Chromatography Forum

Since my 3.2mm id C18 column fouled I am switching to a 4.6mm id C18 column, becoz' it is the only column available to me right now. The flow rate on 3.2mm column was 1.0ml/min. I will be separating the same set of compounds with 4.6mm column. I want to keep the retention time of my analytes same. Please help me in determining the flow rate for 4.6mm id column. Any links or articles towards this will be highly appreciated.

your EXACT rate is 2.066 ml/min but you can use 2 ml/min. This is assuming that you have 3.2 mm column, some manufacturers have slightly smaller diameter 3.08-3.15 mm which they still call 3.2 (depends on a source of SS tubing they are using and ID tolerance from tube manufacturers

How the above answer was reached was by simply working out the cross-sectional area of the two column IDs.
eg 3.2mm ID -> 4.6mm ID = (2.3x2.3xPi)/(1.6x1.6xPi)
Pi can be removed, but I keep it in there just to remind me that the formula uses X-sectional area.

As your flow rate is 1.00, the new flow will be 2.07.

Note that there are several assumptions hiding in there,
the particle size and shape are the same, any wall effects are trivial, etc. etc. The other parameter that is often changed with column diameter is the sample quantity that can be applied to the column, which is covered by a simple formula that also includes column length.

The above simplicity usually works..... Most standard HPLC books and catalogues should have that information.

Good luck

Bruce Hamilton

Hope the column lengths are the same...

If not, you need to change the flow rate to get the same retention time. This may get you into trouble with pressure, if you are going from a 15 cm column to a 25 cm column...

thanks a lot to all you guys for such prompt replies...............I am glad that I am a member of this forum