 by Christa Colyer Department of Chemistry Wake Forest University Winston-Salem, North Carolina |
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by Christa Colyer Department of Chemistry Wake Forest University Winston-Salem, North Carolina |
| Setting the Scene: | Partitioning of the solute between the mobile and stationary phases may require more or less time (the equilibration time) depending upon the nature of the stationary phase, the partition coefficient, the diffusion coefficient of the solute in the stationary phase, and other factors. From a practical point of view, this means that resistance to mass transfer may differ at various sites in the stationary phase, thus leading to peak broadening. |
| Casting: | A minimum of about 20 and a maximum of about 40 cast members are required for this scene. Six cast members will be solute molecules, and the remaining cast members will be split equally amongst stationary phase particles and mobile phase molecules. |
| Action: | Stationary phase particles must appropriately arrange themselves across the front of the classroom so as to form a chromatographic column. They will (as their name implies!) remain stationary throughout the act, but will periodically interact with solute molecules (by grasping arms or hands), as appropriate. Mobile phase molecules will steadily depart from their group, a few at a time, to travel through the "chromatographic column" of stationary phase particles, thereby achieving a constant "flow" through the column. If a mobile phase molecule happens to encounter a solute molecule on its travels, it may "carry" that solute molecule through the column with it (appropriately shown by linking arms or holding hands rather than physically carrying!) for some distance, until such time as the solute molecule "chooses" to interact again with a stationary phase particle. At this point, the mobile phase molecule will continue to pass through the column, alone now, until it may or may not encounter another solute molecule with which it may or may not interact. Solute molecules will begin as a cohesive group of identical molecules at the head or front of the chromatographic column formed by the stationary phase particles. Each solute molecule will be carried onto the column by "linking up" with a mobile phase molecule. After a short distance, the solute molecule will preferentially interact with a stationary phase particle for some short time before linking up again with another mobile phase molecule. This "partitioning" between mobile and stationary phases will continue until the solute molecules have made their way through the column. In this example, the time spent with any given stationary phase particle may differ from the time spent with any other, and likewise, the time spent with any given mobile phase molecule may differ from that spent with any other. Despite these critical variations, each solute molecule should be mindful of the progress of other solute molecules through the column, and should pace their travel accordingly. |
Questions to consider before the scene (Act II):
1. Why is it important for the solute molecules to be identical (i.e., how would the "separation" be affected if they were different)?
2. What determines the nature of interaction between a solute molecule and a stationary phase particle, or between a solute molecule and the mobile phase?
Date Posted: 3/13/02 nas
