a negative peak in anion determination by ion chromatography

[sofia babanova]

The method that I am using for phosphite and phosphate determination is ion chromatographic. The problem is that during the measurement a negative peak appeares with moving retention time, even in the blank sample which is water. What can be the reson?

[tom jupille]

It's called a "system peak". It's caused by the disturbance in equilibrium due to the injection process. Not at all unusual to see it with a conductivity detector.

[sofia babanova]

I don`t think that this is a "system peak" because the retention time of this peak is moving to more negative values when one and the same sample is injected. And this peak appeared only in the last analysis but not in the previos ones.

[Markus Laeubli, Metrohm]

Please give more details e.g. column, eluent ...

[sofia babanova]

We found out the reson. This peak comes from the water, it wasn`t pure enough.

[ivanvins]

Typicaly, negative peaks in ion chromatography (and other modes as well) come from some eluent constituent or contamination, which is giving response on the detector. In case the concentration of such compound in sample is lower than in eluent, you are getting the negative peak. The reason for the retention time shift may be in column or precolumn saturation (some sort of frontal chromatography may take place in case the contaminant has high affinity to the column)