Cleaning is performed usually by rinsing with the same (non-deuterated) solvent used to dissolve the initial sample. Dichloromethane or acetone are good choices because dichloromethane is similar in polarity to chloroform, a common NMR solvent, while acetone dissolves many organic compounds. Sonication and scrubbing with a pipe cleaner may be helpful in removing traces of solid contaminants. If necessary, the tube may be filled with an oxidizing solution of aqua regia or piranha solution (H2O2/H2SO4). Care should be taken with these solutions, as they can unexpectedly and violently erupt from the NMR tube due to pressure build-up (aque regia) or explosion (piranha). Chromic acid solutions are never used, due to traces of paramagnetic chromium left behind on the tubes causing interference with NMR experiments.