I have a drug-loaded np system (simvastatin and oryzanol) but keep getting negative absorbance with uv vis spectrophotometer. The steps I'm using is:

1. Drugs dissolved in NP (100 ppm) by stirrer and ultrasound followed with a centrifuge.

2. Creating a calibration curve with 0.02, 0.04, 0.06, 0.08, and 0.1 ppm concentration with methanol as a solvent. The blank cell is filled with an empty np but with the same dilution as drug-loaded np

3. uv vis spectro 200 - 400 nm.

The problems are:

1. Absorbance value always negative. The higher the concentration, the more negative it became. R>0.999

2. Absorbance only peaked at 230 nm, which is the same peak as an empty np.

3. When using methanol as a blank, empty and loaded np still showing the same peak at 230nm. and when the loaded np subtracted with empty np, still showing negative absorbance.

4. drugs in one of np component showing the correct peak at 238nm for simvastatin and 316nm for oryzanol. but in np system, this result can't be achieved. Absorbance is the same pattern as np absorbance. absorbance above 300nm is 0.

5. when i use the negative value to calculate simvastatin (which i know is not valid). it gives ~7mg/mL simvastatin amount. But, with the previous analysis, I'm certain that simvastatin, just in one np component, reaches 200 mg/mL or maybe more.

Is there something wrong with this method? i'm assuming that the lack of filtration might a factor, but with a saturated system, the same pattern arises.