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)

Abstract

In this work we investigate strontium isotopic ratios in human skeletal remains and reference soil samples from the corresponding archaeological sites. Because 87Sr is spectrally overlapped by 87Rb (m/Δm = 300.000), a separation prior to analysis was a prerequisite for accurate isotope ratio determination 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 determination 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 initial step. A shielded torch system (CD1-option) in combination with ultrasonic nebulization increased the strontium signal intensity by 100-fold in the low mass resolution mode (m/Δm = 300). During separation, the reduction of the HPIC flow rate from 3.0 to 0.5 mL min−1 in combination with successive injections generated a stable transient strontium signal of more than 16 min duration. The precision obtained (RSD for n = 5 measurements) on such transient signals was 0.068% for 5 ng g−1 strontium in the measurement solution deviating by 0.064% from the certified 87Sr/86Sr isotopic ratio value of NIST SRM 987 after dead time and mass bias correction. The optimized procedure for the coupled on-line HPIC-ICP-SFMS system allowed non-interfered Sr isotope ratio measurements with no statistically significant differences in precision and accuracy compared to direct nebulization. The setup was successfully applied to the investigation of bone samples in an archaeological finding. We found that diagenetic influences on Sr isotope ratios in prehistoric human bones were negligible, and grouping of individuals could be performed.