Strontium isotope ratio determination in soil and bone samples after on-line matrix separation by coupling ion chromatography (HPIC) to an inductively coupled plasma sector field mass spectrometer (ICP-SFMS)

In this work we investigate strontium isotopic ratios in human skeletal rem ains and reference soil samples from the corresponding archaeological sites . Because Sr-87 is spectrally overlapped by Rb-87 (m/Deltam=300.000), a sep aration prior to analysis was a prerequisite for accurate isotope ratio det ermination especially in soil samples. The main objective of our study was, therefore, the development of an on-line ion chromatographic method (HPIC) for routine separation of Rb and Sr prior to strontium isotopic ratio dete rmination by sector field inductively coupled plasma mass spectrometry (ICP -SFMS). Operating parameters of the HPIC, including flow rates and eluents, and instrumental bias factors of the ICP-SFMS. were optimized in an initia l step. A shielded torch system (CDI-option) in combination with ultrasonic nebulization increased the strontium signal intensity by 100-fold in the l ow mass resolution mode (m/Deltam = 300). During separation, the reduction of the HPIC flow rate from 3.0 to 0.5 mL min(-1) in combination with succes sive injections generated a stable transient strontium signal of more than 16 min duration. The precision obtained (RSD for n = 5 measurements) on suc h transient signals was 0.068% for 5 ng g(-1) strontium in the measurement solution deviating by 0.064% from the certified Sr-87/Sr-86 isotopic ratio value of NIST SRM 987 after dead time and mass bias correction. The optimiz ed procedure for the coupled on-line HPIC-ICP-SFMS system allowed non-inter fered Sr isotope ratio measurements with no statistically significant diffe rences in precision and accuracy compared to direct nebulization. The setup was successfully applied to the investigation of bone samples in an archae ological finding. We found that diagenetic influences on Sr isotope ratios in prehistoric human bones were negligible, and grouping of individuals cou ld be performed.