Limit to low pressure?

[dkreller]

Hello,

Preamble: I am starting out on an HPLC project in which I am packing my own columns. At this time, I am packing columns with 300 micron quartz sand. As you would expect, the back pressures are very low, ranging from 0-60 psi. I have done some initial experiments in which I am just making injections of 10 mM NaNO3 (as a conservative tracer) over a few identically prepared columns to test for reproducibility in packing. So far, despite the very low backpressures, the column dead times seem to be fairly reproducible.

My question: Is there a limit to how low backpressures can be for good chromatographic work? Is there danger to the pump?

Thanks,

David

[JGK]

It probably depends on the HPLC system you are using.

In the past I have found that back pressures of < 500 psi may result in an unstable flow from the pump as many pumps rely, to an extent on the presence of a level of back pressure to stabilise the flow.

[dkreller]

It is a Waters Delta 600. I think I may have to switch, for future experimental work on the HPLC to a finer grain sand. In the meantime, perhaps I will find some kind of inert filter or guard column to put after the sand-packed column before the detectors.

[JGK]

You could fit a back pressure regulator to the system between the pump and the column.

back pressure regulators set to fixed pressures are available from a number of suppliers

[dkreller]

JGK,

Thank you for the suggestion. I will look in to back pressure regulators. Do you see any reason to prefer placing a regulator before or after the chromatographic column?

Thank you,

DK

ps. I lived in Mississagua 10 years ago. It was growing like gangbusters then, I'm sure I'd barely recognize the place now...

[Hfranz]

JGK: You could fit a back pressure regulator to the system between the pump and the column.

Even more exact, you should fit the regulator between the pump and the sampler or manual injector. If the sample passes through a backpressure regulator, it might lead to band broadening or asymmetric peaks due to additional extra column volume.
Another option is to install the regulator in the waste line of the detector. Of course, this will increase the pressure on all system components such as the flow cell of the detector. If you consider installing the regulator in this position, check the pressure rating of the cell first and then order a suitable regulator that matches this spec.

[dkreller]

Hi,

I haven't any experience with back pressure regulators, and I have seen on the Fisher website that there are several available. My fjuorescence detector has a pressure limit of 145 psi, so I must stay within that if I am going to put it post-detectors. Do you have familiarity with and recommendation of any particular brand names or types of regulators?

Thanks,

David

[HW Mueller]

We have used Upchurch. One has given up the ghost (function). Luckily it happened on the UV detector whose cell can withstand lots of pressure. If it happened on our fluorescence spectrometer it would have cost us a few 1000 $. Especially when using such restrictors at detector outlets I now use a pressure releave valve (also from Upchurch) at the detectors entrance.

[Hfranz]

I did use the Upchurch regulators quite a lot in combination with post-column derivatization. Quite often these applications required flourescence detectors and as far as I can remember, it was trouble free in all the installations I dealt with. However, it is more likely that you built up pressure behind the flow cell when using a back pressure regulator. Therefore, a restrictor between pump and sampler might be the saver option.
This links to Upchurch: http://upchurch.com/Products/specsheet. ... 00&vFrom=L

Cheers,

Holger

[Arodrigues1975]

Hi all.

Why don't you use a peek backpressure tube?