This Recommendation applies to measuring instruments (hereafter referred to as “gas analytical systems”) that serve to determine the volume fraction of carbon monoxide (CO), nitrogen monoxide (NO), nitrogen dioxide (NO2), or sum of nitrogen oxides (NOx) and establishes the conditions and requirements with which such systems shall comply in order to meet the requirements for measurement results of the pollutant content in stationary source emissions within OIML Member States. This Recommendation applies to gas analytical systems intended for measuring (analyzing) industrial emissions, and provides metrological and technical requirements for gas analytical systems, as well as means and methods for checking their normalized characteristics. The principle of operation of gas analytical systems can be based on infrared (IR) and ultraviolet (UV) absorption methods, chemiluminescence, electrochemistry or IR methods with correlation filters, as well as on a combination of the methods mentioned above. Note: This Recommendation is also applicable to analyzers that are capable of analyzing only one or two of mentioned components.