Dear Shazia Kanwal  % DPPH activity increase with increase concentration  in most cases. 

%DPPH in most times increases with increase in concentration.

In general, percentage inhibition of DPPH should increase with increasing plant exract concentration if the extract contains antioxidants.

Yes, % DPPH activity is directly proportional to increasing extract concentrations. I am yet to see an inverse proportionality. However, if after the assay was properly conducted and concluded, the trend negates the norm; then that may be telling/ leading you something/ somewhere... So endeavor to find a suitable explanation for the observation, carefully troubleshooting and logically explaining "the process." Best wishes!

The percentage inhibition of DPPH increases with increasing concentration.

Thanks for your answers. i have performed DPPH assay with 25 plants. almost 22 plants showed regular DPPH inhibition with increasing concentration but 3 plants showed different behavior showing increase DPPH inhibition with decreasing concentration. and there is regular sequential increase in inhibition. What should i do now?

Himanshu Ojha Can you send me the complete paper?

Hi Shazia, agreed to all answers given. However, it is not enough to conclude say saying "it is increases". It need to see how much is optimum concentration or IC 50. 

Sometime, the drugs or extract or sample will remain horizon or decline even if concentrations is increases at a particular point. It is because all element or compounds has their own saturation temperature.

Hope it help!

Dear optimum concentration of plant extract...how to know it?

secondly, your answer sounds interesting but i need to know more. Do you have any refernce of this logic.  How can i know more about these elements or compounds?

Screen a range of concentration as you may wish and see which gives the best r-square. Purification and characterization (GCMS, column chromatography, TLC, FTIR) may help you determine what the active compounds are.

dear collegue we need more information to can aswer logically and surely

Dear Shazia, the difference in behavior of plant extracts may be due to difference in relative abundance of antioxidants agents i.e. proportion of reductants and oxidants although I am not sure about it. Similarly, this may be due to dilution of reaction system after addition of higher volume of extract. To find optimum concentration, you will need to test a series of concentration of extract and see which one gives most (optimum) activity as advised by Ebenezer Idowu Ajayi and others.

 Here is the paper that was refer to by Himanshu Ojha. You cana also access it through digital/online library of HEC, Pakistan.

Regrads

Dear Shazia... from my own experience, in orde rto obtain IC 50 values, we must perform a series of measurements of DPPH inhibition activity covering the range from 20 -80%, then made the linear regression equation and calculated the value of R to see linearity, the equation can be used to calculate the value of IC 50 when the curve generated is linear. Effectiveness among the plant extracts can be seen by comparing the IC50 value, the lower the value of Ic 50 more potent as an antioxidant extract

DPPH free radical scavenging activity is spectrophotometric assay..If the colour of plant extract is darker, then DPPH activity may get affected due to the dark colour. In such cases you can try for lower concentration of plant extracts to get proper results..I agree with the opinion of Rehman sir..

Mohamed Kouidri  i have mentioned concentration of DPPH. should i add the inhibition values too?

Dear collegue 

the DPPH concentration  must  be constant 

Samples were diluted with ethanol as you like; 0.1 mL of the diluted sample is added into 3.9 mL of DPPH solution (0.04 mg/ml). After incubation under dark for 10 min the absorbance was read at 517 nm and the mean absorbance of triplicate were calculated. The absorbance decrease as the concentration of sample increase. But the inhibition percentage increase as the concentration of sample increase

good luck

percentage inhibition of DPPH decreases with increasing plant concentration as the DPPH acts as a radical capture (Antioxidant), as the  plant extract con. increases the DPPH is consumed more hence the DPPH coc. decreases.

Mohamed Kouidri sir my results are in accordance to ur statmnt that inhibition percentage increases with increase in plant extract but Madhukar sir said that inhibition percentage decrease with increasing concentration of plant extract?

The value of the absorbance decrease as the antioxidant concentration increases

But the inhibition percentage increases with increase in plant extract