I first presented this question on the HPLC Message Board, and we are still grapling with it over there. I thought I would try the GC Message Board. Anyone care to take a "swack" at it.

The question is this: As you increase the amount of material you inject on a column (for any type of chromatography) the peak gets bigger (of course). More to the point, it gets both taller and wider. Since the width of the peak changes with concentration, it would seem that plate height would be a function of concentration. But plate height is supposed to be independent of concentration. Can anyone resolve this paradox?

Please note that I am NOT speaking of issues such as column overload and peak tailing due to sorption. These are entirely separate issues.

Any thoughts would be much appreciated.

Dear Anonymous,

Until I read your message, I was confident that, in linear chromatography (where there is no column overloading or other irregularities of similar nature), peak width does not depend on the amount of material injected in a column. My confidence was based on the theory and on a personal experience of measurement of peak widths and studying peak width reports in hundreds (maybe thousands) of chromatograms.

If you have the contradicting data, could you please reveal it.

You are correct. And we came to the same conclusion on the HPLC Board. For some reason I had always thought that peaks got both taller and fatter as the concentration increased. But that's not correct. The kinetic factors that control band broadening are not a function of concentration. So, therefore, neither is peak width or plate height a function of concentration (in the absence of secondary factors, of course).

Cheers