Temperature effects and diffusion

The variation of the capacity factor k with temperature depends upon the heat of transfer of the solute between the mobile and stationary phase and is expressed in the van t Hoft equation  

Since the heat of transfer of solute between mobile and stationary phase is much smaller in LC than in GC, change in temperature has much less effect on the degree of retention and resolution. Nonetheless, an increase in temperature would reduce the mobile phase viscosity giving an increased rate of mass transfer and increased solute solubility. As a result, capacity factors decrease, peaks become sharper, giving better resolution and faster analysis and lower values of H. An important feature in the control of interdiffusion of solute and solvent and of solute mass transfer is the solvent viscosity and where a choice of solvent is possible then that with the lower viscosity should be chosen, e.g. methanol in preference to ethanol since as indicated above mass transfer is optimum at lower viscosity with increase in resolution and column performance. 

In GC capacity factors, retention and selectivity are controlled by adjusting the column temperature and stationary phase characteristics. In LC change in the composition of the eluant serves both purposes more effectively and thus solvent programming (otherwise known as gradient elution) is used in HPLC where temperature programming would be employed in GC. 

Nonetheless, a change in temperature affects both resolution and retention characteristics and a small but significant number of applications use 

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