Elementary Ion Chromatography

[stevel]

Hi All--

Can someone suggest a good resource for learning basic ion chromatography? I have a strong background in HPLC but am feeling a little lost with our IC system. I am trying to develop a separation using a suppressed anion system.

My starting point is an application note that uses a 3.2 mM carbonate/1.0 mM bicarbonate mobile phase. I've tried adjusting these ratios as one might with an HPLC mobile phase composition, but I don't see much happening to improve my resolution.

Can someone explain how the CO3/HCO3 system works to elute the analyte ions? Is one considered the strong solvent and the other the weak solvent?

[tom jupille]

Most basic IC separations are "cookbook". You decide what ions you want to separate and go to Dionex (or other vendor of choice) and they recommend the column and the eluant composition.

If you're trying to separate standard inorganic anions (e.g., things like Cl-, NO3-, etc.), then the CO3= is the "stronger" eluant and HCO3- is the "weaker" eluant (in general, the higher the charge on the driving ion, the more effectively it displaces analyte ions).

In ion exchange, analyte ions of the same charge respond the same way to ionic strength changes, which means that changing the carbonate/bicarbonate ratio usually doesn't change selectivity.

If you want to get more deeply into it, get a copy of the book "Ion Chromatography" by Fritz and Gjerde:
http://www.amazon.com/Ion-Chromatograph ... 785&sr=8-1

[Chris Pohl]

stevel,

Just to add to Tom's post, since carbonate is a much more potent eluent than bicarbonate, the function of bicarbonate is primarily to control the pH. For common anions, the eluent pH has very little effect on selectivity but it does affect the selectivity of phosphate. So if you're trying to adjust the separation of phosphate and sulfate, more bicarbonate will move phosphate earlier and less bicarbonate will move it later without having much effect on the retention time of sulfate. On the other hand, the retention time of monovalent species behaves very differently than divalent species with respect to eluent concentration. If, for example, you cut in half your eluent concentration when you are using only carbonate, the retention time of sulfate should double but monovalent species should only increase by a square root of two factor. As Tom says, the explanation as to why this is so can be found in a number of references including books on ion chromatography by Fritz, Haddad, Weiss or Small. The latter reference is written by the father of modern ion chromatography and focuses mostly on theory with minimal applications information. The book by Weiss has the most comprehensive applications information and is the most up-to-date if you get the third edition.

[Markus Laeubli, Metrohm]

Dear stevel:

Fritz and Gjerde's book is highly recommended.
Anyway, if you do want a short and free of charge introduction ask for Metrohm's Practical Ion Chromatography - An Introduction.