Anal. Chem. 1996, 68, 3250-3257
Effect of Laminar Flow in Capillary Electrophoresis: Model and Experimental Results on Controlling Analysis Time and Resolution with Inductively Coupled Plasma Mass Spectrometry Detection Jeffery A. Kinzer,†,‡ John W. Olesik,*,† and Susan V. Olesik‡
Laboratory for Plasma Spectrochemistry, Laser Spectroscopy and Mass Spectrometry, Department of Geological Sciences, The Ohio State University, 125 South Oval Mall, Columbus, Ohio 43210, and Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210
A capillary electrophoresis model was modified to include the effect of laminar flow. Experimental results using inorganic ions with a wide range of mobilities were compared to model predictions. The effect of laminar flow rate on analysis time, peak widths, peak asymmetry, and electrophoretic resolution is discussed. The laminar flow rate in the electrophoresis capillary was controlled by regulating the sheath flow rate in a concentric nebulizer interface. Laminar flow in the direction of the detector allows the analysis of positive, neutral, and negative species, all in one electrophoretic run, in less than 2 min. By increasing the sheath electrolyte flow rate, laminar flow in the electrophoresis capillary can be eliminated, resulting in increased resolution at the expense of an increase in analysis time. At higher sheath flow rates, laminar flow is generated in the electrophoresis capillary in the direction away from the detector to further retard the migration of charged species. When concentration overload contributes significantly to band broadening, resolution may be enhanced by inducing a laminar flow away from the detector. Mass spectrometry and atomic spectroscopy detectors for capillary electrophoresis require that the sample be physically transported from the capillary to the ionization (atomization) source, in contrast to on-capillary detectors, such as UV-visible absorbance or fluorescence. A pneumatic nebulizer can be used to generate an aerosol that is then transported into an ionization source or into a source for atomic spectrometry. Alternatively, the nebulizer can assist ionization (as in ion spray MS). The rapidly flowing gas that exits through the annular tip of a pneumatic nebulizer produces a suction that, in turn, tends to induce laminar flow in the electrophoresis capillary. Laminar flow produces a parabolic flow profile that results in band broadening. Therefore, laminar flow is normally considered to be undesirable in capillary electrophoresis. However, positive, neutral, and negative species can be detected from a single injection when there is sufficient laminar flow rate toward the electrophoresis † ‡
Department of Geological Sciences. Department of Chemistry.
3250 Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
capillary exit.1 When an element-selective detector is used, it may be advantageous to trade off electrophoretic resolution to obtain analysis times of 2 min or less by using a laminar flow.1 Also, Culbertson and Jorgenson2 showed that resolution could be increased by generating a laminar flow in a direction away from the detector if the capillary i.d. was small enough. An electrolyte sheath is often used to make an electrical connection to the outlet end of the capillary in interfaces to electrospray mass spectrometry3-8 and ICPMS.9,10 Here, by regulating the sheath flow rate, the back pressure produced in the center, liquid-carrying capillary of the concentric, pneumatic nebulizer was controlled. Therefore, the magnitude and direction of laminar flow (or elimination of laminar flow) in the electrophoresis capillary could be selected by proper adjustment of the sheath flow rate. This allowed easy control of trade-offs between resolution and analysis times. THEORY Reijenga and Kenndler11,12 have described a model to predict both CE migration times and band dispersion dependent on Joule heating, injection volume, analyte concentration, buffer concentration, and diffusion in the absence