Anal. Chem. 2001, 73, 3497-3501
Design and Performance of a Universal Sheathless Capillary Electrophoresis to Mass Spectrometry Interface Using a Split-Flow Technique Mehdi Moini
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712
A split-flow capillary electrophoresis electrospray ionization mass spectrometry (CE/ESI-MS) interface is introduced, in which the electrical connection to the CE capillary outlet is achieved by diverting part of the CE buffer out of the capillary through an opening near the capillary outlet. The CE buffer exiting the opening contacts a sheath metal tube which acts as the CE outlet/ESI shared electrode. In cases in which the ESI source uses a metal needle, the voltage contact to the CE buffer is achieved by simply inserting the outlet of the CE capillary, which contains an opening, into the existing ESI needle (thereby greatly simplifying the CE to MS interfacing). As a result of the concentration-sensitive nature of ESI, splitting a small percentage of the CE flow has minimal effect on the sensitivity of detection. In addition, because the liquid is flowing through the opening and out of the capillary, there is no dead volume associated with this interface. Moreover, bubble formation due to redox reactions of water at the electrode does not effect CE/ESI-MS performance, because the actual metal/liquid contact occurs outside of the CE capillary. The sensitivity associated with a sheathless CE/MS interface, the ease of fabrication, universality, and lack of any dead volume make this design a superior CE/ESI-MS interface. The performance of this interface is demonstrated by analyses of a peptide standard and a protein digest using a variety of capillary dimensions. Over the past 15 years, a variety of CE/ESI-MS interfaces have been introduced. These interfaces are divided into two general categories: sheath-flow interfaces and sheathless interfaces.1-4 In the sheath-flow configuration, the electrical connection to the CE outlet is achieved using a liquid sheath that either mixes with the CE buffer at the CE outlet through coaxial tubing5 or enters the ESI emitter6 or nanospray tip7 via a liquid junction. Sheathflow configurations with coaxial tubing in which the outlet of the * Phone: 512-471-7344. Fax: 512-471-1420. E-mail:
[email protected]. (1) Severs, J. C.; Smith, R. D. In Electrospray Ionization Mass Spectrometry: Capillary Electrophoresis-Electrospray Ionization Mass Spectrometry; Cole, R. B., Ed.; John Wiley: New York, 1997. (2) Cai, J.; Henion, J. J. Chromatogr. 1995, 703, 667. (3) Krylov, S. N.; Dovichi, N. J. Anal. Chem. 2000, 72, 111R-128R. (4) Ding, J.; Vouros, P. Anal. Chem. 1999, 71, 378A-385A. (5) Smith, R. D.; Barinaga, C. J.; Udseth, H. R. Anal. Chem. 1988, 60, 1948. (6) Lee, E. D.; Muck, W.; Henion, J. D.; Covey, T. R. Biomed. Environ. Mass Spectrom. 1989, 18, 844. 10.1021/ac010189c CCC: $20.00 Published on Web 05/24/2001
© 2001 American Chemical Society
CE capillary is simply inserted into the ESI emitter (a piece of stainless steel tubing, commonly referred to as the ESI needle) have several advantages, including simple fabrication, reliability, and ease of implementation. These advantages make them the most widely used interfaces for routing CE/ESI-MS analysis. The sheath-flow configurations, however, have several disadvantages: (1) low sensitivity of detection due to dilution of the analyte by the sheath liquid, (2) competition between the species present in the sheath liquid and the analyte for available charges in the ESI process,8 and (3) adverse effects on separation, solubility, and molecular conformation, which vary according to the sheath liquid’s composition.9,10 In sheathless interfaces, the electrical connection to the capillary outlet is achieved by direct metal/liquid contact at or near the tip of the CE capillary outlet.11-21 Because