- Email: [email protected]
Improved microfluidic chip-based sequential-injection trapped-droplet array liquid–liquid extraction system for determination of aluminium
Talanta 77 (2008) 269–272
Contents lists available at ScienceDirect
Talanta journal homepage: www.elsevier.com/locate/talanta
Improved microfluidic chip-based sequential-injection trapped-droplet array liquid–liquid extraction system for determination of aluminium Hong Shen, Qun Fang ∗ Institute of Microanalytical Systems, Department of Chemistry, Zhejiang University, Hangzhou, China
a r t i c l e
i n f o
Article history: Received 15 April 2008 Received in revised form 12 June 2008 Accepted 15 June 2008 Available online 24 June 2008 Keywords: Microfluidic chip Liquid–liquid extraction Trapped-droplet Chemiluminescence detection Determination of aluminium
a b s t r a c t An improved microfluidic chip-based sequential-injection trapped-droplet array liquid–liquid extraction system with chemiluminescence (CL) detection was developed in this work. Two recess arrays were fabricated on both sides of the extraction channel to produce droplet arrays of organic extractant. A chip integrated monolithic probe was fabricated at the inlet of the extraction channel on the glass chip instead of the capillary probe connected to the microchannel, in order to improve the system stability and reliability. A slotted-vial array system coupled with the monolithic probe was used to sequentially introduce sample and different solvents and reagents into the extraction channel for extraction and CL detection. The performance of the system was demonstrated in the determination of Al3+ using Al3+ -dihydroxyazobenzene (DHAB) and tributyl phosphate (TBP) extraction system. The operation conditions, including extraction time, concentration and flow rate of the CL reagents, were optimized. Within one analysis cycle of 12 min, an enrichment factor of 85 was obtained in the extraction stage with a sample consumption of 1.8 L. The consumption of CL reagent, bis(2-carbopentyloxy-3,5,6-trichlorophenyl)oxalate (CPPO), was 120 nL/cycle. The detection limit of the system for Al3+ was 1.6 × 10−6 mol/L with a precision of 4.5% (R.S.D., n = 6). © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the researches on miniaturized total analysis system (-TAS) have made great progress [1–4]. Various microfluidic chip-based analysis systems, including liquid–liquid (L–L) extraction systems, have been developed. Since 2000, Kitamori’s group has reported a series of chip-based L–L extraction systems [5–10] with advantages of extremely low sample and reagent consumption and high extraction efficiency owing to the enhanced interfacing area/volume ratio as well as reduced diffusion distance. These miniaturized L–L extraction systems were applied in the determination of metal ions, such as Fe2+ [5], Ni2+ [6], Co2+ [7–9], Na+ and K+ [10]. Kitamura’s group [11] also reported a microfluidic chip-based extraction system with a Y-shaped extraction channel using Al3+ -dihydroxyazobenzene (DHAB) solution and n-butanol as aqueous sample and organic extractant, respectively. A fluorescence microscope was used to monitor the fluorescence intensity of Al3+ -DHAB extracted into the n-butanol flow to study the effects of extraction time and diffusion distance on the spatially distribution of Al3+ -DHAB in the
∗ Corresponding author at: Institute of Microanalytical Systems, Chemistry Experiment Center, Room 101, Zhejiang University (Zijingang Campus), Hangzhou 310058, China. Tel.: +86 571 88206771; fax: +86 571 88273496. E-mail address: [email protected] (Q. Fang). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.06.016
organic phase both along the flow direction and the channel-width direction. Recently, Bowden et al. reported a microfluidic chip format performing non-aqueous L–L extraction for separation of petroleum composition before GC analysis [12]. In most of these systems, continuous multiphase laminar flow mode was adopted in Y-shaped or -shaped extraction channels, in which higher enrichment factors over 10 were difficult to achieve due to the relatively low aqueous/organic phase ratio under continuous laminar flow mode. In 2004, we developed a chip-based L–L extraction system based on droplet trapping technique and stopped-flow extraction mode, achieving high enrichment factors of 1000–2000 [13]. Based on this work, recently we developed a chip-based sequential-injection droplet array L–L extraction system [14] with chemiluminescence (CL) detection, in which two droplet arrays of extractant on both sides of the extraction channel were produced using the droplet trapping technique. Compared with the single-droplet system [13], more amount of analyte could be extracted into the organic extractant phase in the droplet arrays, thus the detection sensitivity of the chip system was significantly increased. A fluorescent dye butyl rhodamine B (BRB) was used as a model sample to demonstrate the system performance. However, in a prolonged extraction process, the flow rates of the fluids (especially the aqueous fluids) in the extraction channel tended to decrease with the increase of extraction time, due to the change of microchannel surface property resulted from the dissolving of the epoxy glue at
