Direct determination of the helium 3 content of atmospheric air by mass spectrometry

Abstract

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, R A , is established as 1.393×10 −6 by independent measurements of the separate components.