End-column electrical and electrochemical detector for capillary zone electrophoresis

Abstract: Electrical and electrochemical end-column detectors are provided in which a sensing microelectrode 300;530 is placed at the outlet of the separation capillary 100;400 so that the effluent flows out of the capillary between a small gap 330 (1 to 5um) between the sensing microelectrode and the capillary end. The design reduces the electrical interference caused by applied high voltage applied during CZE separations, and provides for detectors that are easy to construct, sensitive, reliable, and easy to operate.

Description: The invention relates generally to capillary electrokinetic devices and in particular to an improved system for detecting electrokinetically separated species with electrical or electrochemical measurement. Zone electrophoresis in capillaries has become an important technique in the repertoire of liquid-phase separations. See Jorgenson et al., Science 222 (1983) 266-272; Gordon et al., Science 242 (1988), 224-228; Ewing et al., Anal. Chem. 61 (1989), 292A-303A; Wallingford et al., Advances in Chromatography 29 (1989), 1-76; and Kuhr, Anal. Chem. , 62 (1990), 403R-414R. Capillary electrophoresis has been used for separations of small and large molecules and comprises several subtechniques including capillary zone electro-phoresis (CZE), capillary gel electrophoresis, micellar electrokinetic capillary electrophoresis, and capillary isoelectric focusing. CZE employs extremely high potential fields, typically 300 V/cm, resulting in highly efficient separations of ionic solutes. A major aspect of CZE in need of new development is detection; specifically, there is a critical need for detectors capable of responding to the small quantity of sample component in the effective detection volume. Detection schemes developed to date include direct and indirect UV absorption (Hjerten, J. Chromatogr. , 347 (1985), 191-198 and Hjerten, et al., J. Chromatogr. , 403 (1987), 47-61), fluorescence (Jorgenson et al., Anal. Chem. , 53 (1981), 1298-1302 and Kuhr et al., Anal. Chem. , 60 (1988), 2642-2644)), and radio-isotope (Pentoney et al., Anal. Chem. , 61 (1989), 1642-1647) as well as mass spectrometric (Smith et al., Anal. Chem. , 60 (1988), 436-441; Lee et al., Biomed. Environ. Mass Spectrom. , 18 (1989), 844-850; Moseley et al., Chromatogr. , 480 (1989), 197-210; Caprioli et al., J. Chromatogr. , 480 (1989), 247-258)), and electrometric (Mikkers et al., J. Chromatogr. , 169 (1979), 11-20; Huang et al., Anal. Chem. , 59 (1987), 2747-2749; and Wallingford et al., Anal.Chem. , 59 (1987), 1762-1766)) detectors. Electrical detectors can be distinguished from electrochemical detectors. Electrochemical detectors involve electric effects due to chemical changes which occur when a particle or species enters the detection zone. Electrical detectors respond to changes in the conductance of current or changes of resistance which result when particles or species enter the detection zone. With electrical detectors no chemical reaction is necessarily associated or required. Existing electrical and electrochemical detectors for CZE use elaborate on-column and post-column detection schemes to prevent the high separation potentials used from interfering with the detection process. One scheme involves construction of 40-µm-diameter holes in the capillary using a laser. Thereafter, small platinum wire electrodes are placed in these holes to carry out on-column conductivity detection. It was demonstrated that the exact placement of these electrodes on opposite sides of the capillary is critical to minimize background noise associated with the high potential field used for separation. (Huang et al., Anal. Chem., 59 (1987), 2747-2749)). In U.S. Patent Application Serial No. 443,059, filed November 28, 1989, by Zare et al., on-column conductivity detectors were disclosed wherein on-column sensing electrodes are located contiguous with the exit of the separation microcolumn. The electrodes are covered on their side surfaces with electrical insulators so that conductivity is measured between their ends alone. Another scheme involves covering a crack in the capillary with a porous glass capillary (Wallingford et al., Anal Chem. , 59 (1987), 1762-1766)) to provide off-column amperometric detection. Although good results have been obtained with current detection systems, they are unsatisfactory in that the structures involved are difficult to fabricate, expensive and, often times, unreliable. This has limited the routine application of both modes of electrochemical detection in CZE. See Ewing et al., Anal.Chem . 