Possible errors in helium analyses of soil gases by mass spectrometers having constant-pressure inlet systems

Published data suggest that soil gas helium concentrations of 5.28–5.34 ppm v/v over uranium and hydrocarbon deposits are significantly anomalous compared to the ambient atmospheric background of 5.24 ppm. However, analyses for helium by mass spectrometers having constant-pressure inlet systems, from which most of these data are derived, are subject to errors of equivalent magnitude. These errors arise when the major component composition of unknown and standard gases differ, for the different gases have different flow rates through the inlet system — relative rates being O₂ < dry air < water-saturated air < N₂ < CO₂ CH₄. Soil gas compositions can vary greatly and, compared to a dry air standard, the flow-rate of a water-saturated gas containing 10% biogenic CO₂ will increase, enhancing the apparent He content to 5.33 ppm. Accurate helium analyses can be achieved by using a constant-volume inlet and integrating the detector response over the period of the samples' passage through the detector.