Isotopic ratio analysis of individual sub-micron particles via spICP-TOFMS

Abstract

We investigate the use of single particle inductively coupled plasma time-of-flight mass spectrometry (spICP-TOFMS) to measure isotopic ratios within individual sub-micron particles and explore the advantages and limitations of this method. Through the analysis of samarium (Sm) isotopes—¹⁴⁷Sm and ¹⁴⁹Sm—in individual monazite particles, and lead (Pb) isotopes—²⁰⁶Pb and ²⁰⁸Pb—in individual galena particles, we demonstrate that isotope ratios recorded by spICP-TOFMS have precision controlled by Poisson statistics. This precision depends on the signal amount measured per isotope from an individual particle: as particle size increases, more counts of each isotope are detected, and the precision improves. In monazite particles with mass amounts of Sm from 0.04 to 4 fg, recorded isotope-ratio precision (relative standard deviation, RSD) ranged from 43% to 5%. However, the average isotope ratio from a particle population is still accurate; the molar ratio determined for ¹⁴⁹Sm/¹⁴⁷Sm was 0.912, which is within 1% of the expected ratio. Lead isotopic composition varies widely in nature because ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb are radiogenic isotopes that decay from thorium (Th) and uranium (U). In the analysis of lead isotopes from galena particles, we found that the RSD for ²⁰⁸Pb/²⁰⁶Pb ratio ranged from 32% to 2% for particles with 1.4 to 80 fg of Pb. We further explore the use of spICP-TOFMS for radiometric dating of monazite particles. Monazite is used in geochronology for radiometric dating based on ²⁰⁸Pb/²³²Th and ²⁰⁶Pb/²³⁸U ratios. spICP-TOFMS analyses of individual monazite particles that contain only 0.02–80 fg of Th and 0.03–30 fg of U showed radiogenic Pb-isotope signatures and a median age of 550 Ma. We also show that the spread of ages from monazite particles is broader than explainable by Poisson statistics, revealing real variation in age or depletion/enrichment of Pb, Th, and/or U in the particles. Overall, we demonstrate that spICP-TOFMS can be used for accurate isotope-ratio analysis with precisions down to a few percent; however, understanding measurement noise is critical to define the significance of isotope ratios measured from individual particles.