An approximation suitable for studying differential electron loss collisions with 'heavy' targets (e.g. Ar) is proposed. It consists of an impulse approximation to singly inelastic (SI) collisions, i.e. those in which the target is not excited, together with a first Born treatment of doubly inelastic (DI) processes, where the target is excited or ionised. Collisions of H(2s), H(1s), He+(1s) and O4+ with a ground-state Ar target are treated. It is assumed that O4+ behaves as if it contained only two 2s electrons, each of which is independently ejected. Comparison is also made between electron loss in light and heavy targets, ground-state He and Ar being taken as examples. In comparison with experiment it is concluded that the approximation works very well for small angles of ejection (<or approximately=15' in the cases studied) but shows some systematic discrepancies at larger angles, particularly towards 180'. It is suggested that these discrepancies may be mainly a result of the failure of the first Born approximation for DI collisions at large angles, a higher-order approximation being needed here.