Investigation of the crucial factors affecting accurate measurement of strontium isotope ratios by total evaporation thermal ionization mass spectrometry

RATIONALE: An absolute method that does not require calibration is very important in several areas of isotope analysis due to the shortage of suitable certified reference materials (CRMs). As total evaporation thermal ionization mass spectrometry (TE-TIMS) theoretically overcomes mass fractionation through integrating the ion currents till the sample on the filament is completely exhausted, it could be an absolute method that does not require the use of CRMs for calibration. However, the lack of reliable method verification and reasonable uncertainty evaluation restricts its extensive application, and these effects need to be quantitatively evaluated. METHODS: A series of different amounts of strontium reference material SRM987 was deposited on both single and triple tantalum filament sources, and analyzed by TE-TIMS. The work function of the tantalum filament loaded with the sample was measured by ultraviolet photoelectron spectroscopy. In order to evaluate the change in ion transmission efficiency, we terminated the total evaporation process manually after running for a certain period of time, and recorded the ion beam intensity before and after this operation. RESULTS: The TE results obtained with the triple filament source agreed well with the reference values of SRM987, while the TE results obtained with the single filament source changed significantly with the amount of sample loading. The crucial effect factors on TE-TIMS, including ionization temperature fluctuation, variation of ion transmission efficiency and ion loss before data acquisition, were quantitatively assessed. These factors were used to correct the single filament TE results and the measurement uncertainty was quantitatively evaluated. CONCLUSIONS: The reliability of strontium isotopic ratio measurement with triple filament TE-TIMS was verified. The mathematical model for correction of each crucial effect factor was successfully established, thus providing a feasible way for the correction of single filament TE-TIMS results.