14. My Daicel column has a high back-pressure and I can’t work at a reasonable flow rate. Why is this?
The pressure limit specified in the Instruction Manual for a Daicel chiral column applies only to the pressure drop across the column itself and not to the rest of the chromatographic system. For example, if the total pressure drop measurement in your system, at normal operating conditions, is 1000 psi, but the system pressure (without column) is 300 psi, then the pressure drop across the column would be 700 psi, which would be acceptable for most columns. A high system pressure probably indicates that there is a partial blockage, possibly in connecting tubing, an in-line filter, or a valve channel. Whenever possible, this blockage should be systematically located and the problem component replaced.
High operating pressures often result from material blocking the frits of the column. In some cases, this material may be removed through the use of recommended washing procedures. To prevent such problems, it is always wise to use a replaceable in-line filter or guard column before the analytical or semi-preparative column.
Flow rates for semi-preparative columns can generally be scaled up from the flow rate developed on an analytical column, by a factor proportional to the volume comparison of the two columns. When operating at the higher semi-preparative flow rate, you may need to increase the diameter of connecting tubing or the volume of the detector flow cell in your system. Alternatively, you may need to slightly reduce the flow rate on the semi-preparative column to stay under the recommended maximum operating pressure.
High back-pressures may result if a column is eluted at normal flow rates with solvents such as pure ethanol or isopropanol, due to the high viscosity of these solvents. If you are experiencing high back-pressure when using these solvents, reduce the flow rate to a level that brings the back-pressure under the recommended maximum operating pressure. Since these solvents are often associated with column-cleaning procedures, you should be able to operate at higher flow rates once you return to your normal operating mobile phase.
A rapid build-up in pressure with the traditional coated columns can be a symptom of a serious column problem. Introduction of incompatible materials (from sample or mobile phase) can be rapidly destructive to a column. When such a problem occurs, chiral stationary phase will dissolve or be lifted into the mobile phase, only to drop out of solution downstream, plugging the flow path, and causing a rapid build-up of column pressure.