Chromatography Forum

How long should I take to change the flow from 0 to 1 ml/min? I usually take 5 mins but perhaps I can do it faster.
I usually use 250 x 4 or 4.6 mm C-18 columns (5 µm)

Unless you have an exotic column (something like a wide-pore material for proteins), just set the flow to whatever you need and be done with it. The system will not pressurize instantly in any case because the mobile phase leaks out (through the column).

HI

Generally in hplc the minumum time for changing the flow rate will take 20 to 30 min in order to stabilze the column as well as flow rate if u change in the increacing order from 0 to 1 ml the column need to stabilize with the flow which u had given in order to get proper base line.


kiran

kiran, are you talking about changing the flow rate only or changing mobile phases, maybe changing both at the same time?

Then, Tom, I program directly 1 ml/min without problems by Pressure shock in the column's head

And, excuseme, thanks a lot to all.
Guillermo

I've never seen anyone bothering with flow gradient for this particular type of column.

Only for 1.8 um columns I use flow gradient and it is set to have pumped volume of approx. 10 column volumes when it goes from zero to desired flow (formulas: flow gradient [mL/min^2] = flow^2 / (2*CV); time = 2*CV / flow).

I will follow your recomendation, thank you.

Pressure shock in the column's head

"pressure shock" is something of a pious superstition among chromatographers. That would only happen if the *outlet* of the column were blocked *and* liquids were absolutely incompressible, *and* tubing did not expand slightly under pressure *and* the pump could change speed instantaneously.

Let me put this in perspective:

In the early 80's I actually developed a method for fast analysis (1 minute) of a simple mixture using a 3-cm guard cartridge (10-micron packing) at a flow rate of 8 mL/min using a single-piston reciprocating pump running at a bit over 1 stroke/second (that *did* make the pressure change quickly ). The cartridges would die over an 8-hour period.

So why am I not concerned about pressurization on start up? That 8 hours represented almost 30,000 pressure pulses. If you start your system once a day, seven days a week, that's 80 years' worth.

This happens to me. I'm an old pious superstitious chromatographer. I really regret the lot of time wasted because I didn't know the existence of this forum.
I'm learning a lot by reading the different topics.
Thanks a lot for it, Tom