Calculation Methods and Mass Fractionation Correction of Spiked Isotopic Ratios with Thermal Ionization Isotope Dilution Mass Spectrometry

Isotope dilution thermal ionization mass spectrometry (ID-TIMS) combined with the advantage of thermal ionization mass spectrometry in isotope measurement with isotope dilution technology is one of primary method of measurement. ID-TIMS has the following advantages: 1) high accuracy; 2) quantitative recovery of measured elements is not required in the process of experiment. Mass fractionation effect is difficult to obtain accurately and it is the main factor affecting the accurate determination of isotope composition. Taking the analysis of strontium isotope ratio as an example, the whole calculation and derivation process were introduced. The method for mass fractionation correction based on exponential approximation was proposed. It could achieve the goal of obtaining isotopic ratio and element content at the same time by weighing one sample and was suitable for isotope with two internal reference isotopic ratios in static multicollector measurements mode. The exponential approximation was similar to the linear correction law formula transformed from the complex exponential correction mode, the calculation method was simple compared to those using Power law and Exponential law. In addition, the influence of isotope ratio in spike on the isotope ratio of dilution analysis was also be discussed. By changing the proportion of standard samples to spikes, a series of strontium isotope ratios in standard samples could be obtained through calculation. The results showed that with the increased of mixing ⁸⁴Sr/⁸⁶Sr ratio, the deviation between the calculated ⁸⁷Sr/⁸⁶Sr in standard samples and the true value increased gradually. Therefore, mass fractionation correction for the isotope ratio of spike was necessary. Theoretically, if isotope ratio of spike was true value, the formula calculated ⁸⁷Sr/⁸⁶Sr of standard samples should be consistent, regardless of the mixing ⁸⁴Sr/⁸⁶Sr ratio. According to this theory, several standard samples were weighed with different proportions of spike, then the mixing isotopic composition and isotopic composition of spike were measured. The quality fractionation correction factor was obtained by mathematical iteration using the “solver” function in Excel software. The isotope ratio of spike was corrected by quality fractionation correction factor. The results of spiked ratios of international standard sample NBS987 using the method of correction for mass fractionation are consistent with the reference value within error range (⁸⁷Sr/⁸⁶Sr=0.710 237±8（1σ）), and the influence of mixture spiking level on the spiked ratios is getting lower.