A miniature Herriott cell consisting of 12.7 mm OD protected gold mirrors with 24 laser passes has been developed for in-situ measurements of temperature and species in a chemical reactor integrated with a nanosecond-pulsed plasma discharge. A wavelength tunable external cavity mode-hop free quantum cascade laser (EC-MHF-QCL) near 7.6 μm accessing the fundamental absorption bands of targeted molecules has been used. Quantitative measurements of temperature, CH4, H2O, and C2H2 in a uniform plasma discharge of both C2H4/O2/Ar and C2H4/Ar mixtures were conducted at 60 torr and 296 K initial temperature as a function of plasma discharge frequency. Wavelength modulation has been employed to achieve high species sensitivity for absorbance less than 0.02. Path-averaged plasma temperature, with an uncertainty of ±12 K, is measured by using two ro-vibrational lines of H2O at 1338.5 cm-1 and 1339.15 cm-1. The results show that there is a decrease in C2H2 and CH4 concentrations in the presence of O2 as compared to the non-oxidative case as the plasma discharge frequency is increased beyond 1 kHz, indicating increased reactivity of the intermediate species possibly due to increased O atom concentration. Comparison with a plasma mechanism can help better understand the plasma combustion kinetics.