UPLC calculator

[gtma]

I'm trying to take a 3 mins isocratic run using conventional column and transfer it to UPLC to get a much shorter run time. However, when I use the UPLC calculator, the minimum retention factor I can enter is 5. Is there a way to enter less than 5 for the retention factor? What impact does retention factor has on the final conditions......other than N and run time? According to the UPLC, the retention factor is the analysis time divided by the column dead time minus 1. Can someone explain how you obtain this equation?

[Mark Tracy]

How curious, I just gave this same answer to another thread on this forum. We recently published an article in LCGC online that describes how to accelerate a method. The theory is rather tedious, so there is a step-by-step description and tutorial. See: http://www.nxtbook.com/nxtbooks/advanst ... tartpage=6

[danko]

Hi gtma,

I never used the calculator so I don’t know whether there is a way of entering a smaller number than 5.
But I’m surprised by the fact that 5 is the smallest value one is allowed to enter. I always go for retention factor (K) between 2 and 10, when developing a new method. And I know many people who will agree that K ~ 2 is just fine. But as I said I find it easier to rely on gut feeling/experience. Especially in your case (isocratic elution) I wouldn’t bother to use the calculator.

K influences – besides N – the resolution and even the sensitivity. The latter is due to peak broadening, which results in lower peaks, when K is too high (e.g. > 10)

Her is the equation: K=(tR – t0)/t0 ~ (tR/t0) – 1

Best Regards

[Uwe Neue]

The scaling method is actually very straightforward, if you use a constant ratio of column length to particle size L/dp, for example going from a 15 cm 5 micron column to a 5 cm 1.7 micron column. Now, you simply need to increase the velocity by the ratio of particle sizes, in this case 5:1.7, and change the corresponding flow rate with the column crossection.

F(UPLC)/F(HPLC)= dp(HPLC)/dp(UPLC) * ID(UPLC)^2 / ID(HPLC)^2

dp is particle size, ID is column internal diameter.

The calculator will do this for you, even for a situation where the ratio of column length to particle size is not constant. It will do so by matching the plate count. The reason that I shied away from very short retention factors is that at short retention, extra-column effects play a role, and I can not calculate extra-column effects for an HPLC system that is unknown to me. The initial version of the calculator allowed the user to input extra-column effects, but based on user feedback, we threw this out again.

The calculation of the retention factor is in textbooks.

[danko]

Mark, please excuse me, your post must have arrived while I wrote mine. I’m sure your link gives more comprehensive answers.

Best Regards