Anal. Chem. 2010, 82, 842–847
Thermal Expansion Pump for Capillary High-Performance Liquid Chromatography Qian Tao, Qian Wu, and Xiangmin Zhang* Department of Chemistry and Institutes of Biomedical Sciences, Fudan University, Shanghai 200433, China A thermal expansion pump (TEP) based on a principle of liquid thermal expansion for capillary high-performance liquid chromatography has been developed. The novel pump is capable of generating a continuous flow at high pressure for constant and stable delivery of binary solvents from nanoliters to microliters per minute without splitting. Theoretical equations for controlling fluidic output of this pump have been established and validated by a series of experiments. Factors affecting flow rate, such as density discrepancy, liquid compressibility, and mass loss in output, were taken into account. An assembly of the pump system employing two groups of thermal expansion pumps (TEPs) working in turns were fabricated, and a controlling strategy for the pump system to maintain a continuous delivery without pressure fluctuation even at switching points was also developed. Both isocratic and gradients of binary solvent delivery by the TEPs were performed. Reproducibility and standard deviation at different flow rates were determined. A capillary high-performance liquid chromatography (µHPLC) system consisting of the TEPs, an injection valve, a homemade packed capillary column (20 cm × 100 µm i.d. with 5 µm C18), and a laser-induced fluorescence detector was set up, and sample separations were carried out. Results of RSD ) 4% for flow and RSD ) 2% for retention times at 500 nL/min were achieved. Such a pump system has almost no moving parts except for the solvent switches. Its overall costs of manufacture and running are very low. It is proven that the TEPs system has great potential and competitive capabilities in capillary liquid chromatography.
gradient delay is also problematic for micro/nanoscale separations. Although mobile phase splitting in gradient elution using a routine HPLC pump system can partially solve these problems, accurate sensor and feedback control of the mobile phase flow are highly demanding and solvents are wasted by splitting, which is also an environmental problem. Up until now, pump devices based on ultrasonic,4 magnetohydrodynamic,5 electro-hydrodynamic,6 electrokinetic,7-9 or electrochemical10 actuations were investigated to deliver solvent in low flow. Unfortunately, most of pumps did not generate enough high pressure for µ-HPLC. An electroosmotic pump could provide high pressure with accurate flow in solvent delivery.11-13 However, it was suffering from risk of bubble formation on its electrodes, and its flow rate was strongly dependent on compositions of the pumped medium. Thermal expansion pumps based on volume expansion at elevations of temperature were studied by several authors.14-16 Pawliszyn et al.14 reported a portable thermal pump design for supercritical fluid delivery, which could generate a 2 mL/min flow at 65 MPa pressure for CO2 as the mobile phase. Miller et al.15 described an electrothermal pump system for liquid chromatography, which was capable of delivering flow rates up to 2 mL/min at pressures up to 40 MPa. Hjerten et al.16 utilized a water-filled stainless-steel tubular spiral immersed into a water bath as a thermal expansion pump, which could produce microliter flow in capillary chromatography. Hjerten’s work demonstrated a great potential application to capillary liquid chromatography. Some special advantages such as pulse-free flow were discussed. This thermal pump was not continuous and delivery was not constant due to the use of simple linear temperature programming.17 In this article, a novel pump based on the liquid thermal expansion principle for capillary liquid chromatography has been
Capillary high-performance liquid chromatography (µ-HPLC) is used extensively in recent years.1-3 A high-performance pump that can deliver flow rate from microliters to nanoliters at high pressure is very important in a µ-HPLC system. The mechanical pumps, such as reciprocating pumps, commonly applied in HPLC, is not suitable for use in µ-HPLC due to inaccuracy in lower than microliter flow. Pulsatile, valve leakage, and dynamic sealing of the piston are problematic in µ-HPLC systems. The solvent
(4) (5) (6) (7) (8) (9) (10)
* Corresponding author. Phone: +86-21-65643983. Fax: +86-65641740. E-mail:
[email protected]. (1) Vissers, J. P. C. J. Chromatogr., A 1999, 856, 117–143. (2) Beisler, A. T.; Sahlin, E.; Schaefer, K. E.; Weber, S. G. Anal. Chem. 2004, 76, 639–645. (3) Song, Y. R.; Feng, Y. Z.; LeBlanc, M. H.; Zhao, S. L.; Liu, Y. M. Anal. Chem. 2006, 78, 8121–8128.
