Plate Theory and Extensions - Temperature Changes During the Passage of a Solute Through a Theoretical Plate in Gas Chromatography > Page 79
When a solute passes through a theoretical plate in a gas/liquid chromatographic column, the solute is first adsorbed and then desorbed from the stationary phase. This adsorption/desorption process is accompanied by the evolution and adsorption of the heat of solution of the solute. Furthermore, this heat exchange continually occurs as it enters and leaves the stationary phase during its progress in and out of the plate. This reversible heat exchange, between solute and solvent and the plate surroundings, causes the temperature of the contents of the plate to rise and fall. Ray (22) suggested that this temperature effect might form the basis of a GC detection system and Klinkenberg (23) thought it might contribute to peak asymmetry. The effect was used by Claxton (24) and Grosek (25) as a detecting system in liquid/solid chromatography and Smith (26) demonstrated that, under certain circumstances, the temperature changes could adversely affect a separation in preparative chromatography.
To examine the thermal changes that take place in a column, an equation that describes the temperature change in a theoretical plate, in terms of the physical properties of the plate and the volume flow of mobile phase that passes through it must be derived.
Consider the (n)th theoretical plate in a GC column, as depicted in Figure 21. The properties of the plate are defined as follows,
|vg||is the volume of gas in the plate,|
|vl||is the volume of liquid (stationary phase) in the plate,|
|vS||is the volume of support in the plate,|
|Sl||is the specific heat of the stationary phase,|
|SS||is the specific heat of the support,|
|rl||is the density of the stationary phase,|
|rS||is the density of the support,|
|Xl(n)||is the concentration of solute in the stationary phase in plate (n),|
|Xg(n)||is the concentration of solute in the mobile phase (gas) in plate (n),|
|q||is the excess temperature of the plate above its surroundings,|
|Q||is the mobile phase (gas) flow rate in ml/sec.|