A device for use in a mass spectrometer allows an ion-optical assembly to be removed, cleaned and reinserted with relatively high positioning accuracy. In particular, the device obviates the need for complex adjustments requiring special knowledge after the reinsertion. The objective is achieved by an arrangement comprising a receptacle and a mount for a removable ion-optical assembly in a mass spectrometer. Favorable implementations provide a mount and a receptacle with three pairs of complementary support elements, the three support elements on the receptacle form a support plane, and, when the mount is inserted into the receptacle, at least two pairs of support elements are engaged and the mount is aligned with respect to the support plane with the aid of the third pair of support elements.