ANALYTICAL SCIENCES JANUARY 2006, VOL. 22 2006 © The Japan Society for Analytical Chemistry
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Notes
Development of Sequential Injection-Lab-at-Valve (SI-LAV) Micro-Extraction Instrumentation for the Spectrophotometric Determination of an Anionic Surfactant Rodjana BURAKHAM,*† Jaroon JAKMUNEE,** and Kate GRUDPAN**† *Department of Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand **Department of Chemistry, Faculty of Science and Institute for Science and Technology Research and Development, Chiang Mai University, Chiang Mai 50200, Thailand
The development of instrumentation for sequential injection analysis with a “lab-at-valve” (SIA-LAV) micro-extraction system is presented. The extractive determination of an anionic surfactant using methylene blue was selected as a model. Sample, reagents and organic solvent were sequentially aspirated into an extraction coil connected to the center of a selection valve, where extraction took place by flow reversal. The aqueous and organic phases were separated in a LAV unit attached to one port of the valve. The LAV unit situated a fiber-optic spectrophotometer to monitor the absorbance change of the extract product in the organic phase. The developed SIA-LAV system offers an alternative micro-total analysis system for automated micro-extraction. (Received September 2, 2005; Accepted November 21, 2005)
Introduction Sequential injection analysis (SIA),1 a second-generation of flow injection analysis (FIA), is generally accepted as one of the versatile techniques for various applications. The possibility of carrying out difficult operations on-line, such as preconcentration, derivatization, and separation, is a positive feature of such a technique. SIA has been miniaturized in a labon-valve (LOV) format, introduced in 2000 by Ruzicka.2 The system serves as a tool for a micro-total analysis system. Various applications of SIA-LOV have been reported.3–5 In recent years, a new concept of SIA, called “lab-at-valve (LAV)”, has been developed.6 The technique is simple and economic, which becomes an alternative cost-effective system for on-line analysis. Various advantages of the SIA-LAV similar to those of SIA-LOV, such as integrated instrumentation, small amounts of reagent consumption, rapidity and automation in analysis are gained. The SIA-LAV approach was successfully demonstrated for the potentiometric determination of chloride with a simply made chloride ion selective electrode (ISE).7 Our group has focused on developing a robust, on-line system for micro-extraction using SIA-LAV. A designed component, a separating chamber, was attached at one port of a conventional multiposition selection valve. The sample, reagents and organic solvent were sequentially aspirated into an extraction coil connected to the center of the selection valve. Good extraction efficiency could be achieved by flow reversal of the pumping system. The aqueous and organic phases were separated in the LAV unit before transport of the organic phase containing the extracted product to the spectrophotometer.8 The system offers † To whom correspondence should be addressed. E-mail:
[email protected];
[email protected]
on-line automated extraction. Less consumption of the sample, reagent and organic solvent was achieved, compared to the conventional batch method. The developed system has been demonstrated for the assays of diphenhydramine hydrochloride in pharmaceutical preparations and anionic surfactant in water samples. However, it is necessary to develop such a system to become a real SIA-LAV for on-line extraction on a micro-scale in order to reduce consumption of the sample and the organic solvent, as well as waste generation. In the present work, the SIA-LAV system was developed further from the previously reported one8 for on-line microextraction. A spectrophotometer was directly plugged at the separating chamber LAV unit via an optic fiber so that the detection process could be performed at the valve instead of transport of the product to the flow cell of the spectrophotometer. An anionic surfactant, which has been studied using flow-based techniques,9–12 was chosen as a model analyte. The detection principle was based on the methylene blue method.13
Experimental Chemicals and reagents All of the reagents used were of analytical reagent grade. Deionized water was used throughout the experiments. A stock solution (1000 mg/l) of sodium dodecyl sulfate (SDS) was prepared by dissolving 0.1087 g of the standard in water and diluting to 100 ml. A stock solution (0.10%) of methylene blue was prepared by dissolving 0.10 g of the standard methylene blue in 100 ml of water. The solution was pre-extracted with three 25-ml aliquots of dichloromethane in order to prevent the background level from partitioning methylene blue in the organic phase. A working methylene blue solution (0.0038%) was prepared according to our previous study,8 using 10 ml of
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Fig. 1
Schematic diagram of SIA-LAV for on-line micro-extraction.
