For twenty years Optical Coherence Tomography (OCT) has been interested in imaging in turbid media. Recently, conventional OCT has been extended to spectroscopic investigation (SOCT). For a full-field OCT configuration in the visible range, we show that the interference conditions are not equal in the whole field of view: the effective numerical aperture depends on the observation point. This results in a spectral shift towards higher wavelengths of the OCT spectra, leading to errors in spectroscopic analysis. We propose a general calibration method for SOCT measurements which has been tested within a protocol, to perform spatially-resolved spectroscopic identification of material by OCT. Firstly we measure the reflectivity of a plane gold sample, corresponding closely to the measurement made with a spectrometer. Then we successfully identify the reflectivity of gold in a mixed sample (silicon and gold).