When samples must be stored for a period of time, they are best stored upright in screw-capped vials. Samples may evaporate through the joint over time, and if a highly volatile sample is used, the joint should be wrapped in Teflon tape (semi-stretchy white film, Figures 1.14 a+b) or Parafilm, (stretchy plastic film, Figures 1.14 c-e) to create a better seal. Teflon tape is less permeable to solvents than Parafilm, and volatile samples wrapped in Parafilm may still evaporate over a period of weeks. If a sample is to be stored in a round bottomed flask for some time, it should be stoppered with a cork stopper or plastic cap. A rubber stopper should not be used as it will tend to swell when exposed to organic vapors, and a glass stopper may freeze.