Chemistry for Everyone edited by
Applications and Analogies
Ron DeLorenzo Middle Georgia College Cochran, GA 31014
A Chromatographic Parable Jon F. Parcher Department of Chemistry, University of Mississippi, University, MS 38677;
[email protected] In thirty years of teaching separations courses, I have often searched for an apt allegory to illustrate the fundamentals of chromatographic processes. The following is one version of such a tale that students seem to find interesting and perhaps even informative. In a small Southern town (it must be a Southern town or the story doesn’t work), the people are planning a Fourth of July race from one end of town to the other. The townsfolk have the commonly observed characteristics that most of them are either Saints or Sinners; however, some of the folks are neither Saints nor Sinners (The Agnostic-Teetotalers) and others are both Saints and Sinners (we’ll call this group the Hypocrites). The race will be conducted along the main street of town, and, as in most Southern towns, the street is lined with a suitable collection of churches and bars. During the race the town folks all run at the same speed, but the Saints cannot pass a church without entering to pray for a while, and the Sinners cannot possibly pass by a bar without pausing for a refreshing beer. The immediate question then is who will win the 4th of July race? Most people want the Saints to win the race, but this is not probable because, while they are in church, the Agnostic-Teetotalers are still running. It is fairly obvious, even to college students, that the Agnostic-Teetotalers will win the race, and, quite deservedly, the Hypocrites will come in last. But what about the Saints and Sinners? Who will come in second or third? And finally, what can be done by the City Fathers to alter the outcome of the race next year? So, what will determine the results of the Saints–Sinners
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race? Let’s say there are ten churches, but only three bars, along the main street. Under these conditions, the Sinners will win the race. Right? Watch out! What if it takes longer to drink a beer than it does to say a prayer? The point of the exercise is to illustrate the concept that the results of this particular race are determined by the amount of time the participants spend not racing, that is, drinking or praying as the case may be. The analogy to chromatographic retention times is obvious if somewhat colloquial. Unfortunately, the analogy between the chromatographic stationary phase and a church or bar is perhaps less exemplary. A secondary effect is possible if not all the racers run at exactly the same speed, if some Saints pray longer than others, or if some Sinners have more than one beer. In this case, not all the Sinners will reach the finish line at the same time. It is even possible that some very fast Saints could reach the finish line (elute) before some of the more tipsy Sinners or vice versa. Thus, there would be a distribution of individuals within a group of townsfolk and possible overlap of Saints and Sinners at the finish line. In chromatographic terms, the distribution is known as dispersion (described by the universally dreaded van Deemter equation) and overlap results in poor resolution. Both effects lead to diminished results for a chromatographic separation. In the 4th of July race analogy, it is possible that all the townsfolk (Saints, Sinners, Agnostics, and Hypocrites alike) would finish the race at the same time. In my experience, this is the most probable outcome for most Southern towns, as well as most chromatographic experiments.
Journal of Chemical Education • Vol. 77 No. 2 February 2000 • JChemEd.chem.wisc.edu
