NO chemiluminescence in exhaled air: interference of compounds from endogenous or exogenous sources

Nitric oxide determination in exhaled air using chemiluminescence analysers is increasingly used, but may be affected by various other components of the air sample. The influence of several compounds originating from endogenous or exogenous sources on NO readings has been studied. Defined amounts of water vapour, carbon dioxide, acetone, heptane, acetonitrile, oxygen, nitrous oxide and enflurane were added to air samples with NO concentrations 0–250 parts per billion. Marked and significant decreases in NO readings, which strongly depend on the concentration of the respective interfering compound, were found for water vapour (0.155% per 1% relative humidity), carbon dioxide (1.97% per 1% CO2 volume/volume (v/v)) and nitrous oxide (0.608% per 1% v/v N2O). While acetone in concentrations up to 8.5% v/v had no measurable effect on NO readings, heptane and acetonitrile led to marked decreases. Oxygen in concentrations of up to 95% (v/v) had no effect on NO determination. NO readings were markedly decreased by >10% per 1% (v/v) of the anaesthetic enflurane. However, due to large variations in NO values, these decreases were not statistically significant. Furthermore, enflurane reacted with the molybdenum converter of the NOx analyser in use, resulting in major damage to the instrument. Eliminating, or at least considering, interferences by compounds present in exhaled air is an urgent prerequisite for reliable and precise nitric oxide determination.