column ovens, column size, and enthalpy of mixing

[DJ]

Are air-heated ovens as useful for controlling temperature of large columns run at 20 mL/min as they are for smaller columns operated at 50 microliters/min (when the mobile phase is at ambient temperature)?

For a 4.6 mm ID column heated in an oven, at what (elevated) oven temperatures should one start to consider externally heating the mobile phase to match column oven temp (to minimize viscosity and temperature differences between the mobile phase and the top of the column?)

Lastly, when is enthalpy of mixing (MeCN + water) a concern in the context of being able to control column temperature? Does heat of mixing become a non-issue when B is pre-mixed (85/15 MeCN/H20, for instance) and used in a linear AB grad where A = water?

[JGK]

I can't answer your question about large columns but, for the 4.6 mm columns, I once had to run a method at 65C.

Rather than heating the mobile phase we ran a length of tubing (1-2 m long) placed between the pump and injector throught the column oven to preheat the mobile phase before it reached the column.

[Uwe Neue]

Let me start at the end: heat of mixing is irrelevant, since you loose this easily in either the pump (with low pressure mixing) or in the injector and the tubing (with high-pressure mixing).

You ALWAYS want to preheat the mobile phase before it enters the column. All column thermostats that I know of have at least a loop for accomplishing this, some have an active heater with associated measurement of the temperature.

You do not really need a bath for the column itself. You could work with solvent preheating and an insulated column. However, a controlled thermal environment around the column will speed up the thermal equilibration of the column.