![]() ![]() ![]() Given that a laboratory's air aliquot size and grain size of standards may be consistent over long periods but likely differ from lab to lab, may help explain internal lab consistency but inter-lab variability. Thus, we believe that this may account for a significant amount of the dispersion observed in the inter-calibration experiment and for a portion of inter-laboratory bias. Given that the 40Ar signals produced by the AC and FC grains used in the EARTHTIME inter-calibration experiment differed by as much as a factor of twenty, a simple linear extrapolation of our results to a 10-20 fold pressure differential in a sample would equate to a 1-2% offset in apparent ages. In our experiment we show a pressure dependency in MD a 5-fold increase in sample pressure results in a ~5% change in MD, this results in about a ~6% shift in the calculated age of AC. We conclude that variation in MD over signal sizes typically analyzed in 40Ar/ 36Ar dating can yield reproducible yet different apparent ages depending on the signal size of the sample gas used to determine MD, that of the standard to determine J, and that of the unknown being analyzed. However, when the MD as determined from the multiple aliquot pipette data (with an 40Ar beam similar to that of the FC analyses) is applied to the FC data a "J" of 1.588☐.001x10 -4 is obtained and an AC age of 1189 ±4 ka the same age at the 95% confidence level, the reported age for AC. When the MD as determined from the single pipette data (which matches 40Ar beam intensity of the AC sample) is applied to both the AC and FC data a "J" of 1.579☐.001x10 -4 is obtained from the FC data and an age of 1183 ±4 ka for AC. EarthTime 5.5.35 Released: 30th Jul 2023 (a few seconds ago) EarthTime 5.5.34 Released: 30th Jul 2023 (a few seconds ago) EarthTime 5.5. Following the analyses, we applied the MD correction to both the FC and AC analyses. The FC grains were about 5-10 times greater (150-600 kcps) than the single air aliquot. The grain sizes for the two mineral standards were chosen such that the AC sample yielded 40Ar signals of about 50 kcps, similar to that delivered by a single aliquot delivered by the air pipette. The air measurements were interspersed with measurements of Alder Creek (AC) sanidine and Fish Canyon (FC) sanidine that were co-irradiated for 0.75 hours. The air 40Ar/ 36Ar aliquots were measured using a MAP 215-50 operating in pulse counting mode. The measured mass discrimination difference (MD) as determined by the 40Ar/ 36Ar ratio between the 1-fold and 5-fold air pipette shots is ~5%. In our experiment, we varied by a factor of five the signal size of aliquots of atmospheric argon delivered from an automated air pipette system. Here we report on the pressure dependency on mass spectrometer fractionation bias. Turrin et al., 2010 and references therein). Temporal variations in mass fractionation are well known phenomena and have been reported in several studies (eg. ![]() Mass spectrometer fractionation bias (mass discrimination) is a ubiquitous phenomenon in noble gas mass spectroscopy and must be corrected for in order to obtain accurate-high precision isotopic ratios that are used in isotopic age calculations. ![]()
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