Role of graphite in isotopic analysis of boron in metal boron alloys by Positive-Thermal Ionization Mass Spectrometry (P-TIMS)

Isotopic composition of boron in alloy samples was determined by Positive-Thermal Ionization Mass Spectrometry (P-TIMS) using Rb2BO2+. The titanium based boron alloy sample was treated with nitric acid to bring it to slurry form which was fused with rubidium carbonate on the high purity Rhenium filament for the formation of borate. Control of pH was found necessary to be necessary for both the formation of alkali borate compound as well as for obtaining good yield Rb2BO2+. Alkaline conditions were required for the formation of rubidium borate during fusion though neutral pH favored formation of Rb2BO2+ ions in the ion source. The sample loading procedure was suitably modified to obtain good ion intensities for Rb2BO2+ for high precision analysis. Steady ion beam was obtained when graphite was loaded on the filament after fusion of alloy carbonate mixture contrary to the reverse procedure that is normally followed. Sequence of graphite addition on the filament plays an important role in the formation of Rb2BO2+ ions. The paper gives the details of the investigations carried out.