I would like to ask for input on the question of open-tubular HPLC, as it pertains to some research I am considering getting into. As I understand it there are two main problems with the technique. The first is that the diffusivity of liquids is low and, therefore, OT HPLC is very slow. The second is that there is a problem with the capacity of the column as it is very easily overloaded. The first issue (low diffusivity) I fully understand. But the second issue (that of capacity) I have some trouble with.

It's obvious enough that since you have to work with small diameter tubing, the stationary phase has to be thin and, therefore, can't handle large amounts of analyte. I guess my primary question is: why is it that this issue doesn't seem to be a problem with gas chromatography? Especially considering that there really isn't much difference between GC and LC with respect to what goes on in the stationary phase (the difference is what goes on in the mobile phase). So how is it that this stationary-phase-related issue is a problem with LC but not GC

I am very interested in hearing any insight that the chromatography guru's out there can provide. And any references that deal specifically with this issue would be of interest as well.

Much Thanks

I suggest that you think about the capacity in the context of detector sensitivity.

Thanks for bringing that up. It's true that the issue of mass overload limits the amount of material that can be placed on column, and therefore reduced sensitivity will be obtained: especially if using a mass-sensitive detector. But it brings me back to my original question: Why is this not a problem in GC, where analysis with OT columns is often done at high ppm levels?

What A.Mouse was saying, I believe, that GC detectors (FID, ECD) are more sensitive than HPLC detectors (UV, RI). So we need much less in GC to detect something, therefore less problems in GC with mass loading.

If OT LC is very slow, it means peaks are very broad. Then what sense does it make? Please explain me as I am completely new to OTLC.

The difference between LC and GC relates to the required capillary diameter for equivalent plate heights. In capillary LC, an ID of 5 microns will give plate count roughly equal to a 5 micron HPLC packed column (on an equivalent length basis). Because of the much greater diffusion coeffients in GC, the capillary ID can be much larger with good efficiency in GC. Larger ID translates to larger surface area and correspondingly greater capacity.

Sorry for having been so short in my message yesterday. Indeed, the combination of the slow diffusion in LC with the requirement for using smaller columns with a smaller i.d. and the wider peaks as well as the lower sensitivity with the LC detectoss all combine. In order to avoid the column overload issue, the detector sensitivity needs to be much higher than in GC. You can probably make an estimate how much mass can be injected by calculating the amount of stationary phase in a 5 micron ID LC capillary with a surface coating of about 0.5 micron to the equivalent condition in GC. I have not done the calculation, so I do not know if modern MS systems get there or not. Let us know, after you have done the calculation.

Difference between LC and GC column construction?

Hi all. Thanks for the responses.

So far the concensus seems to be that the two issues I raised (diffusivity and capacity) are, in fact, related. As I understand it we could sum it up like this: The lower diffusivity of liquids requires the ID of the column to be smaller in order to obtain good efficiency. Because the column ID needs to be scaled down, the thickness of the film needs to be reduced to keep the phase ratio in a workable range. From these factors it follows that the column has very low mass capacity. Sounds good. It all makes sense, but I don't think we have gotten all the way there just yet.

I have seen examples in the literature where people complain of poor retention with OTLC. But if the retention is poor why not just use a thicker film stationary phase. Along the same lines people who do OTLC often used etched columns: the idea being that the surface area is increased and, hence, you get more retention. But I've never heard of the etching technique being used with GC columns (and I did GC for many years): the gas chromatographer simply uses a column with a thicker film if they want more retention. Why is the situation different with HPLC such that one must resort to special techniques such as etching?

It seems to me there is an issue we are still missing. And since there is no real difference between LC and GC in terms of what goes on in the stationary phase, the only thing I can think of is that it has something to do with the difference in the way LC and GC columns are constructed.

This one is not for the faint of heart.

Let me ask you a question: how do you hold the stationary phase in place in LC (if it is a liquid)? If it is not a liquid, how do you generate a large enough surface to get retention?

OK, i'll take a stab at it. Generally it's held in place by virtue of the fact that it's a bonded liquid - like C18 on a silica support.

And generating more surface area can be accomplished by etching as discussed in the previous post. But there again, gas chromatographers don't worry about increasing the surface area, they just use thicker films.

I guess I'm not following very well Uwe. Can you tell me what your thinking is?

You can't simply increase the film thickness without constraint as the diffusion coefficient in the stationary phase will be even lower than that of the mobile phase and as previously mentioned, this limits the range of capillary ID. Thus you must stick with a rather thin film for good performance. You can increase the wall surface area by various means to increase phase mass without increasing film thickness but this will still be far away from the surface area of HPLC packings on a volume basis so the capacity will still be far lower than an equivalent packed column HPLC column even if the injection mass is kept to scale with the column.

