Abstract: Disclosed is a mass selective fluid bandpass filter. This filter provides for selecting gas molecules of a specific mass from a gas sample containing molecules of two or more mass species. This provides for a low power, low cost apparatus for producing 3He from terrestrial sources of helium gas by selective enrichment. This invention further discloses a portable, field deployable means of 3He/4He ratio determination employing one or more of the effects consisting of: statistical linear regression plots of heat ramps, variable emission currents within a quadruple mass spectrometer, use of a quadrupole mass spectrometer in concert with ultrahigh vacuum maintained by non-evaporable getter pumps, and/or construction of vacuum housing structures from non-steel or non-stainless steel alloys, and or non metallic materials selected from a group consisting of: aluminum, titanium, ceramics, or glass. This provides a compact, sensitive field deployable unit with low power consumption.