Help in interpreting a k' plot organic vs buffer

[Mike H.]

This plot describes k' as a function of %organic vs %buffer for the Sigma Aldrich line of Chirobiotic columns when used in reverse phase mode. I don't understand why we get k" maximas at 100% organic and 100% buffer. If this was a C-18 column in RP I would expect to see a linear graph. Does anyone know what mechanism is responsible for this?

One side question. Does the elutropic strength of various organic solvents in chiral RP the same as it would be in normal C-18 RP? ie would ACN decrease k' more than MeOH when using a Chirobiotic T column? I'm new to this technique and the available information is sparse. thx

[tom jupille]

Off the top of my head, I'd say it's a transition from "reversed-phase" to "normal phase" mode. You have to remember that terms like "reversed-phase" and "normal-phase" are mental models that we use to make sense of what we see. To quote from the statistician George Box (admittedly in a different context): "All models are wrong. Some are useful."

If you look at the structure of the Chirobiotic phases, you'll see that they have quite a bit of polar functionality. With a high-water-content mobile phase, the solvent is more polar than the column (i.e. "reversed phase"). At high organic, the column is arguably more polar than the solvent (i.e. "normal phase"). I'll let better organic chemists than me speculate on whether it's actually "HILIC" mode (where the stationary phase presents as a water-rich surface layer) at that point.

As to the relative strength of ACN vs MeOH, that is only an approximation (useful, but not necessarily universal). The differences are more pronounced at low levels; the mobile phase strengths become approximately equal as you approach 100%:

[Mike H.]

This makes sense if I'm understanding what you're saying correctly. Even though separations with ACN and water using a Chirobiotic T column are termed reverse phase, they actually have both RP and NP characteristics based on mobile phase composition.

I took one of your HPLC training courses a couple years ago and was wondering if you guys offered one on chiral chromatography?

[tom jupille]

wondering if you guys offered one on chiral chromatography?

No we don't. That's a very specialized area and we don't have any special expertise there. The best source(s) of information are actually from the vendors like the Supelco/Astec handbook that includes the plot you showed. Regis has lots of information on the Pirkle columns and Chiral Technolgies is the place to go for advice on the Daicel columns.

We have a few slides on chiral in our method development course, but they only present a very broad overview. I usually pass along the advice I got a few years back from a good friend who worked at a big pharma company in New Jersey doing LC in support of process development (so, lots of method development and tweaking). I knew he had a lot of expertise in chiral work, so I picked his brain on how he developed chiral methods. His answer surprised me: "We outsource it. 90% of chiral LC is getting the right column, and the columns aren't cheap. Unless you're doing it full-time, it's way too expensive to keep a big library of columns available. We have CA's and blanket PO's with the major vendors; we send them a sample and a PO for the screening work and then buy the column when/if they show it works."