I have synthesized a Schiff base compound but everytime I do NMR I found an aldehyde peak of intensity 0.20 or 0.25 like that. I have tried column, recrystallization, solvent wash but couldn't get the clean NMR of the final product.
Robin is correct. The Schiff base is always an equilibrium and must be reduced (e.g., cyanoborohydrin) to create a permanent imine. You have to use an aprotic solvent (e.g., acetonitrile) to trap the Schiff base. There is also a byproduct urea that forms that is 1 Da off in mass.