on the absorbance of Se. RS-CPE conditions: Se, 50 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.05% v/v; octanol volume, 1.2 mL; UA-CPE conditions: Se, 50 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.05% v/v; ultrasound time, 20 min; equilibration temperature, 50 °C; Sample volume, 40 mL; dilution solvent, methanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
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Table 1 Tolerant limits of coexisting ions for the determination of selenium (50 lg L Potential interfering ions
K+ Na+ Ca2+ Mg2+ Cl SO24 Co2+ Cu2+ Ni2+ Pb2+ Cd2+ Hg2+ Ag+ Cr3+
1
).
Interferent/metal ratio
Recovery (%)
UA-CPE
RS-CPE
UA-CPE
RS-CPE
7500 7500 5000 7500 7500 7500
8000 10,000 6250 10,000 10,000 10,000
98.6 97.9 99.0 104 97.9 104
106 94.1 98.9 107 94.1 107
625 25 15 500 800 7.5 15 2
94.6 105 96.3 104 96.9 102 99.6 101
92.7 107 98.6 101 103 104 98.0 91.0
375 50 15 400 800 5 10 2
order to obtain the desired complexing and preconcentration efficiencies, the pH values were studied in the ranges of 0.1–4.0 for both methods, adjusted by diluted HCl. The results shown in Fig. 7 indicated that the complex of Se-dithizone required strongly acidic condition. When pH of solution increased, the absorbance signals increased correspondingly in the range from pH of 0.1 to 0.8 for UA-CPE (1.0 for RS-CPE). The weaker acidity induced rapid decreasing of absorbance signals. Hereby, pH of 1.0 was selected as optimal acidity for RS-CPE and pH of 0.8 was chosen for UACPE in the further experiments. The small difference may be due to slightly different solution matrix of RS-CPE and UA-CPE methods. In this section, the same concentration of selenium (50 lg L 1) was used in both CPE methods. The results also indicated that the extraction efficiency of RS-CPE for selenium was higher than that of UA-CPE. Dilution solvent The demand of sampling volume for the conventional spectrophotometer is about 3 mL. The developed pre-treatment methods RS-CPE and UA-CPE were miniaturized extraction techniques. To realize the hyphenation, the proper dilution before detection was necessary. The process would affect the ratios of volumes between water phase and organic phase and sacrificed the enrichment factors of the established CPE methods. In the work, some conventional solvents including methanol, ethanol, acetone and ethyl
acetate were investigated to find out the optimal dilution solvent. The results indicated that methanol was the best choice for UA-CPE and RS-CPE. Conditions of extraction time and phase separation Extraction time is one of the key factors for most of extraction methods. For RS-CPE, extraction time is defined as the time of manual shaking the conical tubes before centrifugation. The results showed the extraction was accomplished in 1 min after the formation of cloudy solution and the equilibrium state was achieved quickly, which was the outstanding advantage of RS-CPE. Thus the extraction time of 1 min was chosen as the optimum condition. For UA-CPE, ultrasound time has been investigated in this work and the optimal extraction time was 20 min. Centrifugation was employed to assist and accelerate phase separation after CPE methods. The experimental conditions were investigated to find out the optimal centrifugation rate and time. It was found that the optimal condition of phase separation was 3000 rpm for 5 min. Interferences Because dithizone could complex with other metal ions besides selenium, interferences may occur due to the competition of other heavy metal ions for chelating agent. The subsequent co-extraction of other formed complex with Se-dithizone could also decrease the extraction efficiency. To evaluate the selectivity of the established RS-CPE and UA-CPE, the effect of typically potential interfering ions was investigated and the results were shown in Table 1 (tolerable limit was taken as a relative error 610%). According to the data, some major matrix ions in natural water samples have no obvious interferences, while some other heavy metal ions have less tolerable limits due to the competition and co-extraction effect. Such as Cr3+, the possible reason for its serious interference maybe it is easy to form complexes with other ligands to affect the extraction efficiency of target analyte. From the results, it can be found the anti-jamming ability of RS-CPE is higher than that of UA-CPE. Analytical figures of merit Analytical figures of merit of the developed RS-CPE–UV–vis and UA-CPE–UV–vis methods were obtained under optimal conditions and compared in Table 2. At least 0.9991 of R2 (correlation coefficient) showed good linearity of the calibration curves. The results