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the interface of the capillary probe and microchannel by organic solvent. In the present work, the stability and reliability of the previous chip-based extraction system was improved by using a chip-integrated monolithic probe instead of the fused-silica capillary probe connected with the chip microchannel. The system performance was demonstrated in the determination of Al3+ using Al3+ -DHAB and tributyl phosphate (TBP) extraction system and CL detection. Such an extraction system for determination of Al3+ was employed in several extraction systems with absorbance [15,16] or fluorescence [11,16] detection systems. In this work, we employed the bis(2-carbopentyloxy-3,5,6-trichlorophenyl)oxalate (CPPO) CL reaction system for the detection of Al3+ -DHAB after the extraction and preconcentration, which has the advantages of high detection sensitivity and simple structure for detection system without the need of light source. Similar CL system has been used for Al3+ detection in conventional analysis systems [17,18], with a sample and reagent consumption in the range 0.1–10 mL. To the best of our knowledge, so far such approach has not been applied in chip-based system. 2. Experimental 2.1. Chemicals and reagents All chemicals used were of analytical reagent grade unless mentioned otherwise. Demineralized water (18 M cm) was used throughout. CPPO was obtained from Senjie Chemical Auxiliary Co. (ZiBo, China), and used without further purification. A 3 mg/mL CPPO solution was prepared every 2 days by dissolving 6 mg CPPO in 2 mL acetonitrile in a borosilicate glass flask. NH4 Ac buffer solution (pH 6.3) was prepared by dissolving 25 g NH4 Ac in water, adjusting the pH to 6.3 using 0.1 mol/L HCl, and made up to 100 mL with water. Aqueous stock solution of 1 × 10−3 mol/L Al3+ was prepared by dissolving 0.028 g Al(NO)3 ·9H2 O (Xinjida Chemical Co., Taiyuan, China) in 100 mL NH4 Ac buffer solution. Aqueous stock solution of 1 × 10−3 mol/L DHAB was prepared by dissolving 0.021 g DHAB in 100 mL buffer solution. The stock Al3+ -DHAB solution was prepared by mixing 25 mL Al3+ solution with 25 mL DHAB solution [11,15]. The series of Al3+ -DHAB working solutions in the range of 1 × 10−4 to 1 × 10−6 mol/L were prepared by sequentially diluting the stock solution with the buffer solution. TBP obtained from Xudong Chemical Co. (Beijing, China) was used as extractant. H2 O2 10% solution was prepared daily by diluting 30% H2 O2 solution with water.
Fig. 1. Schematic diagram of the chip-based sequential-injection trapped-droplet array liquid–liquid extraction system.
shown in Fig. 1) used in this work was similar to those described in the previously reported work [22]. A photomultiplier tube (PMT, CR114, Beijing Hamamatsu Co., Beijing, China) equipped with a luminescence meter (model GD-1, Ruike Electronics, Xian, China) was used for detection of CL light. The PMT window was placed closely beneath the central section of the chip. The chip and PMT were situated within a black light-proof box. The signal was recorded by computer through a data acquisition card (818HG, Advantech Co., Hangzhou, China). For the evaluation of enrichment factor during extraction process, a home-built confocal microscope laser induced fluorescence (LIF) system [13] was used with a detection point as shown in Fig. 1. 2.3. Procedures The six vials in the sample presenting system were filled with 500 L of acetonitrile, TBP and aqueous Al3+ -DHAB (or DHAB blank) solutions, 10% H2 O2 solution, water and CPPO acetonitrile solution, respectively. Gravity driven hydrostatic flows used for sequentially introducing sample and reagents into the chip microchannel were generated by lowering the outlet of the waste tube. Operational program of the system is shown in Table 1. The evaluation for the enrichment factors in different extraction times was performed as described elsewhere [13], except that 1 × 10−5 mol/L Al3+ -DHAB aqueous solution was used as sample and TBP as extractant instead.
