 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: | Diffusion along the length of the chromatographic column leads to peak broadening due to the "spreading out" of the solute zone. Molecules naturally diffuse from regions of high concentration (near the center of a solute zone) to regions of low concentration (expanding the edges of the zone further and further out). The longer the solute zone is in the column the more time there will be for longitudinal diffusion to occur, and hence, the greater the extent of zone broadening. |
| Casting: | A minimum of about 20 and a maximum of about 40 cast members are required for this scene. Cast members should be divided into three equal subgroups, each of which is composed entirely of identical solute molecules with the exception of one member of each subgroup who will represent the mobile phase. |
| Action: | Solute molecules within each subgroup will assemble in a tightly packed (highly concentrated) arrangement, with little space between group members. Action will begin with the first subgroup of solute molecules collectively moving quite rapidly across the front of the room, representing elution of the solute zone through the column. The mobile phase representative will "usher" the subgroup (or zone) of solute molecules along, setting a constant fast pace for elution. As the group travels across the room, they will begin to spread apart from one another. This spreading should be only very slight for the first group, so that by the time they reach the far side of the room they will still be in a fairly tightly packed arrangement. After the first subgroup has "eluted", the second subgroup will likewise begin to travel across the room, beginning in their tightly packed arrangement. However, the collective travel of the second subgroup will be slower than the first subgroup, as established by the slower pace kept by the accompanying mobile phase representative. This slower pace will allow more time for the solute molecules to spread apart from one another, and so by the time they reach the far side of the room, their spreading will be more extensive than that of the first subgroup. Finally, the third subgroup will begin to collectively travel across the room, keeping pace with the slowest moving mobile phase representative yet. Consequently, by the time the third solute zone reaches the far side of the room, they will be more spread out than either of the first two "zones." |
Question to consider before the scene (Act II):
1. Why is it important for the three subgroups to contain the same number of solute molecules (and for the three subgroups to represent the same type of solute) before beginning their travels through the chromatographic column?
Date Posted: 3/13/02 nas
