I am amused by all the discussions about HILIC columns and chromatography. Weren't the same separations principles discovered decades ago and called normal phase chromatography? Perhaps the sole exception being the SeQuant column which they describe as a zwitterionic column.

One point that seems to be ignored in the discussions of these HILIC methods and separations is the need to dissolve the sample in a suitable diluent. Highly polar samples often don't dissolve in organic-rich mixtures and sample diluents with high water content tend to distort the chromatography.

Bob,

Of course you are right.... I did this stuff that is now called HILIC for the first time in 1978 or so, when I tried to figure out why bases tail on the Radial-Pak C18. I was very excited about the beautiful peaks that I got for basic compounds on silica with reversed-phase mobile phases. There were a few publications in the 80's that followed this trend, but it never became very popular back then. Then later, in 1990, Alpert coined the term HILIC, when he was doing peptide separations using the same technique.
It is getting more attention nowadays again. The reason is that you get higher sensitivity with MS detection. Also, if you are using a silica column and not a ZIC, you can take advantage of the ion-exchange properties of the silica for basic compounds. Also, if you do SPE of a urine or plasma sample, you can extract the analytes with acetonitrile, which then makes it ideally suited for injection into a HILIC separation medium.
Yes, there are difficulties with the solubility of compounds in HILIC. But today, we are not looking at sugar separations with a refractometer as the detector. We are using MS detection. Now, you do not have to inject a ton of sample any more, and the solubility is just fine.

Uwe

Looking into the toolbox one can see that instruments and techniques improve. HILIC is an alternative to “old-fashioned” straight phase and the compatibility with reversed phase solvents and the increased sensitivity in MS detection sometimes also makes it a better tool.

Still, aspects on the HILIC mechanism remain to be elucidated. With respect to the use of plain silica as stationary phase one can suspect that the “old” silicas shows another selectivity than todays? Is the ion-exchange mechanism controlable without using salt concentrations that, in the end, are in conflict with MS?

Einar,
One does not need to use table salt to do ion-exchange. Formic acid, ammonium formate, acetic acid, ammonium acetate are all fine things to work with and they are very MS-compatible. In the alkaline pH-range, you can use ammonia or ammoniumbicarbonate, but these are no good for silica and silica-based bonded phases.
Uwe

Uwe,
I don't want to put salt in the cut...
...but isn't the silica ionic interation too strong sometimes?
Einar

Einar - I've never seen a problem. Without a layer of C18 over it, silica works like a charm for all bases, even those that tail a lot on a reversed-phase bonded phase. You should try it at some time...
Uwe

I recall reading somewhere that when a very strong solvent such as H2O is used in normal phase, the mechanism changes from one of adsorption to partition (as a stationary water layer forms on the surface?), and this is why a separate term is used by some for this technique. I'm afraid I can't cite the reference just now. Any comments on this?

Maybe you read it in my book on "HPLC Columns" on pages 217 and 218?

Maybe so. :-)


I looked up that paper from Bidlingmeyer that Uwe mentions, Anal. Chem. 1982, 52, 442-447. He was getting reverse phase behavior on silica. Whoa.