Biomonitoring with mosses: Uncertainties related to sampling period, intrasite variability, and cleaning treatments

To assess the level of sampling period uncertainty and the main components of the measurement uncertainty, such as: sampling, preparation and analysis a total of 63 samples of Pleurozium schreberi (Brid.) Mitt. moss species and their duplicates were collected from the same sampling sites in September of 2014, and March and July of 2015. After each sampling campaign, the samples (primary sample - S1 and its duplicate - S2) were divided into two equal and homogenous samples (P1 and P2) for preparation. The P1 samples were subjected to dry cleaning whereas the P2 samples to wet cleaning. After digestion in a closed microwave system, Cu, Fe, Mn and Zn were determined in all these samples using FAAS and GFAAS techniques. The highest concentrations were noted in July samples. However, the average element contents were as follows: July > March approximate to September for Cu and Mn, July > March > September for Fe, and July approximate to March > September for Zn. The level of sampling period uncertainty was the highest for Fe and Mn, and the lowest for Cu. In 2014 the measurement uncertainty (s(rmeas)) of Fe, Mn and Zn (expressed as a sum of sampling (s(rsamp)), preparation (s(rprep)) and analytical (s(ranal)) uncertainty) was mainly affected by intra-site variability, so the contribution of sampling uncertainty to the srmeas values was the highest. In 2015 the cleaning procedure had the main impact of the measurement uncertainty of Fe and Zn, so the srmeas values were mainly loaded by the srprep values. For Cu and Mn the same relationship between the srsamp and srprep values was maintained in 2014 and 2015. In all cases, the sum of sampling, preparation and analytical uncertainty did not exceed 35%. However, high values of preparation uncertainty of Fe and Zn obtained in 2015 should make us more critical of the results from the analysis of mosses collected within different sampling periods and cleaned using different wet/dry cleaning methods.