there is no sheath liquid to dilute the CE effluent, the major advantage of the sheathless interface is high sensitivity. A variety of sheathless interfaces have been introduced: (1) CE with a metal-coated ESI tip.13,18 The major disadvantage of this design is the short lifetime of the coating due to electrochemical/electrical degradation under CE/ESI-MS. (2) An attached nanospray tip to the CE outlet with a low-dead-volume tee,17 or polysulfone microdialysis tubing.19 Because the capillary inner diameter is usually smaller than the wall thickness, the major disadvantage of this technique is the (7) Figeys, D.; Ducret, A.; Aebersold, R. J. Chromatogr. A 1997, 763, 295306. (8) Gale, D. C.; Smith, R. D. Rapid Commun. Mass Spectrom. 1993, 7, 1017. (9) Foret, F.; Thompson, T. J.; Vouros, P.; Karger, B. L. Anal. Chem. 1994, 66, 4450. (10) Smith, R. D.; Loo, J. A.; Loo, R. R. O.; Busman, M.; Udseth, H. R. Mass Spectrom. Rev. 1991, 10, 359. (11) Olivares, J. A; Nguyen, N. T.; Yongker, C. R.; Smith, R. D. Anal. Chem. 1987, 59, 1230. (12) Wahl, J. H.; Smith, R. D. J. Capillary Electrophor. 1994, 1, 62. (13) Kriger, M. S.; Cook, K. D.; Ramsey, R. S. Anal. Chem. 1995, 67, 385. (14) Fang, L.; Zhang, R.; Williams, E. R.; Zare, R. N. Anal. Chem. 1994, 66, 3696. (15) Cao, P.; Moini, M. J. Am. Soc. Mass Spectrom. 1997, 8, 561-564. (16) Wu, J.-T.; Qian, M. G.; Li, M. X.; Liu, L.; Lubman, D. M. Anal. Chem. 1996, 68, 3388. (17) Tong, W.; Link, A.; Eng, J. K.; Yates, J. R., III. Anal. Chem. 1999, 71, 22702278. (18) Barnidge, D. R.; Nilsson, S.; Markides K. E. Anal. Chem. 1999, 71, 41154118. (19) Figeys, D.; Ducret, A.; Aebersold, R. J. Chromatogr. A 1997, 763, 295306. (20) Severs, J. C.; Harms, A. C.; Smith, R. D. Rapid Commun. Mass Spectrom. 1996, 10, 1175. (21) Herring, C. J.; Qin, J. Rapid Commun. Mass Spectrom. 1999, 13, 1-7.
Analytical Chemistry, Vol. 73, No. 14, July 15, 2001 3497
Figure 1. Schematic representation of the split-flow CE/ESI-MS interface.
existence of significant dead volume where the two capillaries are attached. In addition, there are problems with degradation of the stainless steel (ss) tee, formation of bubbles as a result of the electrochemical reactions at the tee junction under CE/ESI-MS, and detachment of the microdialysis tube from the capillary or the ESI-emitter capillary.20 (3) Insertion of a wire into the CE capillary outlet14,21 or through a small opening near the CE outlet, where it is sealed in place using epoxy.22-25 Wire designs are robust and there is no dead volume associated with them; however, their main disadvantage is the formation of bubbles in the capillary as a result of the electrochemical redox reactions of water. Although remedies for this problem were recently introduced,26,27 a simpler design, which could eliminate this problem and be interfaced to existing ESI sources without any modification, is preferable. In this article, we introduce a simple sheathless CE/MS interface that combines the universality and simplicity of the sheath-flow design with the sensitivity advantage of the sheathless interface. Application of this new design to the analysis of complex biological mixtures is demonstrated. EXPERIMENTAL SECTION Apparatus. The split-flow interface design (Figure 1) is achieved by making a small hole (a few micrometers in diameter) or a crack, using a dentists’ drill (Star-430K, Dental Dynamics; Buffalo, New York) under a microscope, into the wall of the capillary 2-3 cm from the capillary outlet.15 To facilitate observation of the opening in the capillary wall, a solution of methanol was forced through the capillary from the inlet side using an HPLC pump (Perkin-Elmer, Norwalk, Connecticut). The drilling was stopped immediately after observing liquid exiting the opening. A variation of this technique was applied to make a crack in the wall of narrow capillaries (
