A numerical technique called as "adjoint spectrum" has been proposed by the authors in a previous work. Two applications of it are presented here. It is demonstrated that by generalizing the aspect of adjoint spectrum it becomes possible to measure densities of mixed gases simultaneously. An experimental proof of this has been carried on and simultaneous measurement of densities of both methane and dinitrogen oxide was realized. On the other hand, it is also reported that frequency shifting of spectra, which is a cause of error in spectrometric measurements, can be corrected by exploiting the adjoint spectrum technique in connection with the Taylor expansion. Numerical examples showing the success of this method are reported as well.