NO and NO2 measurement by chemiluminescence: effects of carrier gas composition

Accurate monitoring of nitric oxide (NO) and nitrogen dioxide (NO ₂) is of utmost importance during NO inhalational therapy. It is commonly known that NO and NO₂ measurement by chemiluminescence (CL) analysers is affected by high oxygen concentrations. Carrier gas composition and humidity also should affect the precision of NO and NO₂ measurement because of third body fluorescence quenching. Additionally, any change in the sampling flow rate (pre-set by a critical orifice within the analyser) should influence the instruments´ readings. In a laboratory study the authors therefore thoroughly tested the characteristics of NO and NO₂ measurement of two CL analysers (ML9841A and CL700ALmed; these systems are widely used in the US and Europe, respectively) at 0.0<FO₂ <0.95. NO and NO₂ mixtures with stable concentrations were prepared using certified gas tanks. With increasing oxygen, both the ML9841A and CL700ALmed analyser yielded up to 7% lower NO₂ concentration readings than with N₂. With NO, the CL700ALmed showed a smaller cross-interference. The changing sampling flow into the analyser corresponded to the (carrier) gas composition. As the alteration of the NO and NO₂ readings very closely correlated with the sampling flow rate third body fluorescence quenching in the presence of high oxygen appears to be similar as with N₂. The reading error should be taken into account whenever NO or NO₂ are to be precisely measured in gases differing in composition from the calibration gas used