Well, in my opinion I wouldn't make the solution and store it for so long. According to our experience in laboratory, if you want to use it to obtain a calibration line, for example, I recommend that you make your solution almost immediately, not waiting for more than a week. 

Greetings!

Thank you for your guidance, Dear Dr. Sergio Nogales

But I need 120uM Trolox  which it is a  very small mass and probability of  mistake is exists.

How can I reduce this error?

Thank you Dear  Mrs B, Sanaei