This is sort of a followup to the thread on post-column splitting, because it made me think about the advantages of small versus larger bore columns. In addition to the advantages of having more concentrated peaks, having a more MS-friendly flow rate, and using less solvent and ability to use less sample, I had always thought there was a slight efficiency advantage.

There's someone else in my lab who uses strictly micro- and nanobore columns with MS. I asked him if there was much chromatographic advantage over conventional bore columns, and he said there was a huge improvement in theoretical plates. I was skeptical and asked to see a test chromatogram for one of his microbore C18 columns (0.32mm i.d. or less), and plates per meter were about the same as a good quality C18 column of 2.1mm i.d. of the same particle size.

Snyder & Kirkland, 2nd edition, claim just the opposite, that efficiency drops off quickly below 4.6 mm and optimizes at a larger bore than 4.6 (at constant linear velocity). But then I wondered if the experiments they referenced accounted for extra-column effects that would become very important at low flow rates.

Does anyone know what the real answer is? Does column i.d., all other things being accounted for, have much of an effect on column efficiency?

In my expeience, as you scale down in column diameter, columns loose efficiency. In many cases this is not really an issue if your peaks of interest have adequate resolution. The experiment itself is easy to enough to do. Just scale down the flow to give constant linear velocity on 2 columns (a 3.9 mm at 1 mL/min and a 2.1 mm at 0.29 mL/min) and compare the resolution between any pair of peaks.

There is no fundamental reason for the plate count to go up or down with changing column diameter. Of course this assumes that you change flow rate in proportion to the column cross section. On a real HPLC instrument, plate count often drops, as your column dimater decreases. This is due to extra-column effects - tubing, detector cell volume, injection volume etc. If you are running a gradient, only the post column volume plays a role.
In the old times, there were also some difficulties with column packing of smaller i.d. columns. This is primarily a question of column hardware. In today's world, these things should have largely been resolved.

In a very particular instance, I beg to differ. Uwe is right in that theory (van deemter), plate # shouldn't change in regards to column diameter. That is unless you get the column diameter down to about 50 um or less. In this case, the packing of the column becomes more homogenious, because one goes from two packing regions (inner and outer) to just an overlapping inner region. So for capillary columns (which nobody really uses), efficiency is better. I know many people here may disagree, so I found a reference. Its an anal chem 1989 by rt kennedy and jw jorgenson something like "evaluation of microbore capillary columns with 50 um diamater"

Last Anon: Sorry can not find this article on 50µM columns. Can you give more details?

Here's the full reference:
"Preparation and evaluation of packed capillary liquid chromatography columns with inner diameters from 20 to 50 micrometers
Robert T. Kennedy, James W. Jorgenson;
Anal. Chem. ; 1989; 61(10); 1128-1135."
Interesting for those doing capillary LC.

Yes guys, if you are getting down into the range where the column diameter and the particle diameter are only about an order of magnitude different, the rules that I mentioned above do change. You'll get really interesting results, once the column diameter is only about 2x the particle diameter...

Uwe,
I've never heard of anyone getting down to that ratio. Can you fill us in on what happens. I'd imagine that one would constantly run into clogging problems when packing such a column/capillary.
Thanks
Jim
Anon April 21/22

I agree with Uwe
but why should I scale down flow rate? I wont and I'`` get for example (4.6/2.0)^2 shorter run times with a only e bit worth efficiency and the same pressure.
Constantine

to C. Sychov:
Same pressure??? Go try it...

Jim Anon:
In the old times, when people were working with 50 micron particles, it was not difficult at all to get to such a ratio. Today it is a bit more difficult, but people are doing it successfully. People are packing 50 micron capillaries...