Yes, in LC you need a support structure for holding the immiscible liquid phase or a support structure with a large surface area for holding a bonded phase. In GC, you get away with just coating the surface of the capillary with a liquid film, and little support structure is needed. Don't you think that this is making life more difficult for making capillary LC columns, together with the much smaller sizes of then to start with?

Chris, Uwe

My approach to this question is like a dog with a bone. I just won't let go. Let me try once more. I think we are getting there.

Chris - I know what you mean when you refer to the diffusion coefficient in the stationary phase (I guess it's the Cs term of the van Deemter). But the same problem should effect gas chromatographers (stationary phase issues are essentially the same in LC and GC). So I'm wondering if there is still a piece of the puzzle we're missing.

Uwe - What you say about GC columns was true at one point in time; but GC column technology has improved to the point where, in the vast majority of cases, the stationary phase is bonded. But still they are able to use thick films.

In my opinion it's is worth pursuing. If it were possible to do OTLC with decent efficiency and, simultaneously, a larger bore column (never say never) think of the plate counts we could get.

Anon

I admire your dogged persistence but I'm not quite sure why I'm not getting this through to you: the chief distinguishing feature which affects both column dimensions and capacity is the significantly different diffusion coefficients involved for analytes in the two phases in the case of capillary GC versus the case of capillary LC. The choices made in dimensions and film thickness in each technique have been made in order to provide optimum performance taking into consideration the differences in diffusion coefficient. There's really nothing you can do to get around this problem except to raise the diffusion coefficient in capillary LC (i.e. work at very high temperatures).

Chris - I appreciate all the responses, but frankly could do without the condescending tone. I fully understand all the points you've raised, but they do not address the remaining unresolved question of why low retention is more of a problem with OTLC than OTGC. The difference in diffusivities governs the diameter of the column, NOT the thickness of the stationary phase (yes, I know they are related but if the retention is too low than clearly you want a thicker film).

But, there is probably a limit to what can be accomplished in this type of long-distance dialogue. And, I think we have reached that limit. Anyway thanks for your input. It did shed light on some of the issues.

I've been lurking on this thread since the beginning, not thinking I could offer much, but I think it's time to chime in.

First off, let me defend Chris (as well as Uwe and A.Mouse); I didn't find anything condescending in tone from any of them. It's actually quite difficult to explain in words what requires a few equations and a chalk board (your point about long-distance dialog is a good one in that regard). If you've followed the Forum for any length of time, you'll appreciate that all of them have invested a lot of time and expertise in the various discussion threads.

What I think Chris was trying to get across is that similar film thicknesses will *not* give you similar retentions in LC and GC.

Think of retention in terms of k' (we'll stick to isocratic/isothermal separations here for simplicity), which is determined by the distribution coefficient of the analyte between the two phases and by the phase ratio. Other things being equal, liquids are 3-4 orders of magnitude more concentrated than gases (18 mL of liquid water gives 22.4 L of water vapor at STP). Therefore, you need that same 3-4 orders of magnitude more stationary phase for a given column volume in LC to get "equivalent" retention to that of GC.

If you go down from a 50-micron GC capillary to a 2-micron LC capillary, you've picked up a factor of 25 in surface/volume ratio, but that still leaves you a factor of 40 or more short of the same phase ratio.

Making the film 40 times thicker runs you into horrendous diffusivity problems (which is, I think, what Chris was trying to get across).

Hope this helps.

Hmm. Well, I will continue to grapple with it; and think about all the points that were made in this thread.

At any rate I wish everyone a Happy Easter!

Sorry if my response came off as condescending. I sometimes see such in the comments from others and I also find this anoying. I was attempting a play on words re: your "dog with a bone" comment but I guess it came off very badly. My thanks to Tom for tying to bail me out.

No problemo. My bad.

Low retention is caused by an unfavorable phase ratio. Phase ratio is the ratio of surface area (or volume) of the stationary phase to the volume of mobile phase. Due to the constraints of the design of the capillary, I would think that the phase ratio for a capillary is always much lower than that of a packed column, no matter how you twist it.
On the other hand, I am not sure if I want to panick about it. A column packed with a packing with a pore size of 50 nm is still useful for the separation of small molecules.

A. Mouse

I defenitely agree with your point that the phase ratio for an OT column is generally going to be less favorable (i.e. less retentive) than for a packed column. To get phase ratios in an OT column that are comparable to packed columns would require either going to very small column ID's or using very thick films: both of which create huge problems.

In addition to the diffusivity issue - already discussed - another reason that OT columns are much more common with GC is that, due to the compressibility of gases, one is forced to use much larger particles in a packed GC column. Typical particle diameters are 100 um: huge by HPLC standards. So this makes work with packed columns less favorable and hence OT columns become more valuable.