2.2. Microchip and apparatus 3. Results and discussion The schematic diagram of the analytical system with microchip is shown in Fig. 1. The microchip was fabricated using a procedure detailed elsewhere [19,20]. An extraction channel (L8 cm × W90 m × D25 m) was fabricated through the chip (80 mm × 30 mm). Two recess arrays with a length of 10 mm and 134 recesses (L100 m × W50 m × D25 m) in each array were fabricated on both sides of the extraction channel. At the inlet end of the extraction channel, the chip was ground using an emery drill (Shang Gong International Precision Tool Co., Shanghai, China) into a needle-shaped probe with a tip size of ∼200 m (as shown in Fig. 1) to serve as a sampling probe [21]. At the outlet end of the extraction channel, the chip was ground into a cylinder (5 mm o.d., 10-mm long), which was connected with a Tygon tube (5 mm o.d., 100-cm long). The slotted-vial array sample presenting system produced by micropipettes with slotted tip and the CL detection system (as
3.1. System design In our previously reported chip-based L–L extraction system [14], a fused-silica capillary serving as a probe for sample/reagents introduction was connected to the microchannel on the chip and fixed with epoxy glue. During prolonged analysis processes, the epoxy at the interface of the capillary and microchannel was swelled and dissolved by exposure to the organic solvent for L–L extraction, which may partially block the microchannel and change the channel surface property, resulting in the decrease of flow rate for the fluids in the channel using gravity driving system. In this work, a monolithic probe [21] was fabricated at the inlet of the extraction channel to avoid the connecting of a capillary with the microchannel and the subsequent use of epoxy glue at the interface. The working stability and reliability were significantly
H. Shen, Q. Fang / Talanta 77 (2008) 269–272
271
Table 1 Operational program of the system Step
Flow rate (L/min)
Liquid-level differencea (cm)
Time (s)
Function
1 2 3
0.35 0.35 0.18
55 55 25
20 20 600
4 5 6
0.35 0 0.35
55 0 55
20 2 20
Rinsing of the microchannel and recesses by CH3 CN Displacement of CH3 CN by TBP in extraction channel and recesses. Displacement of TBP by Al3+ -DHAB solution in extraction channel. Formation of trapped TBP droplet array in recesses. Continuous extraction of Al3+ -DHAB from sample solution into TBP droplets Displacement of Al3+ -DHAB solution from extraction channel by H2 O2 solution Rinsing of probe tip by water Introduction of CPPO acetonitrile solution; mixing of CPPO, H2 O2 and Al3+ -DHAB in the microchannel initiating CL reaction
a
The liquid-level difference between the sampling probe and the waste tube.
improved due to the good smoothness of the whole extraction channel by the use of monolithic probe. During the experiments over several months, no evident variation on the flow rate of the fluids in extraction channel was observed. In the previous L–L extraction system [14], for simplifying the operation, the peroxyoxalate reagent CPPO for CL detection was pre-dissolved in the organic extractant before the L–L extraction. However, with such an arrangement, a compromise choice for organic solvent had to be made between the optimized L–L extraction and CL detection conditions, which may result in the discount of the system performance. In this work, different organic solvents, TBP and acetonitrile, were employed as the extractant and solvent for CPPO, respectively. Therefore, L–L extraction could be performed under optimized conditions as in conventional L–L extraction systems [16]. The versatility of this system was improved by allowing the use of different solvent for extraction and detection. After the L–L extraction and preconcentration process, H2 O2 aqueous solution and CPPO acetonitrile solution were sequentially introduced into the extraction channel for CL detection (CL reaction equations [23] are shown in Fig. 2). 3.2. Effects of extraction time In this work, the extraction and preconcentration for Al3+ in the extractant droplet array by L–L extraction was carried out before the initiating of the CL reaction, which occupied most of the analysis time. The effects of extraction time in the range of 0–18 min on the enrichment factor of Al3+ were studied using 1 × 10−5 mol/L Al3+ DHAB aqueous solution as sample. The results are shown in Fig. 3. The enrichment factors increased with the time up to about 10 min, beyond which the enrichment factors reduced due to the decrease of TBP droplet volume produced by the dissolving of TBP in the continuous aqueous sample flow. Therefore, an extraction time of 10 min was chosen to obtain the highest enrichment factor of 85.
