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

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- 147 Sm and 149 Sm-in individual monazite particles, and lead (Pb) isotopes- 206 Pb and 208 Pb-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 149 Sm/ 147 Sm was 0.912, which is within 1% of the expected ratio. Lead isotopic composition varies widely in nature because 206 Pb, 207 Pb, and 208 Pb 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 208 Pb/ 206 Pb 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 208 Pb/ 232 Th and 206 Pb/ 238 U 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. Single particle ICP-TOFMS is used to measure isotope ratios within individual sub-micron particles. We explore the advantages and limitations of this method through the analysis of stable and radiogenic isotope pairs in monazite and galena particles.