Chromatography Forum

First of all thanks to all the responses from my first thread. They were a big help.

I have been working with a method to potentially reduce our run time. One idea that I had was to increase the temperature from 30 deg C to 40 deg C. Basically I ran the new method at 40 deg C and presented the results at a meeting this morning. In the meeting I also stated that the increase in temperature decreases the pressure and therefore gives more headroom. One of the consultants was at the meeting and basically asked me how does the pressure decrease if the temperature is increasing. His question was based on the ideal gas law PV=nRT. Obviously this is not an ideal gas and does not follow this law, but unfortunately i could not give him the proper answer as well. I thought maybe it was van der Waals but again was a little unsure. Obviously I have witnessed the drop in pressure, so I know it is an occurrence, but without due explanation. Any help? Thanks

What kind of consultant was this?

Increasing temperature decreases viscosity, therefore reducing pressure.

Hi davidsonchemgrad

I don’t hope this consultant has too much influence on the decisions regarding chromatography in your company.
I can follow his/her logic though. He/she imagines that the system is closed – and then higher temperature equals higher pressure. Unless vacuum of course

Yeah he has really no say at all in chromatography and probably has not had any work with temperature versus pressure problems since he received a degree some 50 years ago. He was just in the meeting and was looking for an explanation. I told him that it would decrease viscosity and therefore decrease pressure, but he was still stuck on the ideal gas law. Obviously this is not going to be true. What would be the mathematical explanation that I could show him? He just likes to understand everything that is going on even if he is not working on it personally. Thanks

Point out that liquid chromatography is run under conditions of constant flow and pressure is proportional to viscosity times flow times some geometrical factors. Gas chromatography is run under constant pressure conditions (usually) where the ideal gas law is a useful guide.

I can't say it better than this:

http://www.despair.com/consulting.html

Davidsonchemgrad,

It seems as though you are training/teaching the consultant.

Perhaps you should consider charging him a consultant fee!

Regards,
Dan