HPLC of quaternary ammonium compunds

[Ben_Zene]

Hello!

I have a pretty basic question about quantitating the amount of a quaternary ammonium compund in a sample. If I use the chloride salt for my standards, but am trying to quantify a carboxylate salt of the ammonium compund, is this valid? The mobile phase being used is 70:30 acetonitrile/water. My guess is it's not valid because of the different degrees of dissociation of the quats in this mobile phase. Maybe a different mobile phase would do?

Thanks!

[Uwe Neue]

The answer to your question may be very simple, but it also can be very complex, depending on what you are seeing.

If the chloride salt and the carboxylate salt elute at the same retention time and with the same peak shape, then I would not worry any more. Then the quantitation of the ammonium compound can be done, assuming that your detection scheme is specific enough for the compounds and does not see the counterion.
If you do not get the same retention time or you get complex peak shapes, then the counterion contributes to retention/elution. If this is the case, you can fix it by adding a background electrolyte to the mobile phase.

[Chris Pohl]

Ben,

Although I have no disagreement with Uwe's suggestions below, I thought it might be worth a clarifying comment and a suggestion.

First, quaternary compounds are fully ionized at any pH so it shouldn't really matter from the point of view of the quaternary compound what the counterion is in terms of ionization of the quaternary compound. On the other hand, if your mobile phase contains no electrolytes, there are some possible complications as suggested by Uwe. There is a simple solution to this problem, though: add an electrolyte to the mobile phase to "normalize" the retention time behavior of the quaternary compound. In this case it won't matter what the counterion was in the sample. For example, if you add a small amount of sodium acetate (say, for example, 10 mM sodium acetate adjusted to pH 5) the quaternary compound will be separated as the acetate salt regardless of the form in which it was injected.