- Email: [email protected]
An electroosmotic pump for packed capillary liquid chromatography
Microchemical Journal 75 (2003) 15–21
An electroosmotic pump for packed capillary liquid chromatography Lingxin Chen, Jiping Ma, Yafeng Guan* Department of Analytical Chemistry and Micro-Instrumentation, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 100, Dalian 116012, China Received 10 December 2002; received in revised form 20 February 2003; accepted 27 February 2003
Abstract An electroosmotic pump (EOP) capable of generating pressure above 3 MPa and mlymin flow rate with reverse phase mobile phases of HPLC was constructed and evaluated. The pump consisted of three parallel connected fused silica capillary columns (25 cm=320 mm I.D.) packed with 2 mm silica materials, hollow electrodes, a high voltage DC power supply, and a liquid pressure transducer. The EOP was applied in a capillary liquid chromatographic system for mobile phase delivery instead of a mechanical pump. Standard samples containing thiourea, naphthalene, anthracene, phenanthrene and acetonitrile were separated on a 15 cm=320 mm I.D. 5 mm Chromasil C18 packed capillary column with acetonitrileywater as mobile phase. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Electroosmotic pump; Micropump; Capillary liquid chromatography
1. Introduction The packed capillary liquid chromatographic columns with internal diameters (I.D.) of 75 to 320 mm are used increasingly in microseparation techniques w1–3x. Pumps that can deliver mlymin and sub-mlymin flow rates at pressure above 3 MPa are necessary in these systems. The mechanical pumps such as reciprocating pumps or syringe pumps will not operate reliably or accurately at such a low flow rate and high pressure, because of the uncontrollable leakage from check valves and dynamic sealing of pistons that constituted the *Corresponding author. Tel.: q86-411-369-3515; fax: q86411-369-3510. E-mail address: [email protected] (Y. Guan).
most important parts of mechanical pumps. For two decades since the first invention of packed capillary liquid chromatography (m-HPLC) w4,5x, the mobile phase delivery at mlymin range under high-pressure condition has been a major obstacle in the instrumental development of capillary HPLC. The electroosmotic pump (EOP) based on the electroosmosis principle has drawn more attention w6–10x in recent years since it could generate pressures up to tens of MPa and flow rates of mly min or sub-mlymin w7,8,10,11x. Previous studies on EOP were limited on the pumping of water or water-based buffers, rather than on mobile phases commonly used in reverse phase HPLC or organic solvents. This is because the EOP originated from
0026-265X/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0026-265X(03)00050-X
16
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
the idea of capillary electro-chromatography (CEC), where only buffers were used as mobile phases. Zeng et al. w8x reported an EOP capable of generating a flow rate of 3.6 mlymin and pressures of 2 MPa for deionized water. Guan et al. w11x utilized 25 cm=0.32 mm I.D. packed column as the electroosmotic channel in their EOP, which could generate pressures up to 20 MPa at flow ™ 0 and flow rate approximately 1.6 mly min at pressure ™ 0 for water buffer. The output flow rate capacity of an EOP is very low at the maximum output pressure condition. When a 320 mm I.D. separation column is employed in a mHPLC system, the mobile phase flow rates are in the range of 1.5–3 mlymin and pressures of 3–8 MPa. The narrow electroosmotic channel in EOP, which is necessary for generating high-pressure output and heat dissipation of Joul heating from the channel, limits the volumetric flow capacity. The real challenges for an EOP to be used in mHPLC systems are the kinds of mobile phases that can be delivered, the volumetric flow rate and pressure values can be generated, and the reproducibility of output flow rate for a given load. In this paper, an electroosmotic pump (EOP) was constructed and evaluated for the delivery of mobile phases in a m-HPLC. The relationships between the maximum output pressure and applied voltage, the maximum output flow rate and the applied voltage, were studied. Unlike mechanical pumps, we found that the output flow rate and pressure of the EOP were inter-related. The EOP was connected to a packed capillary HPLC system for mobile phase delivery instead of a mechanical pump. The results demonstrated that the EOP could be applied in m-HPLC. 2. Experimental 2.1. Electroosmotic pump The electroosmotic pump (EOP) is shown schematically in Fig. 1. The structure of the EOP employs a three-channel-pump system, where three columns (25 cm=320 mm I.D.) packed with 2 mm porous silica grains (Dalian Institute of Chemical Physics, China.) are connected in parallel in
Fig. 1. Schematic diagram of the EOP system: (1) DC power supply; (2) platinum wire; (3) solvent reservoir, which was connected to the anode of the power supply; (4) packed columns, three columns connected in parallel; (5) hollow electrode, which was connected to the cathode of the power supply (the grounded electrode); (6) a liquid pressure transducer and gas-release device; and (7) output of the pump, to which an injection valve and an analytical column can be connected.