Table 2 Analytical figures of merit. Calibration equation
Upper linear rangea (lg L
LODb (lg L
1
1
)
)
R2 (correlation coefficient)
RSD (n = 7, 25 lg L
1
)
Sensitivity enhancement factorc a b c
Without preconcentration (mg L With UA-CPE (lg L 1) With RS-CPE (lg L 1) Without preconcentration With UA-CPE With RS-CPE Without preconcentration With UA-CPE With RS-CPE Without preconcentration With UA-CPE With RS-CPE Without preconcentration With UA-CPE With RS-CPE With UA-CPE With RS-CPE
1
)
A = 0.083 C + 0.0957 A = 0.0086 C + 0.0997 A = 0.0103 C + 0.0925 10,000 100 100 32 0.3 0.2 0.9991 0.9997 0.9991 1.8% 3.2% 4.3% 103 124
Upper linear range, the upper limit of the calibration curve or the linear dynamic range. LOD, limit of detection, was based on 3r criterion for 11 blank measurements. Sensitivity enhancement factor, was calculated by the slope ratio of the calibration curves for Se determination with and without CPE method.
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X. Wen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 200–205 Table 3 Analytical results for selenium in real water samples (Avg. ± SD of three trials). Real sample
Pre-concentration method
Found in sample (lg L
Tap water
UA-CPE RS-CPE UA-CPE RS-CPE UA-CPE RS-CPE UA-CPE RS-CPE UA-CPE RS-CPE
3.0 ± 0.2 2.9 ± 0.2 7.1 ± 0.3 6.9 ± 0.2 4.0 ± 0.2 3.9 ± 0.2 ND⁄ ND⁄ ND⁄ ND⁄
Erhai Lake Xier River Bottled mineral water Bottled purified water
1
)
Spiked (lg L 10 10 10 10 10 10 50 25 50 25
1
)
Recovery (%) 102 95.2 105 103 96.0 95.1 93.1 105 99.5 103
ND⁄, Not detected.
in Table 2 indicated that superior sensitivity of selenium detection and enhancement factor were obtained after RS-CPE preconcentration. For the improvement of analytical performance of spectrophotometric detection, the two CPE methods showed high extraction efficiency with simplicity and rapidity. In addition to UV–vis-RS-CPE/UA-CPE, some other instrumental methods coupled with different extraction methods have been reported for the determination of selenium, such as hollow fiber liquid phase microextraction combined with ET-AAS (LOD of 5 ng L 1 [30]), CPE–FAAS (LOD of 3.81 lg L 1 [31]), hydride generation and gold-coated W-coil trapping ET-AAS (LOD of 39 ng L 1 [32]). Determination of selenium in real water samples Water is one of important pathways of pollutants entering human bodies. Considering the biological and environmental significance of selenium, several real water samples including lake water, river water, tap water and bottled water were analyzed to validate the accuracy and applicability of the established methods. The analytical results were summarized in Table 3. The found values of selenium in environmental water samples detected by RS-CPE and UA-CPE were accurately coincident. The recoveries for the spiked samples were in the acceptable range of 93.1–105%. Conclusions In this work, a comparative investigation of RS-CPE and UA-CPE was carried out. The two CPE methods were firstly coupled with spectrophotometer for trace selenium extraction and detection. The established work expanded the application of the two CPE methods and considerably improved the analytical performance of spectrophotometric determination. For both methods, high enhancement factors were obtained. The comparative investigation indicated that the analytical performance of RS-CPE for selenium was better than UA-CPE. The established methods were applied to the detection of some real water samples with satisfactory analytical results. Acknowledgments Xiaodong Wen gratefully acknowledges the financial support for this project from the National Natural Science Foundation of
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Comparison of rapidly synergistic cloud point extraction and ultrasound-assisted cloud point extraction for trace selenium coupled with spectrophotometric determination Xiaodong Wen ⇑, Yanyan Zhang, Chunyan Li, Xiang Fang, Xiaocan Zhang College of Pharmacy and Chemistry, Dali University, Dali, Yunnan 671000, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A comparative study of RS-CPE and
A comparative study of RS-CPE and UA-CPE was carried out. RS-CPE and UA-CPE were firstly coupled with UV–vis for Se detection.
