Your best overall stability is probably in the range of pH 3 to 4 for any silica-based packing, including hybrids. For elevated temperature work, I would consider acetic acid or an acetate buffer over phosphate or TFA at pH 2 or phosphate at pH 7. I have no solid information about any differences in stability between formic acid and acetic acid, but I also do not see any reason why anybody would prefer formic over acetic. At low pH, even at room temperature, the bonding chemistry is important. You want a phase based on a multi-functional silane, best a trifunctional silane. The only alternative is a phase based on a sterically hindered silane. With respect to the stability of the packing itself, hybrid phases are superior to silica-based phases. Among the polar embedded phases, select one that does not have any residual amine left on the surface. Also here, trifunctionally bonded phases are likely to be better than monofunctionally bonded phases. What I said above for C18 applies equally to C8 or phenyl phases, but the stability declines with the less non-polar bonded phases.
Polar embedded phases are not a single category. There are some which are bonded with a two-step technique which leave amine on the surface. These phases even show stability problems at room temperature. I would not use them at elevated temperature. Other phases are based on a monofunctional silane. They are good to maybe 60 degrees with a reasonable mobile phase.
All of the above applies to silica or hybrid-based phases. Zirconia phases with a polymeric coating can be used at elevated temperature without worry, but you have to relearn your chromatography. I have no experience with the other zirconia-based phases, but I would guess that the story about relearning is similar.
Most people love to stick with mobile phases of more or less the same pH. I suggest doing the same for elevated temperature. Use an acetate buffer or acetic acid, use a trifunctionally bonded hybrid phase, and you can go to about 90 to 100 degrees without worry. For an alternative stationary-phase selectivity, use a phenyl phase, preferentially a trifunctionally bonded phenylhexyl hybrid phase. I think this results in the least amount of disappointment.
Note that the use of elevated temperature is not a substitute for UPLC. If you use very small particles, the minimum of the van-Deemter curve (=maximum performance) happens at about the same pressure independent of the temperature. If anything, elevated temperature results in a HIGHER pressure. So don’t fall into the trap that higher temperature is a substitute for UPLC.
Alfred posted the point that compound stability may be a problem as well. I was concerned about this as well, but I recently got a very good answer from a user: if compound stability is an issue, you should know this already from the stability studies. If indeed it is a problem, you stay away from a high temperature application.
Oh HILIC… I have no information about the stability of different options of phases under HILIC use. I am also concerned that you may not get a lot of retention at elevated temperature in HILIC.
Robert said that retention decreases with increasing temperature. This is of course correct: the rule of thumb is that retention decreases by a factor of 2 to 3 for every 10 degrees C. This is why you can do chromatography with toluene in water at some 200 degrees C. This is the drawback of the use of elevated temperature.