order to increase the output flow rate capacity and improve the heat dissipation property. A platinum electrode is placed in the reservoir of mobile phase, and is connected to the anode of a DC power supply. The reservoir was covered with an insulating sheath to prevent it from accidental electric shock. The ends of the packed columns were immersed in the mobile phase, while the other ends of the columns are connected to a homemade 5-port manifold constructed with PEEK material. A cylindrical Pt electrode is placed inside the manifold and is connected to the outside through one of the ports. This electrode and the cathode of the DC power supply are connected to the ground. The port at the opposite side of the manifold is used as output of the EOP. The mobile phase moves from anode to ground electrodes inside the EOP columns, because of an electroosmotic flow (EOF) when a voltage is applied across the columns. A pressure sensor and a gas-release device are connected to the output end of the EOP in order to measure the output pressure and release the gases produced by electro-chemical reaction occurring at the surface of the cylindrical electrode.
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
17
laboratory. All solvents were of analytical grade and were filtrated over a 0.5 mm filter. 3. Results and discussion 3.1. Evaluation of the EOP
Fig. 2. Schematic diagram of the m-HPLC system: (1) connected to EOP; (2) a 4-port injection valve; (3) the micro-LC column; (4) a UV-VIS detector; (5) Chromatographic Station and (6) waster liquid bottle.
The preparation of the columns was the same as m-HPLC columns, which had been described by several authors w12,13x. 2.2. The m-HPLC system The m-HPLC system is shown in Fig. 2. It consists of a 4-port injection valve with an internal loop of 200 nl (VICI, Switzerland), a 15 cm=320 mm I.D. 5 mm Chromasil C18 packed capillary chromatographic column (homemade), and a JASCO CE-975 on-column UV-VIS detector (JASCO, Japan). An EOP pump, as described in 2.1 was used for mobile phase delivery. A KF-98 Chromatographic Station (Ver. 1.10, Dalian Scien-Tech Instruments Ltd. Dalian, China) was used for data handling. 2.3. Reagents Standard samples of thiourea, naphthalene, anthracene, and phenanthrene were purchased from Shenyang Chemical Reagents Factory (Shenyang, China). Acetonitrile was from Tedia (Tedia Inc., USA). Bi-distilled pure water was prepared in the
The EOP output pressure P and the flow rate F are related to the applied voltage V, the dimensions of electroosmotic columns, the packing material inside the columns, the mobile phase being pumped, and the load w1,5x. The dominating factors behind the phenomena are the resistance and the cross-section area AS of the electroosmotic columns, and the properties of the mobile phases. The flow rate capacity of an EOP is proportional to AS and the V. There should be no limitation to use the column of large AS or apply high voltage in EOP if the joule heating can be removed instantly from the column. We selected a narrow diameter packed column in order to minimize temperature gradient inside the column and to generate higher operating pressure. The AS of the column then limited the volumetric flow rate of the EOP for a given V. In order to increase the AS while maintaining the efficiency of heat dissipation, we used three electroosmotic columns connected in parallel. The deviation of resistance of the columns to be connected in parallel should be less than 10% to prevent the uneven flow distribution of the mobile phase among the columns. This was not difficult since most of the columns prepared under identical conditions had similar resistance value (normally "5%). The P and F values were related to the properties of fluids being pumped when other conditions were identical. The maximum static pressure PM (MPa) and the maximum flow rate FM (mlymin) of pure water, sodium dihydrogen phosphate buffer and some pure polar organic solvents are shown in Figs. 3 and 4, respectively. The data in the figures showed that the pressure and flow rate of the EOP were linearly increased as a function of applied voltage V. When the pumping fluid is a mixture, the P and F of the EOP still increase linearly as a function of V. The PM and FM values of a mixture of water
18
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
Fig. 3. The maximum static pressure of the pure water, 3=10y3 mol ly1 sodium dihydrogen phosphate buffer at pH 7.5, methanol, ethanol, acetone and acetonitrile on the above EOP.
