There many reference for antioxidant activity of DPPH like http://www.ifrj.upm.edu.my/23%20(04)%202016/(50).pdf

http://www.sciencedirect.com/science/article/pii/S0278691510002152

Efficacy of chemically characterized Ocimum gratissimum L. essential oil as an antioxidant and a safe plant based antimicrobial against fungal and aflatoxin B1 contamination of spices

Safety assessment of Zanthoxylum alatum Roxb. essential oil, its antifungal, antiaflatoxin, antioxidant activity and efficacy as antimicrobial in preservation of Piper nigrum L. fruits

You may see this 

http://www.sciencedirect.com/science/article/pii/S2095177915000131

You need to understand the mechanism behind the protocol first then you can apply it judiciously to your plant material.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3551182/

Please read this article. it may helpful for your research

http://dx.doi.org/10.1080/09670262.2016.1218938

Please read this article. 

Regards 

Experiments were performed out according to the standardized protocol]. 3.8ml of methanol was taken as blank, where as 3ml of methanol added in0.3ml of 0.4 mM DPPH solution was taken as control.  Reaction mixture was prepared by taking 3ml of methanol mixed well with 0.5 mL of aqueous & alcoholic extracts and 0.3ml of 0.4 mM DPPH solution. Solution such as blank, control and reaction mixture were allowed to incubate in the dark for 30 min. The colour of the reaction mixture fades as compared to the control and the reduction is observed by the decrease in the absorbance at 517 nm.  The results were expressed in percentage of inhibition by using formula. The results were compared with the positive control i.e. standard Quercetin. The percentage inhibition of the DPPH radical was measured by using the following formula.

Percentage of inhibition = [(absorbance of control- absorbance of reaction mixture)/absorbance of control] X 100

 Please read this article, it may help you about DPPH assy "Antibacterial and Antioxidant Activities of Anethum graveolens L. Dried Fruit Extracts". I attached below.

regard

There many reference for antioxidant activity of DPPH like http://www.ifrj.upm.edu.my/23%20(04)%202016/(50).pdf

http://www.sciencedirect.com/science/article/pii/S0278691510002152

Efficacy of chemically characterized Ocimum gratissimum L. essential oil as an antioxidant and a safe plant based antimicrobial against fungal and aflatoxin B1 contamination of spices

Any standard method procedure for dpph assay in antioxidant activity? - ResearchGate. Available from: https://www.researchgate.net/post/any_standard_method_procedure_for_dpph_assay_in_antioxidant_activity [accessed Nov 18, 2016].

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.678.4869&rep=rep1&type=pdf

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