A new gas-surface scattering model combining the Cercignani-Lampis-Lord (CLL) and Maxwell model has been developed to capture the translational energy transfer and angular distribution of N2 and Ar scattering from a highly oriented pyrolytic graphite (HOPG) surface for direct simulation Monte Carlo (DSMC) simulations. DSMC simulations were used to compute concentration ratios (pressure ratios) and concentration factors (flux ratios) of Ar and N2 for a gas concentrator that can enhance signals in mass spectrometry-based instruments for characterizing atmospheres and plumes of planetary bodies. The deviation between experiments and simulations for Ar was within 5% for all incident translational energies studied, while a purely Maxwellian scattering or purely specular reflection resulted in deviations of ~30%. The validated approach was used to compute the flux across the exit plane of the concentrator with and without the presence of the concentrator, which demonstrated that a prototype gas concentrator could achieve a concentration factor of ~20, which is close to the theoretical limit of 20.6. The deviation between the experiments and simulations for N2 at the lowest incident translational energy used was 1.2%. However, for higher energies, the deviation was about 9.5%. Although the higher deviation could indicate limitations of this approach, N2 can be internally excited at the higher energies, which was not modeled. Future efforts to model the internal excitation of molecules through gas-surface interactions could mitigate errors in the predictions at higher energies for molecular species.