Practical approach for an easy determination of the limit of detection and uncertainty budget associated with on-line measurements of gas and aerosols by ion chromatography

Abstract

The Monitor for Aerosols and Gases in Ambient air (MARGA) is an on-line gas and aerosol sampling and analysis system based on ion chromatography (IC) that allows the monitoring at 1-h time resolution of major water-soluble atmospheric species in the gas phase (NH3, HCl, HONO, HNO3, SO2) and the particle phase (NH4+, Na+, K+, Ca2+, Mg2+, Cl−, NO3−, SO42−). The lack of a common procedure among users to determine the LOD and the uncertainty of measurements for each species led to the comparison of four different operational approaches for the LOD determination, one manual and three automatic. This comparison highlighted the high NO3− blank values around 0.6 μg m−3 when using a HNO3-based cation eluent, so the p-TSA-based eluent was proposed instead. A model for an easy estimation of measurement uncertainties is suggested based on main sources of uncertainty (liquid sample volume, air volume, representativeness of the sample, accuracy of the internal standard preparation, chemical background, repeatability), applied to a real dataset acquired in a rural site in the North of France. It gives relative average uncertainties between 5 and 30% for NH3, HONO, HNO3, SO2, NH4+ NO3− and SO42−, while Na+, K+, Ca2+, Mg2+ and Cl− range between 45% and >100%. A comparison between our dataset and other studies confirmed that the variation of the uncertainty budget depends on environmental concentrations of the species and site typology. The practical recommendations drawn in this paper may be helpful to the user community of gas and aerosol online samplers coupled to IC analysis.