UA-CPE was carried out. RS-CPE and UA-CPE were firstly coupled with UV–vis for Se detection. Conventional UV–vis accomplishes ultra-trace detection via the coupling. Be useful in less developed area without more advanced analytical instruments.
a r t i c l e
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Article history: Received 4 November 2013 Received in revised form 10 December 2013 Accepted 15 December 2013 Available online 21 December 2013 Keywords: Rapidly synergistic cloud point extraction Ultrasound-assisted cloud point extraction Preconcentration Spectrophotometric determination Selenium
a b s t r a c t In this work, rapidly synergistic cloud point extraction (RS-CPE) and ultrasound-assisted cloud point extraction (UA-CPE) were firstly compared and coupled with spectrophotometer for selenium preconcentration and detection. The established RS-CPE pretreatment was simple, rapid and high effective. The extraction time was only 1 min without heating process. Under the effect of ultrasound, UA-CPE accomplished extraction efficiently although the extraction procedure was relatively time-consuming. In this study, RS-CPE and UA-CPE were firstly applied for selenium preconcentration and coupled with conventional spectrophotometer. Their applications were expanded and the analytical performance of spectrophotometric determination for selenium was considerably improved. The influence factors relevant to RS-CPE and UA-CPE were studied in detail. Under the optimal conditions, the limits of detection (LODs) for selenium were respectively 0.2 lg L 1 of RS-CPE and 0.3 lg L 1 of UA-CPE with sensitivity enhancement factors (EFs) of 124 and 103. The developed methods were applied to the determination of trace selenium in real water samples with satisfactory analytical results. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Trace and ultra trace analysis of metal elements is one of important tasks and academic investigation in analytical chemistry. With ⇑ Corresponding author. Tel.: +86 872 2257 393. E-mail address: [email protected] (X. Wen). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.074
the development of analytical instruments, ultra trace analysis has become easier for advanced apparatus such as electrothermal atomic absorption spectrometry (ET-AAS) [1,2], inductively coupled plasma optical emission spectrometry (ICP-OES) [3,4] and inductively coupled plasma mass spectrometry (ICP-MS) [5,6]. Compared with those sensitive but expensive and energy consuming instruments, spectrophotometer is very cost-effective and
X. Wen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 200–205
widely used even in some less developed area. The choke point of spectrophotometric determination is relatively poor sensitivity. If solve this problem, the instrument could be more useful. Through coupling of some advanced preconcentration techniques with traditional spectrophotometer, trace or ultra-trace metal ions could be successfully detected [7,8]. In this work, rapidly synergistic cloud point extraction (RS-CPE) and ultrasound-assisted cloud point extraction (UA-CPE) were firstly coupled with conventional spectrophotometer to pre-concentrate and determine trace selenium. As one of advanced and miniaturized extraction techniques, cloud point extraction (CPE) or micelle-mediated extraction has been well-established and applied in many fields of chemistry, especially in analytical chemistry. Recently, Yazdi [9] reviewed the surfactant-based extraction methods, which introduced relatively detailed researches in this field. Besides that, CPE has been well-reported in the literatures for coupling with a variety of instrumental methods in recent years, including flame atomic absorption spectrometry (FAAS) [10], thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) [11], [12], GF-AAS [13], W-coil ET-AAS [14], ICP-OES [15,16], ICP-MS [17], gas chromatography (GC) [18], high-performance liquid chromatography (HPLC) [19], capillary electrophoresis (CE) [20], etc. On the other hand, the processes and patterns of CPE have been deeply investigated and developed. Traditional CPE depends on heating in water bath to induce cloud point phenomenon and accomplish extraction process, which is time- and labor-consuming. Recently, many researches devoted to improve traditional CPE [21–24]. Rapidly synergistic cloud point extraction (RS-CPE) has been developed by our group and considerably simplified and accelerated traditional CPE procedure [25–27]. As mentioned in our previous work, octanol worked as cloud point revulsant and synergic reagent, which successfully decreased the cloud point temperature (CPT) of surfactant to realize the room temperature CPE without heating. The process of RS-CPE was very rapid at room temperature without heating and the extraction time was only about 1 min. Ultrasound-assisted cloud point extraction (UA-CPE) is