Fluctuating Sulfate Recoveries

[segaloco]

Hey all, we've got a Metrohm 882 Compact Plus system going here with a Metrosep A Supp 5 column, and we are getting weird sulfate fluctuations. We'll run an initial verification and the standard comes up just fine, then two blanks with non-detect levels for all compounds, then we shoot the same standard again and all analytes look exactly the same except for sulfate, which jumps up from about 101% or 102% recovery to 111% or 112%. We've already ordered another column so our current column should be replaced within a week, eliminating the issue, but it's a shame to retire the current column forever given that sulfate just seems to be a bit squirly.

Has anyone seen this happening before? It's usually only on that second standard too, our continuing verification usually works just fine, but consistently that second standard comes up just a tad too high for our acceptance criteria.

Note, I've already tried the regeneration procedure Metrohm suggests; running 10x concentrated eluent through the column then rinsing and running normal eluent again. We've also replaced the CO2 trapping column, I believe, as well as a few other prefilters in the system. If no one has any advice, that's fine, we have another column ready to go, just hoping there might be a simple fix here. Thank you!

[Markus Laeubli, Metrohm]

Dear Segaloco

the effect sounds strange. What I could imagine is a contamination with a coeluting organic acid. Any ideas in that direction?

[Markus Laeubli, Metrohm]

What is your application?

In our files I found two similar cases. Exchanging the sample loop or the injector rotor helped in these cases respectively.
You may try these as well.

I do not think that the column is responable for the effect.

[segaloco]

Our application here is environmental analysis by EPA SW-846 Method 9056 and Clean Water Act Method 300.0. I've looked into coeluting constituents but couldn't find any definitive information. What gets me here is that no unknowns are run between the two standards, and yet sulfate still seems to fluctuate. I will definitely pass on the information with regards to the sample look and injector rotor, thank you!

[James_Ball]

Our application here is environmental analysis by EPA SW-846 Method 9056 and Clean Water Act Method 300.0. I've looked into coeluting constituents but couldn't find any definitive information. What gets me here is that no unknowns are run between the two standards, and yet sulfate still seems to fluctuate. I will definitely pass on the information with regards to the sample look and injector rotor, thank you!

Have you tried running a standard in triplicate to hit all three supressors in sequence?

If you get variation that repeats on a series of 3, then you could have a supressor that is a little dirty. We have had to use the cleaning procedure in the manual where you use H2SO4, Oxalic acid and Acetone pumped through all three supressors in sequence, then flush with DI Water and all returns to normal. Sometimes you just get crud on them when running environmental samples, especially when you run 50-75 waste water samples every night like we do.