I have found the option, the solution was in the modifications tab.
My question now is that I have spiked different internal standard concentrations of the same peptide depending on the type of sample, but in the document grid I can only write the same internal standard concentration for all the samples. I cannot choose in sample A the concentration of the peptide is xx, in sample B the concentration is yy. I dont know how to do this.
Can you help me?