one of improved CPE methods [28]. In this method, ultrasonic wave was used to assist CPE process and the reactions and clouding phenomena were accelerated. Ultrasound can accelerate the interactive rate between the surfactant and aqueous phase so that the target analytes could be well extracted into the surfactant-rich phase. In this work, RS-CPE and UA-CPE were compared and coupled with conventional spectrophotometer to determine trace selenium. The purpose of this work was to compare the analytical performance of the improved two CPE methods. The effect of ultrasound and revulsant/synergic reagent was investigated and compared in this study. To the best of our knowledge, this was the first comparative study for the analytical performance of the two CPE methods. As advanced CPE methods, RS-CPE and UA-CPE have not been widely applied and especially for UA-CPE most of the reported target analytes were organic compounds [29]. In this work, trace selenium was extracted and detected as a target element. Water is the potential source that metal elements enter human bodies, thus the determination of selenium in real water samples could afford some information of significant importance. Without any doubt, the developed couplings could be expanded to the determination of many other metals. The main parameters influencing extraction and determination were investigated in detail. The analytical performance of the two CPE methods was compared in this work. The characteristics and performance parameters of the developed methods RS-CPE and UA-CPE were described below.
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Experimental Apparatus UV–vis Spectrophotometer Model TU-1901 (Beijing Puxi General Instrument Co., Ltd., Beijing, China) was used for the investigation and determination. An ultrasonic cleaner with temperature control Model SB5200DT (Ningbo Xinzhi Biotechnology Co., Ltd., Ningbo, China) was used for ultrasonic extraction for UA-CPE. A centrifuge Model TDL-5-A (Shanghai Anting Scientific Instrument Factory, Shanghai, China) was used for phase separation experiments in 40 mL conical tubes. The pH values were measured by a pH-meter Model pHS-25 (Shanghai Hongyi Instrument Co., Ltd., Shanghai, China). A laboratory pure water system Model DZG-303A (Chengdu Tangshi Kangning Science and Technology Development Co., Ltd., Chengdu, China) was used to prepare ultra pure water. Reagents Selenium standard solution (1000 mg L 1) was purchased from National Standard Material Center (Beijing, China). Working standard solution was obtained daily by stepwise dilution from standard stock solution in ultra pure water. Non-ionic surfactant Triton X-114 (TX-114) (Sigma–Aldrich) was used as extractant. Octanol (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) worked as cloud point revulsant/synergic reagent in RS-CPE. Dithizone (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was used as the chelating reagent for selenium. Other chemical reagents including hydrochloric acid, methanol, ethanol, acetone, ethyl acetate and tetrahydrofuran (THF) were all of analytical grade, as well as the reagents mentioned above. Ultra pure water was used throughout to decrease blank. Procedures of RS-CPE and UA-CPE The experimental procedure of RS-CPE has been well elaborated in our previous work [25]. For each sample, 40 mL analytical solution containing selenium and proper amount of dithizone were mixed in the solution. After adjusting the pH value, the proper amounts of TX114 and octanol were added into the conical tube. After manual shaking the tube, the solution became turbid and micelle occurred by the effect of octanol. The extraction was accomplished rapidly during manual shaking process for 1 min. The complex of Se-dithizone was captured by the micelle and dispersed octanol. For UA-CPE, the procedure was similar with traditional CPE and relatively time-consuming. For each sample, 40 mL analytical solution containing selenium and proper amount of dithizone were mixed in the solution. After adjusting the pH value, the optimal volume of TX-114 was added into the conical tube. Ultrasound was applied to assist and accelerate cloud point extraction under the equilibration temperature of 50 °C for 20 min, which was accomplished in an ultrasonic cleaner with temperature control. After centrifugation at 3000 rpm for 5 min, the turbid solution became pellucid and the micelle rich phase presented deeply color. The water phase was moved carefully and then the proper type (methanol) and volume (3 mL) of solvent was used to dilute the concentrated sample, considering the sampling demand of spectrophotometer. After that, the resultant sample solution was transported to spectrophotometer for determination. Sample collection and preparation Real water samples were all gotten from local place. Lake water was gotten from the famous Erhai Lake (Dali, China).
