For the measurements of the fraction of seed metal (potassium or cesium) in a closed-cycle MHD (CCMHD) power generation system, the reduced absorption coefficients of the 4p-4s line of potassium (766.5 nm) and the 6p-6s line of cesium (852.1 nm) have been determined experimentally for K-Ar, K-He, Cs-Ar and Cs-He mixtures under the condition of the CCMHD working gas. The column density of the alkali metal was measured using the modified hook method which is not affected by the profile and width of the absorption coefficient. The emissivity of the blue wing of the line was obtained by the emission spectrum. From the reduced absorption coefficient, the number density of the seed metal can be measured by monitoring the change of absorption at a fixed wavelength in the wing of the line.
