Abstract: An electrostatic dual-mode lens assembly is provided for selectively transmitting or reflecting an ion beam in a mass spectrometer. The assembly comprises at least one electrode that provides a switchable electric field that, during a first mode of operation, directs an ion beam that enters the assembly along a first path so that the beam is transmitted through the assembly along the first path, and during a second mode of operation, directs an ion beam that enters the assembly along the first path so that the ion beam is reflected by the electric field and exits the assembly along a second path. Methods for operating a mass spectrometer using an electrostatic lens are also provided.