compressibility

Discussions about HPLC, CE, TLC, SFC, and other "liquid phase" separation techniques.

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Hello,

theoretical quastion: A pump is adjusted to the compressibility of water. If I pump a liquid with a higher compressibility with the same settings as water, I will have a reduced flow (compared to water) in an HPLC column because of the elevated pressure.

But if I collect the solvent after the column in a volumetric cylinder I will always get the same volume, no matter what solvent I use, if I always use the same settings. After the column the liquid will expand again because the volumetric cylinder is at atmospheric pressure (like the reservoir).

Is this correct?
I don't think you will get a reduced flow, I think what you would expect is more baseline noise. The pump's pulsations tend to show up in the detector and compensating for the compressibility of a given eluent allows "tuning" of the pump's cycle to help minimize or cancel out those pulsations (I'm not 100% certain on this, but I think that is correct).

It is usually a pretty minor adjustment (ie: one that is seldom altered by most of us).

That said, if you have a very low LOQ or LOD requirement for a peak being eluted using a gradient method, it might pay dividends to set the compressibility to whatever it should be set to given the mobile phase composition at the moment that particular peak elutes from the column.
Thanks,
DR
Image
nice question raised:

from my thoughts, I would agree with the second statement about the similar volumes after the column (if one neglect temperature increase effects by frictional heat within the tubing/column...).

I guess the (simplest) compensation is done in that way as to give the set flow rate at the columns head under the given pressure.
Therefore the stroke frequency will be increased slightly, and with this a higher volume will be delivered, independent of the fluid (stroke volume*frequence)
So I would expect to get the same volumes in my cylinder.

But I'm not that sure about the real flow rate within the column (or at the column head). Probably usually reduced but under some circumstances maybe even an increased flow rate could be possible.

As the compressibility is a function of pressure, and this backpressure depends on the viscosity of the fluid, maybe the actuall compression of the fluid under the particular conditions will be less than that of water and so the pump will deliver a higher flow rate according to the increased pump frequency.

But that's only my theory.

Example (disregarding temperature effect):
Liquid 1: max compressibility is 2%, at 400 bar backpressure (viscosity 1),
therefore the pump delivers 102%, e.g 1020 µL/min as to give 1000 µL/min at column head.

Liquid 2: max compressibilty would be 3% at 400 bar, but its viscosity is only 0.5
With the compressibility settings for liquid1, the pump still delivers 1020 µL/min, but the backpressure would only be about 200bar.
Assuming the actual compression at 200bar is only 1%, then the real flow rate would be 1010 µL/min vs. 1000 µL with liquid 1.
After the column, both liquids will expand to 1020 µL/min.
Let´s assume that a full stroke of the pump is 1mL. During the sucking phase always 1 mL of liquid will be filled into the pump head.

With 0% compressibility I will have a flow rate of 1mL/min on the column and afterwards find 1mL liquid in the volumetric cylinder.

With 1% compressibility the pump will suck 1 mL and compress it to 0.99 mL before the check valve opens. I will then have a flow of 0.99 mL/min on the column and afterwards I´ll find 1mL liquid in the glass.

:?:
I would say: yes
Assuming a 100% efficient pump.

In reality the remaining solvent in the piston head decompresses prior to the inlet check valve opening again and you see this as a repeatable difference in flow.
Hello,
The stroke will be corrected (+additional volume) base on elasticity calibration data and compressibility settings.
Regards,
pksik
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