61 (1989), 292A-303A; Kuhr, Anal. Chem. , 62 (1990), 403R-414R; Huang et al., Anal. Chem. , 61 (1989), 766-770; Huang et al., J. Chromatogr. , 425 (1988). 385-390; Huang et al., J. Chromatogr. , 480 (1989), 285-288; Wallingford et al., Anal. Chem. , 60 (1988), 1972-1975; Wallingford et al., Anal. Chem. , 60 (1988), 258-263; Wallingford et al., J. Chromatogr. , 441 (1988), 299-309; and Wallingford et al., Anal. Chem. , 61 (1989) 98-100. EP-A-295942 describes an on-column conductivity detector for microcolumn electrokinetic separations. The sensing electrodes are placed directly on the column by inserting through the column wall or attaching to the end of the column which is placed in a buffer reservoir. The sensing electrodes present a continuous wall surface on the column to avoid any increase in the cross-sectional flow area of the column. It is an object of the present invention to provide reliable and relatively inexpensive off-column electrical and electrochemical detectors capable of responding to the small quantity of sample components in the effective detection volume in CZE. It is another object of the present invention to provide end-column conductivity and amperometric detectors that eliminate detection problems associated with the high voltage drops across the capillary in CZE separations. A first aspect of the present invention provides a capillary electrophoresis apparatus comprising: a separation capillary having an outlet end through which an electrophoretically separated sample component exits; a reservoir containing a buffer, the outlet end of the separation capillary being submerged in the buffer; and a sensing electrode that responds to the component as it exits the separation capillary, characterised in that the electrode is spaced apart from the outlet of the separation capillary defining a gap through which the component exits. A second aspect of the present invention provides an electrical detection method in capillary electrophoresis, comprising the steps of: positioning the outlet end of a separation capillary into a buffer reservoir; positioning a sensing electrode in the buffer reservoir; placing a second electrode in the buffer; applying an electrical potential between two points along the separation capillary, one point being the outlet end, for electrophoretic separation of a sample in the capillary into its components, and measuring a change in an electrical parameter between the sensing electrode and the second electrode; characterised in that said sensing electrode positioning step positions the sensing electrode so that it is spaced apart from the outlet of the separation capillary defining a gap through which the component exits. The objects of the invention are achieved by providing a design for CZE conductimetric and amperometric detectors in which a sensing microelectrode is placed at the outlet of the separation capillary. These "end-column detectors" are easy to construct. They do not suffer from electrical interference caused by the applied high voltage during the CZE separation. Moreover, end-column detectors demonstrate sensitivities approaching those of previous on-column conductivity and post-column amperometric detectors (Ewing et al., Anal. Chem. 61 (1989), 292A-303A; Huang et al., Anal. Chem. , 59 (1987), 2747-2749; and Huang et al., Anal. Chem. , 61 (1989) 766-770) with only a small sacrifice in resolution. Under typical operating conditions the use of an end-column CZE conductivity detector is found to cause extra zone broadening only about 25% larger than that associated with an on-column CZE conductivity detector. End-column amperometry provides detection limits as low as 56 amol with an efficiency of 143,000 theoretical plates for catechol. Figure 1 is a schematic drawing of a CZE separation device with an end-column conductivity detector. Figure 2 is a cross-sectional view of the plastic jacket assembly. Figure 3 is an enlarged view of the end-column sensing microlectrode. Figure 4 is a schematic drawing of a CZE separation device with an end-column amperometric detector. Figure 5 is a schematic drawing of an end-column amperometric detector. Figure 6 is an electropherogram obtained with an end-column conductivity detector. Figure 7 is an electropherogram obtained with end-column amperometric detection. Figure 8 is an electropherogram obtained with end-column amperometric detection. Figure 1 shows a CZE separation device with an end-column conductivity detector. The separation capillary 100 is a 80 µm i.d., 354 o.d. fused silica microcolumn with a length of 60 cm (Polymicro Technologies, Phoenix, Arizona). A reversible high