(13)
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(14) (15) (16) (17)
Nguyen, N. T.; White, R. M. Sens. Actuators, A 1999, 77, 229–236. Jang, J.; Lee, S. S. Sens. Actuators, A 2000, 80, 84–89. Woias, P. Sens. Actuators, B 2005, 105, 28–38. Lizar, I. M.; Karger, B. L. Anal. Chem. 2002, 74, 6259–6268. Dasgupta, P. K.; Liu, S. R. Anal. Chem. 1994, 66, 1792–1798. Chen, L. X.; Ma, J. P.; Guan, Y. F. Microchem. J. 2003, 75, 15–21. Ni, J.; Zhong, C. J.; Coldiron, S. J.; Porter, M. D. Anal. Chem. 2001, 73, 103–110. Zeng, S. L.; Chen, C. H.; Mikerlsen, J. C.; Santiago, J. G. Sens. Actuators, B 2001, 79, 107–114. Gan, W. E.; Yang, L.; He, Y. Z.; Zeng, R. H.; Cervera, M. L.; Guardia, M. Talanta 2000, 51, 667–675. Chen, L. X.; Wang, H. L.; Ma, J. P.; Wang, C. X.; Guan, Y. F. Sens. Actuators, B 2005, 104, 117–123. Adams, M. A.; Otu, E. O.; Kozliner, M.; Szubra, J.; Pawliszyn, J. Anal. Chem. 1995, 67, 212–219. Miller, T. E.; Davis, C. M. Anal. Chem. 1988, 60, 1965–1968. Ericson, C.; Hjerten, S. Anal. Chem. 1998, 70, 366–372. Chen, L. X.; Ma, J. P.; Guan, Y. F. J. Chromatogr., A 2004, 1028, 219–226. 10.1021/ac901855t 2010 American Chemical Society Published on Web 01/05/2010
demonstrated. A continuous and constant flow at high pressure was realized without splitting. The theoretical equations of flow rate related to temperature change has been derived and validated by experiments. A pump system employing two groups of thermal expansion pumps (TEPs) working by turns was set up and applied to reversed-phase liquid chromatographic separations. THEORY OF THERMAL EXPENSION PUMP A volume expansion of liquid medium mainly depends on temperature elevation. By accurate control of temperature programming for a volume of liquid in a chamber, a stable outflow can be generated. In fact, there are three main factors to influence the accuracy of fluidic output for TEPs: volume thermal expansion, liquid compression, and liquid mass loss in the heating chamber during output. First of all, volume thermal expansion is an essential driving force of the TEPs to output a flow,16
Fexp
δT ) R(T, P)V0 δt
δβ(T, P) (P - P0) δT
(2)
δP δV ) -β(T, P)V0 δt δt
Fcomp ) -β(T)V0
δP δt
(4)
Concerning liquid compression, not only liquid in the chamber is compressed but also liquid in a whole system including connection parts and tubes is compressed. If the liquid volume outside the heating chamber(s) is Vd, this part of liquid can additionally result in a negative flow, Fd, while compressed. Fd can be described at the same formula as eq 4,
∫
t1
t0
F(t) dt )
∆m F(T0, P)
(7)
where the flow rate F(t) is a constant in an isocratic run, whereas it is a function of running time, t, in a gradient run. To carry on a linear gradient run from time t0 to t1, F(t) ) a + bt. The coefficients, a and b, are an intercept and a gradient slope, respectively. In n steps of elution, the Vout is a sum of each step as in eq 8, n
