![]() Finally, the reference gas has the added advantage of facilitating straightforward characterization of electron multiplier dead time over a wide dynamic range. This calibration technique is applicable over a wide range of isotopic ratios and signal sizes. Replicate analyses of 8–12 single-crystal sanidine ages are reproduced to within 1–2‰ (2σ standard error) under optimal analytical conditions. Application of the calibration method to 40 Ar/ 39 Ar multicollection measurements of widely used sanidine reference materials ACs-2, FCs-2, and TCs-2 demonstrate that calculated 40 Ar*/ 39 Ar K can be accurately corrected to yield model 40 Ar/ 39 Ar ages consistent with those reported by Earthtime 40 Ar/ 39 Ar laboratories. Monitoring this variation requires daily calibration of the instrument. The EARTHTIME project has made available to the UPb community common isotopic tracer (or spike) solutions that have been precisely calibrated 28. These correction factors are reproducible to better than ± 0.5‰ (2σ standard error) over intervals spanning ~ 24 h but can vary systematically by ~ 4% over 2 weeks of continuous use when electron multiplier settings are held constant. By combining atmospheric Ar and synthetic reference gas in different ways, we can directly measure 40 Ar/ 39 Ar, 38 Ar/ 39 Ar, and 36 Ar/ 39 Ar correction factors over ratios that vary from 0.5 to 460. We present a new method for calibrating the Noblesse based on use of both atmospheric Ar and the synthetic reference gasses. the EARTHTIME initiative and plan to use the new, internationally calibrated, quadruple (202Pb-205Pb-233U-235U) spike for all of our U-Pb measurements. To accomplish this, we have developed synthetic reference gasses containing 40 Ar, 39 Ar and 38 Ar produced by mixing gasses derived from neutron-irradiated sanidine with an enriched 38 Ar spike. single zircon grain analyses, utilizing both physical abrasion and CA-TIMS and the MIT-1L spike calibrated against the EARTHTIME gravimetric standards. After exploring alternative calibration approaches, we have concluded that calibration of the Noblesse is best performed using exactly the same source, detector, and ion-optic tuning settings as those used in routine 40 Ar/ 39 Ar analysis. Reliance upon an atmospheric Ar standard to calibrate the Noblesse is problematic because there is no straightforward way to relate atmospheric 40 Ar and 36 Ar to measurements of 40 Ar and 39 Ar if they are measured on separate detectors. We have found that detector efficiency and mass discrimination are affected by changes in ion-optic tuning parameters. In particular, use of zoom lens ion-optics to steer and focus ion beams into a fixed detector array (i.e., Nu Instruments Noblesse) makes intercalibration of multiple detectors challenging because different ion-optic tuning conditions are required for optimal peak shape and sensitivity at different mass stations. ![]() 206Pb depending on zircon size and spike weight), with a drop-off below 1 mV. The greatest challenge limiting 40 Ar/ 39 Ar multicollection measurements is the availability of appropriate standard gasses to intercalibrate detectors. 2018, GGR), the Earthtime 2 Ga and Early Time 4.5 Ga synthetic solutions. ![]()
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