GENERAL LAB

between peak broadening at low flow rates from diffusion and

at high flow rates from increased mass transfer effects between

mobile and stationary phases. It also predicts that, for a particular

chromatographic system, an optimal flow rate yields

the best efficiency. Empirically, the effects of flow rate on

efficiency are tested by measuring peak widths at different

flow rates. In practice, there usually is no interest in defining

the complete van Deemter curve, but rather in identifying the

highest flow

 rate that does not result in excessive peak broadening.

The usual goal is to identify conditions for the most

rapid analysis time that still yields adequate separations.

Factors that affect chromatographic efficiency are listed

in Box 13-1. In practice, efficiency is improved by using

(1) smaller particles, (2) nonporous particles, or (3) a coated

capillary surface as the stationary phase to minimize mass

transfer effects (the C constant in the van

 Deemter

 equation).

This allows the use of higher mobile phase flow rates without

peak broadening,

 resulting in faster separations. 

The limiting

factor in reducing particle size is the increased resistance to

flow and the need to operate at higher 

pressures.