voltage power supply 110 (Model R50B, Hipotronics, Inc., Brewster, New York) provides a variable voltage of 0-30 kV with the outlet of the separation capillary at ground potential. The capillary is liquid-filled with a support electrolyte and terminates near a sensing microelectrode inside plastic jacket 200. The jacket is approximately 25 mm long and has a diameter of approximately 6.4 mm. Inlet reservoir 120 and outlet reservoir 130 contain support electrolyte as well, so that the liquid-filled capillary 100 creates a continuous liquid and electrical connection between them. As will be further described below, the electrolyte exiting the capillary flows into the outlet reservoir by way of a hole 290 in the plastic jacket 200. The jacket is inserted part-way into the outlet reservoir through one of the reservoir walls and is held in position thereby. An effective electro-kinetic voltage is supplied from power supply 110 through conductors 140 and 150 and grounding electrode 170 and electrode 160. A sensing microelectrode (described below) is situated at the outlet of the capillary 100 inside plastic jacket 290. The output of the end-column conductivity detector which varies as a function of the material in the electrolyte, is passed through lead 270 to conductivity meter 180, and is recorded. The conductivity measurement is made between the sensing microelectrode and the grounding electrode 170. Figure 2 is a cross-sectional view of the plastic jacket assembly. Lead 270 is positioned inside the 1 cm long fused silica capillary 220 (150 µm i.d. and 355 µm o.d., Polymicro Technologies) that extends midway into the plastic jacket. Similarly, separation capillary 100 extends midway into the plastic jacket from the other direction. Epoxy 210 and Teflon® washer 240 provide support for the capillary 220. Similarly, epoxy 250 and Teflon® washer 260 provide support for separation capillary 100. Around its middle section, the plastic jacket defines channel 280 into which capillary 220 and separation capillary 100 are positioned. The channel is sealed at its ends by the Teflon® washers 240 and 260. A hole 290 on one side of the plastic jacket lines up with the sensing micro-electrode so that fluid flows off the separation capillary 100 and into the reservoir 130. Figure 3 is an enlarged view of the sensing microelectrode 300, which is made of a 50 µm diameter platinum wire (California Fine Wire Co., Grover City, California). The microelectrode is centered in capillary 220, and held in place by epoxy 310 (Torrseal, Varian Corp., Lexington, Massachusetts). The surface of the microelectrode facing the outlet of separation capillary 100 is sanded flat. The end of capillary 220 facing the separation capillary is positioned inside a short but somewhat larger support fused silica capillary 320 (approximately 355 µm i.d.), which resembles a ring or collar structure. The outer surface at the end of capillary 220 is sealed with epoxy to the inner surface of one end of capillary 320. The other end of capillary 320 extends about 1-2 mm over the outlet end of the separation capillary 100, which is nearly butted against the sensing microelectrode 300. This leaves a small path or eluent gap 330, about 1-2 mm long, with a wall separation between separation capillary 100 and capillary 220 of approximately 1-5 µm, and preferably of approximately 1-2 µm, for the eluent to exit into the outlet reservoir 130 via hole 290. The eluent gap can be defined generally as a narrow channel that serves as the detection zone for electrical detectors. The dimensions of the eluent gap depend on, among other things, the amount of sample injected into the separation capillary for analysis. With the inventive conductivity detector, the resistance of each component zone or band is measured as each band flows through the eluent gap. If the gap length is too long, more than one band will be present in the eluent gap at a given time thereby interfering with the measurement. Increasing the wall separation distance reduces the gap length, but has the concomitant adverse effect of creating additional dead volume. Figure 4 is a schematic diagram of the CZE system used for amperometric measurements. The separation capillary 400 is a fused-silica capillary 5 µm i.d./140 µm o.d., Polymicro Technologies (Phoenix, Arizona). The capillary (50-70 cm in length) is positioned in plastic vessel 410 through a bore defined by stainless steel fitting 420 (shown with size exaggerated) that is epoxied to one side of the vessel. The vessel functions as the electrochemical cell. The fitting also serves as the cathode for electrophoresis. The high voltage