Improvement of chemical sample preparation chartfor carbonaceous rocks and high field strength elements (HFSE) analysis by ICP-MS method

Lately, ICP mass spectrometrymethod is the main technique for element and isotope analysis of rare and traced elements. However, there is a problem with the samplepreparation of the geological matrixes for ICP-MS. Carbonaceous rocks are one of the most difficult rocks for acidic decompositionbecause of their high Si-, Al-, Fe-, Mg-, and C-natural content. The existing schemes for making a solution out of such samples do notallow complete sample opening. Therefore, our aim is to optimize the existing schemes of the chemical sample preparation forcarbonaceous rocks and further measurement of HFSE by ICP-MS. In order to do that, we analyzed the non-opening matter by the rasterelectronic microscope TESCAN Vega LMU after each chemical preparation stage in the scanning mode. This microscope has theenergy dispersion spectrometer type to the Oxford Instruments INCA Energy 350. During the experiment, the forming of the fluoricaluminum silicate complexes with the inclusions of petrogenic elements was discovered. Basing on this experiment a conclusion wasmade that the geological matrix does not open at the first stage of decomposition. The used scheme of sample preparation was modifiedby adding a long stage, when a sample is kept in the open system, and gradual adding of HF is for a maximum Si- extraction as afluoride. These modifications let avoid further formation of the compound F-, Al-bearing silicates. All changes made on the samplepreparation have improved the analysis results, but have not still allowed the complete sample opening. Besides, the understating of theREE results was observed. Considering the high C-content in the sample, we assumed that this result interpretation is connected with thepresence of carbonaceous matter. In order to define the temperature, when the thermal extraction of carbonaceous matter is maximum,we have carried out the synchronous thermal analysis of the sample. The obtained data allowed defining the temperature range for themaximum thermal extraction of the carbonaceous component of the sample. These results were approved by the experimental data.Optimal conditions for the thermal preparation process are 850°С during 8 hours. Basing on the obtained data, the prior thermalpreparation stage was included in the preparation scheme of carbonaceous rocks. Thereby, we have obtained information about sedimenttransformation process during opening of a compound C-bearing geological matrix. The non-opening matter with a REE component wasdiscovered, which allowed modernizing the scheme of chemical preparation. An optimum scheme for the prior thermal preparation ofcarbonaceous rocks was also defined. The research has been carried out in the CCU ''Analytic Centre of Natural SystemsGeochemistry''. The study was funded by the Russian Ministry of Education and Science (Federal Program ''Scientific and TeachingPersonnel of Innovative Russia (2009-2013)'').

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