Injection dip and order of elution

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IC newbie here - I have noticed what I believe folks call the “injection dip” or “water dip” in every inject, whether it is a sample or a blank. Why does that dip happen? Why is the peak negative instead of positive?

Additionally, what dictates order of elution between different ions? My focus will be inorganic ion analysis. Why does F- elute first, even though it should have the strongest affinity for the column resin? Does it have to do with size?

Thanks for the help!
See my response to your post in the GPC section for more background. In GPC, elution order is base on size, with small molecules eluting last. The diluent solvent molecules are small, so any peak that they form will be at the end of the chromatogram. In ion exchange, the diluent solvent molecules are usually un- or weakly retained, so any system peak formed will occur at the beginning of the chromatogram -- that's what the "water dip" is. It's usually negative because the diluent at that point no longer contains many ions, hence has lower conductivity.

To a first approximation, affinity for the resin is related to the charge density of the *hydrated* ion; it increases as you go down the periodic table. Visualize it as fluoride having a lot of water stuck to it, chloride less, and so on.
-- Tom Jupille
LC Resources / Separation Science Associates
tjupille@lcresources.com
+ 1 (925) 297-5374
It's a bit complicated. Sometimes the order of elution between analytes can flip if you increase the eluent concentration. On my ICS 2100 Bromide elutes before sulfate at 23 mM, but at 27 mM sulfate comes off the column before bromide. But at 27mM carbonate and nitrite are too close. Even a carbonate removal device will not eliminate enough carbonate to stop that.
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