I am determining ethylene and propylene glycols by HPLC using an ECD detector (Pt working electrode); the mobile phase is 100 mM perchloric acid in water. Injections are performed with an autosampler. I insert a calibration verification standard, containing both glycols in water at 50.0 mg/L, between every 10 unknown samples. All unknown samples are diluted 10,000x in water. The problem I am facing is that there is a significant increase in signal obtained for the standards (all injections of the standard come from the same solution). Back-to-back injections of the standard shows that the signal gradually increases without any apparent leveling-off. Interestingly, changing the waveform of the ECD sometimes results in a downward drift instead of the gradual increase I have been observing.

Things I have already tried:
Replaced the analytical column with a new one
Replaced reference electrode of ECD
Polished the working electrode with polishing powder
Replaced sample injection loop
Modifying the waveform of the ECD

This problem has been going on for months. I have managed to run samples w/ enough accuracy by minimizing the drift by way of varying the waveform of the Ecd. However, the upward drift in signal always returns requiring that I continually search for waveforms that will minimize the drift.

If anyone can point me in a direction or better, tell me what’s going on here, I would appreciate it! Thanks.

NS

N.S.
Have you contacted the manufacturer of the detector for help? Most have an applications person who might be able to help with your specific problem and are generally happy to help.

Regards,
Mark

I have been working on polyethylene gycol recently looking for trace levels in foods. The application that I found to work best was 75mM perchloric acid, AS1-ICE , post column addition of base 400mM NaOH (convert eluent to above pH12) then use a gold working electrode with a carbohydrate waveform.

Although the addition of post column is a bit fiddly it works far better than the the recomended gycol method even in complex food matrixes.

Hope this helps and good luck

NS states that he/she is looking for ethylene and propylene glycol. My lab uses GC for this, either on polar column or on nonpolar column after trimethylsilyl derivatization. NS doesn't state that analyte is in polymeric form....

I am using a DX-500 (equipped w/ ED40 detector) from Dionex; I have been communicating with Dionex technical support about this problem since I started seeing it; the list of things that I have tried, as shown in my first post, are based on suggestions from Dionex (I forgot to add that I had also replaced the signal processing card of the detector module). None of these have solved the problem; varying the waveform of the detector, however, apparently has some effect on it (see first posting).

NS

If selectivity is not a problem, you can try RI detection (10000x diluting an sample means your method is too sensitive)

NS: has your problem been resolved? If not, can you tell me a bit more about your analytical conditions? Which column are you using? Do you know whether the problem is connected to injecting samples or do you see the same phenomenon when you inject only standards? Specifically which waveforms have you been using?