HPLC of Hydrophobic NH-containg analytes at high pH.

[mbreslav]

I need to analyze very hydrophobic compounds with fused aromatic rings which have three pyridine type nitrogens in its structure. I cannot disclose the structure, but think about an antracene with nitrogens in each cycle. It is nicely eluted with acidic mobile phases, but I need to use a basic pH (0.1% ammonium hydroxide would be the best). I tried 2.1x30mm Waters BEH (XBridge) C18 which is stable to high pH, but the compound stuck on a column, and is not eluted even with THF. I cannot raise the temperature of a column.

So, I need a column that will be both stable to high pH, and will have low affinity to hydrophobic compounds. I am not sure that C4 vs. C18 will make a dramatic difference.

Or may be amino or cyano columns will do, but I am concerned about column high pH stability.

Any advice will be appreciated.

[DJ]

Why must you use an alkaline mobile phase?

When you say the solute would not elute from a C18, even with THF, can you describe the elution strategy? (i.e., isocratic at some high % organic? gradient?)

[Gerhard Kratz]

Maybe a HILIC column will do the Job. Elution order will be the other way round and no high pH needed. Or try a polymer based C18 column with very high pH stability.