Comparison Techniques for HPLC Column Performance - - Chromatography Online
Comparison Techniques for HPLC Column Performance

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The practical implementation of this rearrangement is illustrated in the spreadsheet table in Figure 2 and is based on the two following simple equations, translating each data couple in a series of (u 0, H)-data into a new series of t 0-time versus N:

The quantities between brackets are obtained experimentally. The u 0- and H-values vary from row-to-row in the table in Figure 2, while the viscosity value (η), the available column inlet pressure (P) and the column permeability normally remain constant throughout the table. While the column permeability (K v0) is experimentally fixed, the values of ΔP and η can be chosen more freely, but it should be obvious that the most practically relevant system comparison is that where each system is represented by its proper mobile phase viscosity and its proper available maximal pressure drop.

Figure 3
Figure 3 shows the graphical representation of the data rearrangement determined by Equations 1 and 2. As can be noted, each (u 0, H)-data point coming from Figure 3(a) — the same data as in Figure 1(a) is transformed into a unique data (t 0, N)-data point in Figure 3(b). The outcome of the optimal trade-off between permeability and plate height can now be seen at a glance, for each possible plate number required. The support type with the lowest C-term band broadening (red data) can only be expected to provide superior separation speeds for separations having a t 0-time of less than 0.5 min (total analysis time around 5–8 min), while the black data support should be preferred for all applications requiring a higher separation efficiency. Of course, this result assumes that the packing material (support) can be packed as effectively in a column that is shorter or longer than the tested column, but this assumption is also true for any column comparison discussion based on a Van Deemter-curve and a bed permeability.

To really grasp the transformation between the Van Deemter-curve and the kinetic plot in Figure 3(b), it is important to realize that each data point of the kinetic plot relates to a separation performed at the maximal pressure (Equations 1 and 2 are used with a fixed, maximal ΔP). Because each data point of the Van Deemter curve relates to a different velocity, it is, therefore, transformed into a separation performed in a column with a different length. Hence, whereas in a Van Deemter curve the pressure varies from point-to-point and the column length is constant, the opposite is true for a kinetic plot curve: the pressure remains constant, while each data point refers to a different column length. As low velocities allow for the use of long columns, it should be obvious that the data points coming from the B-term dominated region of the Van Deemter curve transform into the long analysis time and high resolution end of the kinetic plot (where long columns are needed), while the data points originating from the C-term dominated region transform into short analysis time and low resolution data points (left hand side of plot). In Figure 3, this transition can easily be visualized by following the position of the numerated points.


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