of laminar flow. The mathematical model does not provide an accurate prediction of the electroosmotic flow velocity, because the effective electrophoretic mobility, µeff, is dependent on the ζ potential, which is not calculated in this model. Therefore, an experimental value of ζ potential is needed as a model input. Modifications were made to the model to include the effect of laminar flow on both analyte migration time and band dispersion. (1) Olesik, J. W.; Kinzer, J. A.; Olesik, S. V. Anal. Chem. 1995, 67, 1-12. (2) Culbertson, C. T.; Jorgenson, J. W. Anal. Chem. 1994, 66, 955-962. (3) Smith, R. D.; Wahl, J. H.; Goodlett, D. R.; Hofstadler, S. A. Anal. Chem. 1993, 65, 574A-584A. (4) Niessen, W. M. A.; Tjaden, U. R.; van der Greef, J. J. Chromatogr. 1993, 636, 3-19. (5) Pleasance, S.; Thibault, P.; Kelly, J. J. Chromatogr. 1992, 591, 325-339. (6) Kostiainen, R.; Franssen, E. J. F.; Bruins, A. P. J. Chromatogr. 1993, 647, 361-365. (7) Smith, R. D.; Barinaga, C. J.; Udseth, H. R. Anal. Chem. 1988, 60, 19481952. (8) Huggins, T. G.; Henion, J. D. Electrophoresis 1993, 14, 531-539. (9) Liu, Y.; Lopez-Avila, V.; Zhu, J. J.; Wiederin, D. R.; Becket, W. F. Anal. Chem. 1995, 67, 2020-2025. (10) Lu, Q.; Bird, S. M.; Barnes, R. M. Anal. Chem. 1995, 67, 2949-2956. (11) Reijenga, J. C.; Kenndler, E. J. Chromatogr. A 1994, 659, 403-415. (12) Reijenga, J. C.; Kenndler, E. J. Chromatogr. A 1994, 659, 417-426. S0003-2700(95)01143-7 CCC: $12.00
© 1996 American Chemical Society
Residence Times. Migration (or residence) time, tm (s), for CE without laminar flow is given by
tm ) Ldfem/Eµeff
(1)
fem ) µeff/(µeff + µeof)
(2)
where
Ld is the capillary length from injection to the detector (m), E is the electric field strength (V m-1), µeff is the effective electrophoretic mobility (m2 V-1 s-1), and µeof is the electroosmotic mobility (m2 V-1 s-1). Equations for calculation of the effective electrophoretic mobility, µeff, and the electroosmotic mobility of the electrolyte solution are discussed in detail elsewhere.11,12 The term fem describes the fraction of analyte ion movement through the capillary that is due to migration in the electric field. The contribution of laminar flow can be included through its velocity in the capillary. The ion migration velocity due to electroosmotic flow and migration is directly dependent on the electric field. Therefore, a modified fem term can be written to describe the fraction of time that ions undergo electrophoretic migration out of the total time ions move through the capillary:
fem )
µeffE µeffE + µeofE + vlam
(3)
where vlam is the velocity of laminar flow (m s-1) in the direction toward the outlet of the capillary. Substituting the modified fem term into eq 1 yields a modified expression of tm that describes the residence time of an analyte in the capillary, including the effect of laminar flow:
tm )
Ld E(µeff + µeof) + vlam
(4)
Band Dispersion. Since the effects of band dispersion due to injection volume, Joule heating, diffusion, and concentration overload are independent of laminar flow, no modifications other than the changes made to the electromigration factor (fem) in eq 3 need to be made to the equations that describe these sources of band dispersion. The contributions of the parabolic, laminar flow profile do, however, need to be considered as an additional source of band dispersion which is not typically present in conventional capillary electrophoresis. The band variance due to the effects of this parabolic, laminar flow profile is given by eq 5:13
σlam2 )
dc2vlam2tm 96D
(5)
where dc is the capillary diameter (m), vlam is the laminar flow velocity (m s-1), tm is the analyte residence time in the capillary (s), and D is the analyte diffusion coefficient (m2 s-1). With this equation, the band variance due to laminar flow can be combined with the variances due to diffusion, concentration overload, injection volume, and Joule heating to yield an overall (13) Golay, M. J. E. Gas Chromatography; Buttersworth: London, 1958.
