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Low energy ion measurements in the vicinity of a comet have provided us with important information about the planet’s evolution. The calibration of instruments for thermal ions in the laboratory plays a crucial role when analysing data from in-situ measurements in space. A new low energy ion source based on carbon nanotube electron emitters was developed for calibrating the ion-mode of mass spectrometers or other ion detectors. The electron field emission (FE) properties of carbon nanotubes (CNTs) for H₂, He, Ar, O₂, and CO₂ gases were tested in the experiments. H₂, He, Ar, and CO₂ adsorbates could change the FE temporarily at pressures from10^−6 Pa to10^−4 Pa. The FE of CNT remains stable in Ar and increases in H₂, but degrades in He, O₂, and CO₂. All gas adsorbates lead to temporary degradation after working for prolonged periods. The ion current of the ion source is measured by using a Faraday cup and the sensitivity is derived from this measurement. The ion currents for the different gases were around 10 pA (corresponding to 200 ions/cm³ s) and an energy of ~28 eV could be observed.