A novel differential concentration modulation spectroscopic technique was demonstrated with the spectrum of N+2 in the near infrared region. Its properties have been studied. Combining the concentration modulation spectroscopy with velocity modulation spectroscopy, the dependence of the signal to noise ratio on the modulation depth and various other factors are obtained. The high signal to noise ratio concentration modulation spectrum of Ar suggests that this spectroscopic technique will be a very sensitive and convenient method for detecting neutral free radicals and molecular ions.