HPLC Calculations

[akthmps]

I was wondering if someone can provide some feedback here... I just want a couple theoretical calculations performed. Assume isocratic elution.

Assume an HPLC column operating at 6000 psi constant flow has now been reduced in pressure to 1000psi. Everything else remains constant except for column pressure. Initially, at 6000 psi, two peaks appeared at 1.5 and 3.0 minutes, both with peak widths of 3 seconds. Is there a way to calculate the new retention times and peak widths with only the pressure having dropped from 6000 to 1000 psi? We can make up a column length, say 4.6 to 150mm C18 column. I was just wondering if there was a simple way to calculate new retention times and peak widths with the pressure change.

Thanks,
Aaron

[DR]

If the relationship between pressure and linear velocity were a truly linear one for liquids, it wouldn't be too difficult.
Given that it is not, and we have no idea what extracolumn effects are in play, it would be tough to calculate in any sort of accurate way. (It would be faster to just compare chromatograms generated at each of the two pressures).

[Mark Tracy]

There are several ways to reduce the pressure while holding everything else the same: 1) a shorter column, 2) larger particle diameter, 3) lower flow rate. Care to speculate on other ways? To a first approximation the effects on retention time would be 1) 6x shorter, 2) the same, 3) 6x longer. To a first approximation the effects on peak width would be 1) sqrt(6) times narrower, 2) fourth root(6) times wider, 3) 6x wider.

I leave it as an exercise for you to figure out where the equations come from. Hint: Van Deemter equation

[HW Mueller]

If I read this correctly, than nothing but a pressure change is postulated. If there is no conformational change due to the pressure change why should there be a change in rt, etc.??

[Mark Tracy]

If for example you set the pressure by a post-column regulator device, then the column geometry and flow rate would be the same. To a good approximation the retention time and peak width would remain the same. Of course there will be the compressibility of the solvent, but that effect is minor. Unless the stationary phase is compressible, then things could get interesting.

The question as stated is quite ambiguous.