Optimum Flow Rate in GPC/SEC

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 (cm³/min)/Cross-sectional area (cm²) [2]