HPLC System Pressure

[Faith]

Hi All,
Recently I purchased a semi-prep flow cell for the DAD, the standard flow cell is 10 mm - 120 bar pressure.
I purchased a 0.3 mm - 20 bar pressure flow cell.

I have been given conflicting advice. Some people argue the 20 bar is the max pressure for the whole system, and that I would have to change the ID of my tubing throughout the system to decrease the back pressure.
Others suggest the HPLC system is an open system, and so if I determine the pressure without the flow cell in series, and then reconnect it, I can determine the pressure across/in the flow cell which should be less than 20 bar.
Is there a way of monitoring this? Should I install an in-line pressure gauge on the other side of the detector?
OR, can I tell by the increase of the pressure registering on the system (the sensor is usually at the pumps). The problem here is: If I am loading a large amount of sample, and have a gradient of three different solvents, then the pressure is going to vary according to the composition of the gradient, and, as the sample passes through the column.
If I have the pressure monitor detection on, and see a line representing the pressure this may help except I may have different samples, and mobile phases etc. and therefore do not know what it should look like.

What I am asking is: Once I have determined the pressure difference with the flow cell in line (i.e. the pressure difference across the flow cell) and that it is less than 20 bar , do I just assume that since the tubing throughout the system is:0.17 mm ID, but the tubing at the flow cell inlet is:0.5 mm, and at the outlet is:0.8 mm, that the pressure should be OK in the flow cell?
What if, there is a blockage at the fraction collector overnight, could this cause back pressure in the flow cell and cause it to crack?
Thank you

[Bruce Hamilton]

Simply put, the 20 bar is the maxium pressure that the cell can see.

As long as the outlet is not blocked, you can put up 20 bar into the cell, and that back pressure will be determined by the flow and viscosity of the mobile phase. If cell outlet lines are larger diameter, and not too long, the pressure will be just caused by the cell..

The backpresure seen at the pump is caused by the injector / column / sample / solvent variations, and will be measured by the pump pressure gauge. It would not be smart to rely on that guage to measure the much, much lower detector cell back pressure.

The large volume of sample will result in an increase in pressure in the detector only if it dramatically increases the mobile phase viscosity. The pressure increase at the pump as the samples passes through the column will not affect the detector pressure if the flow is constant.

So, the simplest way to see if you have a problem is to connect the pump directly to the detector and ramp up the flow of typical viscous solvents to the maxium you will ever use. You can try it with and without the fraction collector working, just to ensure the collector doesn't send pressure pulses back into the cell - most don't.

If the fraction collector fully blocks, or partially blocks during switching, your cell could fracture. If that is a concern, you can put a tee with a small pressure relief device ( tubing popping off is cheap, relief valves slightly more expensive ) in the line between the detector and collector.

Another option may be to use the detector with a tee and restrictor, and have a small bypass flow going to the detector, and the rest of the flow straight to the fraction collector.

Please keep having fun,

Bruce Hamilton

[Faith]

Hi Bruce,
Thankyou for the re-assurance. Just after I posted the question earlier, I went to the system, and determined the max. pressure across the flow cell I would obtain, with the highest flow-rate and the most viscous solvent I envisage using.

You can put a tee with a small pressure relief device ( tubing popping off is cheap, relief valves slightly more expensive ) in the line between the detector and collector.

You brought back memories. I remember obtaining one of these pressure relief valves years ago. What is the other alternative? Tubing that can only take a certain amount of pressure? It would have to be less that 20 bar though if I am understaniding it correctly. Or is it some stopper on the other end of the tee joint?

Another option may be to use the detector with a tee and restrictor, and have a small bypass flow going to the detector, and the rest of the flow straight to the fraction collector.

This would have been great before I spent $4000 on the smaller flow cell. I do not know why I did not think of that, because a while back, I had a DAD in parallell with an ELSD and I had a tee joint with smaller ID tubing going to the ELSD, and it behaved as a restrictor (it caused more back pressure, so a larger flow went to the DAD which is what I wanted). I must be getting old and forgetful!
Thanks Bruce

[Bruce Hamilton]

Hi Bruce,

You can put a tee with a small pressure relief device ( tubing popping off is cheap, relief valves slightly more expensive ) in the line between the detector and collector.

You brought back memories. I remember obtaining one of these pressure relief valves years ago. What is the other alternative? Tubing that can only take a certain amount of pressure? It would have to be less that 20 bar though if I am understaniding it correctly. Or is it some stopper on the other end of the tee joint?

Stoppers, other than PTFE.glass, or metal, are bad news.
They swell and become the last part of a system to fail, except when failure results in valuable product splattered all over the floor, in which case they fail prematurely.

What I have used is a plugged 1/8" ID PTFE tube slid over 1/8" OD stainless tube. Would fail at about 7 bar, but make sure the initial surge goes into a bottle, as the stopper pops off.

Upchurch make/sell a diverse range of microvalves that can be used to split flows, if you don't want to use lengths of narrow bore tubing,

Please keep having fun,

Bruce Hamilton

[Faith]

Bruce wrote

What I have used is a plugged 1/8" ID PTFE tube slid over 1/8" OD stainless tube. Would fail at about 7 bar, but make sure the initial surge goes into a bottle, as the stopper pops off.

This sounds interesting, I have not heard of this before. I love this forum!
However with this plugged tube method, would you get some carry over if you slowed the flow rate down, or stopped it, and therefore some of the liquid that is trapped in this tube may run backwards as the pressure has dropped and then somehow end up on the other side of the tee joint. If it did I, imagine it would be insignificant!

I have to correct something,
Faith wrote

I had a DAD in parallel with an ELSD and I had a tee joint with smaller ID tubing going to the ELSD, and it behaved as a restrictor (it caused more back pressure, so a larger flow went to the DAD which is what I wanted). I must be getting old and forgetful!

The ELSD was in series with the DAD not parallel, it was after the DAD, and I installed the tee joint after the DAD but before the ELSD, with one side going to the ELSD (small ID tubing) and the other going to waste (larger ID tubing). This was for two purposes, firstly if the ELSD blocked , it could divert out to the waste (as there was less pressure). And secondly, I wanted a lower flow rate going to the ELSD.

[Bruce Hamilton]

Bruce wrote

What I have used is a plugged 1/8" ID PTFE tube slid over 1/8" OD stainless tube. Would fail at about 7 bar, but make sure the initial surge goes into a bottle, as the stopper pops off.

However with this plugged tube method, would you get some carry over if you slowed the flow rate down, or stopped it, and therefore some of the liquid that is trapped in this tube may run backwards as the pressure has dropped and then somehow end up on the other side of the tee joint. If it did I, imagine it would be insignificant!

I only used it as a temporary expedient on prep systems, with flows 30 -40 ml/min, because I was manually switching the fraction valve between bottles and didn't want to pop an expensive RI detector.

I'd recommend a proper check valve as the better choice, but cheap disposable plastic spring-loaded check or priming valves can be found on some squirt dispensers, and I've used one of those for a reverse phase system.

Please keep having fun,

Bruce Hamilton