ICP-OES intensity Dropping

Are you using an internal standard?

we used AccuStandard multi and single standards and we prepared different concentrations of QCs 

You can check (depends on your version of ICP Expert) how the argon line as well as the neb pressure are behaving over time if you click on the symbol next to the calibration curve:

You can also check the signal monitor function in conditions on the bottom part of the software. Aspire your QC solution and then let the signal monitor run with your settings for a few minutes. In the plot you will see how the signal behaves, if its stable or of it drifts. When you see drift then you check keep the signal monitor on and go through all the components one by one.

I highly recommend that you use an internal standard to compensate for the drift. An internal standard element is one that is not present in any of your calibration standards and samples, for example Yttrium.  Add the internal standard wavelength in the software and make up a solution of 5 ppm of the single element internal std. You will need a "y-piece" and an extra piece of pump tubing to physically add the Y solution. In the software you can add the Y 371nm wavelength and designate it as the internal std. See the HELP file for additional info in the software.