2.5. Analysis of elements
For the purpose of elemental analysis, Teflon filter paper was cut
using plastic scissors into several small pieces and was placed at the bottom of
round bottom glass digestion vessel. 20ml of concentrated Nitric
acid (65%, GR grade, Merck Suprapure) was poured into the digestion
vessel and for 2 h the digestion vessel was put on hot plate having
180 °C temperature. It was taken care that the digestion vessel did not
completely dry up because of the evaporation of Nitric acid and if required
extra Nitric acid was added during digestion. After digestion
the vessel was allowed to cool down to room temperature. The walls
of the vessel were rinsed with Milli-Q water (resistivity =
18.2 MΩ cm at 25 °C) and kept for 30 min for diffusion of acid from
the filters into the solution, then the solution in the vessel was filtered
through 0.22 μm filters and the final volume was made up to 100 ml
using Milli-Q water. These aqueous samples were then analysed for elements
using ICP-OES (Inductively Coupled Plasma – Optical Emission
Spectrometer, Thermo Fisher, iCAP 6300 Duo) as per the earlier
established procedures (Chakraborty and Gupta, 2010). Blank filters
were also subjected to the same procedure.