post column band broadening

[rs]

Could anyone please explain what is post-column band broadening in context with broad early eluting peaks (other peaks in the chromatogram being normal) and also how it can be avoided ?

Thanks
RS

[DR]

Post column band broadening is a function of the volume of the tubing between your column and detector (assuming that the flow path is reasonably uniform). As a slug of sample moves through the system, diffusion occurs. This results in band broadening (assumes isocratic conditions). Since diffusion occurs as a function of time, and flow is volume per unit time, more volume means more time for diffusion, which means more broadening of peaks.

To minimize the problem, use short runs of fairly narrow bore tubing wherever you can, but especially between the column outlet and detector.

[John]

I would think that post-column broadening would affect all peaks. Your early analyte could be undergoing secondary undesirable interactions w/ your column, overloading the column, or even co-eluting with another analyte.
j

[tom jupille]

John, extra-column band broadening often is more noticeable on early peaks (for isocratic chromatograms, anyway). Late peaks are broader in any case, and independent sources of band broadening are "root mean square" additive, so the additional broadening due to extra-column volume is less noticeable.

There were a couple of threads on this topic earlier this year. Check:
http://www.sepsci.com/chromforum/viewto ... xtracolumn

[Victor]

Hmm- the type and volume of the detector cell you use could have a very significant effect on the narrowness of early eluting peaks. Some "standard" detector cells have volumes of nearly 15ul which will have a significant effect with a high efficiency column. And it's more of a problem to get round than replacing a bit of tubing....