Stability of standard dynamic VOC mixtures for atmospheric measures

Traceable and accurate standard of Volatile Organic Compounds VOC at trace concentration level are necessary for a framework of quality assurance and quality control of atmospheric composition measurements. Dynamic preparation of standard mixtures can fulfill the target accuracy given by the framework. Short-term concentration stability is one of the sources of uncertainty of dynamic preparation and depends on sampling time of the mixture, stability contribution to total uncertainty must be calculated. A method for the calculation of the stability contribution is here proposed. The performance of a preparation device for accurate VOC standard mixtures is here discussed as an application of the method. Two threshold values for stability contribution were set to consider the contribution negligible and relevant respectively. Experimental stability of the dynamic preparation device was measured. The stability of one of the five influence quantities resulted to give the total stability contribution. Stability contribution to total uncertainty resulted to be negligible below 60 min, while its contribution resulted to be relevant over 400 min.