Fig. 2. Equations of CPPO CL reaction.
Fig. 3. Effects of extraction time on enrichment factor.
3.3. Effects of H2 O2 solution concentration The effects of H2 O2 solution concentration were studied in the range 5–15%. The results are shown in Fig. 4. H2 O2 10% solution showed highest CL intensity. In most of the conventional CL reaction systems with CPPO and H2 O2 , the CL intensity usually increases with H2 O2 concentration. In the present system, the CL intensity decreased with the increase of H2 O2 concentration in the range higher than 10%. This may be due to the excessive consumption of CPPO in higher H2 O2 concentration [23] when CPPO solution was mixed with H2 O2 aqueous solution filled in the extraction channel before reaction with Al3+ -DHAB. 3.4. Effects of concentration and flow rate of CPPO acetonitrile solution In CPPO CL reaction system, a higher CPPO concentration will benefit the increase of the reaction speed and detection sensitivity [24]. However, the increase of CPPO concentration is limited by its solubility in acetonitrile and instability in high concentration. Furthermore, it was observed that the blank solutions with DHAB
Fig. 4. Effects of H2 O2 solution concentration.
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H. Shen, Q. Fang / Talanta 77 (2008) 269–272
equation of I = 3.52 × 106 c + 0.65 (r2 = 0.9926) (as shown in Fig. 6). The limit of detection for Al3+ based on three times the standard deviation of the blank signals was 1.6 × 10−6 mol/L, corresponding to an amount of 2.9 × 10−12 mol Al3+ . The total analysis time for one cycle was 12 min, with consumptions for sample and CPPO solution of 1.8 L and 120 nL, respectively. The volume of the organic phase droplet array trapped in the recesses was ca. 17 nL, estimated from the data of number and size of the recesses. 4. Conclusion
Fig. 5. Effect of flow rate of CPPO acetonitrile solution on CL intensity.
In this work, the stability and reliability of the microfluidic chipbased sequential-injection droplet array L–L extraction system was improved by using a chip integrated monolithic probe coupled with a slotted-vial array liquid-presenting system. The versatility of the system was demonstrated in the determination of Al3+ with CL detection using Al3+ -DHAB and TBP extraction system and CPPO CL reaction system. In addition to coupling with on-line spectrometric detection approaches, such as fluorescence, absorbance and CL detection, the chip-based droplet array L–L extraction system also provided a potential sample pretreatment platform for gas or liquid chromatography as well as mass spectrometry, by fabricating multi-channel array with droplet array on one chip or increasing the recess number as well as the recess capacity to increase the treated sample amount. Acknowledgements Financial supports from National Natural Science Foundation of China (Grants 20575059 and 20775071), Ministry of Science and Technology of China (Grant 2007CB714503) and National Education Ministry (Grant NCET-05-0511) are gratefully acknowledged.
Fig. 6. Typical recordings of sequential determination of DHAB blank solution and 5 × 10−6 , 1.0 × 10−5 , 1.5 × 10−5 , 2.0 × 10−5 , 2.5 × 10−5 mol/L Al3+ -DHAB solutions (samples 2, 3, 4, 5 and 6) to show the linear relationship between CL intensity and Al3+ concentration.
