IC50 is the estimation of 50% inhibition by sample or standard.
The minimum amount of sample that cause inhibition of 50% of free radicals is the IC50 of that sample
It can be calculated by using varying concentration of the sample and then from the results of inhibition shown by different concentration the amount exhibiting inhibition of 50% of radical solution is calculated.
First you have to make scatter graph in excel using concentration of extracts in X axis and percenntage of activity in Y axis. then you have to get slop equation on graph which is available in layout- trendline bar-more trendline options- display equation. finally you will get equation (Y=mx+c or Y=mx-c) where Y= 50; m & C value will be in the equation itself. then you have to calculate X that is the IC 50 value.
You may also plot the dose reponse curve (percentage inhibition vs concentration) in excel.. This may or may not be a straight line... Then note the concnetration range within which your 50% inhibition lies... The right click on your plot (line).. then chose select data in the drop down menu.. a new menu appears.. now click on the add tab which opens a new meu with new X and Y data sereis.. Click on the arrow in the for either the X or Y series, then selct the data series from the data which are within your 50% inhibition (may be two each for both X and Y)... click oK, then ok again.. A new plot (having your 50%) will appear on your previous plot... Right click on the new line, add a trendline, then your regression equation... This equation can then be used for the 50% calculation....
Radical scavenging activity was calculated by the following formula:
% Reduction of absorbance = [(AB - AA) /
AB] X 100
where: AB =absorbance of blank sample and AA = absorbance of tested compound (t = 30 min). The concentration of each compound required to scavenge 50% of DPPH (IC50) was determined as well by probit analysis using SPSS software program or any other statistical software program.
you may also use graphPad prism software to plot the values to get graph/curve which represents IC50 based on your concentration on X-axis and %of inhibition on Y-axis.
There are lot of journals published including ACS.