We describe a simple gas expansion ion source based on static discharge voltages and a commercially available pulsed valve. The discharge is initiated by the gas pulse itself between two high voltage electrodes, without the need for fast voltage switches or complex timing schemes. The ion source very reliably produces intense bursts of molecular ions (with currents exceeding 100 μA during the pulse-on phase) with only minor pulse-to-pulse variations in intensity and pulse shape. We …