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"When I am setting up the HPLC systems in the laboratory where I work, the relevant procedure directs that the new mobile phase is flushed through to the purge valve on the pump for 5 minutes at a flow rate of 5mL/min. Is this enough? How do you work out the most suitable purge for an HPLC system?"
"When setting up an HPLC system, the aim of the purge is simply to flush through all the lines so that any remaining solvent in them from a previous analysis or wash is replaced with the new mobile phase. As you would expect, the amount of new solvent required depends on the volume of the tubing in the system. The major contribution to the volume of tubing is usually the on-line vacuum degasser, if installed. The design of these modules is such that they require a relatively large volume to perform well. For example, a typical Agilent 1200 degasser has a volume of ~12 mL for each line. Agilent recommends a purge of at least 30mL in total for the 1200 systems.
Therefore, to estimate how much volume is required for a purge, check the volume specification of the inlet tubing, the degasser and the pump that you are using, then double it to give an approximate purge volume. You may find that the manufacturer of your particular instrument has a recommended amount to use. A flow rate of 5mL/min is commonly used for standard HPLC systems.
If you have a mixture of different brands of HPLC instruments (and degassers) within one laboratory and want the procedure to cover them all then find out which requires the longest purge and use that as the default."