Negative Peak Interfering With Nitrate

Discussions about IC and related topics

8 posts Page 1 of 1
Hi all,

I've spent several years working with liquid chromatography, but have been struggling with an IC issue for several months now and I'm hoping someone here can offer a possible solution.

My lab has a Dionex ICS 1000 and a Thermo Scientific Aquion. Both instruments are used for inorganic anion analysis of drinking water and waste water by EPA Method 300.0. Both use the Dionex IonPac AS22 (4 x 25 mm) analytical column with an AG22 (4 x 50 mm) guard column and an eluent that is
4.5 mM Na2CO3/1.4 mM NaHCO3.

When I first started working with this method a couple of years ago on the ICS-1000, there were no issues. Then all of a sudden (after changing the columns), I started seeing what I assumed was a negative peak eluting approx. 0.5 min after the nitrate peak. A couple of months later, we purchased the Aquion and saw the exact same negative peak appear after nitrate. The Thermo tech who helped with the install did not know what was causing the negative peak as he had never seen it happen before. Since the negative peak was not interfering with nitrate, we decided to just let it be. However...

Recently , that negative peak has started to co-elute with the nitrate peak which is affecting my recovery. I'm seeing this on both of my ICs (different column lot #s), but it is a bigger problem on the Aquion.

I've been trying to work with Thermo to troubleshoot what is happening because I believe the issue was column-related. They are telling me that the negative peak is a carbonate dip. Here are the things I have tried at their suggestion with no success.
- Different water source
- New reagents for eluent prep.
- Weaken the concentration of Na2CO3
- Increase the ratio of Na2CO3 in the eluent to move retention times

I'm just about at my wit's end here. Any help would be appreciated!

Thanks in Advance
It sounds like you are describing the system peak, in your case it is CO3. Is there a water dip as well as the 2nd negative peak you are describing? Can you post a chromatogram?

Page 31 of the attached manual shows what my chromatogram should look like. I have a system peak that appears consistently before the fluoride peak around the two minute mar.k

The negative dip that I'm trying to troubleshoot appears right after nitrate.

On my ICS1000, the retention time of nitrate is ~8.00 min. and the retention time of the negative dip is ~9.00 min. Depending on the characteristics of the sample I'm analyzing, the negative peak may or may not affect my recovery.

On my Aquion, the retention time of nitrate is ~8.1 min. and the retention time of the negative dip is ~8.51 min. which is creating an interference and lowering my recovery. Typically, my nitrate recovery for my standard solution on this instrument is 96% or better. Now my recovery has fallen between 86 - 89% which fails our acceptance limits for this method.

(I'd like to add an attachment of my actual chromatograms, but I'm not allowed to do so in this forum.)
The system peak before fluoride is the water dip. Water has lower conductivity then eluent so that is where that negative peak comes from. The one after NO3 is referred to as the system peak and relates to the carbonate conductivity from the eluent. Sounds like you have made the eluent from fresh sources of Na2CO3/NaHCO3. It could also be your water source, so i would suggest trying a different source of 18Mo water. Have you tried running a standard without the guard? Is the system peak present in a blank?
Yes, I see this peak in blanks as well as standards or samples. I have tried two different water sources and the peak is always there.

I can try an injection without the the guard. Thanks for the suggestion.
Removing the guard column does not remove the negative peak. It is still interfering with nitrate. I will try working with Thermo again.
Hi LabLady,

I manage these columns and would like to help with this. Please contact me directly ( and if possible, include the serial number of the last column that worked well and the serials of the ones giving you problems now. Also, please include details on the various eluent concentrations you have tried. Any cmbx files you can attach would be very helpful, too.
Best regards,

John Guajardo
Product Manager, IC Columns
Thermo Fisher Scientific
Hi Lablady,

By any chance did you get any response from Thermo Fischer regarding the dip?
I am getting the same dip after washing the column with acid and then washing with eluent overnight.

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