Fig. 2 Sequence order of the SIA-LAV system for the microextraction of an anionic surfactant: A, methylene blue solution; B, sample; and C, dichloromethane.
the methylene blue stock solution, 4.25 g KH2PO4, 2.18 g K2SO4, 0.5 ml H2SO4 and diluting to 250 ml with water. Fig. 3
Apparatus The schematic diagram of the SIA-LAV system (Fig. 1) was modified from our previously reported system.8 It consisted of a 2.50 ml syringe pump (Cavro, USA), a 10-position selection valve VICI with a microelectric actuator (Valco Instruments, USA), a D1000 CE UV-Vis light source and a USB2000 spectrometer. The syringe pump was connected to the center of the selection valve by means of an extraction coil (0.79 mm i.d. × 400 cm PTFE tube) and a separating unit (8 mm i.d. of the wider end × 7 cm long, modified from a 1 ml pipette tip (Eppendoft, Germany)) situated a fiber-optic spectrophotometer; a LAV module was placed at port-1 of the selection valve. Both instrumental control and data acquisition were manipulated via the FIAlab software (FIAlab Instruments, USA).
Results and Discussion Optimization of the experimental parameters Some experimental parameters that affected the extraction efficiency were studied. They included the operational sequence, the number of flow reversals, and the volumes of the sample and the organic solvent. The sequence order of operation and the number of flow reversals are important factors that determine the time and efficiency that the aqueous and
Effect of flow reversal.
organic phases are in contact for improving the extraction efficiency. Two sequence orders, as shown in Fig. 2, were examined. The results are shown in Fig. 3. It can be seen that the operational sequence in which the sample, reagent and organic solvent were sequentially aspirated as small segments (see Fig. 2(b)) provided a higher slope of calibration (a plot of absorbance vs. SDS concentration). This could be due to a higher degree of contact between the two phases. In addition, the sensitivity increased with increasing number of the flow reversals. However, a higher slope of calibration could be reached by using a repeatedly segmented sequence, although only 1 cycle of flow reversal was performed. Therefore, the selected sequence order of the operation was as follows: the sample, reagent and dichloromethane were sequentially repeatedly introduced into an extraction coil. The extraction step was then performed using flow reversal of the syringe pump by programming in the aspiration and dispensed modes. The extraction process took place in the extraction coil. After that, the aqueous and organic phases were propelled into the separating unit where separation of the two phases occurred. The solution was left for 5 s before spectrophotometric detection (at 650 nm) of the organic phase containing an ion
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139 Table 1 Operational sequence of the proposed SI-LAV for online micro-extraction of an anionic surfactant Sequence 1 2 3 4
Fig. 4
Effect of the sample volume.
5 6
association product. It should be noted that both separation and detection were performed at the valve (“lab-at-valve” concept). The separating LAV unit was then cleaned before starting the next determination. In order to gain better preconcentration efficiency, the volume ratio of the aqueous-to-organic phases was studied. It was found that the sensitivity, indicated by the slope of the calibration graph, increased with decreasing the volume of the extracting solvent. A higher standard deviation was obtained using dichloromethane of less than 300 µl as it was difficult to reach complete separation for the two phases, i.e. aqueous droplets being contaminated within the organic phase during the detection process. Therefore, an organic solvent of 300 µl was then selected. The effect of the sample volume on the sensitivity was studied in the range of 250 – 1000 µl. The results are shown in Fig. 4. The sensitivity increased with increasing sample volume. A sample volume of 1000 µl was used in this work because of the limitation of the bed volume of the separating chamber. Table 1 summarizes the operation steps under the optimized conditions. Analytical characteristics Using the