Abstract: A tandem time-of-flight mass spectrometer includes an ion source, a velocity selector downstream of the ion source, a dissociation cell downstream of the velocity selector, and an ion accelerator downstream of the dissociation cell, the accelerator being capable of focusing ions at a space focal plane, and an ion-reflector (reflectron) downstream of the accelerator. The accelerator allows the ions to subsequently separate according to their m/z ratios and the reflectron to subsequently space-focus the ions over a broad mass range at a detector. A velocity selector includes a pair of ion defectors each having multiple electrically conductive strips, the strips including alternate positive voltage strips and alternate negative voltage strips.