Flow Velocity and Column Backpressure

It is known that increasing the linear flow velocity of the mobile phase will lead to faster separations. But since H is dependent on u, what velocity is needed to maximize the resolution per unit time (minimize /// ) Using the van Deemter equation, Hlu may be expressed as 

From this equation, Hlu approaches its minimum value of C as m becomes large. In other words, the separation should be performed at the fastest velocity possible. (Note also that this represents mathematically what was presented in the previous section that is, in the case of optimizing for speed, the separation is dominated by the C-term.) This doesn t mean that the resolution itself will improve—on the contrary, since H generally increases with velocity when u Mopt, resolution will worsen—but that the resolution per unit time is improving. Again, since the quality of the separation must not be sacrificed, the speed of analysis can be improved only to the point where resolution can no longer be sacrificed. 

Of course, the ability to increase u depends on the pressure capabilities of the instrument, since pressure is directly proportional to velocity  

Decreasing the column length lowers the pressure requirement proportionally, allowing use of the available pressure to gain an advantage in speed. Column efficiency, however, drops with use of a shorter column and at faster velocities. Care must therefore be taken to ensure that resolution between peaks is not lost when decreasing analysis time in this manner. 

Lowering the viscosity of the mobile phase is another way to lessen the required pressure. This may be accomplished by raising the column tempera- 

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