Overcoming humidity dependence of the chromium trioxide converter used in luminol-based nitric oxide detection

The surface-atmosphere exchange of gaseous NOx (NO + NO2) is commonly measu red by chamber methods that employ a portable luminol-based NO2 detector. D etection of NO depends on its prior oxidation to NO2 by CrO3 with efficienc y that depends on sample relative humidity. We found that inadequate drying of wet chamber air samples, and consequent changes in NO oxidation efficie ncy, often contributes to observed apparent shifts in the response of comme rcially available detectors in the field. Our instrument's performance was improved when the water vapor gradient across its membrane dryer was enhanc ed by replacing the restrictor on the air inlet of the dryer's shell with a desiccant column, and then increasing ambient air flow through the shell t o approximately equal the sample air flow rate. We also demonstrated that p umice is the preferred CrO3 support material for luminol-based detection of NO in chamber air samples; advantages include low cost, superior physical properties, and the simplicity of fabricating a CrO3/pumice converter by th e user. (C) 1998 Elsevier Science Ltd. All rights reserved.