In the Classroom edited by
Applications and Analogies
Ron DeLorenzo Middle Georgia College Cochran, GA 31014
The Escalator—An Analogy for Explaining Electroosmotic Flow Andrew J. Vetter and Garrett J. McGowan* Division of Chemistry, Alfred University, Alfred, NY 14802
In recent years, a number of articles in this Journal have discussed the use of capillary electrophoresis (CE) in undergraduate instrumental analysis laboratories for the analysis of a variety of analytes (1–9). Instrumental analysis texts now include a discussion of CE in the chapters following classical separation methods such as gas and liquid chromatography (10, 11). As CE becomes more routine in the undergraduate laboratory, it will become necessary for professors to incorporate a discussion of CE into their courses. In our Instrumental Analysis course, CH 423, approximately 2–3 days of lecture are now devoted to CE and modified versions of CE, such as micellar electrokinetic electrophoresis. Background The separation of species in a CE system depends primarily on two factors: electrophoretic mobility and electroosmotic flow. Electrophoretic mobility is based on both the size and charge of the given species and electroosmotic flow is the bulk flow of solution through the capillary. Electroosmotic flow results from the partially negative capillary wall (Si–O᎑ groups) attracting a fixed layer of low-density cations. Because there are still available negative charges on the capillary wall, a second more loosely bound layer of cations is also present, the mobile layer. When an electric field is applied, the mobile layer is pulled toward the negatively charged electrode, the cathode, and since these cations are solvated, they pull with them the bulk solution within the capillary, thus creating a bulk flow of solution, known as electroosmotic flow (12). We have developed an effective analogy using people, who represent analytes, on an escalator to illustrate the principle of electroosmotic flow (bulk flow of solution). The Analogy Suppose there are six people (A–F), all with slightly different interests, standing on the bottom step of an escalator. Table 1 describes the people in terms of their interests and gives the corresponding CE analogy. With the people and interests portion of the table exposed to students via overhead projection, allow the students to predict what would happen if the escalator were turned on,
going up, if hardware were located on the second floor and jewelry were located on the first floor. It may help to have a drawing of an escalator with the letters A–F on the bottom step and then draw an arrow showing the escalator motion going up directly above the steps of the escalator drawn on the board. Position an onlooker near to the second floor. Now ask the students “What would be observed from the viewpoint of the onlooker?” At this point, you could show an overhead of Figure 1. Figure 1 shows the “separation” some time, t, after the voltage is applied, with the crude image of an eye representing the position of an onlooker. After the students’ (correct, we hope) predictions, draw an electropherogram of the predicted separation, showing time along the x-axis and intensity along the y-axis as in Figure 2. Figure 2 clearly shows that compound A will be the first to elute and compound F will be the last. Now, apply the CE analogy and allow hardware (upstairs) to represent the negative electrode, jewelry (downstairs) the positive electrode, and the onlooker (eye) to be the detector. As soon as the voltage in the CE system is applied (escalator turned on), there is an overall net flow toward the top due to the motion of the escalator; point out again the arrow drawn to represent the motion of the escalator. Explain that this corresponds to what is happening within the capillary. There is a bulk flow of solution toward the negative electrode (electroosmotic flow). The smallest, most highly charged cations (A) would be accelerated toward the top and effectively look as if they “ran up the escalator”. These would be detected first; the larger, lesser-charged cations (B) appear to walk up the escalator and would be detected next. The two neutral species (C and D), with an interest in neither the cathode nor the anode (hardware or jewelry), remain stationary on the step and simply get carried up the escalator; they would be detected simultaneously (co-elute, the smaller neutral species eluting earlier). The larger, lesser-charged anion, labeled E, which in our analogy would be walking down the escalator (toward the positive electrode, jewelry) is not walking fast enough. The rate of the escalator (bulk flow of solution) overcomes the interest of E in going down; therefore E would be carried upward and is detected next. And finally, F (which is a small, highly charged anion), in an attempt to run down the esca-
Table1. The Escalator–CE Analogy Person Interests
CE Analogy
A
strong interest in hardware, no interest in jewelry
small, highly charged cations
B
mild interest in hardware, no interest in jewelry
larger, lesser-charged cations
C
no interest in hardware or jewelry
neutral species
D
no interest in hardware or jewelry
neutral species
E
mild interest in jewelry, no interest in hardware
larger, lesser-charged anions
F
strong interest in jewelry, no interest in hardware
small, highly charged anions
JChemEd.chem.wisc.edu • Vol. 78 No. 2 February 2001 • Journal of Chemical Education
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In the Classroom
A A
B B
C
C
D
D
E E
F F
Figure 1. An illustration of the escalator after some time, t, into the separation. The eye represents the location of the detector.
