Chromatography Forum

I am using an ICS-1100 with carbonate/bicarbonate eluent for anion analysis. On blank samples I am getting a small dip at the end of the water dip as it should be returning to the baseline. This bottom of the second dip is where the detected baseline starts, but then the conductivity rises back to the original baseline. This action also shows up in the low calibration standard since fluoride comes off so early.

Any thoughts as to what might be causing this?

Does the baseline return to normal before the first peak of interest elutes?

It really shows up on blank samples, and when I compare those to calibration samples it looks like the dip actually lines up with where the fluoride peak would be. It doesn't seem to show up when fluoride is present. We have 2 identical machines and it is more noticeable on one of them.

You should check purity of your eluent and try prepare it from different source than usually. You should also check quality of water which you use for eluent preparation

Thanks. We dilute the eluent 10x from the manufacturer, and have been through several bottles. The water is 17.95 and was serviced not long ago. Also, we have two machines and this really shows up on only one of them.

What about columns? Have you tried to check other column the same type in "bad machine"? What's happened when you injected sample in mobile phase? Can you swap injection valves (try valve from "good" set in "bad" machine)?

Thanks for the thoughts. It happens a little bit on the other machine (I suspect that might be from the water) but is quite a bit worse on this machine and has been for a while.

A little more info. I remade some of the lines and replaced the sample loop but still seeing the same thing. Today I injected a sample of the eluent and the dip was there, even stronger than the blank.

Another thing to mention is I think our pressure is slightly out of calibration. A service tech thought it wasn't getting all the way to zero so he reset the offset. After using it for a while it seems like it takes longer to register any pressure at start up than it should.

I tried some different water, seemed to be better for a few injections but is now back. Does this sound like a problem with the injector at this point?

Hi
If this this occurs on two different instruments the common is the DI water. Without seeing a chromatogram, Its likely acetate, which elutes immediately before fluoride and after the water dip. I have seen acetate removed from blanks by changing the di water cartridge (pm the di water system)
You can prove or disprove this by running blanks with a different water source

It is definitely a lot more noticeable on this machine, don't really see it on the other one at this point. On the report that came with the column it shows acetate eluting right after fluoride rather than before it. I tried some different water but I'll try it again in case I didn't stick with it long enough.

Thanks!

If it uses a switching valve in the injector it could be some contamination in the rotor of the valve. The fact that injecting eluent instead of water and the peak is worse would say that the eluent is pulling more of the contaminate into the flow than the water does.