and acetonitrile are shown in Table 1. It is very important to pump solvent mixture rather than water-based buffer because the mobile phases used in HPLC are organic solvent and water mixtures in most cases. Both the PM and FM values of the EOP were also dependent on the relative percentage of each solvent in the mixture (Table 1) under
identical V, and set the limit for the EOP. The results showed that when the concentration of a solvent in a mobile phase was changed, the applied voltage must be adjusted accordingly to keep the flow rate constant. Unlike PM and FM where the values were obtained either at Fs0 or Ps0, a given flow at a
Fig. 4. The maximum flow rate of pure water, methanol and acetonitrile on the EOP.
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
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Table 1 The maximum static pressure PM (MPa) and flow rate FM (mlymin) of the mixture with water-acetonitrile mixture of different proportion Applied voltage (kV) Acetonitrileywater (80y20, vyv) Acetonitrileywater (50y50, vyv)
Pressure P (MPa) Flowrate F (mlymin) Pressure P (MPa) Flowrate F (mlymin)
pressure above 2 MPa was required in practical application. To evaluate the EOP at such a condition, the EOP was connected to the m-HPLC system to deliver mobile phase instead of a mechanical pump. The sample containing thiourea, naphthalene, anthracene and phenanthrene were separated on the separation column and their chromatograms are illustrated in Figs. 5–7. The retention times of peaks were good markers for the measurement of flow rate reproducibility and stability, and are shown in Table 2. The RSD% of the retention times in Table 2 was less than 1% for the 4 peaks, proving that the reproducibility of the EOP was excellent even at such a low flow rate. Comparing the chromatograms in Figs. 5–7, where the applied voltage of EOP was increased from 7 to 8 kV and finally to 9 kV, we found that
5
10
15
20
2.8 2.53 2.1 1.93
4.7 4.53 3.7 3.93
7.7 6.82 6.0 6.21
9.8 8.86 7.6 8.24
the output flow rate of the EOP was proportional to the applied voltage, as evidence of shorter retention time and faster separation. The retention times of the peaks vs. the applied voltage showed a good linear decrease relationship (Fig. 8), which reflected a linear dependence of the EOP output and the applied voltage when a load was connected to the EOP. 4. Conclusion The above-described EOP can deliver mobile phase continuously and steadily under high pressure and mlymin flow range. Connection of several narrow diameter electroosmotic columns in parallel is an effective way to increase the volumetric flow capacity of an EOP while maintaining the output pressure value. The liquid that the EOP can deliver
Fig. 5. Chromatogram of a test mixture. Mobile phase: acetonitrileywater (70:30, VyV); applied voltage of EOP: 7 kV; separation column: 15 cm=320 mm I.D. (C18, 5 mm); detector: UV at 254 nm; peaks (from left to right): thiourea, naphthalene, phenanthrene, anthracene.
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
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Fig. 6. Chromatogram of a test mixture. Applied voltage of EOP: 8 kV; peaks (from left to right): thiourea, naphthalene, phenanthrene and anthracene; other conditions are the same as in Fig. 5.
Fig. 7. Chromatogram of a test mixture. Applied voltage of EOP: 9 kV; peaks (from left to right): thiourea, naphthalene, phenanthrene and anthracene; other conditions are the same as in Fig. 5. Table 2 The repeatability of the retention time of peaks in the chromatograms Retention time (min) Sample
1
2
3
4
Mean
RSD%
Thiourea Naphthalene Phenanthrene Anthracene
3.136 12.491 18.762 20.103
3.164 12.606 19.124 20.440
3.115 12.532 18.901 20.324
3.123 12.424 19.021 20.312
3.134 12.513 18.952 20.295
0.69 0.61 0.82 0.69
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
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Fig. 8. Retention times of the compounds vs. applied voltage (All the data were abstracted from the Figs. 5–7. S – thiourea, N – Naphthalene, Ph – Phenanthrene, A – Anthracene.
