5.1 Proton exchange membranes (PEM) used in fuel cells are susceptible to contamination from a number of species that can be found in hydrogen. It is critical that these contaminants be measured and verified to be present at or below the amounts stated in SAE J2719, ISO 14687-2 and ISO 14687-3 to ensure both fuel cell longevity and optimum efficiency. Contaminant concentrations as low as single-figure ppb for some species can seriously compromise the life span and efficiency of PEM fuel cells. The presence of contaminants in fuel-cell-grade hydrogen can, in some cases, have a permanent adverse impact on fuel cell efficiency and usability. It is critical to monitor the concentration of key contaminants in hydrogen during the production phase through to delivery of the fuel to a fuel cell vehicle or other PEM fuel cell application. In ISO 14687-2 and ISO 14687-3, the upper limits for the aforementioned contaminants are specified. Refer to SAE J2719 and the California Code of Regulations (see 2.3) for example specific national and regional requirements. For hydrogen fuel that is transported and delivered as a cryogenic liquid, there is additional risk of introducing impurities during transport and delivery operations. For instance, moisture can build up over time in liquid transfer lines, critical control components, and long-term storage facilities, which can lead to icing up within the system and subsequent blockages that pose a safety risk or the introduction of contaminants into the gas stream upon evaporation of the liquid. Users are reminded to consult Practice D7265 for critical thermophysical property such as the ortho/para hydrogen spin isomer inversion that can lead to additional hazards in liquid hydrogen usage. In addition to this test method employing CRDS, test methods such as D7649 (carbon dioxide, argon, nitrogen, oxygen and water) and D7653 (IR active species) are used to measure impurities in hydrogen fuel.