I'm fairly new to ion pair chromatography and would like to know if anyone has an idea of an ion pair reagent suitable for ELSD. A volatile cationic ion pair reagent is what I'm really looking for. Is there such a compound?

How about ammonium hydroxide?

I'll give it a try!

Thanks

It might be helpful to know what kind of system (column, MP, etc.) you're using and what sort of compounds you're trying to separate and detect.

I'm working with ionic surfactants on a C18 column. The mobile phase is methanol:water (40:60) at flowrates of 0.5mL/min.

Ammonium hydroxide did help in retention, but there is considerable peak tailing now. Perhaps a little method tweaking will improve the peak shape.

The volatile ion pair reagents I know of are all perfluorinated acids, which will not help to retain or separate the negatively charged sample components. Suggestions of other volatile ion pair reagents (not acidic) would be helpful.

There's a little confusion in terminology here. It's not your fault -- it's been going on for years. True "ion pairing" is the explanation for purple benzene. If you shake an aqueous solution of (purple) potassium permanganate with benzene, nothing happens. If you add an ion pairing reagent (a tetraalkyl ammonium ion, if I recall correctly) and shake, the purple color (KMnO4) can move completetly into the benzene. What has happened is that you've formed an ion pair of tetraalkylammonium + permanganate which is soluble in the benzene.

Ion pairing can occur in normal phase chromatography, but the term really "does not compute" in reversed-phase. Ionic compounds already freely dissolve in water and even in methanol. Pairing can occur temporarily, but this is probably not the retention mechanism in "ion pairing" HPLC.

The point of this diatribe is that you really aren't seeking an ion-pairing agent at all, even though all the literature you've been reading may have said so.

This gets us around to the real question: What is the problem you're trying to remedy? You don't provide any information as to what's happening and what it is you want to happen.

I infer from your last posting that you are getting insufficient retention of your analytes (ionic surfactants) on a C18 column. This is not particularly surprising. The concept of using an "ion pairing" reagent is one correct response, except for the nomenclature. It has been shown that when "ion pairing" reagents are used in reversed phase HPLC, the nonpolar part of these reagents clings to the nonpolar column rather tightly and the polar part swings freely in the mobile phase. What you've created is an ion-exchange column.

Therefore the logical solution is to use an appropriate ion-exchange column instead of the C18 column. This completely avoids the problem of interference with the ELSD. Ion-exchange columns have may have a bad reputation because formerly some were not very stable. I understand that this problem has been solved and stable columns are on the market now.

Thanks Bruce, I understand what you're saying...here's what I have seen.

I have tried a few ion exchange columns, but have observed extremely high baselines with the ELSD. Another colleague explained that as stipping off the amino-propyl groups from the column and eventually it would equilibrate and decrease the baseline. After more than 72 hours of equilibration, I didn't see much change - over 800mV reading on the ELSD output. I also tried an Absorbosphere cation exchange column with Methanol and water mobile phase (70:30), but got no retention (void time was 3.8min, and retention was around 2-3min). Are higher aqueous mobile phases the way to go with the ion exchange columns with ionic surfactants? Also, is the use of a column heater very important in stability with an ion exchange column?

Basically, I am getting adequate retention of the compounds with a RP C18 column using an ion pair reagent (ammonium hydroxide or perfluorinated acid), but they all co-elute. It is a very similar situation to a previous posting from "Laura" about sodium compounds. I probably just don't understand the chemistry behind the interactions with the mobile phase and compounds in the ion exchange mode of HPLC. I'm not even sure whether to use anion or cation exchange columns.

Thanks again!
Liz

I have no experience with such a separation. I glanced at my Alltech Associates catalog and find that they do list columns for the separation of ionic surfactants. Since Alltech also manufactures an ELSD, you might want to try contacting them directly.