A closed system digestion and purification procedure for the accurate assay of chlorine in fossil fuels

This paper describes a robust and quantitative method to separate, purify, and assay the amount of chlorine in fossil fuels using isotope dilution mass spectrometry. The digestion/extraction process uses Carius tubes containing the fossil fuel samples in the presence of HNO3, AgNO3 and a 37Cl spike. The closed system oxidation permits complete equilibration of the sample and spike Cl. The evolved chlorine is trapped as AgCl within the Carius tube, and can then be separated and cleaned. The purified AgCl is also amenable to isotopic analysis by solid or gas source isotope ratio mass spectrometry. For isotope dilution mass spectrometry, the chlorine can be readily measured in a thermal ionization mass spectrometer in a negative ion mode as 37Cl/35Cl ratios with an approximate detection limit (LOD) of 0.3 μg Cl and a precision of 0.2%, relative. This technique may also be used to produce samples suitable for high precision measurements of chlorine isotopic variations in fossil fuels and chlorinated hydrocarbons.