could also produce a CL signal when mixed with H2 O2 and CPPO solutions. Such background signals maybe caused by the catalyzing effect of the impurity in CPPO solution on the reaction of H2 O2 and CPPO, showed increasing trend with the increase of CPPO concentration. Therefore, a medium CPPO concentration of 3 mg/mL was chosen. The effect of flow rates of CPPO solution on CL intensity was investigated within a range of 0.13–0.60 L/min. The results are shown in Fig. 5. Flow rate of 0.35 L/min demonstrated the highest CL intensity, and was adopted in the following experiments. 3.5. Analytical performance The performance of the system was demonstrated in the determination of Al3+ aqueous sample under optimized conditions. The reproducibility of repetitive measurement for a 1.0 × 10−5 mol/L Al3+ solution was 4.5% (R.S.D., n = 6). A linear response was obtained in the range of 5 × 10−6 to 2.5 × 10−5 mol/L Al3+ , with a regression
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Contents lists available at ScienceDirect
Talanta journal homepage: www.elsevier.com/locate/talanta
Improved microfluidic chip-based sequential-injection trapped-droplet array liquid–liquid extraction system for determination of aluminium Hong Shen, Qun Fang ∗ Institute of Microanalytical Systems, Department of Chemistry, Zhejiang University, Hangzhou, China
a r t i c l e
i n f o
Article history: Received 15 April 2008 Received in revised form 12 June 2008 Accepted 15 June 2008 Available online 24 June 2008 Keywords: Microfluidic chip Liquid–liquid extraction Trapped-droplet Chemiluminescence detection Determination of aluminium
a b s t r a c t An improved microfluidic chip-based sequential-injection trapped-droplet array liquid–liquid extraction system with chemiluminescence (CL) detection was developed in this work. Two recess arrays were fabricated on both sides of the extraction channel to produce droplet arrays of organic extractant. A chip integrated monolithic probe was fabricated at the inlet of the extraction channel on the glass chip instead of the capillary probe connected to the microchannel, in order to improve the system stability and reliability. A slotted-vial array system coupled with the monolithic probe was used to sequentially introduce sample and different solvents and reagents into the extraction channel for extraction and CL detection. The performance of the system was demonstrated in the determination of Al3+ using Al3+ -dihydroxyazobenzene (DHAB) and tributyl phosphate (TBP) extraction system. The operation conditions, including extraction time, concentration and flow rate of the CL reagents, were optimized. Within one analysis cycle of 12 min, an enrichment factor of 85 was obtained in the extraction stage with a sample consumption of 1.8 L. The consumption of CL reagent, bis(2-carbopentyloxy-3,5,6-trichlorophenyl)oxalate (CPPO), was 120 nL/cycle. The detection limit of the system for Al3+ was 1.6 × 10−6 mol/L with a precision of 4.5% (R.S.D., n = 6). © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the researches on miniaturized total analysis system (-TAS) have made great progress [1–4]. Various microfluidic chip-based analysis systems, including liquid–liquid (L–L) extraction systems, have been developed. Since 2000, Kitamori’s group has reported a series of chip-based L–L extraction systems [5–10] with advantages of extremely low sample and reagent consumption and high extraction efficiency owing to the enhanced interfacing area/volume ratio as well as reduced diffusion distance. These miniaturized L–L extraction systems were applied in the determination of metal ions, such as Fe2+ [5], Ni2+ [6], Co2+ [7–9], Na+ and K+ [10]. Kitamura’s group [11] also reported a microfluidic chip-based extraction system with a Y-shaped extraction channel using Al3+ -dihydroxyazobenzene (DHAB) solution and n-butanol as aqueous sample and organic extractant, respectively. A fluorescence microscope was used to monitor the fluorescence intensity of Al3+ -DHAB extracted into the n-butanol flow to study the effects of extraction time and diffusion distance on the spatially distribution of Al3+ -DHAB in the
∗ Corresponding author at: Institute of Microanalytical Systems, Chemistry Experiment Center, Room 101, Zhejiang University (Zijingang Campus), Hangzhou 310058, China. Tel.: +86 571 88206771; fax: +86 571 88273496. E-mail address: [email protected] (Q. Fang). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.06.016
