In this work, the ionizations involving the charge transfer and single capture processes in low charge state ion-atom collisions in low-to-medium energy region, are investigated by the position sensitive and time-of-flight techniques. For a given charge state of the incoming projectiles, the dependence of the ratio RTS of the transfer ionization to the single capture changing with the projectile velocity is obtained, and the conclusion that the ionized electron in the transfer ionization process comes from the outermost sub-shell of the target atom is confirmed by comparing our theoretical results with the experimental data.