Identification of an Organic Impurity Leaching from a Prototype ISS Water Container 2001-01-2125
Collapsible bladder tanks called Contingency Water Containers (CWCs) have been used to transfer water from the Shuttle to the Mir and the International Space Station (ISS). Because their use as potable water storage on the ISS is planned for years, efforts are underway to improve the containers, including the evaluation of new materials. Combitherm®, a multi-layer plastic film, is a material under evaluation for use as the CWC bag material. It consists of layers of linear low density polyethylene, ethylene-vinyl alcohol copolymer, nylon, and a solvent- free adhesive layer. Long term studies of the quality of water stored in Combitherm bladders indicate a gradual but steady increase in the total organic carbon value. This suggests a leaching or breakdown of an organic component of the Combitherm. The NASA Johnson Space Center Water Laboratory’s routine set of chemical analyses was not identifying significant portions of this total organic carbon, implying that the compounds leaching into the water were escaping detection. Liquid chromatography/mass spectrometry (LC/MS) analysis suggested the presence of no major contaminants other than caprolactam, the monomer of Nylon 6. The LC/MS analysis led to a more direct and accurate method of quantification of caprolactam, using direct injection high performance liquid chromatography (HPLC). Re-analysis of the water samples by the HPLC method seemed to completely account (within experimental error) for the total organic carbon in the water samples. Shortcomings of the previous liquid-liquid extraction method for quantifying caprolactam by gas chromatography / mass spectrometry are discussed. Drawbacks of the new method are also described along with approaches to resolving general problems with carbon accountability in recycled water.