Chromatography Forum

I am conducting troubleshooting on my IC system (Thermo Dionex ICS-6000) for anions analysis. My calibration verification QC's are failing low for Nitrate and high for Nitrite and I have run out of ideas. I have used the same vendors and standards with no issues in the past, what could be different all of a sudden?

My standards:

Calibration: Dionex Seven Anion STD II (Product: 057590)

( I have tried different lots)

Calibration Verification: Inorganic Ventures (IV-stock-64)

ICV/CCV- should be running at 5 ppm

(I have also tried different lots)

What else am I missing?

My latest run shows nitrite at 5.7566 ppm and nitrate at 3.9793 ppm

It is possible you are seeing nitrification in your calibration standards, but I wouldn't expect that across multiple lots as long as they are unexpired and stored refrigerated.

How far apart were the different lots prepared by the vendor?

How does the bias on your calibration curves look?

Prep a 5 ppm calibration standard with your ICV and run it directly after your ICV. That should help narrow it down to system/cal differences vs standard differences.

If it looks like standards, get a third source.

I use Dionex seven Anion Standard II as a QC solution and noticed that lot 23-12JKS had far too much Nitrate back in June 2024. The Nitrite levels were low but not unacceptable - about 94% of what was on the COA vs 113% for Nitrate. The next lot I got was better but far from ideal. That one had 108% of what the COA said for Nitrate. I suffered with it for a few months and then ordered another bottle. Finally lot 23-87JKS was great.

I have had this happen before from a different company so I order stock well before it will be needed and test it as soon as I get it. I would prefer to make my own standards but the bosses won't let me because everyone wants to show auditors a shiny COA. I can make better standards than any company and I suggest people have their own chemicals on hand so they can in a case like this. ERA is the most reliable manufacturer of standards that I have seen. Try getting QC solutions from them.