has been extended to pure organic solvent and RPHPLC mobile phase in our study. The EOP is applicable in reverse phase m-HPLC instead of mechanical pumps. It offers a number of advantages over mechanical pumps, including ease of fabrication, the absence of moving parts and dynamic sealing, no frictional wear or material fatigue, pulsation-free and no noise. The EOP will find application in other micro-fluidic systems. Acknowledgments We wish to thank the Chinese Foundation of Natural Science for financial support. References w1x J.P.C. Vissers, A.C. Henk, C.A. Cramers, J. Chromatogr. A 779 (1997) 1–28. w2x J.P.C. Vissers, J. Chromatogr. A 856 (1999) 117–143.
w3x T. Jiang, Y. Guan, J. Chromatogr. Sci. 37 (1999) 255–265. w4x T. Tsuda, M. Novotny, Anal. Chem. 50 (1978) 271–275. w5x D. Ishii, K. Asai, K. Hibi, T. Jonokuchi, M. Nagaya, J. Chromatogr. 144 (1977) 227–232. w6x P.H. Paul, Studies of hydrostatic pressure effects in electrokinetically driven m-TAS, Micro Total Analysis Systems, Kluwer Academic Publishers, 1998, pp. 49–52. w7x P.H. Paul, Electrokinetic pump application in micrototal analysis systems, mechanical actuation to HPLC, Micro Total Analysis Systems, Kluwer Academic Publishers, 2000, pp. 583–590. w8x S.L. Zeng, C.H. Chen, J.C. Mikerlsen Jr, J.G. Santiago, Sens. Actuators B 79 (2001) 107–114. w9x L.X. Chen, Y.F. Guan, Chin. J. Chromatogr. 20 (2002) 115–117. w10x L. Chen, J. Ma, F. Tan, Y. Guan, Sens. Actuators B 88 (2003) 260–265. w11x Guan, Y., Chen, L. Chinese Patent, CN01110506.2 (2001). w12x Y. Guan, L. Zhou, Z. Shang, J. HRC and CC. 7 (1992) 434. w13x J.C. Gluckman, A. Hirose, V.L. McGuffin, M. Novotny, Chromatographia 17 (1983) 303–309.
An electroosmotic pump for packed capillary liquid chromatography Lingxin Chen, Jiping Ma, Yafeng Guan* Department of Analytical Chemistry and Micro-Instrumentation, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 100, Dalian 116012, China Received 10 December 2002; received in revised form 20 February 2003; accepted 27 February 2003
Abstract An electroosmotic pump (EOP) capable of generating pressure above 3 MPa and mlymin flow rate with reverse phase mobile phases of HPLC was constructed and evaluated. The pump consisted of three parallel connected fused silica capillary columns (25 cm=320 mm I.D.) packed with 2 mm silica materials, hollow electrodes, a high voltage DC power supply, and a liquid pressure transducer. The EOP was applied in a capillary liquid chromatographic system for mobile phase delivery instead of a mechanical pump. Standard samples containing thiourea, naphthalene, anthracene, phenanthrene and acetonitrile were separated on a 15 cm=320 mm I.D. 5 mm Chromasil C18 packed capillary column with acetonitrileywater as mobile phase. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Electroosmotic pump; Micropump; Capillary liquid chromatography
1. Introduction The packed capillary liquid chromatographic columns with internal diameters (I.D.) of 75 to 320 mm are used increasingly in microseparation techniques w1–3x. Pumps that can deliver mlymin and sub-mlymin flow rates at pressure above 3 MPa are necessary in these systems. The mechanical pumps such as reciprocating pumps or syringe pumps will not operate reliably or accurately at such a low flow rate and high pressure, because of the uncontrollable leakage from check valves and dynamic sealing of pistons that constituted the *Corresponding author. Tel.: q86-411-369-3515; fax: q86411-369-3510. E-mail address: [email protected] (Y. Guan).
