Isotope ratio measurements by MC-ICPMS below 10 μL min−1 under continuous sample flow conditions. Exploring the limits with strontium

Abstract

Preliminary investigations have shown that performing Sr isotope ratio measurements at 15 μL min⁻¹ (continuous flow, for 300 μL large samples) by multi-collector Inductively Coupled Plasma Mass Spectrometry using a heated torch integrated spray chamber (TISIS) was possible at the same level of uncertainty as for measurements at 200 μL min⁻¹ (using a mini-cyclonic chamber). The objective of the current work was to achieve the same performance when working below 10 μL min⁻¹, and to further reduce the sample consumption to no more than 100 μL. The TISIS was modified to allow the additional introduction of a sheathing gas flow, and a semi-automated system was developed using syringe pumps and multi-port valves for the sample delivery. The influence of the liquid flow rate, combination of gas flow rates and temperature of the TISIS on different figures of merit was studied, and the relative combined uncertainty (k = 2) on measurement results was estimated for the method validation. The signal stability was brought to less than 5% relative standard deviation at sample flow rates as low as 5 μL min⁻¹. 7 μL min⁻¹ was identified as a threshold below which the exponential model used to correct for mass discrimination effects cannot be applied without the need to account for a relative bias of more than 200 parts per million, ‘ppm’ (500 ppm at 5 μL min⁻¹). The best compromise corresponded to 9 μL min⁻¹ when heating the spray chamber walls at 65 °C. It required 2.6 times less analyte overall compared to the results at 15 μL min⁻¹, the repeatability on the ⁸⁷Sr⁺/⁸⁶Sr⁺ ratio measured in the SRM 987 was around 40 ppm (and around 20 ppm after internal correction with the ⁸⁸Sr⁺/⁸⁶Sr⁺ ratio, from the application of the exponential model), and relative combined uncertainties (k = 2) on stand-alone results were around 180 ppm at maximum. This method may be particularly useful for samples only available in small sizes and/or not allowing any dilution, and will depend in particular on the analyte mass fraction and the target uncertainty of the application.