Reason is as below. If you ae using .
RI detector: refractive index of the analyte lower than that of the mobile phase
reverse detector polarity to obtain positive peaks
UV detector: absorption of the analyte lower than absorption of the mobile
use a mobile phase with lower UV absorption; if recycling solvent, use fresh HPLC grade eluent when the recycled mobile phase starts to affect detection
A technique in which a mobile phase additive causes a positive detector signal output. When a solute elutes from the column, it dilutes the signal and gives a negative peak or a vacancy. The technique has been most recently applied to single column ion chromatography, in which mobile phases that absorb in the UV region, such as citrate and phthalate buffers, are used. When a nonabsorbing anion elutes, it dilutes the UV absorbing background and causes a negative peak. The detector output leads are usually reversed to make the chromatogram look normal.