For the calibration of the TXRF spectrometer, single element stock
solutions of La, Pr, Nd, Sm, Eu and Gd were prepared by dissolving
their respective high purity oxides in suprapure nitric acid in such a
way that the concentration of each rare earth element was 100 μg/mL.
Merck certiPUR Cobalt ICP standard having a concentration of
1000 μg/mL diluted to 50 μg/mL using 1.5% suprapure nitric acid in
Milli-Q water was used as internal standard.