Cyclohexane pressure problems with Hitachi pump

[tooreck]

Hello,

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).

Interestingly - the problem immediately went away when I employed a different solvent (such as Methanol, Acetone, n-Propanol), either through the same line or another line in my ternary Hitachi L-6200 pump. The flow rate was set to 1 mL/min and only cyclohexane or hexane did not demonstrate constant flow rate (measured over 5 minutes) while the other solvents did well. It must be added here that the problem was permanent whether I was degassing them with He sparge (constantly, by the way) or not, even when the bottles were pressurized with He (10 psi).

At first, I thought the check valves were wrong (while cyclohexane was there) and, as a result, I practically, step-by-step, have rebuilt the whole pump: changed both check valves, piston seals and at the end I also changed the pistons to brand new ones. The result was… no improvement whatsoever! Still the fluctuation with these two solvents was very high and it was impossible to develop any gradient run with them (my setup was NPLC gradient from 100% Cyclohexane or Hexane to 100% n-Propanol or Acetone). Then I replaced them (the same solvent line A) with Methanol and … voila! The flow rate and pressure stabilized immediately. Also, when cyclohexane or hexane were mixed (during the gradient runs) with either of two other solvents (by the pump mixer) the problem went away (with higher %B content) and came back again upon column re-equilibration with 100% hexanes again.

Is this about compressibility only?

I will be grateful for any suggestions about practical solution to this problem.

Regards!

Maciej Turowski

[Chris Pohl]

tooreck,

Maybe this is just legend since I couldn't find any reference to it but I've been told that cyclohexane can solidify at room temperature and elevated pressure. It's melting point is ~ 6C but supposedly the melting temperature increases substantially when using pure cyclohexane at elevated pressure. Maybe you should stay away from cyclohexane or at least mix it with another solvent.

[Poskrobko Jan]

Maciej!
Hexane is very volatile, thus problem with saucing by pump plunger is typical. There is the good solution to start gradient from 5% propanol. In my case (HP10500) similar is impossible to work with 100% hexane and very good works if I start from 5% 2-propanol.
Jan

[Poskrobko Jan]

Ones more to explanation: volatility of hexane and cavitation caused by low viscosity of hexane.
Jan

[Alex Buske]

Hi Maciej,

We use normal phase HPLC sometimes. It is always problematic to change from RP to NP mode. NP solvents have high vapor pressure and low viscosity. So the plunger seals seem to "dry out". There are different seals on the market, some are better for NP ( black or gray with our equipment).
It always takes some hours to equilibrate the system to NP solvents. seals have to swell.... Plunger seal washing can be helpfull, but I can't remember if it is possible with the 6200.
Does the pressure fluktuation influence your chromatography? if not I would ignore it.

Alex

[WK]

Maciej,
Have you got a compressibility compensator on the Hitachi?
I had a HP1050 once and the compressibility compensator worked well.
It is difficult to get no ripple with non-polar like hexane though.
I had trouble with it too.
With the HP1050 you could have different seals for NP than those for RP.
(Luckily for me my method changed from NP to RP!!)
WK

[tooreck]

Everybody,
Many thanks for your replies and valuable answers!
My Hitachi is pretty old and does not support compensation of compressibility.
100% CH was not generating reproducible results, however, mixing it up with 5% n-propanol seems to help a bit, at least for initial few hours. After that, the fluctuation comes back - usually found out next morning - but then I purge the system at 10 mL/min for ca. 5 minutes, while the solvent bottles are pressurized under Helium. This arrangement works OK; perhaps it's not perfect, but provides resonably stable pressure, followed by reproducible retention.
I do tend to believe also, that the pump head cavitation (perhaps combined with a high level of air dissolution) may have caused the problem.

Once again, thanks much for your kind attention.

Regards,

Maciej

[Perkh]

I agree with a former post - hexane is difficult because it has a high vapor pressure and because of dissolved air. If the hexane is not free of air, you will cavitate - Sometimes 5 minutes with Helium is not enough. For Hexane or even pentane you might do better stirring and pulling a vacuum - be careful - you will lose some (use a cold trap) - and do not walk away from it and let it go . Most of the air will be removed in the first couple of minutes - then stop. Keeping a headspace of helium above the hexane helps keep it from reabsorbing air. Do you have a eluent bottle that can support 2-3psi helium? The slight pressure will also help it feed your pump.

[zuabedin]

Hexane and Cyclohexane are volatile solvents which prevent the check valve mechanism to work properly. If you sonicate check valve it can give a solution (I think not a permanent solution).