One aspect of the Department of Energy's National Uranium Resource Evaluation (NURE) program sponsored through Bendix Field Engineering Corp. involves evaluation of instrumentation used for uranium exploration. In recent years ⁴He has been useful in exploring for both uranium ore and petroleum deposits. As a consequence, mass spectrometers (leak detector type) have been used to seek small (a few percent) variations of ⁴He in soil gases and aquifers. We modified one of these instruments to ascertain its reliability regarding resolution (the ability to separate adjacent mass peaks). We used the Veeco MS 17 AB because it provides twice the resolution of comparable instruments. An electromagnet was substituted for one of the permanent magnets to enable s anning the 0 to 5 amu mass range.

Acquired spectra allowed the determination of the contribution to mass three (HD + H₃ + ³He) from mass two (H₂). Interference is significant at the 1% level (the size of helium anomalies usually sought) when the abundance ratio (M = 2)/(M = 3) >= 75. This value is valid for the H₂/⁴He ratio for an equivalent system with 50% lower resolution.

Additional experiments identified dissociated water or hydrocarbons as the principal source of H₂. Correlations of ⁴He with soil gas moisture found in prior studies may not be due to true variation in ⁴He but are ascribable to artifacts resulting from H₂ contributions to ⁴He. Corrections for such artifacts can be made by converting field instruments to a scanning mode.

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