The homochirality of biomolecules is a signature of life on Earth and has significant implications in, for example, the production of pharmaceutical compounds. It has been suggested that biomolecular homochirality may have arisen from the amplification of a spontaneously formed small enantiomeric excess (e.e.). Many minerals exhibit naturally chiral surfaces and so adsorption has been proposed as one possible mechanism for such an amplification of e.e. Here we show that when gas-phase mixtures of D- and L-aspartic acid are exposed to an achiral Cu(111) surface, a small e.e. in the gas phase, e.e._g, leads to an amplification of the e.e. on the surface, e.e._s, under equilibrium conditions. Adsorption-induced amplification of e.e. does not require a chiral surface. The dependence of e.e._s on e.e._g has been modelled successfully using a Langmuir-like adsorption isotherm that incorporates the formation of homochiral adsorbate clusters on the surface.