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 t he air sample. The influence of several compounds originating from endogeno us or exogenous sources on NO readings has been studied. Defined amounts of water vapour, carbon dioxide, acetone, heptane, acetonit rile, 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 t he concentration of the respective interfering compound, were found for wat er vapour (0.155% per 1% relative humidity), carbon dioxide (1.97% per 1% C O2 volume/volume (v/v)) and nitrous oxide (0.608% per 1% v/v N2O). While ac etone in concentrations up to 8.5% v/v had no measurable effect on NO readi ngs, heptane and acetonitrile led to marked decreases. Oxygen in concentrat ions of up to 95% (viv) had no effect on NO determination. NO readings were markedly decreased by >10% per 1% (v/v) of the anaesthetic enflurane. Howe ver, due to large variations in NO values, these decreases were not statist ically significant. Furthermore, enflurane reacted,vith the molybdenum conv erter of the NO, analyser in use, resulting in major damage to the instrume nt. Eliminating, or at least considering, interferences by compounds present in exhaled air is an urgent prerequisite for reliable and precise nitric oxid e determination.