Negative peak

[vicou_chen]

I am a new user of RP-HPLC,in many cases,I always find a negative peak prior to the eluting of the analytes.So ,my question is the mechanism that the negative peak occurs,could anybody help me answer this question?

[chromatographer1]

A negative peak indicates a change in refractive index, or UV absorption.

It is usually caused by the change in the mobile phase composition entering the detector from the solvent that was injected or the salts or water present in the dissolved sample.

You can minimize this by injecting your sample only after dissolving it into the mobile phase.

best wishes,

Rod