Single and multiple spike procedures for the determination of butyltin compounds in sediments using isotope dilution GC-ICP-MS

The combined use of single and multiple spike solutions for the determination of butyltin compounds in sediments by species-specific isotope dilution mass spectrometry has been evaluated in this work in order to validate an analytical procedure proposed for routine determinations. For this purpose, a test sediment sample was analysed during the course of an intercomparison exercise using both a single isotope spike solution (a mixture of mono-, di- and tributyltin enriched in 119Sn) and a triple spike solution containing each butyltin species enriched with a different tin isotope. The triple spike methodology (able to correct for interconversion reactions) was employed for the optimisation of the microwave assisted extraction procedure that was subsequently applied for the analysis of the test sediment. The determination of butyltin compounds in the test sediment was performed using both spike solutions under the same extraction conditions. An excellent agreement between the results obtained using both approaches and those provided by all laboratories participating in the intercomparison exercise was obtained, validating in this way the methodologies employed in this work. Uncertainty budgets were calculated for both spiking methodologies. The complexity of the triple spike methodology, in which the simultaneous measurement of nine tin isotope ratios must be carried out, provided additional sources of error in obtaining degradation-corrected concentrations of the three butyltin species. Additionally, uncertainty budgets were obtained also for the calculation of degradation factors F1 (degradation of TBT to DBT) and F2 (degradation of DBT to MBT). The results demonstrated that their uncertainty was independent of the uncertainty of the measurement of the concentration of the butyltin species, both in the spike solution and in the sample, when using a multiple spiking approach, and depended only on the measured isotope ratios and on the uncertainty in the isotopic composition of the spike.