The primary advantages of constant pressure pumps are simplicity and freedom from pulsations, resulting in smooth baselines. The most simple of these are usually inexpensive, easy to operate, and easy to maintain. They suffer from several disadvantages, however. Flow rate must be monitored carefully and constantly, especially when performing either qualitative or quantitative analysis. Flow rates can and do change! This can happen if the solvent viscosity changes due either to a temperature or composition change.
Changes in flow rate can influence both qualitative and quantitative analysis. In
qualitative analysis, component identification is primarily based on matching retention volumes. If these change as a result of flow
rate changes, especially during the analysis of complex mixtures, component location and
identification becomes difficult. In quantitative analysis, the most common detectors (UV and refractive
index), are concentration sensitive. Changes in flow rate affect the time that a
chromatographic band passes through the detector cell, which in turn show up as changes in
peak area, which is used for quantitation.