Disclosed are systems and methods of using integrated computational elements to determine unknown interferents in a fluid being monitored. One method includes monitoring a fluid with an optical computing device comprising at least two integrated computational element (ICE) cores configured to optically interact with a fluid and detect a corresponding at least two characteristics of the fluid, each ICE core being designed and manufactured with reference to known spectra related to the at least two characteristics of the fluid, generating output signals corresponding to the at least two characteristics of the fluid with the optical computing device, wherein an intensity of each output signal corresponds to a concentration of the at least two characteristics of the fluid, and calculating a representative spectrum of the fluid with a signal processor based on the known spectra of the at least two characteristics and the intensity of each output signal.