Relation between column length and flow rate

[doc21]

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

[danko]

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

[Bluesman75]

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.