Chromatography Forum

I know that by changing column dimensions, we have to adjust flow rate to maintain the retention time.

However, this might sound a little silly, but wanted to know if there is any relationship between column length and flow rate too.

example,

If my 1st column is 4.6 x 50mm x 5um with flow rate of 2ml/min and then 2nd column I choose will be - 2.5 x 100mm x 3um --> how will the longer column effect the flow rate that I should use?

Sorry if this has been asked before, but I searched and din't find anything very specific

The column length per se shouldn’t necessitate a flow rate modification in relation to its performance. If you employ a longer column than the initial one, you’ve probably needed higher efficiency and then you’ll have to accept the longer retention time. However, from your description I can see that you’ve changed/reduced the column diameter as well. I haven’t seen a column with a diameter of 2.5 mm (usually they are 2.1 mm) but in this context the exact dimensions are just for illustration purposes – right? OK, so because of the reduced diameter you’ll need to set the flow rate down by a factor of 4 – 5. But because of the reduced particle size you’ll be able to reduce the flow rate by factor of say 2 and still achieve the desired efficiency.
So a concrete suggestion from me would be, to set the flow rate to 1 mL/min and see if there would be needed some minor adjustments according to the obtained results.
All that, if your system can tolerate the resulting pressure. From the described column, I would expect a pressure of about 3000 psi - all depending on the nature of the mobile phase and the column operating temperature.

Best Regards

Hi, Danko is correct as far as isocratic methods are concerned, but for gradient methods there are additional changes to be made.
This is probably a good guide http://www.usp.org/pdf/EN/USPNF/PF35(6)_Stim3.pdf and shows you the maths
Most column suppliers also have guides or web applications to help you make the changes.