Proposal of an empirical model with flow velocity to improve time-weighted average concentration estimates from the Polar Organic Chemical Integrative Samplers

It is now widely recognized that the sampling rate of Polar Organic Chemical Integrative Samplers (POCIS) is significantly affected by flow velocity, which can cause a consequent bias when determining time-weighted average concentrations (TWAC). Besides, in previous works, we observed the desorption of a compound such as deisopropylatrazine (DIA) over time when added to the receiving phase of a POCIS. In particular, it was found in situ that its desorption rate was also influenced by flow velocity, in an agitated water environment. In the method presented here, we calibrated 30 pesticides under controlled laboratory conditions, varying the flow velocity over four levels. At the same time, we studied the desorption rate of DIA-d5 (a deuterated form of DIA) over time. The information provided by the accumulation kinetics of the compounds of interest on the one hand, and by the elimination of DIA-d5 on the other hand, was processed using an empirical model based on a power law involving flow velocity. With this type of model, it appeared possible to take into account the effect of this crucial factor on exchange kinetics, and then to obtain more accurate TWACs, with reduced bias and acceptable dispersion of the results.