The samples are dried then cleaned under the binocular microscope, crushed with the agate mortar.
50 mg samples are weighed exactly, then, digested in Teflon bakers on sand bath, by reagents HNO3/HF/H2O2 ultra pure.
The solutions obtained are analyzed by ICP MS (Series X7-Thermo electron) as well as a sample of lichen reference (IAEA 336) treated in the same way.
18 elements were retained Mg, Al, Ti, V, mn, Fe, Co, Cu, Zn, Rb, Sr, Zr, Cd, Sb, Pb, Ag, Th, U. To establish the initial content (base line, before exposure) these elements were proportioned in 20 taking away of lichens of Boulogne, each taking away corresponding to an individual. In this way, the homogeneity of the population collected was tested.
The preliminary results confirm part of the conclusions advanced in preceding publications. Namely that pollution is not a continuous process but which it operates by jerks and which the cumulative effect is not easy to see even over such a long period.
From a methodological point of view, we also confirm that the sheltered situation and the horizontal position offer the best chances of accumulation.
In detail, the elements considered of origin lithophile, like Al accompanied by Fe and Mg show during the analyzed period a considerable reduction recorded in the four described situations.