most important parts of mechanical pumps. For two decades since the first invention of packed capillary liquid chromatography (m-HPLC) w4,5x, the mobile phase delivery at mlymin range under high-pressure condition has been a major obstacle in the instrumental development of capillary HPLC. The electroosmotic pump (EOP) based on the electroosmosis principle has drawn more attention w6–10x in recent years since it could generate pressures up to tens of MPa and flow rates of mly min or sub-mlymin w7,8,10,11x. Previous studies on EOP were limited on the pumping of water or water-based buffers, rather than on mobile phases commonly used in reverse phase HPLC or organic solvents. This is because the EOP originated from
0026-265X/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0026-265X(03)00050-X
16
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
the idea of capillary electro-chromatography (CEC), where only buffers were used as mobile phases. Zeng et al. w8x reported an EOP capable of generating a flow rate of 3.6 mlymin and pressures of 2 MPa for deionized water. Guan et al. w11x utilized 25 cm=0.32 mm I.D. packed column as the electroosmotic channel in their EOP, which could generate pressures up to 20 MPa at flow ™ 0 and flow rate approximately 1.6 mly min at pressure ™ 0 for water buffer. The output flow rate capacity of an EOP is very low at the maximum output pressure condition. When a 320 mm I.D. separation column is employed in a mHPLC system, the mobile phase flow rates are in the range of 1.5–3 mlymin and pressures of 3–8 MPa. The narrow electroosmotic channel in EOP, which is necessary for generating high-pressure output and heat dissipation of Joul heating from the channel, limits the volumetric flow capacity. The real challenges for an EOP to be used in mHPLC systems are the kinds of mobile phases that can be delivered, the volumetric flow rate and pressure values can be generated, and the reproducibility of output flow rate for a given load. In this paper, an electroosmotic pump (EOP) was constructed and evaluated for the delivery of mobile phases in a m-HPLC. The relationships between the maximum output pressure and applied voltage, the maximum output flow rate and the applied voltage, were studied. Unlike mechanical pumps, we found that the output flow rate and pressure of the EOP were inter-related. The EOP was connected to a packed capillary HPLC system for mobile phase delivery instead of a mechanical pump. The results demonstrated that the EOP could be applied in m-HPLC. 2. Experimental 2.1. Electroosmotic pump The electroosmotic pump (EOP) is shown schematically in Fig. 1. The structure of the EOP employs a three-channel-pump system, where three columns (25 cm=320 mm I.D.) packed with 2 mm porous silica grains (Dalian Institute of Chemical Physics, China.) are connected in parallel in
Fig. 1. Schematic diagram of the EOP system: (1) DC power supply; (2) platinum wire; (3) solvent reservoir, which was connected to the anode of the power supply; (4) packed columns, three columns connected in parallel; (5) hollow electrode, which was connected to the cathode of the power supply (the grounded electrode); (6) a liquid pressure transducer and gas-release device; and (7) output of the pump, to which an injection valve and an analytical column can be connected.
order to increase the output flow rate capacity and improve the heat dissipation property. A platinum electrode is placed in the reservoir of mobile phase, and is connected to the anode of a DC power supply. The reservoir was covered with an insulating sheath to prevent it from accidental electric shock. The ends of the packed columns were immersed in the mobile phase, while the other ends of the columns are connected to a homemade 5-port manifold constructed with PEEK material. A cylindrical Pt electrode is placed inside the manifold and is connected to the outside through one of the ports. This electrode and the cathode of the DC power supply are connected to the ground. The port at the opposite side of the manifold is used as output of the EOP. The mobile phase moves from anode to ground electrodes inside the EOP columns, because of an electroosmotic flow (EOF) when a voltage is applied across the columns. A pressure sensor and a gas-release device are connected to the output end of the EOP in order to measure the output pressure and release the gases produced by electro-chemical reaction occurring at the surface of the cylindrical electrode.
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
17
laboratory. All solvents were of analytical grade and were filtrated over a 0.5 mm filter. 3. Results and discussion 3.1. Evaluation of the EOP
Fig. 2. Schematic diagram of the m-HPLC system: (1) connected to EOP; (2) a 4-port injection valve; (3) the micro-LC column; (4) a UV-VIS detector; (5) Chromatographic Station and (6) waster liquid bottle.
