Chromatography Forum

Hello everyone:

I use a Dionex DX-120 IC with a AERS500 (4 mm) conductivity suppressor and AS-14 (4 mm) column for anion determinations (F , Cl , NO2 , Br , NO3 , PO4 , SO4) with CO3/HCO3 as eluent.

Normally I calibrate with 7 stds at the start, I run about 80 samples of waters and in the end I pass my QC sample of recovery with very good results for years.

It recently started to happen that the QC sample began to get very high, so after that I re-injected the stds and observed that the sensitivity has increased for all anions by 40-50%

I thought it was a problem in the detector electronics inside the mainboard so we changed the board, but the problem persists.

Also note that the conductivity of mobile phase changes from the typical 17 uS to 20-25 uS.

Has anyone had a similar problem or can you think it may be happening?

Thanks and best regards
Flavio

You do not mention if you tried replacing the column itself. We have seen similar behavior after running a series of highly acidic samples (mining waste) and no amount of backflushing restored column performance. Upon replacing the column and running eluent for a bit our calibration standard came back to within 5-10% of its prior values.

Thanks for your answer, it's true, it does not probe changing the column, however, this happens systematically whenever the equipment is used, at first the peaks have a certain area and with the passage of the hours the area increases. When I have another column available I will try. Thanks and regards.

I had a very similar experience, although in the opposite direction!

I use the same suppressor with an AS14A column. 8 mM sodium carbonate, 1 mM sodium bicarbonate eluent. The normal background conductivity is around 24 uS.

Once the background conductivity started dropping into the 23's and standards were coming back lower, especially phosphate (>10%).

The background conductivity and standard recovery would slowly increase over time, however the next time I start the instrument, low conductivity again, low recovery to start with again, and then gradually increase.

Oxalic acid clean up of the suppressor followed by chemical regeneration (as described in AERS500 manual), and replacing the sample loop, solved it for me. Hopefully this can help others.

Also passing eluent through the suppressor without power can cause small, increasing peak size.