Predicting / modelling Chiral separations

[Rob Burgess]

Hi there,

I'm wondering if anyone agrees with me that sometimes chiral separations are some of trickiest to solve and get right (especially trying to baseline separate at the 0.05% level to a main peak!).

Is there any predictive modelling tools out there that could help in this respect rather than just trial and error? In this day and age it seems that a chiral method developer doesn't have the same tools available as there are for reversed phase separations e.g. Drylab, Chromsword and Fusion QbD

[tom jupille]

Actually, the same predictive tools *can* be used for chiral separations. The catch is that they are fundamentally interpolation programs: you give them retention data under defined conditions and they interpolate to predict what will happen under intermediate conditions. Chiral separations differ from "conventional" reversed-phase or HILIC in that most of the selectivity is "baked in" to the stationary phase, and stationary phase chemistry is a discontinuous variable, so the interpolation programs don't help.

Basically, the trick is to find the right column. Once you have done that, optimization of the mobile phase can be done in the same way and using the same tools that would be used in any other type of separation.

Back when I was supporting DryLab, we actually used it to optimize separations on OVM columns (mobile phase has more impact on selectivity on protein columns than on most of the other types).

[M_Farooq]

Chiral separations are tricky because of multi-point interaction of the analyte with the stationary phase. The efficiency is often not so great. With some core-shell chiral columns it is now conceivable to see 15000-20000 plates per column. I think your problem is more complicated by overloading. In order to see the 0.05% impurity, you might be injecting relatively high concentrations. It has been observed many times that the high concentration peak not only broadens the adjacent trace peak but also shifts the retention time (in either direction, early elution or late elution) by the so-called displacement effect and tag-along effects. If the main peak tails, one effect is seen, if it fronts another effect is seen (very rare to see fronting in chiral modes). These effects are perhaps not predicted by softwares.

As others have said, the best way is to begin with a high resolution selector for that particular application.

Regards,
Farooq