The preparation of the columns was the same as m-HPLC columns, which had been described by several authors w12,13x. 2.2. The m-HPLC system The m-HPLC system is shown in Fig. 2. It consists of a 4-port injection valve with an internal loop of 200 nl (VICI, Switzerland), a 15 cm=320 mm I.D. 5 mm Chromasil C18 packed capillary chromatographic column (homemade), and a JASCO CE-975 on-column UV-VIS detector (JASCO, Japan). An EOP pump, as described in 2.1 was used for mobile phase delivery. A KF-98 Chromatographic Station (Ver. 1.10, Dalian Scien-Tech Instruments Ltd. Dalian, China) was used for data handling. 2.3. Reagents Standard samples of thiourea, naphthalene, anthracene, and phenanthrene were purchased from Shenyang Chemical Reagents Factory (Shenyang, China). Acetonitrile was from Tedia (Tedia Inc., USA). Bi-distilled pure water was prepared in the
The EOP output pressure P and the flow rate F are related to the applied voltage V, the dimensions of electroosmotic columns, the packing material inside the columns, the mobile phase being pumped, and the load w1,5x. The dominating factors behind the phenomena are the resistance and the cross-section area AS of the electroosmotic columns, and the properties of the mobile phases. The flow rate capacity of an EOP is proportional to AS and the V. There should be no limitation to use the column of large AS or apply high voltage in EOP if the joule heating can be removed instantly from the column. We selected a narrow diameter packed column in order to minimize temperature gradient inside the column and to generate higher operating pressure. The AS of the column then limited the volumetric flow rate of the EOP for a given V. In order to increase the AS while maintaining the efficiency of heat dissipation, we used three electroosmotic columns connected in parallel. The deviation of resistance of the columns to be connected in parallel should be less than 10% to prevent the uneven flow distribution of the mobile phase among the columns. This was not difficult since most of the columns prepared under identical conditions had similar resistance value (normally "5%). The P and F values were related to the properties of fluids being pumped when other conditions were identical. The maximum static pressure PM (MPa) and the maximum flow rate FM (mlymin) of pure water, sodium dihydrogen phosphate buffer and some pure polar organic solvents are shown in Figs. 3 and 4, respectively. The data in the figures showed that the pressure and flow rate of the EOP were linearly increased as a function of applied voltage V. When the pumping fluid is a mixture, the P and F of the EOP still increase linearly as a function of V. The PM and FM values of a mixture of water
18
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
Fig. 3. The maximum static pressure of the pure water, 3=10y3 mol ly1 sodium dihydrogen phosphate buffer at pH 7.5, methanol, ethanol, acetone and acetonitrile on the above EOP.
and acetonitrile are shown in Table 1. It is very important to pump solvent mixture rather than water-based buffer because the mobile phases used in HPLC are organic solvent and water mixtures in most cases. Both the PM and FM values of the EOP were also dependent on the relative percentage of each solvent in the mixture (Table 1) under
identical V, and set the limit for the EOP. The results showed that when the concentration of a solvent in a mobile phase was changed, the applied voltage must be adjusted accordingly to keep the flow rate constant. Unlike PM and FM where the values were obtained either at Fs0 or Ps0, a given flow at a
Fig. 4. The maximum flow rate of pure water, methanol and acetonitrile on the EOP.
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
19
Table 1 The maximum static pressure PM (MPa) and flow rate FM (mlymin) of the mixture with water-acetonitrile mixture of different proportion Applied voltage (kV) Acetonitrileywater (80y20, vyv) Acetonitrileywater (50y50, vyv)
Pressure P (MPa) Flowrate F (mlymin) Pressure P (MPa) Flowrate F (mlymin)
pressure above 2 MPa was required in practical application. To evaluate the EOP at such a condition, the EOP was connected to the m-HPLC system to deliver mobile phase instead of a mechanical pump. The sample containing thiourea, naphthalene, anthracene and phenanthrene were separated on the separation column and their chromatograms are illustrated in Figs. 5–7. The retention times of peaks were good markers for the measurement of flow rate reproducibility and stability, and are shown in Table 2. The RSD% of the retention times in Table 2 was less than 1% for the 4 peaks, proving that the reproducibility of the EOP was excellent even at such a low flow rate. Comparing the chromatograms in Figs. 5–7, where the applied voltage of EOP was increased from 7 to 8 kV and finally to 9 kV, we found that
5
10
15
20
2.8 2.53 2.1 1.93
4.7 4.53 3.7 3.93
7.7 6.82 6.0 6.21
9.8 8.86 7.6 8.24
the output flow rate of the EOP was proportional to the applied voltage, as evidence of shorter retention time and faster separation. The retention times of the peaks vs. the applied voltage showed a good linear decrease relationship (Fig. 8), which reflected a linear dependence of the EOP output and the applied voltage when a load was connected to the EOP. 4. Conclusion The above-described EOP can deliver mobile phase continuously and steadily under high pressure and mlymin flow range. Connection of several narrow diameter electroosmotic columns in parallel is an effective way to increase the volumetric flow capacity of an EOP while maintaining the output pressure value. The liquid that the EOP can deliver
Fig. 5. Chromatogram of a test mixture. Mobile phase: acetonitrileywater (70:30, VyV); applied voltage of EOP: 7 kV; separation column: 15 cm=320 mm I.D. (C18, 5 mm); detector: UV at 254 nm; peaks (from left to right): thiourea, naphthalene, phenanthrene, anthracene.
