Ion chromatography - Anion detection

[Zydrunas Stanius]

Dear colleagues,


I have some thing to ask, maybe somebody is more expierenced than me in ion chromatography.

some conditions:
Anion exchange column Shim pack IC-SA2;
electrochemical suppression HIC-20A (Shimadzu);
Detection - Conductivity;
Mobile phase - 1,8 mM Na2CO3 + 1,7 mM NaHCO3;

Problem:

In some samples instead of NO3(-) peak I have negative peak, while analysing stanard solution there is no problems like this and calibration curve is perfect even in low concentrations (ppb's). Also the negative peak appears just in some samples - in others it's no problem (like in pure water it's just flat baseline at NO3 RT.


What could cause the phenomena. Something what is in some samples and suppresses the NO3 conductivity? lack of ideas because I am quite new in IC. Maybe somebody has faces same problem?




will be very thankfull


zydrunas

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[krickos]

Hi

Been awhile since I done IC and perhaps someone else more "up to date" can help you, but it sounds like you might have a interfeering "water dip" caused by low (variations of) concentrations of CO3 and HCO3 in your samples.

You can google Ion chromatography and water dip or for example read about it in:
Chapter 4.6 from "ion chromatography principles and applications" (J of Chrom. Libery, vol 46, P.R. Haddad & P.E. Jackson, 1990).

If it is the "water dip" issue I seem to recall that you usually solve it by adding 1ml of concentrated eluent to 100ml standard and samples.

www.epa.gov/glnpo/lmmb/methods/methd200.pdf

[Markus Laeubli, Metrohm]

it sounds like you might have a interfeering "water dip" caused by low (variations of) concentrations of CO3 and HCO3 in your samples.
...
If it is the "water dip" issue I seem to recall that you usually solve it by adding 1ml of concentrated eluent to 100ml standard and samples.

Usually the water dip is that negative peak at t0 (early in the chromatogram). The carbonate interference (I call it system peak) might be the problem. But just adding eluent concentrate will not really help. As you might turn a negative system peak into a positive one. Anyway it will interfere with your nitrate peak and therefore with the result.

There are two ways of solving the problem:
1. You may change the eluent composition. A different carbonate/bicarbonate ratio (e.g. a different pH) might move the system peak.
2. Removal of the CO2 after the suppression step almost removes the system peak. We published a Poster on this issue.

[Jade.Barker]

Removal of the CO2 after the suppression step almost removes the system peak.

I am a novice, so forgive me if this is an elementary question: Is there a good way to remove disolved CO2 in the sample? We test fermentation samples from different points in fuel ethanol plants. I suspect the CO2 could be the interfering substance in some problem chromatograms.

[Markus Laeubli, Metrohm]

For samples we use a Sample degasser (which is virtually the same as an eluent degaser).
For the Eluent after suppression we use a slightly different setup, but again almost the same as a degassing tool.
In a batch procedure applying vaccum might help.

[Jade.Barker]

In a batch procedure applying vaccum might help.

Thats a good idea, I think I have a set up I could try for that. It would be easy enough to test, I would just have to pick up some beer and try to make it "flat" on purpose. I had also thought about vortexing some in a tube, but I'm not sure how much that would help. Thanks for the suggestion.