Systems and methods for implicit chemical resolution of vacuum gas oils and fit quality determination are disclosed. The systems and methods include utilizing an FT-IR spectrum of an unknown VGO composition, and a database of FT-IR spectra of known VGO compositions, to determine a model of composition for the unknown VGO composition. Additionally, the fit quality for the model of composition is determined by performing a partial least squares analysis on specific spectral regions of interest in the FT-IR spectrum of the unknown VGO composition.