The authors describe a spectroscopic technique to investigate the transport processes of electrons and the quenching processes of excited metastable molecules under the condition of non-self-sustained Townsend discharges. The technique is applied to pure N2, and N2 including NO as an impurity at E/N=282.5 Td and currents less than 10-9 A. The quenching factor of N2(A3 Sigma u+) metastable by NO, defined by (1-(density of N2(A3 Sigma u+) in N2 with NO)/(density of N2(A3 Sigma u+) in pure N2)), is measured as a function of total gas density from the emission spectrum both of the second positive bands of N2 and of the gamma system of NO. The spatial profile of N2(A3 Sigma u+) is also monitored and estimated by the emission of the gamma system. It will be noted that half of the N2(A3 Sigma u+) metastables are deactivated with only 20 PPM of NO at N=1.94*1017 cm-3 in the low pressure ionisation growth chamber. The results give a quantitative explanation for the rapid decrease of the secondary ionisation coefficient in N2 in the presence of small amounts of the impurity. Also the radiative lifetime and the self-quenching rate coefficient are estimated in the present experimental conditions.