Calculate column void volume

[thohry]

Hi all,

Anyone knows how to calculate the column void volume. For example my column is 250 x 4.6 and 5um.

Thanks

[grzesiek]

viewtopic.php?t=11735

[thohry]

Thank you grzesiek,

That means we accept the approximation that the bed volume is 1/3 of the whole. So it does'nt depend on the particle size.

[grzesiek]

It is a good approximation, you can also measure your t0 and multiply it by the flow

[Uwe Neue]

It does not depend on the particle size, but it does depend on the specific pore volume of the packing. I would never "calculate" it because it is so easy to measure with an unretained peak.

[thohry]

It does not depend on the particle size, but it does depend on the specific pore volume of the packing. I would never "calculate" it because it is so easy to measure with an unretained peak.

Your way is the best and correct one. But not all the time, I think we can find out unretained components and in case we need just roughly figures.

[tom jupille]

This is a "perennial" topic.

From the archives in 2000:
http://www.lcresources.com/discus/messa ... 20000443pm

And from 2004:
http://www.lcresources.com/discus/messa ... 20041222pm

[Garlarg]

Try this:
http://www.chromacademy.com/Gradient-Calculator.html

[HPLCCONSULT]

Here is one from the web:

http://www.hplctools.com/columnvolume.htm

[Proton]

Hello,

When I tried measuring column Vo empirically by injections of unretained solutes:
1) Acetone (0.2% v/v in water)
2) NaNO3 (10 ppm aqueous)

The results were as follows:





Why then the discrepancy in retention times if both solutes are unretained?

Conditions:
Column: 2.1 x 100mm 1.7um BEH ShieldRP18 (embedded polar group)
Isocratic Mobile Phase: 95% (aqueous H3PO4 pH ~ 3.0) / 5% ACN
Flow Rate: 0.5 mL/min
Column Temperature: 45C
Injection Volume = 1.0 uL

[HW Mueller]

Since acetone would not be expected to exclude here it appears that your data obviously shows retention of something, provided that the flow rate was the same, the detector setup was the same.
What is your phosphate concentration? What is the detection?

[Proton]

H.W. Mueller,
Thanks for your response. Both chromatograms were run under the same conditions, where the detection was UV (PDA) and the amount of acid in the mobile phase A was approximately 150 X (0.85) mg/L, a concentration which yields a pH of ~ 3.0.

What is surprising to me is that the NaNO3 was retained relative to the acetone.

[HW Mueller]

If it is not an artefact you probably see NO3-, not NaNO3. One can get some interesting chromatography near tm (to), especially in low ionic strength solvents.

[Bruce Hamilton]

One possibility may be that your mobile phase has insufficient acetonitrile.

I usually use Uracil, and once noticed that it appeared retained on a C18 column when the mobile phase acetonitrile content was only about 5%. Increasing CH3CN to 70% reduced retention from about 3.8 to the expected 2.7 min.

Not sure if that applies in your case, but I'd try using the same CH3CN content as used on the original column certificate.