by
lmh » Tue Jul 02, 2019 11:17 am
... it's a tool in the toolbox. Whether it's the right tool depends on the column and the nature of the contamination. With a modern C18 column, probably the bulk of what Phenomenex sell, the packing is so tight and reliable that reverse flushing won't cause any disturbance, and is perfectly safe. Phenomenex are column manufacturers - they're not going to recommend any procedure that causes their columns to perform worse. But on an old-fashioned irregular particle column, the column bed is less stable, and back-flushing might be a bad idea.
Similarly the nature of the contamination matters: if you are working on complex biological extracts then it is quite possible there will be components that are extremely strongly retained - to the extent that they take hours to move a few mm onto the column even in 100% strong solvent, and would take many days to get all the way through. In this situation, washing forwards with a strong solvent won't help (unless the wash keeps going for days...) - in fact if the dirt is stuck on the guard column and someone washes forwards without first changing the guard, the wash may merely move the contamination from the guard to the analytical column. Reverse flushing helps with very strongly-retained, but chromatographically well-behaved contaminants, because it reduces the distance the dirt has to travel, which means they come out quicker than in an isocratic wash.
Of course, ideally, samples containing that sort of dirt could be cleaned up by a quick SPE before analysis, but since the SPE will probably cost vastly more, per sample than a guard column, and will take extra sample-prep time, it's often more cost-effective to accept the contamination.