DC power supply 430 provides the electric field for electrophoresis. Electrode 440 situated in buffer reservoir 470 is connected to the power source by conductor 450. Similarly, steel fitting 420 is connected to the power source by conductor 460. The end of the microelectrode 530 (described below) of the detector 480 is manipulated through a slot cut into the opposite side of plastic vessel 410 and up against the end of the separation capillary 400 with a micromanipulator (Newport, Model 422) while viewed under a microscope. Figure 5 is a schematic top view of the amperometric detector. In constructing the detector, a single carbon fiber, 10 µm diameter, Amoco Performance Products, Greenville, South Carolina, is aspirated into glass capillary 500. The capillary was pulled around the fiber 530 with a vertical microelectrode puller. The fiber 530 serves as the microelectrode. Under an optical microscope, a drop of epoxy 540 is applied to the area where the fiber entered the glass capillary. After curing, the fiber is cut with a scalpel to an exposed length of 0.1-1 mm. The open end of the glass capillary is filled with mercury, a segment of nichrome wire 590 placed into it, and sealed with a drop of DUCO cement. The carbon fiber detector is cemented onto a microscope slide 510 so that the end containing the exposed fiber protruded from the edge of the slide. The entire detector assembly is then placed onto a micro-manipulator 520, Oriel Corp., Stratford, Conn. The tip of the carbon fiber is aligned with the bore of the separation capillary 400. The separation between the tip of the carbon fiber and the separation capillary is approximately 1-5 µm. The carbon fiber electrode 530 and outlet of the separation capillary 400 are submerged in the buffer solution of electrochemical cell 410 wherein a reference electrode 550 is also positioned. Reference electrode 550 is connected to potentiostat 570 by lead 560; nichrome wire 590 is connected to the potentiostat by lead 580. EXPERIMENTAL Conductivity Detection . For conductivity measurements, the end-column conductivity detector was placed directly at the outlet of the CZE capillary (as shown in Figure 3). Samples were introduced by gravity at the cathodic or the anodic end of the capillary by raising the inlet a known height (7-12 cm) with respect to the outlet for a fixed period of time (5-10 s). Amperometric Detection . For amperometric measurements, the cell was filled with 0.1 M KCl as supporting electrolyte. Detection was performed in a 2-electrode configuration with a sodium saturated calomel reference electrode (SSCE). Electrochemical detection was carried out at 0.8 V SSCE. The low currents measured required that the detection end of the system be housed in a Faraday cage in order to minimize the effects of external noise sources. Injection was by electromigration. Chemicals . All chemicals were from Sigma Chemical Corp. (St. Louis, Missouri) and were used without further purification. Conductivity detection runs were carried out with a buffer solution consisting of 20 mM morpholinoethanesulfonic acid (MES)/histidine at pH 6.0 to which a cationic surfactant, tetradecyl-trimethylammonium bromide (TTAB), is added (1 mM final concentration). Water used to prepare solutions is freshly deionized and distilled with a water purifier (Model LD-2A coupled with a Mega-Pure Automatic Distiller, Corning Glassworks, New York). Amperometric detection runs were also carried out with MES, but these buffers were adjusted to the desired pH by addition of solid NaOH. Catecholamine stock solutions were prepared as 0.01 M solutions in 0.1 M perchloric acid and diluted to the desired concentration with operating buffer. RESULTS AND DISCUSSION End-Column Conductivity Detection . Figure 6 shows an electropherogram obtained when a mixture containing six different carboxylic acids at 5 x 10⁻⁵ M each is injected onto the CZE setup illustrated in Figures 1-3. The acids were: formate (peak 1), acetate (peak 2), propanoate (peak 3), butanoate (peak 4), pentanoate (peak 5), and hexanoate (peak 6). The CZE is operated at 20 kV and 8.8 µA. The buffer is MES/HIS (20 mM each) at pH 6 with 1 mM TTAB. Injection was by gravity. The sensitivity and the resolution are quite similar to what has been observed previously (see Figure 1 of Huang et al., Anal. Chem. , 61 (1989), 766-770). Because the TTAB in the buffer reverses the electro-osmotic flow direction, the polarity of the power supply was also reversed so that the most mobile anion arrives first at the detector. This end-column conductivity detector has more dead volume than an on-column conductivity detector. This