A trace gas sensing apparatus includes a cathode, an anode, a vacuum enclosure, and a membrane. The anode coaxially surrounds the cathode, wherein the cathode and the anode define an annular ionization chamber. The vacuum enclosure surrounds the cathode and the anode and includes a gas inlet fluidly communicating with the ionization chamber. The membrane is coupled to the gas inlet in a sealed manner and is permselective to trace gas. The apparatus may further include circuitry for applying a negative voltage potential to the cathode and for measuring an ion current signal generated by the cathode, and a magnet assembly for generating a magnetic field in the ionization chamber. The cathode may include an elongated member located along a longitudinal axis, and first and second end plates orthogonal to the longitudinal axis.