The Column www.thecolumn.eu.com Tips & Tricks: GPC/SEC height for all column dimensions is at a linear flow of 2 cm/min. 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 (Column) (cm2) [2] Table 1 summarizes the column dimensions, their application fields, the optimum linear flow and the resulting optimum flow-rate. Are There Any Exceptions to the Recommended Flow-rate? The values in Table 1 can be taken as a rule of thumb to choose the best flow-rate for GPC/SEC experiments. However, there are definitely two important exceptions to this rule: (a) High molar mass samples As mentioned before, the plate height will be influenced dramatically by the mass transfer, which is a function of the velocity. Figure 3 shows the influence on the peak height as a function of the velocity (flowrate) for three different molar masses. While the plate height stays almost constant for low molar masses, it decreases strongly with higher flow-rate for high molar mass compounds. Therefore, the flow-rate has to be reduced with increasing molar mass, to get close to the minimum of the van Deemter plot. The separation efficiency for large molecules can be improved by Figure 1: Construction of a Van Deemter plot: black eddy diffusion; red longitudinal diffusion; green mass transfer; blue summation of all three curves, minimum of the Van Deemter plot shows the best flow-rate. Van Deemter plot 200 Figure 2: Plate height as a function of the linear flow for various column dimensions; the minimum indicates the optimum linear flow. 0.034 Column dimension 0.032 0.030 0.028 8 300 mm 0.026 0.024 0.022 0.020 20 50 mm 0.018 0.016 0.014 4.6 250 mm 0.012 0.0 0.5 1.0 1.5 2.0 2.5 Optimum 100 0 Plate height (mm) Plate height 0 2 4 Flow-rate 6 8 10 12 Linear flow (cm/min) 3.0 10 1 9 Contents Tips & Tricks 2 13 Vosloo Rainville and Mather 6 17 News Meeting Review 8 20 Market Trends & Analysis Events & Contacts