PNNL

Abstract

Chemical processes using cryotrapping are found to have fundamental limitations on transfer efficiency due to the presence of small amounts of non-condensable impurities. The impurities may be derived from contaminants in the actual sample, leaks into the transfer apparatus, or residual pressure when pumping out the cryotrap and manifold. When cryotransfer occurs, the non-condensable impurities will collect in the trap and will eventually stall out the cryotransfer by effectively filling the cryotrap and not allowing additional analyte mixture into the trap. After this occurs, analyte can still be trapped out but on a diffusion-limited timescale. For practical cryotransfer, it is preferable to design the process so that the necessary amount of analyte is transferred over at the stall pressure and diffusion is not relied upon. For some cases, it will be difficult or impossible to have an efficient transfer based on required sizes of the cryotrap, manifold, or amount of impurities. In those cases, a much higher efficiency may be obtained by breaking the process up into multiple steps—that is, to transfer to an intermediate volume(s). By stepping the volume down incrementally instead of a single stage, higher levels of impurities can be tolerated while maintaining high transfer efficiency. In fact, the product of individual efficiencies of a multiple stage cryotrap can drastically exceed the efficiency of a single stage.