Vout )
∑∫ i)1
ti
ti-1
Fi(t) dt
(8)
An output flow from eqs 2 and 4 is a flow at a heating temperature T. It should be converted to a real flow at room temperature T0 by eq 9,
F)
(3)
If water was used as expansion medium, its compressibility is a constant in a wide pressure range, δβ/δP ) 0.20 Thus, eq 3 can be written as
(6)
where Tini is the initial temperature in chamber (in our experiment, the Tini ) 60 °C). Actually, liquid flowing out from the chamber(s) is quickly cooled down to room temperature, T0. The volume of this portion of liquid, Vout, is flowing out. The liquid mass loss is completely converted into a flow rate, F, and it can be calculated with eq 7,
(1)
where P0 is an initial pressure in the chamber. The β(T,P) is a compressibility of the liquid.19 Second, liquid compression with pressure would lead to a decrease in outflow. This negative flow is proportional to liquid compressibility and a pressure gradient. The β is also a function of the T and P.19 So, we have
Fcomp )
∆m ) V0F(Tini, P) - V0F(T, P)
Vout )
where V0 is the heating chamber volume, R(T,P) is the cubic thermal expansion coefficient,18 which is a function of temperature, T and pressure, P,
R(T, P) ) R(T, P0) -
where T0 is the room temperature. In a heating chamber, the liquid expansion either generates a flow or compresses as a pressure increase. The total flow is a sum of expansion and compression. Third, the liquid mass loss, ∆m, in the heating chamber during outflow should be taken into account. ∆m is equal to a liquid density change. It can be written as follows,
[
]
Vout F(Tini, P) (Fexp + Fcomp) F(T0, P) V0
(9)
In eq 9, a ratio of water density at initial temperature, F(Tini,P), vs density at room temperature, F(T0,P), is constant (0.9860 ± 0.0001) in an applied pressure range. This can be approximated to be (F(Tini,P))/(F(T0,P)) ≈ 0.986. The Fd (in eq 5) is not included in the calculation by eq 9 because this compressed part of the liquid is never heated. With combination of eqs 9 and 5, the total flow rate Ftotal can be computed at any T and P using eq 10,
[
]
δβ(T) δT [0.986V0 - Vout] δT δt δP + Fd (10) β(T)[0.986V0 - Vout] δt
Ftotal ) R(T, P0) - (P - P0)
(5)
The R(T, P0) at P0 and β(T) at different temperature can be obtained from refs 18 and 19.
(18) Handbook of Chemistry and Physics, 70th ed.; Weast, R. C., Ed.; CRC Press, Inc.: Boca Raton, FL, 1987. (19) Lange’s Handbook of Chemistry, 12th ed.; Dean, J. A., Ed.; McGraw-Hill Book Co.: New York, 1978. (20) Martin, M.; Blu, G.; Guiochon, G. J. Chromatogr. 1973, 11, 641–654.
EXPERIMENTAL SECTION Reagents and Materials. Fused silica capillaries (100/250 µm i.d. × 375 µm o.d.) were purchased from Yongnian Optical Fiber Factory (Yongnian, Heibei, China). Packing materials of C18
Fd ) -β(T0)Vd
δP δt
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Figure 1. A diagram of the TEP’s liquid heating chamber assembly.