additional dead volume estimated to be about 5-6 nL arises from the eluent gap 330 between capillaries 100, 220, and 320 as shown in Figure 3. To investigate the effect of this dead volume on the separation efficiency, the following experiment was carried out. Benzoate anion (C₆H₅COO⁻) was injected at 1 x 10⁻⁵ M and the peak profile was recorded using on-column UV absorption and end-column conductivity detection. The UV absorption detector was 9 cm upstream from the end-column conductivity detector, so that the additional zone broadening in traveling this distance is minimal (Huang et al., J. Chromatogr. , 480 (1984), 95-110). It was found that the peak from the end-column conductivity detector is 23% broader than that for the UV absorption detector, at an injection volume of 23 nL. This extra zone broadening caused by the additional dead volume of the end-column conductivity detector compares well with the estimated magnitude of the extra broadening. For typical injection volumes (20 nL), and for a 80-µm-i.d. capillary, this extra zone broadening is less than 25%, which is modest compared to the advantage of such a simple construction. Moreover, the end-column conductivity detector can be readily fitted on the outlet of any capillary electrophoresis system, suggesting its use as a simple universal second detector for CZE separations. Furthermore, CZE employing the inventive conductivity detectors can yield quantitative measurements on an absolute basis when an internal standard is used. End-Column Amperometric Detection . As described above, the CZE system for amperometric detection comprises essentially a fused-silica capillary and a carbon fiber microelectrode. With the inventive end-column amperometric detector, there is no need to isolate the sensing electrode from the high electric field needed for electrophoresis because the voltage drop outside the bore of the separation capillary is negligible. The microelectrode is aligned with the bore of the capillary and positioned up against but not into the capillary, thereby creating a thin layer cell at the capillary outlet. Because the diameter of the micro-electrode in this embodiment is about twice the internal diameter of the capillary, good oxidation efficiency is obtained. Detector sensitivity is further increased by approximately one order of magnitude when a cylindrical carbon fiber electrode is used, as shown in Figure 5, relative to a disk-shaped electrode. Apparently the eluent from the capillary forms a sheath-like flow around the electrode, providing more efficient oxidation of solutes at the detector. The separation conditions for the electropherograms in Figures 7 and 8 were: separation potential, 20 kV; electrochemical detector potential, 0.7 V; injection, 20 kV for 5 s. The electropherogram in Figure 7 shows the separation of equimolar (1 x 10⁻⁵ M) concentrations of dopamine (peak 7), isoproterenol (peak 8), catechol (peak 9), and 3,4-dihydroxyphenylacetic acid (peak 10) obtained with a 5 µm i.d. capillary that was 56.6 cm long. It is clear that cations, neutrals, and anions are readily separated using this technique. Figure 8 shows an electropherogram obtained in a separate study of the linearity of the detection system. An equimolar (5 x 10⁻⁶) mixture of dopamine (peak 11), isoproterenol (peak 12), and catechol (peak 13 ) was injected onto a 5 µm i. d., 5 6. 6 cm long capillary. The peaks correspond to 960 amol dopamine, 870 amol isoproterenol, and 560 amol catechol. No attempt was made to optimize these separations. Standard calibration curves for test solutes were computed. In one set of experiments, detection of catechol was examined for total injection amounts ranging from 1.13 fmol to 0.113 pmol (10⁻⁵ to 10⁻³ M). Linear regression analysis provided values for the slope, and correlation coefficient of 7.7 x 10⁻³ and 0.991, respectively. In another set of experiments, detection of dopamine, isoproterenol, and catechol was examined for average total injection amounts ranging from 80 amol to 4.0 fmol (5.0 x 10⁻⁷ to 2.5 x 10⁻⁵ M). Linear regression analysis provided correlation coefficients of 0.998 for DA, 0.999 for IP, and 0.993 for CAT. The detection limits computed for these compounds are 64 amol for DA, 70 amol for IP, and 56 amol for CAT (S/N=2, peak-to-peak noise evaluated over 10 peak widths). These values were calculated based on the lowest amount injected, which was 5 x 10⁻⁷ M (80 amol). It is to be understood that while the invention has been described above in conjunction with preferred specific embodiments, the description and examples are intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims.

