I'm used to synthesize peptide using the Fmoc-based Solid Phase Peptide Synthesis, with HATU/HOAt as coupling reagents. This time, after the cleavage from the 2Cl-Trt resin, I see the peaks of HOAt in the NMR. How is it possible? Did I Do some mistakes in the washing step? I usually performed 3 washings with DMF and 3 with DCM, and I've never observed HOAt impurities.

Thank in advance for your suggestions!