Ion Chromatography-High backgr. conductivity in Dionex DX500

[Vangelis]

We recently followed the cleanup procedure suggested by the Dionex manual, and we passed through our CS12 (configured with a CG12) solutions of 1M HCl (for 60') and 0,01M HCl (for 15' before and after the 1M solution). After that, the system was left to equilibrate with eluent running through for 1 hour (the manual suggests 30 mins), and then the suppressor was again connected, the guard column was placed before the column and the system was turnt to work again.

While the peaks look almost ideal, and separation seems to take place as it should (checking standards' labelled concentrations are verified with a discrepancy within 5%), the background conductivity remains as high as 6.0us (while it should normally be less than 2.0us), with suppressor working at 100mA. At the same time, the H20 dip at the beginning of the chromatogram has grown since earlier.

What would cause that high background conductivity? Could it be any leftovers of HCl as we suspect - but if it's this how can it be faced? Could it be any HCl leftovers that passed through the suppressor and contaminated it?

Any ideas/suggestions would be highly appreciated.

[Chris Pohl]

Vangelis,

There are several possibilities depending upon the specifics of your system. Generally, the most common cause of elevated background in a cation system is contamination of the eluent. But, presumably, in your case this possibility can be ruled out since most likely the eluent you are using was exactly the same as before you observed the problem and the source of the high background is coming from elsewhere. One possibility is that the problem originates from system contamination which occurred during the time you were running the cleaning solution through the column. Did you use your analytical pump for the column regeneration protocol? If so, did you make sure that the pump was thoroughly rinsed and that the eluent container was either replaced or thoroughly rinsed? Residual chloride in your eluent arising from contamination due to the prior regeneration protocol may be responsible for this elevated background. Another common cause of elevated background is carbonate contamination. You might want to try a quick regeneration of the suppressor with 0.5 molar sodium hydroxide (it's important, to make sure that you use good-quality hydroxide free from carbonate contamination in order for this to be helpful) to see if this helps bring your background back into line.

[Vangelis]

Chris:

thank you for the reply. We eventually solved the problem by simply letting eluent run through the system for almost 24 hours. Normal background conductivities were again achieved and the peaks we get look almost ideal. It was obviously a result of Chloride remaining somewhere in the system after the cleanup procedure.

However, since you referred to it, do u think it's a good idea to implement this quick regeneration technique you suggested with the use of NaOH instead of using the oxalic acid solution that is suggested by the manual, in case i observe any problems with the suppressor?