flow direction on the HPLC column

[jduan]

hi, there, I have one question. Usually, when we get the column, they will have flow direction labelled on that. I once opened one column, I found the filter at the both ends and from the outlook, I could not see any difference.Could anyone tell me why there will be a flow direction? Thanks,

jin

[tom jupille]

Two reasons:

1. To keep users from calling up to ask "which end to I connect to the injector?".

2. Once a column has been in service, "junk" can build up on the head of the column. A flow direction arrow lets you connect the column the same way every time.

[Mark Tracy]

I don't know about anyone else's products, but Dionex Acclaim columns use 0.5 micron frits at the outlet and 2 micron at the inlet.

[tom jupille]

Actually, Mark, that's a very good point. Most, but not all, columns are symmetrical, so always check with the manufacturer first!

[Uwe Neue]

The Waters columns with a 3.5 micorn or smaller particle size are also not symmetrical, having a filter with a larger pore size at the inlet than at the outlet. At least some if not all of the smaller-particle Zorbax columns are built the same way.

[jduan]

The Waters columns with a 3.5 micorn or smaller particle size are also not
^^
symmetrical
^^^^^^^^

Sorry, I am a little bit confused, what do you mean "not symmetrical"? Do you mean that the packing is different at the inlet from the outlet?

thanks,

jin

[Mark Tracy]

What he/we mean in this case is symmetrical with respect to flow direction. The outlet frit has small enough porosity to retain sub-3.5 micron particles, but the inlet frit has a larger porosity that may not retain the packing well.

To my knowledge, no commercial column is deliberately packed with one end different than the other.

[HW Mueller]

Strange new custom? In this lab about 90% of plugging is due to the frit, therefore, (if body fluids are not involved) I use a pre-frit only. Plugging is then almost always alleviated by flushing the pre-frit backwards (without the column). With these unsymmetrical columns I would then expect the main plugging problem at the outlet frit, unless one put a pre-frit, identical or even finer than the outlet frit, on the column (or a corresponding guard). So why use a coarser inlet frit? Now even with a guard or pre-frit the inlet frit on symmetrical columns sometimes plugged. Usually reversing the column fixed this. Not possible with the new unsym. columns. Of course, one does not get repair kits (0.5g stat. phase, mostly) anymore either, nice new world.

[Victor]

HWM- I'm not exactly sure how you are using a "pre-frit"? However, I guess the assumption is that rubbish from the sample, the mobile phase and the pump itself is more likely to block the frits than the particles of the packing itself. I guess this rubbish is what blocks inlet frits, so they are larger porosity. A small outlet frit is necessary to stop bits of packing getting into the detector where they give noise spikes -or worse.

During packing, the inlet of the column must experience higher pressures than the outlet end. Thus I doubt whether the column is symmetrical. I did think once that this might cause instability in the packed bed if the column was used in the wrong direction. However, despite the above, I have often reversed the flow in apparently dead columns and used them for several more months with excellent results. However, this only applies to 5um columns- I assume these probably have frits of the same porosity at either end. I don't know what particle size the Dionex acclaim column is though. Obviously , great care may be necessary if reversing columns, in the light of this information, due to the possibility of damaging the detector with column particles.

[HW Mueller]

Victor, thanks for restating, sort of, my reasons given to justifying my question "Strange new custom?".
On pre-frits: There are all kinds of holders for frits or sieves that can be attached ahead of the column, some are made to have very little volume between the extra frit and the column, similar to some guards.

[jduan]

hi, there, I also checked with one of my friends who has done chromatography work for long. She told me that HPLC column is packed under much higher pressure, much higher than the operation pressure. And it is better to keep the flow in the same direction as it is packed. Using in the reverse direction may change the column stability and reproducibility because it may move the bed.

jin

[HW Mueller]

OK, looks like we have to repeat an earlier discussion again. To old chemists, like me, it was one of the great advantages of modern columns that they could be run backwards (the other being the reduction of secondary retention effects). This way you could easily remove plugging particles from the inlet frit and back out crud from the head end of the bed without passing it through the entire column. Now with the "assymetric" columns one can´t do this anymore.
Most likely the manufacturers still suggest to use a guard or pre-frit (Alltech calls this "column prefilter"), the frit(s), of course, being of the size of the outlet frit of the assym. column (otherwise one would have to use a larger grain stat phase in the guard, etc, etc). So now you have columns which allow more particulates to pass to the stat phase, and to prevent that, you then put a guard on it...... If you don´t, then you got the particulates (or possibly precipitated proteins) on the stat phase, plugging the column there, irreversibly.

[tom jupille]

to jduan: if changing the flow direction can make the bed shift, then the column was poorly packed, period.

to the column manufacturers: if the recommendation is to use an in-line filter anyway, why not make both the inlet and the outlet frits on the column 0.5 micron?

[Cliff Mitchell]

It sounds like the discussion thus far has been about the asymmetry of the column frits. Is there any asymmetry in the density of the packing? I read an article by Dolan (septerber 1 2004 LCGC) that said they found columns performed better if they were ran in the opposite direction that they were packed.

http://www.lcgcmag.com/lcgc/article/art ... ?id=119846

If you follow this link, look at the "What about that arrow" section (although the whole article is pretty good).

Cliff

[ananda]

Hi All,

We are using a TosoHaas, SEC column these days and we had high back pressure indicating cloged frit or column. Manufacuter suggests to back flush and also to remove the inline frit and or if necessary to replace with a new inline frit.
So the bottom line is, if manufacturer suggests that you can revers the flow and can even replace the inline frit, we should be able to do it without damaging the column. If they dont recommend, don;t do it.

Specially this is important in the biological molecules due to the lasrge size,.

Ananda