[en] Reference materials of natural and isotopically enriched or depleted Li₂SO₄ are used for neutron cross section measurements. The detection limits of Sm, Eu, Gd, Dy and Cd in Li₂SO₄ by atomic emission spectroscopy (AES) and instrumental photon activation (IPA), correspond to a total contribution equivalent to the natural lithium (σ=70.5 barns), of 0.160% and 0.0213% respectively. MnSO₄ solutions are used for the calibration of neutron sources, e.g. ²⁵²Cf. The direct analysis of Sm, Eu, Gd, Dy, and Cd in MnSO₄ by AES and IPA, provides detection limits from which the total contributions equivalent to the manganese (σ=13.3 b) are respectively 2.38% and 0.395%. In order to improve the detection limits of Rare Earths, a method had been developed for the R.E/Mn chemical separation which is based on the coprecipitation of Rare Earths as fluorides by HF in the presence of ammonium after the addition of very pure Y as a carrier. A solution of sup(152+154)Eu(1.11x10⁵Bq) is used to monitor the recovery of the lanthanides. The Sc and Y are determined directly by AES or IPA. As an example, the contributions of Sm, Eu, Gd and Dy obtained by AES after the chemical separation of a solution containing 300 mg MnSO₄ per gram, are reduced to 0.0114%. The concentration of Cd found in this solution is 3.7 μg/g MnSO₄ and the total contribution of the 16 Rare Earths and Cd is <=0.107%