The analysis of a plasma impurity composition in many plasma systems (such as coaxial plasma accelerators, magnetic-plasma compressors, plasma-focus devices, and others) is of great importance for plasma physics. In this work we have proposed a new spectral method for the determination of impurity concentrations in relation to the concentration of the main plasma (made of hydrogen in our case). This technique can be used e.g. for diagnostics of a peripheral zone in Plasma-Focus (PF) discharges, where LTE conditions must be fulfilled. Advantages of this method in comparison with other methods, as well as possible errors, are considered and discussed.