A determination of the helium 4 content of near‐surface atmospheric air within the continental United States

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Abstract
The Bureau of Mines, Helium Field Operations, in Amarillo, Texas, has determined the helium 4 content of near‐surface atmospheric air within the continental United States. Analysis of 40 atmospheric samples obtained from 12 locations during a 1‐year period showed the helium 4 content of dry air to be constant at 5.2204 ± 0.0041 ppm by volume (ppmv). The reported accuracy of ±0.0041 ppm is at the 68% confidence level. The mean helium concentration obtained is in agreement with the concentration of 5.222 ± 0.017 ppmv reported by Oliver et al. (1984) but is 0.36 ± 0.08% lower than the currently accepted value of 5.239 ± 0.004 ppm reported by Glueckauf (1946). A helium‐4‐tuned mass spectrometer having a precision of ±0.0011 ppm was used to analyze the air samples. Helium‐4‐in‐nitrogen gravimetric standards accurate to ± 0.003 ppm were used for calibration of the mass spectrometer.

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