The negative absorbance means that you have the calibration problems of zero baseline: 1) is the solvent the same both in reference and working samples? 2) is the both cuvettes' material the same? (you need quartz for UV) 3) is the reference cuvette clean? 4) probably, your solvent has low transparency in the UV region, and the calibration program’s mistake takes place. Try to calibrate your device 1) without cuvette, 2) with empty cuvette, and repeat getting spectrum.