data chromatography

[lotussss]

Hello, I would like to know where to find or how to calculate the following data:
- porosity of a column
- optimal flow of a column
Thanks.

[Kostas Petritis]

Lotussss,

Assuming that you are not trying to synthesize your own stationary phase, the porosity (pore size) of the stationary column is almost always given from your column manufacturer.

There is an equation that calculates the optimum linear velocity of a column according it's caracteristics (and you can deduct from that your optimum flow rate) which I do not have handy right now, but I can dick it up (if Uwe or someone else won't pitch in).

[Uwe Neue]

The porosity of a column can vary from a bit less that 40% for a non-porous packing to maybe around 75% for a fully porous packing.

The optimal flow rate of a column depends on mobile phase and type of sample. If you are more specific, I can give you a better answer.

[lotussss]

Hello,
I work with mobile phase of acetonitril/water/acetic acid(30/67/3) and with a lichrospher column : 4*250mm C18e at 0.7ml/min. I think that it is the optimal flow because all the tests show a better separation with this flow. But I want right knowledge if there is there an equation or data of manufacturer to determine this. Obviously porosity intervenes in this calculation. I would like to also know how to obtain this data. Another thing, what corresponds to the following data: carbon load?
Thanks.

[Uwe Neue]

I expect the maximal platecount for this column (if it is a 5 micron material) to be between 0.2 to 0.4 mL/min. You would get a bit more perfroamcne, but you will sacrifice a lot of time. Therefore your 0.7 ml/min are not bad at all.

The equation that allows you to do such calculations is the van-Deemter equation. You need to know the particle size, the column dimensions, and the diffusion coefficient of your compound. The latter can be estimated from the Kozeny-Carman equation. You can find the background on this in my book "HPLC Columns".

Actually, if I have some time and you are interested, I can put a spreadsheet together that allows you to estimate these things. Send me an e-mail, if you are interested.