Chromatography Forum

Hi,

I have just analysed a number of solutions prepared from the same stock, but diluted with solutions containing different amounts of NaCl (from 0 to 150 mM).

Standard RP-method, full loop injection on Waters Acquity (10 µl). Luna C18 column.

The samples were randomised during the sequence. The content result should be the same for all solutions, but there is a 100% correlation between the salt content and the result. The more salt, the lower the content of the active (about 10% difference between highest and lowest)
I see no degradation peaks in any of the samples.

If it had been adsorption, the salt should give higher results? Do you have any idea what has happened here?

Some thoughts...

- How many samples / injections are we talking about? Our brain tends to see more correlations than there are in reality...

- Are the dilutions homogenized? I remember having odd results when I diluted a calibration series in-vial and forgot to shake/vortex the vials.

- Did you check if there's any difference between injecting a blank without salt, and with salt? Perhaps the salt contains some analyte, who knows.

Thank you for your reply!

There were 14 different dilutions so the chance that this would happen by coincidence is very low.

I did not prepare the dilutions myself, but I will ask if they mixed everything properly.

I have not injected only salt solution, but since the content is lower with salt present, there should not be an issue (unless there is a negative peak of course).

My gut feeling is that this has something to do with the chromatography or the instrument. My analyte is very basic in nature. Could the chloride ions play some part here? Some strange ion-pairing effect?