Chromatography Forum

can I trust flow shown in the chemstation software?

Yes, until you have evidence that it is wrong.

Peter

I too have found the flow calculations to be quite reliable.

You can always go "old school" and inject something that's unretained on your column and see if it comes out where it should.

I agree. Whenever I do injection port maint. or install a new column, I go to manul tune. Turn off the cal valve and set the ions to 28, 16, and 44. I hand inject 1 cc of air and using the gc stopwatch I time when the air peak elutes. I average 3 injections and enter this into the adjust column length in the software.

The column flow is a calculation, not an actual reading. GC/Chemstation uses the Poiseuille equation to determine the pressure needed in the inlet to produce the flow rate you set. The GC handles most of the calculation parameters, but part of the calculations is the diameter and length of the flow path. The GC user needs to put in accurate column parameters if they want accurate flow rates. Assuming the correct gas is selected and the correct outlet is selected (vacuum vs atmospheric), the Poiseuille equation can be stripped down to the following.

Flow Rate = (r^4)/L

r= column inner radius
L = length of the colum

If you input a 30 meter column but physically have a 28 meter column, your actual flow rate will be higher than the calculation you see on the GC/Chemstation. Many places are not worried about the slight difference in retention time that will cause, but if you need accurate flows you will need to follow rb6banjo and Bigbear's recommendation of using an unretained peak. In Chemstation where you configure the column you will see the option to calibrate the true length of the column.