As mentioned by the Peers, you need to inject a range of concentration of the pure standard compound, develop a standard curve and this curve will help you to estimate the % Concentration.
Using the same instrument and qualified method, create a calibration curve using pure standards which meets good chromatography fundamentals. Once the calibration table has been created and is repeatable, you can attempt to analyze your actual sample to determine the concentration.
Then you will have a concentration vs area graph, and by interpolate you will be able to, by determining the peak area corresponding to the compound, estimate its concentration.