Missing a peak in Gel Filtration Standard (SEC HPLC)?

Hi Everyone. I ran GFS on Agilent 1100 HPLC, and the profile is atypical. It seems to be missing a peak and there is also a negative peak. Does anyone know what might be going on?

Mobile phase: 25 mM Ammonium Acetate, pH 8 (not filtered)

Flow rate: 0.3 mL/min

Wavelength: 280 nm

Column: AdvanceBio SEC 130Å, 7.8 x 300 mm, 2.7 µm (brand new)

GFS preparation: Reconstituted 1 vial of Bio-Rad GFS with 1 mL of HPLC Grade water, centrifuged, and injected supernatant at 5 uL.

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