particles (Kromasil, 5 µm, 300 Å) and 5 µm spherical silica gel were obtained from Eka Nobel (Switzerland). Fluorescein isothiocyanate (FITC), trifluoroacetic acid (TFA), and methyltriethoxysilane (MTES) were purchased from Sigma-Aldrich (St. Louis, MO). Methanol and acetonitrile were HPLC grade, and methylene chloride (CH2Cl2) was an analytical grade reagent purchased from Fisher Scientific Co., Ltd. (Fairlawn NJ). Pure water was produced using a Milli-Q device from Millipore (Bedford, MA). Amino acids, serine (Ser), proline (Pro), and valine (Val) were purchased from Aldrich Company and are all analytical grade. Sample Preparation and Detection. A derivation procedure was as follows. Dissolve 1 mg of FITC in 1 mL of acetone to get 2.5 mM solution. Then, dilute to 1.25 mM with 20 mM borate buffer (pH 9.2) before use. Dissolve the amino acid standards in water to obtain solutions of 3 mM Ser, 0.5 mM Pro, and 1 mM Val. A volume of 40 µL of amino acids containing 10 µL of Ser, 5 µL of Pro, and 10 µL of Val solution together with 15 µL of water was mixed with 40 µL of FITC solution for reacting for 10 h at room temperature in the dark. A 1000 time dilution with 20 mM borax buffer (pH 9.2) of this sample was injected and analyzed. Fluorescence was monitored at an emission wavelength of 520 nm by an excitation with a solid blue laser at 473 nm (15 mW). Fabrication of the Pump System. An accurate volume with 1.000 mL of the pump’s heating chamber was fabricated using stainless steel. This volume and its heating velocity were optimized for the µ-HPLC system fitted to 50-5000 nL/min output flow. The pump heating part included a pressure-resist chamber, 56.20 mm × 4.84 mm i.d. (59.00 mm × 8.88 mm.o.d.) stainless steel cylinder, and a platinum resistance thermometer, and a ceramic tube coiled by a heating cord served as the heating part as shown in Figure 1. Then, the chamber was filled with air-free water. The filling must be careful to eliminate air bubbles in the chamber by repeating several times. 844
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A prototype of TEPs system was set up as shown schematically in Figure 2. It is comprised of four TEPs, two 10-port electrically actuated valves, V1 and V2 (Valco Inc.), and two pressure gauges, PG1 and PG2. The four TEPs were divided into two groups working alternatively. With the valve, V1, switched, the two groups alternated to keep a continuous output flow. While the working group of TEPs was delivering solvents, the free group was refilled by negative pressure generated during liquid cooling. The valve V2 was employed not only to switch for filling up solvents but also to control the pressure balance of the switching to avoid any flow fluctuation. For instance, while the working pressure was keeping P (indicated by PG1), the other group had to be preheated until its pressure (indicated by PG2) approached the same pressure as P to start switching. Then the free group was replaced with the working group. In order to isolate solvent from expansion water in each TEP, the 100 µL capillary coil reservoir, indicated as S in Figure 2, was used immediately outside each TEP. Also, a 5 mm segment of mercury was filled between the water and solvent interface. Homemade software was developed to control all of the switching and temperatures of the four TEPs. The pressures of the two gauges were also read out by the software to control synchronous actuations. A strategy of a proportional integral derivative (PID) was adopted for stable temperature controls. An initial temperature of the TEPs was actually set at 60 °C for obtaining a stable state of the temperature control. In order to minimize the dead volume of the pump system, all stainless steel tubes used to connect flow ways were 170 µm .i.d. Wider bore PEEK tube, 0.5 mm i.d., was used between the switchport and solvent reservoirs for low resistance in solvent refilling. A µ-HPLC equipped with the pump system, a 6-port injection valve with a 200 nL loop, a reversed-phase column of 20 cm × 100 µm i.d. packed with 5 µm Kromasil C18 as made in our previous work,21 and a homemade laser-induced fluorescence detector22 as reported before were set up for the thermal expansion pump performance evaluations as schematically shown in Figure 2. Accurate Measurements of the Pump Flow. A homemade flowmeter was employed to measure the flow rates accurately. It consist of an orderly coiled transparent fused-silica capillary (250 µm i.d. × 3 m) mounted on a cylinder with diameter 20 cm. The volume scale was marked on the cylinder. The volume was directly read out by the marks for capillary length. The capillary was washed by water and CH2Cl2 and then dried by N2 before being filled up with water and a short plug (3-5 mm) of mercury as an observed label inside the capillary. The flow rate was calculated according to a moving distance of the mercury label and a elapsed time. The inlet of the flowmeter capillary was connected to the outlet of the pump, and the outlet end was connected to pressurized gas with a back pressure of 0.4-1 MPa to avoid any influence of air bubbles during measuring. RESULTS AND DISCUSSION Expansion Media and Parameter Confinement for the Pump. Liquid medium used for TEPs should be thermal inert, high expansion rate, high boiling point, and low compressibility. (21) Yang, X. H.; Zhang, X. M.; Li, A. Z.; Zhu, S. Y.; Huang, Y. P. Electrophoresis 2003, 24, 1451–1457. (22) Zhang, X. M.; Huang, S. J. Chromatogr., A 2001, 910, 13–18.
