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December 8, 2009

E-Separation Solutions

Volume 0, Issue 0

*How do I find the optimum flow rate for a GPC/SEC separation?*

The following question and answer were taken from an article by Thorsten Hofe and Daniela Held (“Tips & Tricks: GPC/SEC”) that appeared in the June 2009 issue of LCGC Europe’s “The Column.”

**Q:** How do I find the optimum flow rate for a GPC/SEC separation?

**A:** It is a well-known fact that there is an optimum flow rate for the mobile phase at which the best resolution is achieved. The well-known van Deemter equation helps to select the proper flow rate in GPC/SEC. It is used to explain band broadening and relates to the height of a theoretical plate (which can also be seen as 1/plate number, *N*) to the flow. At the lowest point of the van Deemter curve (lowest plate height, *H*), the diffusion behavior of the sample through the column is at its optimum and the band broadening is minimized. The height *H* of the theoretical plate is given by:

*H* = A + B/*n* + C • *n* [1]

Where *n* is the velocity.

Three terms (A, B, and C) contribute to the van Deemter equation: The eddy diffusion (or eddy dispersion), the longitudinal diffusion of the solute while passing through the column, and the (interphase) mass transfer.

While eddy diffusion does not depend on *n*, longitudinal diffusion decreases with increasing *n* and mass transfer increases linearly with *n*.

However, the flow rate at the pump itself is not universal enough for a general description. To compare different column dimensions, the flow rate has to be normalized by the cross-sectional area of the column. Then the linear flow can be discussed. The following equation is used for transferring the linear flow into the flow rate set at the pump:

Linear flow (cm/min) = Volumetric flow rate (cm^{3}/min)/Cross-sectional area (cm^{2}) [2]