Chromatography Forum

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?

thx

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,

aicmm

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.

Imagine the end of the school day, when a large number of kids are released into the street simultaneously, and start to walk away.

On an average street full of bus-stops, fire-hydrants, litter-bins, lamp-posts and street-signs (an average column full of particles) some kids will, by chance, go fairly straight, while others will get caught up circling round street furniture. That's A, the eddy-diffusion bit.

As the kids walk along, they will tend to spread out along the street. The longer you leave them, the more they'll spread. If they're marched along at a fairly rapid rate, they may get quite some distance from the school while still being a coherent bunch. If you let them dawdle and drift, and take twice as long to cover a street-length on average, you'll probably find that they will spread out twice as far along the street. That's B, the straightforward diffusion bit.

If the street is lined with sweetshops and newsagents, some kids will go in and interact with the sweetshop. This will delay them relative to others who continue along the street. When a sweetshop kid comes back out, they will find themselves behind the kid that kept walking, and the faster the kids are walking, the more they'll find they've spread out. That's C, the resistance to mass transfer.