Figure 2. Schematic diagram of the TEPs system. It contains four TEPs, P1 and P3 is a group while P2 and P4 is another group. V1 and V2 are the 10-port electrically actuated valves; CS is a 4 channel temperature control system; PG1 and PG2 are pressure gauges. Vi is a injection valve, and S is the capillary coil reseviors to store mobile phase. CC is a capillary RPLC column, and D is a laser-induced fluorescence detector. SV is the solvent vessels; He is the helium gas source. Table 1. Calculated Flow Rates from Equation 10 Corresponding to Various Chamber Volumes V0 and Temperature Programming, δT/δt (°C/min) with the Assumption of P ) P0, δP/δt ) 0a chamber volume (mL) F(nL/min)
δT/δt ) 0.4
δT/δt ) 1
δT/δt ) 2
δT/δt ) 3
50 100 250 500 750 1000 1250 1500 1750 2000 2250 2500
0.181 0.362 0.906 1.812 2.717 3.623 4.529 5.435 6.341 7.246 8.152 9.058
0.072 0.145 0.362 0.725 1.087 1.449 1.812 2.174 2.536 2.899 3.261 3.623
0.036 0.072 0.181 0.362 0.543 0.725 0.906 1.087 1.268 1.449 1.630 1.812
0.024 0.048 0.121 0.242 0.362 0.483 0.604 0.725 0.845 0.966 1.087 1.208
a
The R(T, P0) values were from ref 18.
Variety of liquids could be expansion media used in TEPs, such as water, mercury, and silicon oil, etc. Water is an excellent medium for use in TEPs because its thermal behavior is intensively studied and it is absolutely nontoxic. A thermal expansion below 210 °C is easily managed to apply to the TEPs system since its vapor pressure is lower than 20 bar at this temperature.18 Concerning the pump chamber volume, a larger chamber could generate a higher flow rate but it would result in a low precision of output in a low flow rate, vice versa for a smaller volume chamber. A small chamber demands a high temperature ramp rate to produce a flow. However, a high temperature ramp rate would lead to a short ramping time and a dynamic temperature delay. Actually it is necessarily demanded to balance and compromise in confining the chamber volume. Table 1 listed the
chamber volumes calculated from eq 10 for a temperature ramp range of 0.4 to 3 °C/min and for an outflow range of 50-2500 nL/min. Accuracy of the TEP flow output solely depends on precision of the dynamic temperature control. For current TEPs system, the temperature ramp rate was generally confined within 0.3-3 °C/min to achieve the best heating balance and temperature precision. According to the result listed in Table 1, 0.966-1.087 mL chamber volumes for TEP cover wider outflow ranges. Therefore, the 1 mL volume was selected. Calculation and Measurement of Flow Rate. A synchronous calculation from eq 10 during pumping would be the best way for accurate control of real-time flow rate. A simplified software is also working well for isocratic and gradient runs. A database consisting of a series of precalculated temperature programming curves corresponding to their flow rates was established. Parts of the database are made plots as shown in Figure 3. In the calculation of temperature programming curves, two approximations were made: (1) A accurate fitting relationship, R(T,P) ≈ 5.181T + 3500.5/T + 251.4, was obtained and applied to the calculation; (2) (P - P0)δβ(T)/δT ≈ 17 was employed. In the R(T,P) calculation, this approximation may lead to 1% deviations. Because P is unknown in calculation, P ) 100 bar