Negative Peaks

[Gemma]

I would welcome anyones help on this if possible.

This is the scenario...........
HPLC
Mobile phase: Ethanol/water
Diluent: Ethanol

Known source of contamination in the diluent gives two seperate positive peaks.

If this contamination gets into the mobile phase and not the diluent, the same peaks are negative rather than positive.

I Understand that the negative peaks will be due to the absence of the contaminant at that RT but do not understand the mechanism behind it, i.e. what is happening on the column.

Thanks

[Mark Tracy]

When the contaminant is in the mobile phase, it establishes an equilibrium concentration in the stationary phase. When you inject a sample with a solvent different than the mobile phase, it alters the equilibrium temporarily. The negative peaks are "holes" in the equilibrium concentration in the stationary phase that propagate much like peaks.