What causes negative chromatographic peaks using fluorescence detection. The peak interferes with the analyte, chloroquine (retention time = 10 min.). I've tried altering the mobile phase composition (35% methanol, 1% TEA) to get separation to no avail. The neg peak is never consistent. Sometime it is large and sometimes it is small. Any suggestions?

Thanks!

Could it be that your mobile phase fluoresces or scatters light, and a quenching (etc.) unknown (dirt) does not chromatograph correctly (consistently)?
What happens when you inject water? Is your baseline smooth and level? What does a dummy injection (actuating the valve .... without injecting anything)do? What happens to the base line when you let your different mobile phases pass the column?