Chromatography Forum

The equation for column efficiency shows that increasing plate count results in increased retention and/or narrower peaks. Does dropping particle size, for example, from 5µm to 3µm typically result in a change in retention time? I am aware of the peak sharpening affect but unsure about whether I should expect retention to change.

No unless you want to operate at the optimun linear velocity. The optimum linear velocity increases as particle size decreases. In the case of sub-2u it is mostly flat around it's optimum. For example for 1.8 um particle size you get about the same efficiency for linear velocities spanning from 0.3 to 1.5 cm/sec... unlike 3.5 and 5 um particle sizes...

The optimum linear velocity affects HETP which translates to N. Which brings me back to the question rephrased; "Does an increase in N usually affect retention time or peak width?"

To be absolutely clear: under constant operating conditions, an increase in N means a narrowing of the peak width with constant retention. This is the common meaning of N.

In practice, I would expect an increase in retention for anything that has any appreciable retention under the original conditions.

To maximize the increase in theoretical plates available with smaller particles, you have to optimize the linear velocity - but the slope of Van Deemter plots for this situation are generally not too steep to the right of their minima, so you can see a difference without going to too much trouble with respect to optimizing flow rates.

The only thing that could fundamentally change the retention with a smaller particle diameter is a change in the thermodynamics of the phase transfer, i.e., the phase ratio. The initial question is really, "Do smaller particles have a greater phase ratio?"

I think we can safely accept that the phase ratio should be larger with smaller particle diameters, depending on particle porosity.

Edit#1: According to Unger, the surface area should not change by more than 0.1% with a decrease in particle size of porous silica, and therefore should not see any increase in retention due solely to the decrease in particle size.

Edit #2: This should clear it up

PS - Dorsey lab rules[/url]