  The dependence of the avalanche charge gain in Thick Gas Electron Multipliers
(THGEM) on the purity of Ne, Ar and Xe filling gases was investigated. The
gain, measured with alpha-particles in standard conditions (atmospheric
pressure, room temperature), was found to considerably drop in gases purified
by non-evaporable getters. On the other hand, small N2 admixtures to noble
gases resulted in high reachable gains. The results are of general relevance in
the operation of gas-avalanche detectors in noble gases, particularly that of
two-phase cryogenic detectors for rare events.
