This paper describes a detailed theoretical analysis of the ionization lineshape based on numerical integration of coupled differential equations of motion for an optical double-resonance (ODR) ionization scheme. The results of the optimization studies, to identify laser intensities of both the excitation steps for efficient and selective ionization of 168Yb isotope are presented. The evaluated saturation intensities are found to agree well with earlier reported experimental results. Quantitative estimation of interference of the neighboring isotopes on the selective ionization is carried out by taking into account the finite laser bandwidths. The dependence of Autler-Townes doublet peaks of neighbouring isotopes on the spectral bandwidth of the excitation lasers and their consequent influence on the ionization lineshape is discussed elaborately. The studies indicate that accurate and quantitative prediction of the selectivity and ionization efficiency is feasible by inclusion of realistic experimental conditions. The theoretical results indicate that the degree of enrichment of 91 is feasible for the low abundant 168Yb isotope with an ionization efficiency of ≤ 32%.