A range of copper nanopowders (BET particle size 20-70 nm) has been prepared through a DC thermal plasma synthesis under varying operating conditions. A thermal evolution analysis was performed on these materials to quantify the nonmetallic impurities (O, C, N, H) which originated from the working and ambient environments. A temperature-programmed oxidation-infrared detection (TPO) was used for carbon and hydrogen determinations, an inert gas fusion-infrared/conductometric detection (GFA) for oxygen and nitrogen determinations. The TPO and GFA evolution patterns provided an insight into impurity speciation in terms of the free and chemically bound carbon, oxidic and adsorbed oxygen, as well as the possible types of organic functionalities. The impurity uptake and speciation were found to be a strong function of the operating conditions, including the choice of the plasma gas and the Cu feedstock. The operating conditions were identified under which the oxidation of the product nanopowder was minimized upon exposure to ambient laboratory environment. The product passivation during the synthesis was attributed to the presence in the working environment of the gaseous species (CO, CN), which are capable of strong chemisorption on the freshly prepared copper surface. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.