organic phase both along the flow direction and the channel-width direction. Recently, Bowden et al. reported a microfluidic chip format performing non-aqueous L–L extraction for separation of petroleum composition before GC analysis [12]. In most of these systems, continuous multiphase laminar flow mode was adopted in Y-shaped or -shaped extraction channels, in which higher enrichment factors over 10 were difficult to achieve due to the relatively low aqueous/organic phase ratio under continuous laminar flow mode. In 2004, we developed a chip-based L–L extraction system based on droplet trapping technique and stopped-flow extraction mode, achieving high enrichment factors of 1000–2000 [13]. Based on this work, recently we developed a chip-based sequential-injection droplet array L–L extraction system [14] with chemiluminescence (CL) detection, in which two droplet arrays of extractant on both sides of the extraction channel were produced using the droplet trapping technique. Compared with the single-droplet system [13], more amount of analyte could be extracted into the organic extractant phase in the droplet arrays, thus the detection sensitivity of the chip system was significantly increased. A fluorescent dye butyl rhodamine B (BRB) was used as a model sample to demonstrate the system performance. However, in a prolonged extraction process, the flow rates of the fluids (especially the aqueous fluids) in the extraction channel tended to decrease with the increase of extraction time, due to the change of microchannel surface property resulted from the dissolving of the epoxy glue at
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the interface of the capillary probe and microchannel by organic solvent. In the present work, the stability and reliability of the previous chip-based extraction system was improved by using a chip-integrated monolithic probe instead of the fused-silica capillary probe connected with the chip microchannel. The system performance was demonstrated in the determination of Al3+ using Al3+ -DHAB and tributyl phosphate (TBP) extraction system and CL detection. Such an extraction system for determination of Al3+ was employed in several extraction systems with absorbance [15,16] or fluorescence [11,16] detection systems. In this work, we employed the bis(2-carbopentyloxy-3,5,6-trichlorophenyl)oxalate (CPPO) CL reaction system for the detection of Al3+ -DHAB after the extraction and preconcentration, which has the advantages of high detection sensitivity and simple structure for detection system without the need of light source. Similar CL system has been used for Al3+ detection in conventional analysis systems [17,18], with a sample and reagent consumption in the range 0.1–10 mL. To the best of our knowledge, so far such approach has not been applied in chip-based system. 2. Experimental 2.1. Chemicals and reagents All chemicals used were of analytical reagent grade unless mentioned otherwise. Demineralized water (18 M cm) was used throughout. CPPO was obtained from Senjie Chemical Auxiliary Co. (ZiBo, China), and used without further purification. A 3 mg/mL CPPO solution was prepared every 2 days by dissolving 6 mg CPPO in 2 mL acetonitrile in a borosilicate glass flask. NH4 Ac buffer solution (pH 6.3) was prepared by dissolving 25 g NH4 Ac in water, adjusting the pH to 6.3 using 0.1 mol/L HCl, and made up to 100 mL with water. Aqueous stock solution of 1 × 10−3 mol/L Al3+ was prepared by dissolving 0.028 g Al(NO)3 ·9H2 O (Xinjida Chemical Co., Taiyuan, China) in 100 mL NH4 Ac buffer solution. Aqueous stock solution of 1 × 10−3 mol/L DHAB was prepared by dissolving 0.021 g DHAB in 100 mL buffer solution. The stock Al3+ -DHAB solution was prepared by mixing 25 mL Al3+ solution with 25 mL DHAB solution [11,15]. The series of Al3+ -DHAB working solutions in the range of 1 × 10−4 to 1 × 10−6 mol/L were prepared by sequentially diluting the stock solution with the buffer solution. TBP obtained from Xudong Chemical Co. (Beijing, China) was used as extractant. H2 O2 10% solution was prepared daily by diluting 30% H2 O2 solution with water.
Fig. 1. Schematic diagram of the chip-based sequential-injection trapped-droplet array liquid–liquid extraction system.
shown in Fig. 1) used in this work was similar to those described in the previously reported work [22]. A photomultiplier tube (PMT, CR114, Beijing Hamamatsu Co., Beijing, China) equipped with a luminescence meter (model GD-1, Ruike Electronics, Xian, China) was used for detection of CL light. The PMT window was placed closely beneath the central section of the chip. The chip and PMT were situated within a black light-proof box. The signal was recorded by computer through a data acquisition card (818HG, Advantech Co., Hangzhou, China). For the evaluation of enrichment factor during extraction process, a home-built confocal microscope laser induced fluorescence (LIF) system [13] was used with a detection point as shown in Fig. 1. 2.3. Procedures The six vials in the sample presenting system were filled with 500 L of acetonitrile, TBP and aqueous Al3+ -DHAB (or DHAB blank) solutions, 10% H2 O2 solution, water and CPPO acetonitrile solution, respectively. Gravity driven hydrostatic flows used for sequentially introducing sample and reagents into the chip microchannel were generated by lowering the outlet of the waste tube. Operational program of the system is shown in Table 1. The evaluation for the enrichment factors in different extraction times was performed as described elsewhere [13], except that 1 × 10−5 mol/L Al3+ -DHAB aqueous solution was used as sample and TBP as extractant instead.