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River water was collected from Xier River (Dali, China). Tap water was collected from our lab. Bottled mineral water and purified water were purchased from local market. All the environmental water samples were filtered through a 0.25 lm micropore membrane prior to use. The resultant samples were then subjected to the established RS-CPE and UA-CPE and subsequently analyzed by spectrophotometer. Results and discussion Study on the absorption spectra of complex In this work, conventional spectrophotometer was coupled with the advanced RS-CPE and UA-CPE preconcentration methods. The maximum wavelength of absorption for the resultant samples was the basis of the quantification analysis. In the preliminary experiment, the resultant samples were conveyed to the spectrophotometer to measure the absorption curves, according to the experimental procedures described above. For direct detection of selenium, the complex of Se-dithizone was determined in the range of wavelength from 400 nm to 600 nm. During the experiment, the concentration of 1000 lg L 1 selenium was used for wavelength scan without preconcentration. For RS-CPE and UACPE, 50 lg L 1 and 100 lg L 1 of selenium were respectively used to be pre-concentrated and detected (without optimization for both methods). From the results in Fig. 1, the preliminary enhancement of sensitivity for both methods was obvious and the extraction efficiency of RS-CPE for selenium maybe higher than that of UA-CPE. The results indicated that the maximum absorption wavelength after RS-CPE and UA-CPE was 420 nm, and the wavelength without extraction was 522 nm (moving to the short wavelength after extraction). During the determination, the blank absorbance of all reagents was corrected. Investigation and comparison of RS-CPE and UA-CPE parameters Effect of equilibration temperature and ultrasound time For UA-CPE, the procedure was similar with traditional CPE, which was accomplished by the effect of heating in water bath. In this method, ultrasonic wave was used to assist CPE process and the reactions and clouding phenomena were accelerated. Ultrasound can accelerate the interactive rate between the surfactant
Fig. 1. Absorption spectra for the determination of Se with/without extraction. RSCPE conditions: Se, 50 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.05% v/v; octanol volume, 1.0 mL; pH, 1.4; sample volume, 40 mL; dilution solvent, ethanol. UA-CPE conditions: Se, 100 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.10% v/ v; ultrasound time, 15 min; equilibration temperature, 50 °C; pH, 1.0; sample volume, 40 mL; dilution solvent, ethanol. Without extraction conditions: Se, 1000 lg L 1; dithizone, 1.25 10 5 mol L 1; pH, 1.0.
