Using EVA and Polyethylene NMR Tube Caps with Organic Solvents

While preparing a sample for NMR, a Laboratory Technician noticed that his sample solution became colored after shaking with CDCl3 while in the NMR tube. The question was directed to our Technical Service Department, as how to prevent this from occurring when making up new samples. Upon questioning, the Technician mentioned that he was using standard EVA NMR tube caps that were supplied with his NMR tubes. He would prepare his samples for NMR by adding solid sample directly to the tubes with Chloroform-d, and then inverting the tubes to facilitate mixing. After further questioning he mentioned that the solvents were in contact with the caps. We informed him that in using halogenated solvent with EVA or Polyethylene, there is a risk of dissolving the cap and/or leaching out monomers, plasticizers, curing agents, and color components from the plastic. Our recommendation was to only use Teflon® caps, TC-5-PTFE or TC-10-PTFE, to avoid any possible contamination of sample solution from leaching effects of the solvent.