2.2. Microchip and apparatus 3. Results and discussion The schematic diagram of the analytical system with microchip is shown in Fig. 1. The microchip was fabricated using a procedure detailed elsewhere [19,20]. An extraction channel (L8 cm × W90 m × D25 m) was fabricated through the chip (80 mm × 30 mm). Two recess arrays with a length of 10 mm and 134 recesses (L100 m × W50 m × D25 m) in each array were fabricated on both sides of the extraction channel. At the inlet end of the extraction channel, the chip was ground using an emery drill (Shang Gong International Precision Tool Co., Shanghai, China) into a needle-shaped probe with a tip size of ∼200 m (as shown in Fig. 1) to serve as a sampling probe [21]. At the outlet end of the extraction channel, the chip was ground into a cylinder (5 mm o.d., 10-mm long), which was connected with a Tygon tube (5 mm o.d., 100-cm long). The slotted-vial array sample presenting system produced by micropipettes with slotted tip and the CL detection system (as
3.1. System design In our previously reported chip-based L–L extraction system [14], a fused-silica capillary serving as a probe for sample/reagents introduction was connected to the microchannel on the chip and fixed with epoxy glue. During prolonged analysis processes, the epoxy at the interface of the capillary and microchannel was swelled and dissolved by exposure to the organic solvent for L–L extraction, which may partially block the microchannel and change the channel surface property, resulting in the decrease of flow rate for the fluids in the channel using gravity driving system. In this work, a monolithic probe [21] was fabricated at the inlet of the extraction channel to avoid the connecting of a capillary with the microchannel and the subsequent use of epoxy glue at the interface. The working stability and reliability were significantly
H. Shen, Q. Fang / Talanta 77 (2008) 269–272
271
Table 1 Operational program of the system Step
Flow rate (L/min)
Liquid-level differencea (cm)
Time (s)
Function
1 2 3
0.35 0.35 0.18
55 55 25
20 20 600
4 5 6
0.35 0 0.35
55 0 55
20 2 20
Rinsing of the microchannel and recesses by CH3 CN Displacement of CH3 CN by TBP in extraction channel and recesses. Displacement of TBP by Al3+ -DHAB solution in extraction channel. Formation of trapped TBP droplet array in recesses. Continuous extraction of Al3+ -DHAB from sample solution into TBP droplets Displacement of Al3+ -DHAB solution from extraction channel by H2 O2 solution Rinsing of probe tip by water Introduction of CPPO acetonitrile solution; mixing of CPPO, H2 O2 and Al3+ -DHAB in the microchannel initiating CL reaction
a
The liquid-level difference between the sampling probe and the waste tube.
improved due to the good smoothness of the whole extraction channel by the use of monolithic probe. During the experiments over several months, no evident variation on the flow rate of the fluids in extraction channel was observed. In the previous L–L extraction system [14], for simplifying the operation, the peroxyoxalate reagent CPPO for CL detection was pre-dissolved in the organic extractant before the L–L extraction. However, with such an arrangement, a compromise choice for organic solvent had to be made between the optimized L–L extraction and CL detection conditions, which may result in the discount of the system performance. In this work, different organic solvents, TBP and acetonitrile, were employed as the extractant and solvent for CPPO, respectively. Therefore, L–L extraction could be performed under optimized conditions as in conventional L–L extraction systems [16]. The versatility of this system was improved by allowing the use of different solvent for extraction and detection. After the L–L extraction and preconcentration process, H2 O2 aqueous solution and CPPO acetonitrile solution were sequentially introduced into the extraction channel for CL detection (CL reaction equations [23] are shown in Fig. 2). 3.2. Effects of extraction time In this work, the extraction and preconcentration for Al3+ in the extractant droplet array by L–L extraction was carried out before the initiating of the CL reaction, which occupied most of the analysis time. The effects of extraction time in the range of 0–18 min on the enrichment factor of Al3+ were studied using 1 × 10−5 mol/L Al3+ DHAB aqueous solution as sample. The results are shown in Fig. 3. The enrichment factors increased with the time up to about 10 min, beyond which the enrichment factors reduced due to the decrease of TBP droplet volume produced by the dissolving of TBP in the continuous aqueous sample flow. Therefore, an extraction time of 10 min was chosen to obtain the highest enrichment factor of 85.
