Development of methods for Mg, Sr and Pb isotopic analysis of crude oil by MC-ICP-MS
Fecha
2021-09-17Primeiro coorientador
Mello, Paola Azevedo
Segundo coorientador
Mesko, Márcia Foster
Primeiro membro da banca
Vanhaecke, Frank
Segundo membro da banca
Mester, Zoltan
Terceiro membro da banca
Müller, Edson Irineu
Quarto membro da banca
Bizzi, Cezar Augusto
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In this work, the determination of isotope ratios of Mg, Sr and Pb in crude oil by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) was performed. Two sample preparation methods were evaluated: (i) microwave-assisted wet digestion with a pressurized digestion cavity (MAWD-PDC); and (ii) solubilization of inorganic solids after the ASTM D4807 test method. Using MAWD-PDC, up to 0.5 g of crude oil was efficiently digested using just 6 mL of 14.4 mol L-1 HNO3 (75 min, temperature up to 250 °C). MAWD-PDC was shown to be a suitable sample preparation method for subsequent determination of Mg, Sr and Pb concentration and their isotope ratios. On the other hand, the ASTM based method does not require any sophisticated equipment or the use of halogenated solvents. With this method up to 10 g of oil can be dissolved in toluene and filtered through a nylon membrane. Elements present as inorganic solids, such as Mg and Sr, are retained on the nylon membrane and can be easily recovered in water. However, Pb was not recovered using this method, possibly because this analyte is present in crude oil as organic complexes that are not retained on the membrane. Four isolation protocols using column chromatography were evaluated for Mg, Sr and Pb isolation from crude oil matrix. For Mg, isolation was successfully carried out using the cation exchange resin AG 50W-X8. The isolation of Sr and Pb was performed using a sequential isolation protocol relying on the use of the Sr-spec resin. Both isolation protocols were characterized by quantitative yields and matrix elements removal. No statistical difference was observed between the results for Mg and Sr isotope ratios obtained using both sample preparation methods (MAWD-PDC and solubilization). Thus, both sample preparation methods can be used for Mg and Sr isotopic analysis of crude oil.
The Mg-Sr-Pb isotopic composition of the Brazilian crude oils evaluated in this work was within the range observed for seawater and the deposit bedrock. The methods developed in this work can be considered as promising tools to decipher the formation history of oil reservoirs.
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