relate the a b c terms in van deemter physically?

Basic questions from students; resources for projects and reports.

3 posts Page 1 of 1
hey! so if i make a van deemeter equation i have a b c terms, and i know waht they mean like eddy diffusion etc etc but is there a way to relate them physically? like what would the a b c terms be in the components of the gc?

I'm gonna take the easy one - c. Applies to packed columns only, goes away in capillary. Think of the carrier gas swirling around a particle, particularly an odd shaped one. Your create an eddy on the backside just like you do a rock in the river. if your analyte gets caught in the eddy....

Best regards,

Uhh, actually, that's the A term (eddy diffusion). Good explanation though.

The B term is identified with "molecular diffusion". Imagine injecting a sample and then stopping the flow entirely. The sample molecules would gradually diffuse until they filled the entire column (making for a very wide peak). At low flow rates, you are allowing too much time for this diffusion to occur.

The C term is identified with "mass transfer delay". That reflect the fact that equilibration between the two phases is not instantaneous; it takes a finite time for a molecule to diffuse into/out of the stationary phase. If equilibration is very slow, at high flow the main peak will have moved on before a give molecule gets out of the stationary phase, or else a molecule will be swept forward further during the time it takes to diffuse to the mobile phase surface.
-- Tom Jupille
LC Resources / Separation Science Associates
+ 1 (925) 297-5374
3 posts Page 1 of 1

Who is online

In total there are 10 users online :: 0 registered, 0 hidden and 10 guests (based on users active over the past 5 minutes)
Most users ever online was 599 on Tue Sep 18, 2018 9:27 am

Users browsing this forum: No registered users and 10 guests

Latest Blog Posts from Separation Science

Separation Science offers free learning from the experts covering methods, applications, webinars, eSeminars, videos, tutorials for users of liquid chromatography, gas chromatography, mass spectrometry, sample preparation and related analytical techniques.

Subscribe to our eNewsletter with daily, weekly or monthly updates: Food, Environmental, (Bio)Pharmaceutical, Bioclinical, Liquid Chromatography, Gas Chromatography and Mass Spectrometry.

Liquid Chromatography

Gas Chromatography

Mass Spectrometry