- Method development
The relationship between optimal resolution capability (HETP), particle size, and flow rate (linear velocity) is summed up by Van Deemter plots. The lower the curve, the higher the efficiency; the higher the curve, the lower the efficiency.
HETP = Height Equivalent to a Theoretical Plate. This term stems from old distillation setups and separate volatile components; the wider or larger the plate width, the less efficient the separation. The smaller the plate width, the more plates can fit in a given length, and the more efficient the separation.
Different compounds have optimal flow rates, but every separation is a balance of physical limitations (system pressure, system plumbing, loading capacity).
As you go down in particle size, the optimal place, where the HETP is lowest (highest resolution), starts to flatten out for small molecules and remains flatter at higher flow rates. 10 µm cannot run at high flow rates without compromising resolution capability; 5 µm is better than 10 µm.
The following poster depicts the relationships described above in a Van Deemter plot —in Figure 1 in the lower-left of the poster, comparing curves of 10, 5, 3.5, and 1.7 µm:
For full calculations, see the textbook (WAT038216 - HPLC COL THEORY TECH/PRACTICE BOOK) by Dr. Uwe D. Neue.
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