No abstract

No abstract

This study characterises for the first time isotope ratio measurements by multi-collector ICPMS when performed at liquid flow rates as low as 10-15 mL min À1 . An evolution of the torch integrated sample introduction system (TISIS) was employed, which combined an OpalMist nebulizer and a heated single pass spray chamber, allowing the transport of almost 100% of the solvent. The different factors potentially influencing the isotope ratio measured values and the associated sources of uncertainty (sensitivity and interferences, mass discrimination effects, repeatability of isotope ratio measurement, and rinsing time and memory effects) were investigated systematically under varying experimental conditions. Results showed that the liquid flow rate has a large impact on mass discrimination effects, making the control of this variable critical. From 5 to 30 mL min À1 , when using the exponential model, the mass discrimination per mass unit changed from À1.6 to À2.1 and from À1.1 to À1.9, for 88 Sr + / 86 Sr + and 208 Pb + / 206 Pb + ratios respectively. Moreover, extrapolations from these results lead to the conclusion that a syringe pump may be required instead of a free aspiration regime to control the liquid flow rate and eliminate the possibility of undesired variations of isotope ratio results (typically, 0.05& error for 1-2% fluctuations at 10 mL min À1 ). The validity of the exponential model also depends on the experimental conditions selected. When working at 15 mL min À1 and heating the chamber walls at 60-80 C, the performance was as good as it could be with a MicroMist/water cooled cinnabar combination operated at 200 mL min À1 (thus, the efficiency was 12 times better for the TISIS). Both systems were compared for the measurement of the Sr isotopic signature in honey samples. Since the TISIS allowed for a preconcentration of samples by a factor 5, the combined uncertainty on results could be improved by 1.6 to 4.4. The main drawback was, however, the longer rinsing time required to reach a stable background signal (10-12 minutes rather than 4).

The aim of this work (from the FP6 project TRACE) was to develop methods based on the use of geochemical markers for the authentication of the geographical origin of cereal samples in Europe (cf. EC regulations 2081/92 and 1898/06). For the first time the potential usefulness of combining n(87 Sr)/n(86 Sr) and δ 13 C, δ 15 N, δ 18 O and δ 34 S isotopic signatures, alone or with key element concentrations ([Na], [K], [Ca], [Cu] and [Rb], progressively identified out of 31 sets of results), was investigated through multiple step multivariate statistics for more than 500 cereal samples collected over 2 years from 17 sampling sites across Europe representing an extensive range of geographical and environmental characteristics. Both models compared involved three sample classification categories (north/south; proximity to the Atlantic Ocean/to the Mediterranean Sea/to else; bed rock geologies). The first two categorisations were the most efficient, particularly when using the ten variables selected together (with, in some instances, element concentrations making a greater impact than the isotopic tracers). Validation of models included external prediction tests on 20% of the data randomly selected and, rarely done, a study on the robustness of these multivariate data treatments to uncertainties on measurement results. With the models tested it was possible to individualise 15 of the sampling sites.