Fig. 2. Equations of CPPO CL reaction.
Fig. 3. Effects of extraction time on enrichment factor.
3.3. Effects of H2 O2 solution concentration The effects of H2 O2 solution concentration were studied in the range 5–15%. The results are shown in Fig. 4. H2 O2 10% solution showed highest CL intensity. In most of the conventional CL reaction systems with CPPO and H2 O2 , the CL intensity usually increases with H2 O2 concentration. In the present system, the CL intensity decreased with the increase of H2 O2 concentration in the range higher than 10%. This may be due to the excessive consumption of CPPO in higher H2 O2 concentration [23] when CPPO solution was mixed with H2 O2 aqueous solution filled in the extraction channel before reaction with Al3+ -DHAB. 3.4. Effects of concentration and flow rate of CPPO acetonitrile solution In CPPO CL reaction system, a higher CPPO concentration will benefit the increase of the reaction speed and detection sensitivity [24]. However, the increase of CPPO concentration is limited by its solubility in acetonitrile and instability in high concentration. Furthermore, it was observed that the blank solutions with DHAB
Fig. 4. Effects of H2 O2 solution concentration.
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equation of I = 3.52 × 106 c + 0.65 (r2 = 0.9926) (as shown in Fig. 6). The limit of detection for Al3+ based on three times the standard deviation of the blank signals was 1.6 × 10−6 mol/L, corresponding to an amount of 2.9 × 10−12 mol Al3+ . The total analysis time for one cycle was 12 min, with consumptions for sample and CPPO solution of 1.8 L and 120 nL, respectively. The volume of the organic phase droplet array trapped in the recesses was ca. 17 nL, estimated from the data of number and size of the recesses. 4. Conclusion
Fig. 5. Effect of flow rate of CPPO acetonitrile solution on CL intensity.
In this work, the stability and reliability of the microfluidic chipbased sequential-injection droplet array L–L extraction system was improved by using a chip integrated monolithic probe coupled with a slotted-vial array liquid-presenting system. The versatility of the system was demonstrated in the determination of Al3+ with CL detection using Al3+ -DHAB and TBP extraction system and CPPO CL reaction system. In addition to coupling with on-line spectrometric detection approaches, such as fluorescence, absorbance and CL detection, the chip-based droplet array L–L extraction system also provided a potential sample pretreatment platform for gas or liquid chromatography as well as mass spectrometry, by fabricating multi-channel array with droplet array on one chip or increasing the recess number as well as the recess capacity to increase the treated sample amount. Acknowledgements Financial supports from National Natural Science Foundation of China (Grants 20575059 and 20775071), Ministry of Science and Technology of China (Grant 2007CB714503) and National Education Ministry (Grant NCET-05-0511) are gratefully acknowledged.
Fig. 6. Typical recordings of sequential determination of DHAB blank solution and 5 × 10−6 , 1.0 × 10−5 , 1.5 × 10−5 , 2.0 × 10−5 , 2.5 × 10−5 mol/L Al3+ -DHAB solutions (samples 2, 3, 4, 5 and 6) to show the linear relationship between CL intensity and Al3+ concentration.
could also produce a CL signal when mixed with H2 O2 and CPPO solutions. Such background signals maybe caused by the catalyzing effect of the impurity in CPPO solution on the reaction of H2 O2 and CPPO, showed increasing trend with the increase of CPPO concentration. Therefore, a medium CPPO concentration of 3 mg/mL was chosen. The effect of flow rates of CPPO solution on CL intensity was investigated within a range of 0.13–0.60 L/min. The results are shown in Fig. 5. Flow rate of 0.35 L/min demonstrated the highest CL intensity, and was adopted in the following experiments. 3.5. Analytical performance The performance of the system was demonstrated in the determination of Al3+ aqueous sample under optimized conditions. The reproducibility of repetitive measurement for a 1.0 × 10−5 mol/L Al3+ solution was 4.5% (R.S.D., n = 6). A linear response was obtained in the range of 5 × 10−6 to 2.5 × 10−5 mol/L Al3+ , with a regression
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