The present invention provides an ion selector gate having a first deflection zone 32 and a second deflection zone 34 spaced from the first deflection zone, wherein in use, a single low voltage trigger pulse generated by timing electronics with a width in time equal to or proportional to the width of the gate pulse Tgd and at a position in time that the ion gate is required to be open to allow through ions of the correct nominal mass. The trigger pulse may go through an inverter 38 and then to two high voltage pulsers 40 and 42. The high voltage pulsers each produce simultaneously a signal that switches to ground and back on again from the same amplitude high voltage but the opposite polarity. The outputs of the high voltage pulsers are connected to interleaved wires of both ion gates 32, 34. Thus, the high voltage pulsers are applied simultaneously to both ion gates.