Why Should Dispersion be Measured?

Why Should Dispersion be Measured?

As previously described, dispersion impacts on the efficiency of a separation, where it can lead to analyte band spreading. The expected % efficiency yield for different column formats with 1.7 μm packing material were compared over a range of dispersion values. The 10 and 17 μL dispersion calculated in Figure 1 were marked on the plots to illustrate the impact on the efficiency yield.

For the different column lengths with constant ID, the 17 μL dispersion saw all three columns below 90% of the expected efficiency for that column. At 10 μL, the 100 and 150 mm column lengths achieved at least 90%, whilst the 50 mm achieved just over 80% of the expected efficiency which may still be deemed acceptable for applications.

When the column length was kept constant and the ID changed, it was noted that the 50 x 4.6 mm was the only column to achieve greater than 90%, whilst the 50 x 3.0 mm achieved greater than 85% with dispersion of 17 μL and the 2.1 mm ID achieved approximately 60% of the expected efficiency. Both the 50 x 3.0 and 50 x 4.6 mm columns were greater than 90% with the lower dispersion system, whilst the 50 x 2.1 mm possessed greater than 80% of the expected efficiency. Both of these examples illustrate the need to match the column parameters to the instrument capabilities.

Figure 1 Operating conditions for system dispersion and example chromatograms of two different systems.

Figure 2 Theoretical comparison of different column formats packed with 1.7 μm material on the effect of dispersion versus % efficiency yield. The expected efficiency yield for bandwidths of 10 and 17 μL (black and pink trace, respectively) were added to illustrate the necessity for the right instrument for the right column.