Chromatography Forum

Hello,
I know that I need to gradually increase the rate of my column flow until I get to my test method flow rate (i.e. increase rate of 0.2 ml/min until I reach my flow of 2.0 ml/min) to prevent column damage. Our SOP states to gradually decrease the flow back to 0 ml/min at a rate of 0.2 ml/min. I can see why we gradually increase the rate but I don't know why just stopping the flow to returning to 0 ml/min would cause fissures or some other sort of damage to the column. Can someone please explain? Thanks

I don't know why just stopping the flow to returning to 0 ml/min would cause fissures or some other sort of damage to the column

It shouldn't. Actually, even the "ramping up the fow rate gadually" business is based more on pious superstition than actual evidence. It may have made sense 45 years ago when column technology was in its infancy, but is nothing but a time-waste today. Quite aside from the fact that today's columns are quite robust, the system can't pressurize abruptly in any case, because there is a huge leak -- out through the column.

Awesome! Thank you for your help Tom!!

agreed. this is a myth spread worldwide through QC labs. Put the column on and set it to 1.0ml/min
Shut the pump off or enter the flow as zero.
When a system overpressures at its pressure max, the pump shuts off, the column does not explode, perfectly safe

Thank you BostonFSE for your help as well!!

^... unless you're running GPC, then it may pay to go at them a bit more gently.


or am I showing my age?

^... unless you're running GPC, then it may pay to go at them a bit more gently.

No, you're not (at least not from that statement; I can't tell about anything else! ). GPC or GFC columns with large pore volumes are probably the worst-case scenario in terms of physical ruggedness of the packing. Even there, however, I think "ramp the flow" would be overkill.

I just want to follow up this thread by another question. Could the flow ramping be relevant dependent on HPLC-system? I remember using Shimadzu instruments on my last job and those pumps went from 0 to set flow value instantly, causing big pressure peaks. I remember having to ramp the flow in the software to avoid hitting the set pressure limits.
Nowadays I'm using Thermo isntruments and I've noticed these actually ramp the increase in flow internally in-pump.

So, maybe this could be an issue at least when using Shimadzu instruments?Any thoughts?

Ramping the flow on start up certainly can't hurt the column (neither can sprinking holy water on it). "Back in the day" we used to run single-piston reciprocating pumps without all the fancy electronics and simple hydraulic pulse dampeners. In effect, that was like start-up with each and every stroke; the columns did not die in droves.

Now kiddies, go play so grandpa can have his morning nap.

I once talked with a representative of a large column manufacturing company and he said that he recommends ramping up the flow rate gradually when the column is used for the first time however after that, he said there is no problem going at it directly, i.e. at the working flow.

When the column is first used? That is somewhat of a silly suggestion. Don't stress it. When HPLC columns are packed, they are put under packing pressures that are 1.5 to 2.0 times the max used pressure, so no bed distortion should occur "at first use" under normally rated pressures. They are also run in mobile phase for QC, so already "first used". Some columns packed back in the 70/80's could show bed disruption if the pressure changed suddenly and shocked the column, but I was told this was more common with the 10 and 20 u particles used at the time (and poorly packed beds).

My suggestion would be to just flush out the shipping solvent and start equilibrating the column as usual. Flow ramping is fine (and will not hurt anything), to gradually bring things up to pressure (the column buffers the pressure change), but should not present an issue.

The only time I have found it necessary is when changing mobile phases with small particle size columns. If I stored it in 80% methanol and switch to 100% water rapidly at full flow it will over pressure the pumps when the huge change in solvent composition tries to pass through the column. If I ramp it slowly it will have time to wash out the previous mobile phase with the new one and not create an overly high back pressure. But this is a unique situation, if you are just starting up after a shutdown with the same mobile phase then it shouldn't be a problem.

What is your impression for a semi-prep or prep HPLC system? For instance if your working flow rate is 20 ml/min, would you set it straight to this flow or would you ramp up? I am more thinking of the pump integrity than that of the column.

Re: Prep Pumps.
Just set it to the desired flow rate and wait for it to stabilize. No pump can just turn on and run at the initial set flow anyway. They all have to start at zero and ramp up to the requested flow rate (simple physics). If the pump can not handle it because it was not properly primed or the wrong settings were input, then you will know in short order.