B
C D
E
F
F
E
C D
B
A
Intensity
Intensity
A
Figure 3. An illustration of the escalator after some time, t, into the separation while the system is operating in reversed polarity mode.
Time
Time Figure 2. A sample electropherogram with the bulk electroosmotic flow towards the negative electrode.
Figure 4. A sample electropherogram with bulk electroosmotic flow towards the positive electrode.
lator, also cannot overcome the upward movement of the escalator, and is also carried upward by the net electroosmotic flow. F would be detected last. The concept of electroosmotic flow is demonstrated by the fact that even though both E and F have an interest to get to the bottom floor, the overall net flow of all species is toward the top owing to the direction of the escalator’s movement. This is analogous to what is observed in a CE separation, where the escalator represents electroosmotic flow and carries neutral as well as negative species toward a negatively charged electrode. We have found that this is the most difficult part of CE separations for students to understand.
placed near the jewelry counter at the base of the escalator. Again, in this mode, students should be able to correctly predict the resulting electropherogram. Figure 3 represents the separation that would be observed at some time into a “run” in reverse polarity mode and Figure 4 is the corresponding electropherogram in reversed polarity mode. The interest in reversing polarity arises because when analyzing for anions in normal mode, the anions may take a long time to elute. By running the system in reversed polarity mode, anions elute first, thus reducing analysis time. This analogy also enables students to grasp the fact that CE is able to separate cations, neutral species, and anions in the same run as a direct result of electroosmotic flow. We hope that this analogy enhances students’ comprehension and they will be better able to retain the knowledge of the fundamental processes that affect a CE separation.
The Reversed Polarity Analogy This analogy can easily be applied to running the separation in reversed polarity mode as well. When running the system in reversed polarity mode, the overall net flow is toward the positively charged electrode, the anode, and small, highly charged anions are the first to elute (12, pp 33–37). Using the escalator analogy, people start at the top of the escalator, the escalator moves downward, and the onlooker is 210
Acknowledgment We wish to express our deepest gratitude to Amy A. Glossner for providing us with the illustrations that accompany this text.
Journal of Chemical Education • Vol. 78 No. 2 February 2001 • JChemEd.chem.wisc.edu
In the Classroom
Literature Cited 1. Hage, D. S.; Chattopadhyay, A.; Wolfe, C. A. C.; Grundman, J.; Kelter, P. B. J. Chem. Educ. 1998, 75, 1588. 2. Valenzuela, F. A.; Green, T. K.; Dahl, D. B. J. Chem. Educ. 1998, 75, 1590. 3. Janusa, M. A.; Andermann, L. J.; Kliebert, N. M.; Nannie, M. H. J. Chem. Educ. 1998, 75, 1463. 4. Williams, K. R. J. Chem. Educ. 1998, 75, 1079. 5. McDevitt, V. L.; Rodriguez, A.; Williams, K. R. J. Chem. Educ. 1998, 75, 625. 6. Copper, C. L.; Whitaker, K. W J. Chem. Educ. 1998, 75, 347.
7. Thompson, L.; Veening, H.; Strein, T. G. J. Chem. Educ. 1997, 74, 1117. 8. Contradi, S.; Vogt, C.; Rohde, E. J. Chem. Educ. 1997, 74, 1122. 9. Conte, E. D.; Barry, E. F.; Rubinstein, H. J. Chem. Educ. 1996, 73, 1169. 10. Skoog, D. A.; Holler, F. J.; Nieman, T. A. Principles of Instrumental Analysis, 5th ed.; Saunders: New York, 1998. 11. Harris, D. C. Quantitative Chemical Analysis, 5th ed.; Freeman: New York, 1999. 12. Baker, D. R. Capillary Electrophoresis; Wiley: New York, 1995; pp 19–52.
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