Chromatography Forum

Working with 100% cyclohexane and n-hexane (and only then), I have observed (ternary Hitachi L-6200 pump) that the pressure drop was severely fluctuating (e.g. from 0, 10, 12 bar minimum to 80 bar maximum, every few seconds), with the column. When the column was removed, I observed the same pressure variation (naturally, smaller range 0-7 bar) and flow rate variance as well (i.e. when pressure went down to 0 there was no flow while it was at 7 the flow was too fast, almost overflowing).

It seems that there's an obstruction somewhere and you already narrowed it down between the mobile phase bottle and the part that connects to the column.

I suggest to narrow it down further by decoupling more tubings step by step.

There are several possibilities, one thing i had lately that gave the same symptoms was a pump line filter frit where something got stuck.

peterjack wrote:
Working with 100% cyclohexane and n-hexane (and only then), I have observed (ternary Hitachi L-6200 pump) that the pressure drop was severely fluctuating (e.g. from 0, 10, 12 bar minimum to 80 bar maximum, every few seconds), with the column. When the column was removed, I observed the same pressure variation (naturally, smaller range 0-7 bar) and flow rate variance as well (i.e. when pressure went down to 0 there was no flow while it was at 7 the flow was too fast, almost overflowing).

This looks typical of a check valve problem. Happens pretty often with hexane. Was the system used in a polar solvent before? If is is the case make sure to wash with lowering polarity solvent before going to hexane, otherwise you will get check valve problems.