A numerical simulation of the separation of gaseous species through a microfluidic gas preconcentrator has been performed. The gas preconcentrator is a curved converging/diverging nozzle that operates like a gas centrifuge. The preconcentrator is being developed to enhance the sensitivity of microscale gas sampling devices for fast response, on-site environmental monitoring. The numerical model solves the Euler equations and the Maxwell-Stefan diffusion equation to provide an exit mass flux profile for a particular set of inlet conditions. From this profile, the efficiency of the preconcentrator is determined by computing the separation factor, which measures the ability of the device to separate the gas flow into two streams that are depleted and enriched in the heavy gas molecule. The results of the simulations are compared to experimental measurements of the separator's performance and provide insight into the underlying physics of the operation of the preconcentrator.