Fig. 2. Effect of equilibration temperature for UA-CPE. UA-CPE conditions: Se, 100 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.10% v/v; ultrasound time, 20 min; pH, 1.0; sample volume, 40 mL; dilution solvent, ethanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
and aqueous phase so that the target analytes could be well extracted into the surfactant-rich phase. Besides ultrasound, the lowest possible equilibration temperature is very desirable in the CPE process. When the temperature was lower than cloud-point temperature, no cloud-point phenomenon occurred. So, it was important to investigate the two key factors for UA-CPE. The results in Fig. 2 indicated that 50 °C was the optimal equilibration temperature. As shown in Fig. 3, ultrasound time was optimized from 2 min to 30 min. In the range of 2–20 min, the signals increased dramatically, which demonstrated the maximum extraction efficiency was obtained at ultrasound time (incubation time) of 20 min. Volume of octanol As cloud point revulsant and synergic reagent in RS-CPE, octanol efficiently lowered the cloud point temperature (CPT) of TX-114 below room temperature and assisted the subsequent extraction process. The developed RS-CPE procedure could be accomplished rapidly without heating or adding any salts. In this section, the volume of octanol was investigated from 0.2 mL to 1.5 mL. As shown in Fig. 4 , 0.2 mL of octanol was not enough to accomplish room temperature extraction effectively and the absorbance increased with the increasing of octanol volume. It was
Fig. 3. Effect of ultrasound time for UA-CPE. UA-CPE conditions: Se, 100 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.10% v/v; equilibration temperature, 50 °C; pH, 1.0; sample volume, 40 mL; dilution solvent, ethanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
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Concentration of chelating reagent As a principle reagent in RS-CPE and UA-CPE, dithizone was used to form selenium chelate and the concentration of dithizone was carefully studied in the range from 5.0 10 6 to 2.0 10 5 mol L 1, as shown in Fig. 6. It was found that the absorbance for both methods increased rapidly with the increasing of dithizone concentrations from 5.0 10 6 to 1.25 10 5 mol L 1 and at higher concentrations, the absorbance remarkably decreased. The excessive dithizone could be co-extracted into the micelle rich phase, thus decreased the extraction efficiency of target analytes. Therefore, 1.25 10 5 mol L 1 of dithizone concentration was chosen as the optimal condition of RS-CPE and UA-CPE for further experiments. In this section, the same concentration of selenium (50 lg L 1) was used in both CPE methods. The results well demonstrated that the extraction efficiency of RS-CPE for selenium was higher than that of UA-CPE. Fig. 4. Optimization of octanol volume. RS-CPE conditions: Se, 50 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.05% v/v; pH, 1.4; sample volume, 40 mL; dilution solvent, ethanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
Effect of pH For the extraction of metal ions, the complexing and extraction efficiencies are closely related to the pH of the solution system. In
found that 1.2 mL was the optimal volume, and the signals were decreased when octanol volume beyond 1.2 mL. The extraction efficiency basically kept constant with the increasing of octanol volume. The reason of decreasing of absorbance may be the increased viscosity of resultant samples. Effect of Triton X-114 concentration In this work, the extraction was accomplished in different patterns. For RS-CPE, the micelle-rich phase was induced under the effect of revulsant and synergic reagent octanol. For UA-CPE, the micelle of surfactant was occurred by the effect of heating and ultrasound in water bath. For the two methods, careful attention should be paid to the concentration of TX-114, which remarkably affected the extraction efficiency of the developed methods. As shown in Fig. 5, for RS-CPE it was found that the absorbance of selenium increased with the TX-114 concentration increasing from 0.0125% to 0.05% v/v. When TX-114 was excessive in the system, the absorbance rapidly decreased. The excessive surfactant could affect the efficiency of cloud point extraction. For UA-CPE, the trend was similar and 0.05% v/v of TX-114 was the optimal condition. After optimization, the best concentration of TX-114 in both systems was found to be 0.05% v/v.
Fig. 5. Optimization of Triton X-114 concentration. RS-CPE conditions: Se, 50 lg L 1; dithizone, 1.25 10 5 mol L 1; octanol volume, 1.2 mL; pH, 1.4; UACPE conditions: Se, 100 lg L 1; dithizone, 1.25 10 5 mol L 1; ultrasound time, 20 min; equilibration temperature, 50 °C; pH, 1.0; Sample volume, 40 mL; dilution solvent, ethanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
Fig. 6. Optimization of dithizone concentration. RS-CPE conditions: Se, 50 lg L 1; TX-114, 0.05% v/v; octanol volume, 1.2 mL; pH, 1.4; UA-CPE conditions: Se, 50 lg L 1; TX-114, 0.05% v/v; ultrasound time, 20 min; equilibration temperature, 50 °C; pH, 1.0; Sample volume, 40 mL; dilution solvent, ethanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
Fig. 7. Effect of pH on the absorbance of Se. RS-CPE conditions: Se, 50 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.05% v/v; octanol volume, 1.2 mL; UA-CPE conditions: Se, 50 lg L 1; dithizone, 1.25 10 5 mol L 1; TX-114, 0.05% v/v; ultrasound time, 20 min; equilibration temperature, 50 °C; Sample volume, 40 mL; dilution solvent, methanol. The error bars standard for ‘‘± one standard deviation of three trials’’.
