The Bureau of Mines, Helium Field Operations, in Amarillo, Texas, has developed a method for direct measurement of the helium 3 content of air by mass spectrometry. The helium is preconcentrated by passing samples of air through a charcoal trap at liquid nitrogen temperature. Sample measurements are compared with identically made measurements of gravimetrically prepared standards. The helium 3 content of near‐surface atmospheric air is determined to be 7.27±0.20 parts per trillion by volume (pptv). The accuracy of 0.20 pptv is at the 68% confidence level. The helium 4 content of near‐surface atmospheric air has recently been determined at this facility to be 5.2204±0.0041 parts per million by volume (ppmv). The helium 3/helium 4 ratio for atmospheric helium, RA, is established as 1.393×10−6 by independent measurements of the separate components.
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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