A colorimetric method for the measurement of ppb-level NO in exhaled air using porous glass analytical chips

Patients with asthma exhale high concentrations of nitric oxide (NO), making the measurement of NO clinically important. Although we previously developed a 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO) chip that relied on the reaction between NO gas and PTIO in the chip, the sensitivity and accuracy were insufficient for measuring NO in air exhaled by asthma patients. In this study, we focus on the conversion of NO to NO2 in the reaction with PTIO, and devise a new method to measure the converted NO2 using our previously developed NO2 analytical chip. NO is converted to NO2 in the PTIO chip and immediately released upon exposure to the atmosphere. This released NO2 is trapped on the adjacent NO2 analytical chip and reacts with diazo-coupling reagents to form an azo dye; a linear relationship was identified between the NO concentration and the change in absorbance at 525 nm due to production of the azo dye. The NO concentration could be measured in the 0−142 ppb range with a maximum error of ± 10 ppb at 142 ppb using the devised method, demonstrating sufficient sensitivity and accuracy for measuring NO in exhaled air. Furthermore, we successfully measured NO in the exhaled air of seven healthy adults, and detected ~19 ppb of NO, thus illustrating its feasibility for clinical use.