Claims: A capillary electrophoresis apparatus comprising: a separation capillary (100;400) having an outlet end through which an electrophoretically separated sample component exits; a reservoir (130;410) containing a buffer, the outlet end of the separation capillary (100;400) being submerged in the buffer; and a sensing electrode (300;530) that responds to the component as it exits the separation capillary (100;400), characterised in that the electrode is spaced apart from the outlet of the separation capillary (100;400) defining a gap through which the component exits. An apparatus as claimed in claim 1, characterised in that the distance between the sensing electrode (300;530) and the outlet is between 1 and 5 microns so that as the component exits out of the separation capillary (100;400) the component is not substantially diluted by buffer when it contacts the sensing electrode. An apparatus as claimed in claim 1 or 2, characterised by a second capillary (220;500) into which the sensing electrode (300;530) is inserted, said second capillary (220;500) being spaced from the outlet end of the separation capillary (100;400) to define an eluent gap through which the component flows. An apparatus as claimed in claim 1, 2 or 3, characterised by means (520) for establishing an eluent gap through which the component flows. An apparatus as claimed in any one of claims 1 to 4, characterised in that the distance between the sensing electrode (300;530) and the outlet end is between 1 and 5 microns so that as the component flows out of the separation capillary (100;400),the component comes in substantially continuous contact with the sensing electrode (300;530). An apparatus as claimed in any one of claims 1 to 5, characterised by a grounding electrode (170) in the reservoir, wherein the grounding electrode (170) and the sensing electrode (300) form an electrode pair for detecting the resistance of the component. An apparatus as claimed in any one of claims 1 to 5, characterised by a reference electrode (550) in the reservoir (410), wherein the reference electrode (550) and the sensing electrode (530) form an electrode pair for detecting the electrochemical reactions at the sensing electrode (530). An apparatus as claimed in claim 2 or 5, characterised in that the distance is from 1 to 2µm. The apparatus of claim 1, characterised by means (110, 160, 170, 420, 40, 430) for applying an electrical potential along the capillary between first and second points for causing electrophoretic separation, wherein the second point is at the outlet end. The apparatus of claim 9, characterised in that said electrical potential applying means comprises: a second electrode (160,440) electrically coupled to the first point of the capillary; and a third electrode (170,550) electrically coupled to the outlet end. The apparatus of claim 10, characterised in that said third electrode is a reference or grounding electrode (170,550) in the buffer. The apparatus of claim 11, characterised in that said electrical potential applying means includes a power source (110,430) for applying a current between the second and third electrodes. An electrical detection method in capillary electrophoresis, comprising the steps of: positioning the outlet end of a separation capillary (100) into a buffer reservoir (130,140); positioning a sensing electrode (300) in the buffer reservoir (130,410); placing a second electrode (170,550) in the buffer; applying an electrical potential between two points along the separation capillary (100,400), one point being the outlet end, for electrophoretic separation of a sample in the capillary (100,400) into its components, and measuring a change in an electrical parameter between the sensing electrode (300,530) and the second electrode (170,550); characterised in that said sensing electrode positioning step positions the sensing electrode so that it is spaced apart from the outlet of the separation capillary (100,400) defining a gap through which the component exits. The method of claim 13, characterised in that said sensing electrode positioning step positions the sensing electrode so that it is at a distance of 1 to 5 microns from the outlet end.

Inventors: EWING ANDREW G, HUANG XIAOHUA, SLOSS SANDRA E, ZARE RICHARD N, EWING, ANDREW G., HUANG, XIAOHUA, SLOSS, SANDRA E., ZARE, RICHARD N.
Applicants: UNIV LELAND STANFORD JUNIOR, THE BOARD OF TRUSTEES OF THE LELAND STANFORD JUNIOR UNIVERSITY
Application year: 1991
Main classification: G01N27
IPC codes: G01N27