MC-ICPMS isotope ratio measurements using an ultra-low flow sample introduction system

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).

Soil properties, strontium isotopic signatures and multi‐element profiles to authenticate the origin of vegetables from small‐scale regions: illustration with early potatoes from southern Italy

“…Multi‐element measurements were carried out by quadrupole inductively coupled plasma mass spectrometry (ICPMS), with an Elan 6000 mass spectrometer (Perkin‐Elmer Belgium, Zaventem, Belgium) equipped with a pumped micro‐concentric nebuliser (MicroMist, Glass Expansion, West Melbourne, Australia) and a mini‐cyclonic spray chamber (Cinnabar, Glass Expansion). Sr isotope ratio measurements were carried out by multi‐collector (MC)‐ICPMS with a Nu Plasma mass spectrometer (Nu Instruments, Wrexham, UK), fitted with an OpalMist perfluoroalkoxy nebuliser (Glass Expansion) and a home‐made single‐pass spray chamber which acted as an evaporation cavity according to the principles of the ‘torch integrated sample introduction system’ . The chamber was heated at 60–80 °C and the liquid flow rate was 15 μL min −1 .…”

“…Typical settings and parameters resulting from daily optimisations of the two ICPMS instruments can be found in reports from studies conducted with the same machines under similar conditions by Brach‐Papa et al . and Paredes et al …”

“…Calculations were made on a dry mass basis (Mmc = M(1 – mc) where Mmc = dry mass (g) of the sample, M = mass of the sample (g), and mc = relative moisture content of the sample). Sr isotope ratio measurements were carried out according to the procedure described in Paredes et al …”

“…The variability was successively multiplied by 2 and rounded to the closest multiple of 5 (and to the multiple of 5 immediately above for Cu and Zn): 20% for Mn, 15% for Cu, 15% for Zn, 30% for Rb, 55% for Sr and 25% for Cd. For the n ( 87 Sr)/ n ( 86 Sr) measurements the validation scheme applied was based on the approach described by Paredes et al . It included a systematic assessment of the factors influencing the results, a full mathematical description of the measurement procedure and the estimation of uncertainties associated to measurement results.…”

Lead isotopic analysis of Antarctic snow using multi-collector ICP-mass spectrometry

“…Paredes et al investigated the performance of a TISIS coupled with MC-ICP-MS for the determination of Pb 14 and Sr 15,16 isotope ratios. Accurate data were obtained at a very low sample uptake rate (5-30 mL min À1 ) and at a total sample consumption of less than 0.3 mL of solution.…”