The problem with negative peaks can occur due to following reasons:
a) Recorder lead reversed. Solution: Check polarity. b) Refractive Index of solute less than of mobile phase (RI detector). Solution: Use mobile phase with lower refractive index or reverse recorder leads. c) Sample solvent and mobile phase differ greatly in composition (UVdetector). Solution: Adjust or change sample solvent. Dilute sample in mobile phase whenever possible. d) Mobile phase more absorptive than sample components to UV wavelength (vacancy peaks). Solution: Change UV wavelength or use mobile phase that does not adsorb chosen wavelength.