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Table 1 Tolerant limits of coexisting ions for the determination of selenium (50 lg L Potential interfering ions
K+ Na+ Ca2+ Mg2+ Cl SO24 Co2+ Cu2+ Ni2+ Pb2+ Cd2+ Hg2+ Ag+ Cr3+
1
).
Interferent/metal ratio
Recovery (%)
UA-CPE
RS-CPE
UA-CPE
RS-CPE
7500 7500 5000 7500 7500 7500
8000 10,000 6250 10,000 10,000 10,000
98.6 97.9 99.0 104 97.9 104
106 94.1 98.9 107 94.1 107
625 25 15 500 800 7.5 15 2
94.6 105 96.3 104 96.9 102 99.6 101
92.7 107 98.6 101 103 104 98.0 91.0
375 50 15 400 800 5 10 2
order to obtain the desired complexing and preconcentration efficiencies, the pH values were studied in the ranges of 0.1–4.0 for both methods, adjusted by diluted HCl. The results shown in Fig. 7 indicated that the complex of Se-dithizone required strongly acidic condition. When pH of solution increased, the absorbance signals increased correspondingly in the range from pH of 0.1 to 0.8 for UA-CPE (1.0 for RS-CPE). The weaker acidity induced rapid decreasing of absorbance signals. Hereby, pH of 1.0 was selected as optimal acidity for RS-CPE and pH of 0.8 was chosen for UACPE in the further experiments. The small difference may be due to slightly different solution matrix of RS-CPE and UA-CPE methods. In this section, the same concentration of selenium (50 lg L 1) was used in both CPE methods. The results also indicated that the extraction efficiency of RS-CPE for selenium was higher than that of UA-CPE. Dilution solvent The demand of sampling volume for the conventional spectrophotometer is about 3 mL. The developed pre-treatment methods RS-CPE and UA-CPE were miniaturized extraction techniques. To realize the hyphenation, the proper dilution before detection was necessary. The process would affect the ratios of volumes between water phase and organic phase and sacrificed the enrichment factors of the established CPE methods. In the work, some conventional solvents including methanol, ethanol, acetone and ethyl
acetate were investigated to find out the optimal dilution solvent. The results indicated that methanol was the best choice for UA-CPE and RS-CPE. Conditions of extraction time and phase separation Extraction time is one of the key factors for most of extraction methods. For RS-CPE, extraction time is defined as the time of manual shaking the conical tubes before centrifugation. The results showed the extraction was accomplished in 1 min after the formation of cloudy solution and the equilibrium state was achieved quickly, which was the outstanding advantage of RS-CPE. Thus the extraction time of 1 min was chosen as the optimum condition. For UA-CPE, ultrasound time has been investigated in this work and the optimal extraction time was 20 min. Centrifugation was employed to assist and accelerate phase separation after CPE methods. The experimental conditions were investigated to find out the optimal centrifugation rate and time. It was found that the optimal condition of phase separation was 3000 rpm for 5 min. Interferences Because dithizone could complex with other metal ions besides selenium, interferences may occur due to the competition of other heavy metal ions for chelating agent. The subsequent co-extraction of other formed complex with Se-dithizone could also decrease the extraction efficiency. To evaluate the selectivity of the established RS-CPE and UA-CPE, the effect of typically potential interfering ions was investigated and the results were shown in Table 1 (tolerable limit was taken as a relative error 610%). According to the data, some major matrix ions in natural water samples have no obvious interferences, while some other heavy metal ions have less tolerable limits due to the competition and co-extraction effect. Such as Cr3+, the possible reason for its serious interference maybe it is easy to form complexes with other ligands to affect the extraction efficiency of target analyte. From the results, it can be found the anti-jamming ability of RS-CPE is higher than that of UA-CPE. Analytical figures of merit Analytical figures of merit of the developed RS-CPE–UV–vis and UA-CPE–UV–vis methods were obtained under optimal conditions and compared in Table 2. At least 0.9991 of R2 (correlation coefficient) showed good linearity of the calibration curves. The results
