In general sense, if current height is different for each CV under the same concentrations that is likely because of ion pairing issues on the electrode surface during the redox process. It makes your electrode passivated.

If this is the case, you may try polishing (or cleaning) electrode after each CV scan. It could be tedious but worth doing it.

At one point, you may add a standard (non reactive with your analyte) material into your electrolyte solution and run CVs.

Reversibility will be changed, redox peak potentials will be shifted and higher peak separations etc.  

Hope it helps!  

Thanck for your answer, but I change electrodes every time

If you change electrode, you may not expect the same or identical responses.

Surface area/roughness factor and diffusion rates might not be the same, unless you use the identical conditions!

Thermodynamic factors would be similar (only if you used similar types of electrodes) but I do not believe kinetic factors are not identical!

Hope it helps!

 SPEs are in fact 3-electrode cells and each one contains a REF electrode. This REF electrode is not complete, it contains only Ag/AgCl, the necessary Cl- in solution is missing.  When SPE is used with a very small sample (1 drop), AgCl can saturate the solution and you may have a stable REF potential. When you use SPE in a large volume sample, AgCl can't saturate the sample solution and the REF potential is unstable (continuous variation).

SPEs are commonly used for quantitative analyses. You may use SPE only according to manufacturer instructions, but also you have a lot of  choices:

- use only small volume samples

and for larger volumes:

- add Cl- (as much as possible, e.g. 3.5 M) in the sample composition

- even better, saturate the sample solution with AgCl (using AgCl pp.)

- disconnect the REF of SPE and use a common REF electrode

Thank you very much for your answer. I tried changing the electrolyte from KNO3 0.1M to KCl 3M, both using 50 ul sample volume directly on the SPEs surface or 10 ml volume using an electrochemical cell. Concentrations of monosodium glutamate varied between 25 uM and 500 uM and the electrods were DS-DRP 110 (carbon WE). Still, the signals obtained look almost identical for all concentrations. I attached beneath the resulting plots.  

Dear Dora Popescu, can you describe (or note a link) your electrodes (and cell).

In the case(s) of bigger volume(s) : your electrodes are (exactly) the same ?

(FYI, I could not open your files!)

Can you also try different scan rate, maybe increasing scan rate in the same scan range?

Theoretically, you may see higher current height with increasing scan rate.

You also expect higher separation of redox peak potentials due to ohmic loss at higher scan rate, if your system is a diffusion control mode. 

If you don't see any changes, you could have a proceeding chemical reaction and followup electrochemical reactions. If this is a case, your electrode surface is already passivated. Then, you will observe sort of similar current height.

What you might try to do is that measure the rest potential and keep there for a minute or more, then try CV scans to see what's different.

(Or run multi scans instead of single scan!)

What I am saying is to try the electrode surface cleaning using indirect controlled potential electrolysis just before CV scans. When you do multi scans, I expect passivating and stripping electrode surface repeatedly during the positive and negative scans.

Let's see how it is going on!

Hope it helps!

Increasing scan rates gave good results and electrodes are changed after every use, so I don't think it'is pasivation. Thanks for response