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
20
Fig. 6. Chromatogram of a test mixture. Applied voltage of EOP: 8 kV; peaks (from left to right): thiourea, naphthalene, phenanthrene and anthracene; other conditions are the same as in Fig. 5.
Fig. 7. Chromatogram of a test mixture. Applied voltage of EOP: 9 kV; peaks (from left to right): thiourea, naphthalene, phenanthrene and anthracene; other conditions are the same as in Fig. 5. Table 2 The repeatability of the retention time of peaks in the chromatograms Retention time (min) Sample
1
2
3
4
Mean
RSD%
Thiourea Naphthalene Phenanthrene Anthracene
3.136 12.491 18.762 20.103
3.164 12.606 19.124 20.440
3.115 12.532 18.901 20.324
3.123 12.424 19.021 20.312
3.134 12.513 18.952 20.295
0.69 0.61 0.82 0.69
L. Chen et al. / Microchemical Journal 75 (2003) 15–21
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Fig. 8. Retention times of the compounds vs. applied voltage (All the data were abstracted from the Figs. 5–7. S – thiourea, N – Naphthalene, Ph – Phenanthrene, A – Anthracene.
has been extended to pure organic solvent and RPHPLC mobile phase in our study. The EOP is applicable in reverse phase m-HPLC instead of mechanical pumps. It offers a number of advantages over mechanical pumps, including ease of fabrication, the absence of moving parts and dynamic sealing, no frictional wear or material fatigue, pulsation-free and no noise. The EOP will find application in other micro-fluidic systems. Acknowledgments We wish to thank the Chinese Foundation of Natural Science for financial support. References w1x J.P.C. Vissers, A.C. Henk, C.A. Cramers, J. Chromatogr. A 779 (1997) 1–28. w2x J.P.C. Vissers, J. Chromatogr. A 856 (1999) 117–143.
w3x T. Jiang, Y. Guan, J. Chromatogr. Sci. 37 (1999) 255–265. w4x T. Tsuda, M. Novotny, Anal. Chem. 50 (1978) 271–275. w5x D. Ishii, K. Asai, K. Hibi, T. Jonokuchi, M. Nagaya, J. Chromatogr. 144 (1977) 227–232. w6x P.H. Paul, Studies of hydrostatic pressure effects in electrokinetically driven m-TAS, Micro Total Analysis Systems, Kluwer Academic Publishers, 1998, pp. 49–52. w7x P.H. Paul, Electrokinetic pump application in micrototal analysis systems, mechanical actuation to HPLC, Micro Total Analysis Systems, Kluwer Academic Publishers, 2000, pp. 583–590. w8x S.L. Zeng, C.H. Chen, J.C. Mikerlsen Jr, J.G. Santiago, Sens. Actuators B 79 (2001) 107–114. w9x L.X. Chen, Y.F. Guan, Chin. J. Chromatogr. 20 (2002) 115–117. w10x L. Chen, J. Ma, F. Tan, Y. Guan, Sens. Actuators B 88 (2003) 260–265. w11x Guan, Y., Chen, L. Chinese Patent, CN01110506.2 (2001). w12x Y. Guan, L. Zhou, Z. Shang, J. HRC and CC. 7 (1992) 434. w13x J.C. Gluckman, A. Hirose, V.L. McGuffin, M. Novotny, Chromatographia 17 (1983) 303–309.