To prove the results for FPAAS, we checked the mercury concentration
in selected ND samples (with high concentrations of Hg obtained
using FPAAS) with wavelength-dispersive XRF. In all the selected samples,
we obtained strong, reliably identified, and reproducible peaks of
Hg (see Fig. 4). Peak heights are in good concordancewith the Hg values
from FPAAS measurements. Nevertheless, the accuracy of quantitative
XRF determination of Hg in NDs was far beyond the scope of this manuscript
and is a theme for a separate research, so we used XRF here for
qualitative confirmation of our results only. Quantitative comparison
of FPAAS and ICP-AES data for samples UDA-S-GO, UDA-S, and UDAGS
can be found in [