Additive and Subtractive HODS

It may be of interest to determine whether or not there is a physical or chemical interaction between two substances. This question can be answered by additive HODS, i. e., by comparing the derivatives of a mixture with the sum of the derivatives of the pure substances (Fig. 4-36). (Sec. 2.6.3.1) If they are congruent, no interaction takes place, and vice versa. 

Subtractive HODS enables one to determine an unknown substance from the derivative of a mixture. For example, in a mixture of two components only one of the substances and its concentration is known. Its derivative must then be subtracted from the derivative of the mixture. The remaining derivative belongs to the unknown substance which has to be identified by comparing it with standard spectra in a catalog generated under the same conditions (Fig. 4-37). 

In Sec. 2.6.4.1 we were able to show that the logarithm of different concentrations of the same substance leads to congruent spectra which are only shifted along the j-axis. Their shape is undependent of the concentration [10, 29]. If they are differentiated, they are superposed upon each other and no differences should be observed (Fig. 4-38). This type of derivative is used especially for comparing and identifying substances, for standardization, and for creating spectra catalogs and data banks [33]. 

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