Determination of ultralow iridium concentration in small geological samples using isotope dilution coupled with multiple ion counting inductively coupled plasma mass spectrometry†

Concentrations of Ir in natural samples can provide useful information for tracing a variety of geological and planetary processes; however, efficient, sensitive, and precise analysis of Ir contents remains challenging, especially for crustal samples that are highly depleted in Ir (i.e., at the pg g⁻¹ level). Here we report an analytical method for determining ultralow Ir contents (pg g⁻¹ level) in small geological samples (<1 g) by the isotope dilution method (ID) using a multiple ion counting inductively coupled plasma mass spectrometer. An ¹⁹¹Ir-enriched spike was mixed with the sample during sample digestion, followed by the separation and purification of Ir from the rock matrix using AG MP-1 anion exchange resin. Ir isotope ratios were analyzed on a Nu 1700 Sapphire MC-ICP-MS using the multiple ion counting. Our tests indicated that the use of collision cell mass spectrometry with helium and hydrogen as the collision/reaction gases did not offer benefits in removing isobaric interferences for Ir isotope analysis. However, through the combination of chemical purification with conventional wet-plasma mass spectrometry, we attained sufficient accuracy for Ir analysis at ultralow levels. The total procedural blank and detection limit for this method were determined to be 7.6 ± 3.5 pg (2σ, N = 10) and 0.35 pg g⁻¹, respectively. To validate the accuracy of this analytical method, a K-Pg boundary reference sample (DINO-1) and six USGS reference materials were analyzed, and the obtained results were consistent with previous studies. Furthermore, we report Ir contents in other 11 international geological reference materials with low Ir abundance, demonstrating the applicability of this method in studying ultralow Ir content samples associated with magmatic processes, supergene processes and impact events.