description of the band variance observed in electrophoresis with the addition of laminar flow. A complete derivation of the band variances due to factors other than laminar flow was presented elsewhere.11,12 From these equations, the theoretical plate height for each process can be assessed individually. Once the theoretical plate height (H) values are calculated, it is then possible to calculate the contribution to full width at half-maximum (fwhm) for each of the processes responsible for band dispersion in capillary electrophoresis:14
fwhm ) 2xHtm2/0.72Ld
(6)
EXPERIMENTAL SECTION Capillary Electrophoresis. Fused silica capillaries (Polymicro Technologies; 50 µm i.d., 300 µm o.d., 40-60 cm long) were used. Either a Hipotronics Model 10B or a Spellman Model CZE 1000R dc power supply was used. Electrical connection of the electrolyte and the high-voltage power supply was maintained with a nickel-chromium ribbon electrode. The electrolyte inlet was held at positive potential, while the capillary outlet was grounded through a sheath liquid. A Fluke 8020B digital multimeter between the capillary and the ground terminal of the high-voltage power supply was used to measure current. Calcium chloride (13 and 2.7 mM) solutions were used (pH of 6.0 and 5.9, respectively; conductivities of 2980 and 616 µS, respectively), with applied voltages of 10 (19 µA) and 30 kV (22 µA), respectively. Experiments were performed at 15 kV with both 13 and 2.7 mM electrolyte concentrations (28 and 16 µA, respectively). Chemical Reagents. All reagents used were analytical grade unless stated otherwise. The electrolyte was prepared from calcium chloride (99.9%, Fisher). Analytes were prepared from solids and included the following: K2Cr2O7 (Mallinckrodt), CoCl2‚ 6H2O (Aldrich), metallic chromium (99%, Fisher), YCl3 (Aldrich), and LiCl (Fisher). The sample solution contained either 1 or 0.1 µg/mL each of K, Co, Cr(III), Cr(VI), Y, and Li in deionized water. The conductivities of the samples were 65 and 6.6 µS, respectively. CE-ICPMS Interface. A schematic of the CE-ICP interface, similar in concept to that described previously,10 is shown in Figure 1. The interface was constructed with a stainless steel tee (Upchurch Scientific Model U428 with Model F230 PEEK sleeve for capillary). The electrophoresis capillary was threaded through the collinear ends of the tee and sealed in place with ferrule fittings. A peristaltic pump (Gilson Minpuls III, with 0.19 mm i.d. (Cole Parmer) tubing) delivered sheath electrolyte (composition identical to that of the electrophoresis electrolyte) through the lower arm of the tee. Calibration of liquid flow rate was linear with pump revolutions per minute across the range used in these experiments (from 20 µL/min at 5 rpm to 110 µL/ min at 30 rpm). Stainless steel LC tubing (Upchurch Scientific, Catalog No. U138, 0.04 in. i.d., 0.06 in. o.d., 5 cm long) was used to connect the stainless steel tee to a Teflon union (Upchurch Scientific Model U402). The concentric glass nebulizer was connected directly to the opposite end of the Teflon union. The high-voltage power supply ground was connected to the stainless steel tubing that carried the electrolyte solution. Nebulizers. Most of the data were acquired using a Meinhard high-efficiency nebulizer (HEN) with an inner capillary dimension (14) Giddings, J. C. Unified Separation Science; Wiley-Interscience: New York, 1991; pp 99-101.
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Figure 1. Diagram of the interface between the electrophoresis capillary and the pneumatic, concentric nebulizer for CE-ICPMS. Insets show position of end of capillary inside nebulizer for HEN and SB-30-A3 nebulizers.
tapering from 100 µm at the inlet to 56 µm at the nebulizer tip over a distance of 45 mm. For comparison, a Meinhard SB-30A3 nebulizer with an inner capillary tapering from 320 µm at the inlet to 200 µm at the tip over a distance of 45 mm was used. The argon nebulizer gas flow rate (0.8 L/min unless noted otherwise) was controlled with a mass flow controller (Porter Model 201USVC). Spray Chambers. An open, conical spray chamber1 was used with the Meinhard HEN because the total primary aerosol volume produced per minute was small (
