while performing DPPH antioxidant assay for my samples I am getting a lower absorbance value for blank when compared to my extracts. DPPH solution- 24mg DPPH mixed with 100mL methanol.

Blank- 50% acetone+ DPPH working solution (10uL+190uL)

Sample- Extract+DPPH working solution (10uL+190uL)

Standard- Trolox 20-250mg/L

I am subtracting blank absorbance from extract absorbance (Blank- Extract) to get the final value for extract absorbance and then using this value to calculate antioxidant capacity in terms of Trolox equivalence.

Could someone please advice why is this happening?

Thanks

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