1. Dilution of plant extracts (to very lower concentrations) would help get more stable results and lower CVs. Could be delays, mixing (uneven), higher concentrations leading to local activities and so on!

2. Luminol is another robust approved assay system and many commercial kits are available (still biochemical!): https://www.researchgate.net/post/Is_luminol_a_good_assay_to_detect_ROS_quenching_by_an_antioxidant

3. Several (non-chemical) ways to test for antioxidants would be: Just search them on web:

(I) DNA-damage protection (calf-thymus or plasma DNA would do; very visual and convincing).

(II) DPPH-TLC Bioautography (I am big fan of those and have used them a lot!)

(III) Cell lines and ROS production inhibition after UV/H2O2 treatment etc.

Thanks,

1. No answer without knowing all details. Thousands reasons.

2. Plz, define what is "the antioxidant capacity?"

Dear piravin

Usually a dose dependent reaction was seen in both DPPH and ABTS methods.

Which method and solvent was used to extraction. you should considered that some solvents such as hexane can affect on your results in compare with methanol or ethanol.

Before starting the tests, calibrate the spect., refresh the solutions.

Another tests such as FRAP ,Hydrogen peroxide scavenging (H2O2) assay, Nitric oxide scavenging activity, Superoxide radical scavenging activity (SOD), Reducing power method, β-carotene linoleic acid method/conjugated diene assay, Metal chelating activity and ... can help you to evaluate your results.

good luck