Table 2 Analytical figures of merit. Calibration equation
Upper linear rangea (lg L
LODb (lg L
1
1
)
)
R2 (correlation coefficient)
RSD (n = 7, 25 lg L
1
)
Sensitivity enhancement factorc a b c
Without preconcentration (mg L With UA-CPE (lg L 1) With RS-CPE (lg L 1) Without preconcentration With UA-CPE With RS-CPE Without preconcentration With UA-CPE With RS-CPE Without preconcentration With UA-CPE With RS-CPE Without preconcentration With UA-CPE With RS-CPE With UA-CPE With RS-CPE
1
)
A = 0.083 C + 0.0957 A = 0.0086 C + 0.0997 A = 0.0103 C + 0.0925 10,000 100 100 32 0.3 0.2 0.9991 0.9997 0.9991 1.8% 3.2% 4.3% 103 124
Upper linear range, the upper limit of the calibration curve or the linear dynamic range. LOD, limit of detection, was based on 3r criterion for 11 blank measurements. Sensitivity enhancement factor, was calculated by the slope ratio of the calibration curves for Se determination with and without CPE method.
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X. Wen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 200–205 Table 3 Analytical results for selenium in real water samples (Avg. ± SD of three trials). Real sample
Pre-concentration method
Found in sample (lg L
Tap water
UA-CPE RS-CPE UA-CPE RS-CPE UA-CPE RS-CPE UA-CPE RS-CPE UA-CPE RS-CPE
3.0 ± 0.2 2.9 ± 0.2 7.1 ± 0.3 6.9 ± 0.2 4.0 ± 0.2 3.9 ± 0.2 ND⁄ ND⁄ ND⁄ ND⁄
Erhai Lake Xier River Bottled mineral water Bottled purified water
1
)
Spiked (lg L 10 10 10 10 10 10 50 25 50 25
1
)
Recovery (%) 102 95.2 105 103 96.0 95.1 93.1 105 99.5 103
ND⁄, Not detected.
in Table 2 indicated that superior sensitivity of selenium detection and enhancement factor were obtained after RS-CPE preconcentration. For the improvement of analytical performance of spectrophotometric detection, the two CPE methods showed high extraction efficiency with simplicity and rapidity. In addition to UV–vis-RS-CPE/UA-CPE, some other instrumental methods coupled with different extraction methods have been reported for the determination of selenium, such as hollow fiber liquid phase microextraction combined with ET-AAS (LOD of 5 ng L 1 [30]), CPE–FAAS (LOD of 3.81 lg L 1 [31]), hydride generation and gold-coated W-coil trapping ET-AAS (LOD of 39 ng L 1 [32]). Determination of selenium in real water samples Water is one of important pathways of pollutants entering human bodies. Considering the biological and environmental significance of selenium, several real water samples including lake water, river water, tap water and bottled water were analyzed to validate the accuracy and applicability of the established methods. The analytical results were summarized in Table 3. The found values of selenium in environmental water samples detected by RS-CPE and UA-CPE were accurately coincident. The recoveries for the spiked samples were in the acceptable range of 93.1–105%. Conclusions In this work, a comparative investigation of RS-CPE and UA-CPE was carried out. The two CPE methods were firstly coupled with spectrophotometer for trace selenium extraction and detection. The established work expanded the application of the two CPE methods and considerably improved the analytical performance of spectrophotometric determination. For both methods, high enhancement factors were obtained. The comparative investigation indicated that the analytical performance of RS-CPE for selenium was better than UA-CPE. The established methods were applied to the detection of some real water samples with satisfactory analytical results. Acknowledgments Xiaodong Wen gratefully acknowledges the financial support for this project from the National Natural Science Foundation of
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