selected conditions, the analytical characteristics of the proposed SIA-LAV system were evaluated by examining the linear range, precision, limit of detection (LOD) and sampling frequency. The linear calibration graph of SDS was obtained in the range of 0.10 – 1.00 mg l–1 with the regression equation y = 0.3182x – 0.0031 and a correlation coefficient of 0.9998. The LOD, defined as the concentration of the analyte giving signals equivalent to three-times the standard deviation of the blank signals, was found to be 0.01 mg l–1. The relative standard deviation (RSD) was less than 6% (n = 11, 0.50 mg l–1 SDS) with a sampling frequency of 12 h–1. All of the figures are better than that of the previous work8, i.e., calibration: y = 0.2248x – 0.1517, r2 = 0.9992, LOD of 1.90 mg l–1 and a sampling frequency of 5 h–1. Effect of foreign ions The effects of common ions contained in water samples were examined. A foreign ion of a known concentration was added to a solution containing 0.20 mg l–1 SDS before determination. Signals were observed and the recoveries were compared with those obtained using the standard without the foreign ion. The highest concentration studied of the common ions that caused a deviation of less than ±5% were: 5.64 × 10–4 mol l–1 Cl– (NaCl), 1.04 × 10–3 mol l–1 SO42– (Na2SO4), 2.17 × 10–4 mol l–1 NO2– (NaNO2), 8.06 × 10–6 mol l–1 NO3– (NaNO3), 8.60 × 10–7 mol l–1 SCN– (NH4SCN), 1.25 × 10–5 mol l–1 Br– (KBr), 2.53 × 10–4 mol l–1 K+ (KCl) and 2.08 × 10–3 mol l–1 Na+ (NaSO4). The
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Valve position
Mode
Volume/ µl
Description
Aspirate 200 µl standard into EC 5 Aspirate 50 Aspirate 50 µl methylene blue into EC 10 Aspirate 75 Aspirate 75 µl CH2Cl2 into EC 2 Aspirate 200 Aspirate 200 µl standard into EC 5 Aspirate 50 Aspirate 50 µl methylene blue into EC 10 Aspirate 75 Aspirate 75 µl CH2Cl2 into EC 2 Aspirate 200 Aspirate 200 µl standard into EC 5 Aspirate 50 Aspirate 50 µl methylene blue into EC 10 Aspirate 75 Aspirate 75 µl CH2Cl2 into EC 2 Aspirate 200 Aspirate 200 µl standard into EC 5 Aspirate 50 Aspirate 50 µl methylene blue into EC 10 Aspirate 75 Aspirate 75 µl CH2Cl2 into EC 2 Aspirate 200 Aspirate 200 µl standard into EC 5 Aspirate 50 Aspirate 50 µl methylene blue into EC 1 Dispense 1500 Extract 1 Aspirate 1300 Extract 1 Dispense 1300 Extract 1 Aspirate 200 Extract 1 Delay 5 s — Absorbance detection 1 Aspirate 1700 Clean LAV unit 6 Dispense Empty Clean LAV unit Valve in Aspirate 2500 Fill syringe with DI water 1 Dispense 1500 Clean LAV unit with DI water 1 Aspirate 1500 Clean LAV unit 6 Dispense Empty Clean EC The system is now ready for the next determination 2
Aspirate
200
EC: extraction coil.
concentrations of foreign ions in natural water are normally less than these concentrations. The results indicate that the studied foreign ions should not cause any interfering in the proposed method. Application to water samples The applicability of the developed system was investigated for the extractive determination of anionic surfactants in natural surface water samples. The samples, taken from different reservoirs, were filtered through a 0.45 µm membrane filter before being introduced into the SIA system. Table 2 summarizes the results together with the %recoveries. The results were compared to those obtained by the standard batch method.14 An evaluation by the t-test at the 95% confidence level showed that there is no significant difference between the two methods.
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Table 2 Recoveries and analysis of an anionic surfactant in water samples by the proposed and standard methods Found/mg l–1 (n = 3) Sample
%Recoverya
A B C D E F
97 107 97 106 93 100
Proposed method
Standard method14
0.37 ± 0.02 0.20 ± 0.01 0.30 ± 0.01 0.19 ± 0.00 0.25 ± 0.01 0.22 ± 0.01
0.40 ± 0.01 0.16 ± 0.01 0.35 ± 0.01 0.14 ± 0.00 0.27 ± 0.04 0.18 ± 0.01
a. Determined by the proposed method
Acknowledgements The Thailand Research Fund (TRF) and the Commission on Higher Education are gratefully acknowledged for financial support of this research. Thanks are due to the Postgraduate Education and Research Program in Chemistry (PERCH).
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