Some modern HPLC columns do not have a truly symmetrical design which may lead to problems if they are reversed.
My copy of "Practical HPLC" (V. K Meyer) suggests the following for regenerating C18, C8, Phenyl and Nitrile columns:
Flow in the range 0.5 - 2 mL/min
75 mL water + 4 x 100 µL injections of DMSO during this cycle;
75 mL methanol;
75 mL Chloroform;
Water
![right arrow :rarrow:](https://www.chemicalforums.com/Smileys/classic/rarrow.png)
0.1 M sulphuric acid
![right arrow :rarrow:](https://www.chemicalforums.com/Smileys/classic/rarrow.png)
water (is another option).