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Sanidine from the Fish Canyon Tuff is regularly used as a neutron flux monitor by 40Ar/39Ar geochronologists. A new sampling of the Fish Canyon Tuff (denoted FCs-EK) has yielded sanidine that relative to FC-2 is confirmed here to have an R value of 0.9997±0.0100, which is indistinguishable from 1. The new sample will allow for continuity as many 40Ar/39Ar laboratories have exhausted their supplies of FC-2. FCs-EK is now available in significant quantities from the Scottish Universities Environmental Research Centre.Supplementary material:A table of Ar isotopic data is available at http://www.geolsoc.org.uk/SUP18632.

In 2013, NPL, SUERC and Cranfield University published an estimate for the Boltzmann constant [1] based on a measurement of the limiting low-pressure speed of sound in argon gas. Subsequently, an extensive investigation by Yang et al [2] revealed that there was likely to have been an error in the estimate of the molar mass of the argon used in the experiment. Responding to [2], de Podesta et al revised their estimate of the molar mass [3]. The shift in the estimated molar mass, and of the estimate of kB, was large:-2.7 parts in 10 6 , nearly four times the original uncertainty estimate. The work described here was undertaken to understand the cause of this shift and our conclusion is that the original samples were probably contaminated with argon from atmospheric air. In this work we have repeated the measurement reported in [1] on the same gas sample that was examined in [2, 3]. However in this work we have used a different technique for sampling the gas that has allowed us to eliminate the possibility of contamination of the argon samples. We have repeated the sampling procedure three times, and examined samples on two mass spectrometers. This procedure confirms the isotopic ratio estimates of Yang et al [2] but with lower uncertainty, particularly in the relative abundance ratio R38:36. Our new estimate of the molar mass of the argon used in Isotherm 5 in [1] is 39.947 727(15) g mol-1 which differs by +0.50 parts in 10 6 from the estimate 39.947 707(28) g mol-1 made in [3]. This new estimate of the molar mass leads to a revised estimate of the Boltzmann constant of kB = 1.380 648 60 (97) × 10 −23 J K −1 which differs from the 2014 CODATA value by +0.05 parts in 10 6 .

Lasers are fundamental tools for sampling in geochemical studies and have found wide application in mass spectrometric sample introduction systems. Here we describe an isotope extraction method for 40Ar/39Ar geochronology using a new scanning CO2 laser system. This method can partially un-mix radiogenic (40Ar*) from trapped argon components and provides an alternative to furnace step-heating methods. A key advantage of the laser scanning method developed at the Scottish Universities Environmental Research Centre (SUERC) is the ability to step-heat samples as large as 100 mg to fusion using low raster speeds, although care must be taken to avoid self-shielding of grains and proper laser targeting. The scanning laser extraction system has the potential for lower overall blanks and the ability to run blanks and calibrations between steps of a heating sequence. This provides better control on system performance and characterization during sample measurement and can result in improved data quality.Supplementary material:Ar/Ar data presented in Figures 6a–c and 7 (obsidian) is available at http://www.geolsoc.org.uk/SUP18694.