On eliminating matrix influence on rare-earth elements determination in geochemical species by mass spectrometry with inductively connected plasma

The urgency of the reliable determination of rare-earth elements (REE) trace quantities in the geological species is caused by the fact that they are sensitive geochemical markers, they accompany the elements of the platinum group. The appearance of a highly sensitive method of analysis, mass spectrometry with the inductively connected plasma (ISP MS), ensured the determination of all fourteen rare-earth elements at the level of 10 g/ml; however, for eliminating the influence of matrix background dilution is used which reduces sensitivity by 2-3 orders, and/or the expensive internal standards are added into the solution. The basic rock-forming elements of geological objects are silicon, iron, and aluminum. The influence of silicon is minimized in the process of sample preparation by treatment with hydrofluoric acid, as a result of which it is driven off in the form of tetra fluoride. A reduction of the iron content in the analyzed solution is especially urgent in the case of lowest possible rare-earth elements concentrations in the sample. In the present work for decreasing the content of iron (III) in the solution its selective absorption on the AV-17-8 anionite (Cl-f.) is proposed. Iron absorbs from the acid media in the form of anionic complexes with Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt). Fe (III) ions and rare-earth elements have close chemical properties; therefore the conditions for complete Fe (III) absorption in the presence of Tiron and some lanthanide (Pr, Sm, Nd, Eu, Yb) on the ion-exchange column with AV-17-8 (Cl-f.) were established experimentally. These are: Fe/Tiron ratio is 1/3; flow - 0.15 ml/min, pH=2. In this case iron concentration in the solution is reduced by an order. For the quantitative assessment of heavy rare-earth elements the correction factors of 1.15-1.22 were introduced which consider their insignificant absorption (20-25)% calculated as the relation of the declared and obtained result of analysis. Separation of Al(III) from other components of species is carried out in the alkaline medium in the form of soluble HO-complexes. The proposed procedure of sample treatment includes stages: the removal of silicon in the process of the acidic "opening" of species by distillation in the form of tetrafluoride; separation of Al(III) in the alkaline medium (pH 10-12) in the form of HO-complexes from other hydroxides by filtration through the Blue Ribbon Filter. The sediment of metal hydroxides on the filter (iron, rare-earth elements and other) was dissolved with 0.01 M of HCl (pH=2), then Tiron was added and Fe (III) in the form of anionic complexes was retarded on the column packed with AV-17-8 anionite (Cl-f.); REE content in the eluate was analyzed by ICP-MS. The developed procedure of the matrix elements removal is worked out in the model solution and is tested thoroughly with the analysis of the geological standard models: Gbpg-1, BHVO-2, BCR-2. Its correctness is proven by the method of mathematical statistics.

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