Disposable NMR Tube Caps are manufactured from molded PVC or Polyethylene depending on the tube OD. The different colors help track samples based on various reference solvents.
When using Chloroform or Acetone as reference solution, please avoid (1) direct contact between the sample and the disposable cap, (2) long term storage over 2 hours. If these conditions are inevitable, please use PTFE NMR tube caps.