Oxygen determinations are required for a broad range of applications, including process control, health and safety issues, and environmental concerns. For gas-phase applications, the oxygen concentration of interest can range from trace contaminant levels to nearly pure oxygen. For example, hydrogen production via water electrolysis generates hydrogen in the

and Process Control

cathode stream, but will also produce a stoichiometric amount of nearly pure oxygen at the anode. Indeed, water electrolysis is one means to produce oxygen for breathing air in confined environments with finite resources, such as in submarines or in the International Space Station [300]. There are applications that require high oxygen levels and purity; oxygen cylinders for welding contain a minimum of 99.5 vol% O2. At the other extreme, oxygen can be viewed as an impurity with acceptable tolerances in the low ppm range. The maximum oxygen content in hydrogen is rigorously regulated for many applications. The current hydrogen fuel quality standards for PEM fuel cell vehicles restrict the maximum allowable oxygen level to 5 ppm [301,302]. The allowable oxygen content is regulated for other grades of hydrogen, some of which are presented in Table 4.1. In addition to nearly pure oxygen or for oxygen present at trace concentrations, there is often a need to quantify oxygen concentrations at intermediate levels between these two extremes. The oxygen content in air is typically given as 20.9 vol%. To guard against asphyxiation, the U.S. Occupational Safety and Health Administration (OSHA) mandates that the oxygen content in breathing air must be above 19.5 vol% [303]. The verification that oxygen is present at a safe concentration is mandated for a variety of activities, such as confined space entry. Thus, depending on the application, oxygen measurements can be required from low ppm levels to nearly 100 vol% and at concentrations in between these two extremes. There are some commercial sensors specified for 0 to 100 vol% O2, but the low-level accuracy or ability to resolve small changes in oxygen concentration in these broad range sensors may be inadequate for some applications. These broad range sensors have a typical lower quantification limit (LQL) not better than approximately 0.05 vol%. To improve performance specifications, some oxygen sensors have been optimized to operate over a smaller concentration range. However, some trace level oxygen determinations remain challenging and may be beyond the current capability of sensor-based analytical methods.