Plate Theory and Extensions - Temperature Changes During the Passage of a Solute Through a Theoretical Plate in Gas Chromatography > Page 89
Thus, by plotting the logarithm of the plate temperature against (w), a straight line will be produced, the slope of which will give a value for and, thus, will allow the heat loss factor to be evaluated. In practice, (b) ranges from 0 to about 0.2 and, thus, using equation (74) and values for (b) of 0, 0.5, 0.1, 0.15, 0.17, and 0.2, the temperature curves for the (n) th plate of a column having 2500 plates can be calculated. The results obtained are shown in Figure 22. It is seen that the expected S-shaped curve is produced. As the solute dissolves in the stationary phase, and the heat of solution is evolved, the temperature rises. After the peak maximum is reached, the solute desorbs from the plate, the heat of solution is absorbed, and the temperature falls below that of its surroundings. This effect can be simply demonstrated by inserting a thermocouple into a column and monitoring the temperature as the solute band passes. An example of a set of such curves (27) is given in Figure 23. It is seen that the expected temperature profiles are realized. It is also interesting to note that the front of the peak is eluted at a higher temperature than the back of the peak throughout the whole length of the column.
This, as is shown by the theory, is due to the evolution of the heat of absorption, during solute absorption at the front part of the peak. Conversely, the back of the peak is eluted at a lower temperature than the surroundings throughout the length of the column due to the absorption of the heat of solute desorption. As a result, the distribution coefficient of the solute at the front of the peak, and at a higher temperature, will be less than the distribution coefficient at the back of the peak, at the lower temperature. Consequently, the front of the peak will travel more rapidly through the column than the back of the peak, which will result in an asymmetric peak. Such asymmetry is shown by the concentration profiles given in Figure 23.