monolith high resolution

[jbland]

All I see the silica monolith columns used for is high-speed separations. My MS will not handle fast, thin peaks. Does anyone have any info on their use at normal speeds to increase resolution? I need to know how much of an increase in resolution is possible.

[Uwe Neue]

Monoliths have a lower backpressure. If this is not important to you (i.e. for slow analyses), you can get into the same spot by using a small particle column, 3 or 3.5 micron.

[Einar Ponten]

The benefit of the monolithic (silica) column is that it has a flat HETP curve, i.e., the effiency is more or less independent of the flow rate as the resolution will be.

To optimise the resolution you need to do the same as with a packed column e.g., have a k' 2-5, modify the selectivity by the mobile phase solvent, pH, ion strength, buffer type etc

[Uwe Neue]

To be clear: a monolith has an HETP curve that is as flat as a 3 micron packing. No more, no less.

[jbland]

I guess I didn't state my case correctly. Is there any info on using a monolith at high flow rates that mimics low flow rates but with high resolution? By this I mean doing a run with increased flow rate but backing off on the solvent strength and to give a retention time similar to a run at a low flow rate. This should give higher resolution compared to the original low flow rate run at higher solvent strength.

[Uwe Neue]

OK.

First of all, with respect to resolution capabilities, there is no essential difference between a monolith and a standard column.

If you are talking about isocratic separations, the monolith has a roughly two-fold, maybe three-fold lower backpressure compared to an HPLC column of equal capability. At the same time, it has a roughly two-fold lower pressure limitation compared to a steel column. To me, this looks like only a mild advantage, maybe even a wash.

For gradient separations, I can't give you an answer without a more detailed analysis, which I have not yet done.

[kiknos]

just do the test and then you can get to know.

if i were you, i may try the easiest compounds with the method to evaluate the column efficiency. try as what you mentioned, and to see whether higher plates can be gotten. if it works, then go ahead to some other more "complicated" compounds/standards.

[MG]

I've not used a monolith, but...

At the same time, it has a roughly two-fold lower pressure limitation compared to a steel column.

...that seems like a big deal. You are saying I couldn't use a monolith above about 175 bar? If so, I'm glad I found that out now before I bought one.

[adam]

Can anyone explain why a monolith column allows lower backpressure. I know that there is a different geometry, vs particles, but something doesn't seem to add up.

If we think about perfusion particles for a minute, my understanding was that it was a good idea in theory, but didn't work very well in practise. And the main reason for this was that only a very small fraction of mobile phase actually went through the particles: most of the mobile phase went between the particles. A simple "path of least resistance" phenomenon.

Well, the channels in monolith columns are basically the same as the intraparticle channels in perfusion particles. So shouldn't the same 'resistance' and hence pressure, be a problem here?

Thanks in advance for you input.

[Einar Ponten]

The Chromolith columns are beautiful! They combine the efficency of a conventional high performance column with a low back-pressure i.e., they can be run at high flow rates ->-> high sample throughput!

The higher molecular weight of the analyte the better relative performance of the monolith due to its mass transfer properties.

The inherent risk of a monolith is that the ratio of flow through the "particles" (Barbapapas) is too low and also that dead ends of some channels creates bandbroadening. In particular, the current polymeric monoliths may show dead ends that are devastating for the plate number in the separation of small molecules. [Object if you disagree]

In conclusion, if you intend to run "old fashioned" standard chromatographic separations of small molecules there is no benefit of using a monolith. If you buy it, run at high flow rate!

[Chris Pohl]

Adam,

The answer to your question as to how monoliths can potentially (this property is certainly not inherent in a monolith but only a capability) have lower backpressure than a conventional column packed with spherical particles is related to simple geometric considerations. A column packed with spherical particles is roughly two thirds filled with the particles while the remaining third is interstitial volume. In a monolith it's possible to create stable media with the ratio reversed (for example). Of course under these conditions, the pressure will be considerably lower.

Einar,

Dead end channels are not a given with monoliths although they are certainly something to worry about. The real challenge with monoliths is something entirely different: reproducibility.

[Uwe Neue]

The monoliths are made first, and then a tubing has to be made around them. The selected technology is to use a PEEK tubing. This limits the pressure to about 150 or 200 bar (initially, it was 150, but I believe that they raised the value later). At higher pressures, the PEEK separates from the monolith, and you get a dead column.

So... the pressure is lower than packed beds, but the useful pressure is also lower. Then what is the point of these things?

Einar: mass transfer properties in the pores are determined by the ratio of the size of the analyte to the size of the pores. The monolith has a standard roughly 10 nm pore size, and with respect to this aspect, it has no advantage over a porous particle. Actually, you are likely to get better results from a 30 nm packing than from a 10 nm monolith for a protein or other large molecualr weight compound.

[Einar Ponten]

Chris & Uwe,
I agree!

[adam]

Uwe and Einar

The problems you are describing do not sound like fundamental problems. They sound more like design/engineering problems.

In other words, it seems to me that it's only a matter of time before these issues are solved. And, at that point, I don't see how standard columns (with spherical particles) will be able to compete.

Adam

[Einar Ponten]

To some extent I agree to that as well. I did spend years on this and we are certainly still very interested in the matter.

For small molecules, the technical problem is to create a monolith that more or less act as a bundle of parallell sub-nano columns. Some day we will see it. Still the major benefit will be the flat HETP curve that enable high flow rate and throughput?

Or low price due to ease of production?