Bound OH- is one major unwanted absorbing impurity causing significant attenuation for certain wavelengths of laser radiation when propagating through fused silica. A wavelength tunable laser calorimeter (LCA) is used to detect dopants and other impurities in optical material. Also, a technique for photothermal absorption measurements is combined with the LCA set-up allowing for a lateral scan of the sample retaining an absolute calibration. Regarding inhomogeneities within the distribution of impurities the combination of both techniques provides a highly sensitive method to investigate in the lateral distribution of OH- content in fused silica.