[harmonslide]

I'm trying to find a marker (unretained analyte) for an ion chromatography analysis, with little luck. We are using a NaHCO₃/Na₂CO₃ buffer as the eluent on an anion-exchange column (quaternary ammonium) and utilizing eluent suppression at the end to replace all the cations with hydronium.

The system uses a conductivity detector, which picks up the analytes of interest (fluoride, bromide, phosphate) nicely, but it would be helpful to have an unretained marker here to be able to calculate t_m and hence retention factors, selectivity factors, etc. Cations are exchanged, anions are detected but also are retained by the column; I don't know any neutral compounds that might be useful for the conductivity detector.

Is anyone here familiar with IC methods? Is there a marker that I'm just missing here? How is the void time usually calculated if not? The only other thought I have would be finding the column's void volume and flow rate and estimating from there, but that seems more cumbersome than necessary.

[MOTOBALL]

Can you continuously pass a low-level solution of a salt that would give a constant signal at the conductivity detector---then inject a purely covalent compound (e.g., acetone) that is not expected to be retained (and will not precipitate the inorganic salt), and that would not have a response.  Your chromatogram would then show the compound as a negative peak on the trace.

If you try this (or any variant) please let us know if it works or not.

Motoball

[harmonslide]

That's a good idea --- I might try that if I have a chance. We did end up observing a negative peak for the solvent front when the sample matrix composition (it was pretty simple, just various salts in a carbonate buffer) was sufficiently different from the eluent; problem *apparently* solved!