Hi, we use EC detection for few years to analyze our drug, without any important problems.Now, when injecting our samples (SPE extraction) or drug in pH 3.0 ammonium phosphate buffer ,or especially in Krebs buffer pH 7.4 (new introduction in our method), the baseline drops abrutly to a very negative signal,at 6.4 min. The running time is 15 min. We can't see the internal standard peak since the signal is too low. It takes many zero adjustments getting the zero back to normal. And after some other injections, the problem comes back again.
Mobile phase:phosphoric acid:ACN:MeOH, recycled with Alltech Recycler.
Detector: Coulochem II, ESA, Sensitivity:5µA
Integrator: Shimadzu CR601, signal should be -1000 to +5000, and it goes down to -5000! during the injection.
We taught the Krebs buffer responsible for that, but after returning to samples injection from SPE it does the same.

Does anyone have an idea how to solve that problem?

Johanne

Have you tried to inject a reference solution diluted in mobile phase? It sounds like a system peak due to pH changes. EC is very sensible to pH changes. You should not see the neg. peak if EC detector were OK.
Are you using oxidative or reductive monitoring?
Potentials?
It would be helpful as much information as possible.
Regards

What is the injection volume and flow-rate of your separation?
Do you dilute the Krebs buffer samples to an apparent pH close to that of the mobile phase?
Krebs buffer cannot be injected as such, since you would be eluting a solvent plug at pH 7.4 in a very acidic environment: this would cause disturbance to the column equilibration. In addition, if you are measuring ionizable compounds, the peak shape at pH 7.4 would be very likely different from the one you observe normally with your mobile phase.
What is the effect of calcium and other cations/anions present in Krebs buffer on your separation?

To both anonymous persons

We use oxidative monitoring. Potentiels applied are 450 and 750 mV.
The injection volume is 50 µl and the flow-rate 1.8ml/min
We didn't notice any difference in peaks shape from one to the other.
With samples diluted in mobile phase,we still have the same problem.
We changed sensitivity from 500nA to 5 µA and got better results, but still samples got down again.
Johanne

Well, I'm the first anonymus and my English is noy very good but I'll try to help you. EC detection is a problem, in my experience I've had problems with:
Passivating: Sometimes stainless steel form the injector loop can release Fe or others metals, or maybe organic contaminants which can be electroactives. Try to inject the final solvents alone and mobile phase alone. Is the peak still there?. If so, it is highly probable that injector is contaminated. Try another injector, in the same conditions
Chek your EC manuals for passivating stailess steel tubes. If you don't find how to do it, write a message here. Also, organic compounds could be bonded to the staionary phase at the column head. Have you tried a new column?
We have also had problems with some batches of SPE columns, in which we found contaminants.
The second step is to chek salts (ammonium phosphate)and the H3PO4. Change them for new ones.
The 3rd step could be organic solvent. We had problems with ACN from Fisher for EC.
I hope to be helpful.
Good Luck! Johanne.
Marcelo