was assumed, and the δβ(T)/δT value was computed as from 0.05 to 0.35 in the whole operating temperature range. An average value, δβ(T)/δT ) 0.17, was used. So, the term (P - P0)δβ(T)/ δT was approximated to 17. This approximation might lead to a maximum deviation of ±1.5% of the total flow. The TEPs system flow rate was controlled by readout of the database. Both of the flow rates generated by the single TEP and TEPs system were measured by the flowmeter and listed in Table 2. It is validated that the calculated values from eq 10 are very close to the measured ones. A larger RSD% at low flow rates was probably due to a low precision in temperature control. While with high flow rates, it might be due to dynamic temperature delay/ Analytical Chemistry, Vol. 82, No. 3, February 1, 2010
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Figure 3. Temperature programming curves corresponding to different flow rates: 0-0.1 µL/min with an interval 0.025 µL/min; 0.1-1 µL/min with an interval 0.05 µL/min; and 1-2.5 µL/min with an interval 0.1 µL/min.
Figure 4. System pressure increase as a function of time at different invented flow rates on a stop-flow status.
Table 2. Comparison of Calculated and Measured Flow Rates (µL/min)a measured calculated
TEP
RSD%
TEPs
RSD%
0.05 0.1 0.2 0.5 1 2 3 4 5
0.043 0.094 0.175 0.448 1.021 1.826
14.6 10.5 8 6.1 2.9 2.5
0.048 0.098 0.196 0.5 0.992 2.017 2.999 4.007 4.772
13.0 8.5 5.0 4.0 2.1 1.8 2.2 3.9 5.6
a TEP refers to a determination from single chamber; TEPs is from the dual TEPs system.
unbalance in the heating chamber at a high heating rate. Increasing the temperature precision and improving the heatingequilibration will further enhance the TEP and TEPs system performance. Pressure Raise for TEPs Switching. It is necessary to preheat the free group of TEPs for generating enough high pressure to carry out a switching for alternating the working group of TEPs. Actually, if the TEPs are closed (stop-flow) and the temperatures are increased δT, the pressure inside the chambers will be raised δP according to the following equation, R(T,P)δT ) β(T)δP. In this case, the temperature ramp up is completely converted into a pressure increase. Considering the R(T,P) is approximately a linear increase with T in a temperature range above 60 °C,18 the pressure increase with the temperature increase can be calculated. As a result of the experiment, plots of pressure increases with the invented flow rates (stop-flow) were shown in Figure 4. From Figure 4, we know that the TEPs system would be promising for ultrahigh pressure liquid chromatography (UPLC). As an estimate, it is easy to achieve as high as 1000 bar by raising the temperature from 25 to 95 °C. Constant Flow for Different Columns. A high-performance pump produces an accurate delivery which is independent of column back-pressure. For columns with different flow resistances and for mobile phases with different compositions/viscosities, the 846
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Figure 5. Plots of backpressure as a function of flow rate for capillary columns with different flow resistance: (A) 29 cm × 100 µm i.d. (5 µm Kromasil C18) packed capillary columns and (B) 15 cm × 100 µm i.d. column packed with same batch of particles. Mobile phase: 100% methanol.
