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Working with a silver working electrode

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Characteristics for analysis of iodide (212_003) using a flow cell with silver working electrode.

Summary

A summary is given of some practical issues when working with a silver working electrode. These are:

  1. Silver always reacts with iodide between -100 and +100 mV working potential. The output signal of an ECD is inverted at a potential below zero, resulting in negative peaks. This is common practice in any ECD, to enable reductive detection. In a DECADE II or Elite the polarity can be switched to have positive peaks. For an Intro and DECADE, simply invert the output wiring.
  2. The mobile phase should never contain halogens, such as chloride, bromide, iodide etc. The same holds for other substances that are known to react with silver. This will lead to serious contamination of the WE, a high background current and loss of sensitivity.
  3. It is strongly advised not to recycle the mobile phase when injecting samples. Especially in case of urine samples the buffer will be contaminated already after 1 injection!
  4. Characteristic for ion chromatography is the occurrence of system peaks which are caused by displacement of ions on the column. These system peaks are minimized by adapting the sample buffer to the mobile phase (same pH, same ionic strength etc.). Dilution of samples with a high ionic strength (urine) may improve the chromatography.
  5. Certain inorganic ions may give a sharp positive peak followed by a sharp negative peak. This is normal and due to oxidation followed by reduction processes at the working electrode.
  6. Depending on the sample composition, it may be necessary to polish the working electrode each day. This takes usually only 5 min, after installation the cell should be stable within 15 min.
  7. At a working potential of above 250 mV the oxidation of silver occurs, resulting in a very large cell current. It is therefore strongly advised not to work at a potential above 200 mV vs. Ag/AgCl.
  8. In our system the pressure was about 40 bar and therefore the pulse dampener is not much effective (the best performance  for a pulse damper is at a pressure above 80 bar). We therefore increased the pressure by placing a restrictor between the pulse dampener and injector (do NOT place this restrictor AFTER the injector!).

Another crucial point in the analysis of iodide is the quality of the anion exchange column. If a column is used other than recommended by Antec, make sure that the column supplier guarantees the performance for iodide.  

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