Metrohm 881 baseline problem

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6 posts Page 1 of 1
We have a Metrohm 881 compact IC which in the past has run mainly Chlorite, Bromate and Chlorate by EPA300.1 by conductivity and some samples for Hexavalent Chromium by EPA 218.7 UV/Post column derivatization.

I started last week to set up to run the main Anion list by 300.1 Fluoride, Chloride, Nitrite, Bromide, Nitrate, Phosphate, and Sulfate (listed in elution order) using the same Supp7 column as for the other 300.1 analytes. The first injections gave very very broad peak for Fluoride, massive tailing, Chloride, Nitrite, had only a tiny amount of fronting, Bromide was perfect, Nitrate had a tiny amount of tailing, and Phosphate was so broad it was barely detectable as a peak and bleed over into Sulfate which was also tailing very badly and very broad round top peak.

I did all of the suppressor cleaning procedures twice and it helped with peak shapes somewhat. I also ran the oxalic acid and acetone through the MCS and that also improved peak shapes. The peaks are now only a little misshapen and could be usable except I have an undulating baseline which has been present for a long time even when doing the Chlorate, Bromate, Chlorite analysis. This hump in the baseline did interfere with the Chlorate and DCAA surrogate before but makes finding low level peaks for Phosphate and Nitrate very difficult now. The top of the hump seems to stay at about 24 minutes. I switched to a Supp5 column and also found the hump at the end of the run which was 18 minutes and sometimes carried over into the next run with a drop at the beginning of the run.

Image

From bottom to top is three blanks and a second level calibration standard, the next in that series showed that the shape of the humps follows the suppressor segments as it also had the double hump, but I didn't include that one.

Could this be related to a problem in the suppressor? If it is prior to the suppressor it would have to be in the injection valve, but it is odd that the retention time of the hump doesn't change when changing column and eluent mixture. I have been trying to get rid of this for a long time now, just wondering if anyone else has seen this problem.

Thanks.
The past is there to guide us into the future, not to dwell in.
Dear James_Ball

So far I never saw such an effect coming from the suppressor. A tailing fluoride peak might result from an (unknown) contamination of the detector.
But just from your description it is not clear where the problem comes from.

I would recommend to check back with your Metrohm sales or service person to test your system by running it with a different suppressor roter and a different detector. Also the injector should be checked.

Please do not hesitate to contact me directly under lae@metrohm.com.
Dr. Markus Laeubli
Manager Marketing Support IC
(retired)
Metrohm AG
9101 Herisau
Switzerland
Thank you very much for the reply. My next step is to change the PEEK rotor in the injection valve since it looks to be discolored and did not clean up when I sonicated it.

Can this detector be cleaned using the dilute nitric acid mentioned in the manual for the older 761 compact unit? I noticed that passivation procedure had been removed from the 881 user manual so I wanted to be sure before using it.

When I can get a day without many samples I plan to swap the suppressor rotor from our other instrument and see if the problem remains or moves to the other unit. That will either confirm or eliminate the suppressor as the problem.
The past is there to guide us into the future, not to dwell in.
Dear James_Ball

Yes, passivation of the detector is still a valuable procedure.

As "last chance" passivation some times the following procedure might be applied:

1) 30 mL 0.02 mol/L potassium permanganate (pH=3, adjusted with HNO3)
2) 50 mL 0.2 mol/L HNO3 / 5% H2O2; (H2O2 is added to remove the manganese dioxide)
3) rinse with 50 mL 0.2 HNO3
4) rinse with ultra pure water
5) rinse with eluent
Dr. Markus Laeubli
Manager Marketing Support IC
(retired)
Metrohm AG
9101 Herisau
Switzerland
James_Ball wrote:
Thank you very much for the reply. My next step is to change the PEEK rotor in the injection valve since it looks to be discolored and did not clean up when I sonicated it.

Can this detector be cleaned using the dilute nitric acid mentioned in the manual for the older 761 compact unit? I noticed that passivation procedure had been removed from the 881 user manual so I wanted to be sure before using it.

When I can get a day without many samples I plan to swap the suppressor rotor from our other instrument and see if the problem remains or moves to the other unit. That will either confirm or eliminate the suppressor as the problem.



Hi,

I have a similar issue to this. Did you find out what was behind the problem?

Thanks,
G
Dear glj

quite often a broad peak occurs when the regenerant is too week. Depending on the way of delivering the regenerant, we recommend to use 100 mml/L sulfuric acid or 500 mmol/L sulfuric acid for peristaltic pump and Dosino regeneration rspectively. Initially we recommend to to run 6 times a regeneration without an injection.
Dr. Markus Laeubli
Manager Marketing Support IC
(retired)
Metrohm AG
9101 Herisau
Switzerland
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