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.
| Original language | English |
|---|---|
| Pages (from-to) | 1874-1884 |
| Number of pages | 11 |
| Journal | Journal of Analytical Atomic Spectrometry |
| Volume | 39 |
| Issue number | 7 |
| DOIs | |
| State | Published - May 30 2024 |
| Externally published | Yes |
Funding
The authors would like to acknowledge Iowa State University for funding through a faculty start-up grant, and funding though NSF CAREER grant CHE-2237291. We would also like to thank Dr Ralf Kaegi at EAWAG for the monazite sample, as well as the Iowa State University Department of Geological and Atmospheric Sciences for the galena mineral sample. We would like to thank Hark Karkee, Aishwarya Mantravadi, and Dr Yulia Zaikina from the Department of Chemistry at Iowa State University for ball-milling of the galena mineral. We also want to acknowledge the Iowa State University Glass Shop and Chemistry Machine Shop for fabrication of our microdroplet introduction system.