We have seen that cyanopropyl colums change from reversed phase to normal phase when organic concentrations in aq. mixture increase from about 40 to 60%. Has anyone seen something similar with phenyl columns?

Please explain "change from reversed phase to normal phase" My understanding of normal phase was "column is polar, eluent is non-polar" and reverse phase was the opposite.

When ACN concentration increases from 0 to 40%, RT becomes shorter and shorter (reversed phase behavour). After certain points, let's say 50% ACN, RT increases as organic concentration increases (normal phase behavour). We have seen it a few times on CN columns.

I have found some info confirming this at the Keystone site:
www.thermohypersil.de/csg_uk/csg/index.html
Look in the Betamax link

But this doesn't answer your original inquiry regarding phenyl columns.

All silica-based packings contain silanols. These silanols are generally responsible for the increase in retention as the organic concentration increases. It is commonly observed with samples with strong polar groups or amino groups. It is more pronounced, if the column is not endcapped.

"reversed-phase" and "normal-phase" are shorthand terms that we use to quickly characterize complex phenomena. John B's definition is correct, but sidesteps the ambiguous situations where "polarity" of the two phases may be comparable.

Anonymous #2 gave the "operational" definition (as you add more ACN to the mobile phase, if retention decreases, it's reversed-phase; if it increases, it's normal-phase).

And I think Uwe hit the nail on the head (as usual!) by pointing out that silanols can and do contribute to retention. This means that the "polarity" of the stationary phase as seen by an analyte that interacts with silanols can be quite a bit different than the "polarity" as seen by an analyte that doesn't.

Bottom line: this "change of mechanism" is not uncommon with cyano columns ("gee, the polarity of the cyano group is similar to that of ACN, imagine that!"). I've never seen it on phenyl columns, but I wouldn't rule it out.

-- Tom Jupille / LC Resources Inc.

No Name,
As you have seen, it happens. The analyte determines if and at what solvent proportions it will occur. Changing from cyano to phenyl columns may change the behavior for your analyte enough to be useful depending on what you want.
Jim