flow rate should be kept constant. For a given column, its flow rate should be a linear change with pressure. The TEPs system was employed for such tests on mobile phase deliveries. Two columns were employed to measure the relation between flow rate and back-pressure. As shown in Figure 5, one of them gave 46 bar and the other gave 63 bar at the same flow rate of 500 nL/min. The pressure is well-proportional to the flow rate for each of the columns. Gradient Elution. A gradient generation from the dual TEPs was performed. Solvent A was water with 20 mM borate (pH 9.2), solvent B was water with a 2.5 × 10-4 M FITC. A three-way joint was acting as a mixer of binary solvents. The solvent gradient was generated by changing the flow rate of the two TEPs while maintaining the total output constant at 500 nL/min. The gradient was monitored by the laser induced fluorescence detector. As shown in Figure 6, both linear and stepwise gradients were realized. No fluctuation was observed when switching between the two groups of TEPs.
Figure 7. Chromatograms of continuous injections; peak identities, Ser, Pro, Val, and FITC as labeled. The arrows are the injection starts.
Table 3. Reproducibility of the Retention Times from Peaks of the Amino Acids in the Chromatograms retention time (min) sample
1
2
3
mean
RSD%
serine proline valine
5.438 6.507 8.164
5.535 6.487 8.013
5.448 6.488 8.348
5.474 6.494 8.175
0.97 0.17 2.05
CONCLUSIONS
Figure 6. Binary solvents gradients run at a constant flow at 500 nL/min: (A) stepwise gradients, 0% B for 20 min, then goes up to 50% B for 20 min, then ramp to 100% B for 20 min; (B) a linear gradient, 0%B for 10 min, then linear gradient from 0% B to 100% B in 40 min. Solvent A, H2O with 20 mM borate (pH 9.2); solvent B, H2O with 2.5 × 10-4 M FITC.
In Figure 6B, the last 10 min was set as 100% solvent B in a constant flow. It could be observed that the baseline linearly went up with a longer time to be flat. The signal noise might be a result of the rough mixing of the binary solvents. A precise temperature control system and better blender are still in development for achieving better performance. Software that could calculate the flow synchronously with a real time pressure and temperature is in development. Performance Validation of the TEPs System in Capillary Chromatography. A stable and constant flow is demanded for capillary liquid chromatography to obtain reproducible results. To validate the actual performance of the TEPs system, we set up a µ-HPLC system to evaluate the pumping delivery stability and reproducibility as the chromatograms illustrated in Figure 7. During a typical elution period (over 2 h), the output pressure at the flow rate of 500 nL/min was kept at 73 bar placidly. No pressure fluctuation was observed even when the valve switches occurred. Furthermore, chromatogram reproducibility of three repeated runs was fairly identical. The RSD% of the retention times in Table 3 was less than 2% for these peaks. It indicates that the reproducibility of this thermal expansion pump system was excellent in performance even at a submicroliter flow rate.
We have demonstrated that a thermal expansion pump, TEP, was able to deliver micro/nanoflow (from 50 nL/min to 5 µL/ min) for stable and constant output in high pressure. The TEPs system’s theoretical equations, device configuration, solvent delivery precisions, and performance were validated through a series of experiments. It has been proven that the pump would have great potential as use as a µ-HPLC for a variety of applications, especially in coupling to mass spectrometry (MS). A very important feature to use this pump is the small amount of samples needed to carry out an analysis. Also, the sample dilution in the separation is less, which is much more favorable for highly sensitive detection by MS, UV absorbance, and fluorescence. In addition, such a pump system has almost no moving parts and no mechanical consumptive hardware to change frequently. It runs in silence. Its overall costs of manufacture and routine running are very low. It is also easy to fabricate and assemble without complicated technologies. ACKNOWLEDGMENT This study has been supported financially by the National Basic Research Priorities Program (Grant 2007CB914100/3), the 863 Project (Grant 2006AA02A308), the National Natural Science Foundation of China (Grant 20475011), and the Shanghai Leading Academic Discipline Project B109.
Received for review August 17, 2009. Accepted December 19, 2009. AC901855T Analytical Chemistry, Vol. 82, No. 3, February 1, 2010
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