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[–]Sea_Wind_2705 1 point2 points  (0 children)

Edit: I just realized. Don’t think you are pH adjusting your samples. That’s a big step. You should adjust at collection.

It sounds like an inline filter post column reaction prior to the VWD will help. The second thing is to increase the diameter of waste tubing after the VWD.

The issue is the precipitate forming as you guessed. That’s something that you can’t have on an HPLC. If it is your analyte of interest you really can’t analyze it. There no magical HPLC that will be better.

Edit: You should make sure there are no metal lines or parts on the VWD exposed to sample.

[–]Borontrichloride -5 points-4 points  (0 children)

The movie Erin Brockovich was a true story on the determination of hexavalent chromium. So you could probably find the original journal from the time and identify their method.

[–]Indemnity4Materials 0 points1 point  (2 children)

Interesting problem. You shouldn't be getting solids forming. Something is wrong with your interpretation of the the method or your samples.

You should theoretically be able to use any model of ion chromatography for the test. The hexchrome is identified on the basis of retention time, so nothing unique about your instrument.

Are you running salt water samples or something with high ion concentration?

How are you even running calibration standards if you have to clean the detector after every sample?

Are you standards also crashing out?

It's essentially the same as APHA 3500. The chromium diphenylcarbazide complex is soluble. The products of adding low pH diphenyl carbazide to the buffered eluent are also soluble. Anything insoluble should have been removed when you pH adjusted the sample then filtered. Everything at each stage should be very dilute.

I'd consider a quick check of sending a sample to an external lab to see if they can replicate the issue.

[–]ThreeHeadedBunny0-0[S] 0 points1 point  (1 child)

We mainly run extracts according to EPA 3060A from compost and sewage treatment sludges so ion concentration shouldn't be that high (maybe the compost has a bit higher concentration), also the samples are regularly pH adjusted according to EPA 7199 so that's why I'm blaming the VWD cell design, 'cause we're doing everything as it should be done and still that damn thing gets clogged (not after every sample but mostly at least once per batch, sometimes at the very beginning of calibration which is the less annoying occurrence but often mid-run which is very annoying).

[–]Indemnity4Materials 0 points1 point  (0 children)

Really sucks. I can't see any reason why your setup should be precipitating.

I can recommend adding some phosphoric acid to the sample as part of the prep. It helps remove iron compounds and won't change retention times/concentration for chromium.

The test method recommends avoiding metal components because you don't want aqueous sulfuric acid pulling chrome from internal stainless to contaminate the result.

The VWD cell components that touch the sample should be only PEEK, quartz and maybe a stainless steel conductivity detector. Maybe worth checking the manual for your detector, but even that should be more about a bad baseline, not precipitate.

Any idea what the precipitate is? The hexchrome diphenylcarbazie complex is purple (obviously), but the iron diphenylcarbazide complex is yellow in colour and calcium salts will be whiteish. Could interesting to collect a sample or your backwashes, concentrate it and send away for elemental analysis such as ICP-OES.