Anal. Chem. 2002, 74, 3931
Response to Comment on “Electrokinetic Stacking Injection of Neutral Analytes under Continuous Conductivity Conditions” The Chien commentary has brought to light an important issue in present day sample stacking methods; which is, the time that one chooses to call “the injection time”. That injection time is at the heart of Chien’s commentary, our paper and, indeed, the root of questions being asked by the scientific community interested in stacking phenomena. The authors chose to define the injection time from the point where the sample front reaches the detector while the sample vial is still interfaced with the column. The separation time is then deemed to start the moment the sample vial is replaced with the run buffer. Thus, as long as the sample vial is connected to the column, sample is being injected into the column. Equation 1 in ref 1 gave the amount of material that can be physically introduced into the column before it passes the detector. At that point, there is still column length remaining to separate the sample from the sample plug. Once separation time starts, stacking is still occurring as the micelle boundary interacts with the sample. If the EOF is greater than the sample complex, there can, in fact, be an injection time in which the sample is pushed past the detector. At that point, the sample complex is never seen to be separated from the sample plug and no peak detection is possible. In eq 3 of ref 1, we had discussed the time for the sample plug to move past the micelle boundary but had not explored it to the level detailed in Chien’s commentary. This brings us to some of the work we have done and are currently investigating concerning the boundary conditions of the various sample stacking methods reported over the last couple of years. If the EOF is greater than the velocity-induced vector, then the vector is a stationary boundary relative to the sample plug. The sample plug then moves toward the detector causing the stationary boundary to change the neutral compounds into complexes with velocity. Those complexes form a new stationary boundary until the sample plug moves past
10.1021/ac020355w CCC: $22.00 Published on Web 07/03/2002
© 2002 American Chemical Society
them. The balancing of the velocity of the EOF relative to the velocity of the vector will allow large volumes of sample to be introduced into a column until the concentration is high enough to change the electric fields in the column. This boundary study applies to several methods described in the literature today. The commentary by Chien questions the “definitions” used or the basis for certain “assumptions” made in our interpretation of the data associated with stacking under continuous conductivity conditions. In addition, Chien views the same problem that we tackled but from a different, yet valid, perspective yielding an alternative interpretation of the data that does not deviate too extensively from that in the original work. This alternative view is not only invaluable but is exactly what is needed in this field, a free exchange of ideas and new interpretations of the sometimes simple data that is generated from complex electrokinetic phenomena occurring in capillaries and microchips. It is interesting that a diligent survey of the CE stacking literature can, in fact, identify similar questions with the “definitions” used and assumptions made with most, if not all, stacking phenomena. However, work from our group,1-4 Terabe’s group,5,6 Thromann,8 and others9-13 is beginning to dissect out some unifying concepts common to all stacking phenomena. These will be of critical importance, since they will not only form the foundation upon which all stacking phenomena can be interrogated and understood but will provide tools through which they can be exploited for realworld sample analysis.
Dean S. Burgi and James P. Landers*
Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, Virginia 22901 AC020355W (1) Munro, N.; Palmer, J. F.; Oda, R. P.; Stalcup, A. M.; Landers, J. P. J. Chromatogr. 1999, 731, 369-381. (2) Palmer, J.; Landers, J. P. Anal. Chem. 2000, 72, 1941-1943. (3) Palmer, J.; Munro, N.; Burgi, D.; Landers J. P. Anal. Chem. 2001, 73, 72531 (4) Palmer, J.; Burgi, D.; Landers, J. P. Anal. Chem. 2002, 74, 632-638. (5) Terabe, S. J. Chromatogr., A 1999, 850, 339-344. (6) Quirino, J. P.; Terabe, S. Anal. Chem. 1999, 71, 1643. (7) Quirino, J. P.; Terabe, S. Science 1998, 282, 465-468. (8) Thormann W. J.; Wey, A. B. J. Chromatogr. A 2001, 924 (1-2), 507-518. (9) Bocek, P. Electrophoresis 2000, 21, 2797-2808. (10) Shihabi, Z. J. Chromatogr., A 1999, 853, 3-9. (11) Weiss, D. J.; Sanders, K.; Lunte, C. E. Electrophoresis 2001, 22, 59-65. (12) Cao, C.-X.; Zhou, S.-L.; He, Y.-Z.; Zhang, X.; Chen, W.-K.; Qian, Y.-T. J. Chromatogr., A 2000, 891, 337-347. (13) Britz-McKibbin, P.; Bebault, G. M.; Chen, D. D. Y. Anal. Chem. 2000, 72, 1729-1735.
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