A new technique to study the entrainment of air into the inductively coupled argon plasma is presented. The combination of vibrational Raman scattering and Rayleigh scattering enables measuring absolute particle densities of air and argon. The measurements show the entrainment of air into the plasma. At an axial position of 2 mm above the end of the quartz torch it is found that at 90% of the plasma radius 55% of the particles originate from air and by exponential extrapolation towards 70% of the radius about 1% entrainment of air is predicted to be present. Furthermore, a comparison with a cold argon flow shows that due to the higher viscosity the entrainment in the plasma is lower than in the cold argon flow.