Chromatography Forum

Can anyone explain to me the idea behind pressure ramps? And what is a ballpark pressure to run GC at (ex. 105m x 0.25mm) and is there a max when using pressure ramps?


Thanks ahead of time.

Most GCs give you the option of running in constant pressure or constant flow mode. If you run in constant pressure or use pressure ramps, the flow rate in your column will change throughout the analysis to adjust to your desired pressure. Ideally we run a 30 meter with constant flow (1 ml/min) with 15-20 psi. We have also ran samples with pressure ramps but have not noticed a significant difference in either method. The issue of pressure is more beneficial in the inlet, where 15-25 psi can help with VOCs.

As far as flow rate you want to look at a van deemter plot: http://faculty.sdmiramar.edu/fgarces/La ... terEqn.gif

The optimal flow rate depends on what carrier gas, and also whether you are using a mass spec (which due to pumping capacity has restrictions on higher flow rates depending on what kind of pump you have.) Another consideration is your resolution: if you have good resolution you can generally increase flow rate in order to decrease the total run time.

I've only run into one person using pressure ramps and they were kind of a nut, they also had their GC set to kPa for pressure units .

Problem solved, thanks guys!