We disclose a gas injection system having at least one crucible, each crucible holding at least one deposition constituent; at least one transfer tube, the number of transfer tubes corresponding to the number of crucibles, each transfer tube being connected to a corresponding crucible. There is at least one metering valve, the number of metering valves corresponding to the number of transfer tubes, each metering valve being connected to a corresponding transfer tube so that the metering valve can measure and adjust vapor flow in the corresponding transfer tube. A sensor is provided capable of sensing reactions between deposition constituents and a focused ion beam A computer is connected to receive the output of the sensor; the computer is also connected to each metering valve to control the operation of the valve, and the computer is programmed to send control signals to each metering valve to control the operation of the valve; the control signals being computed responsive to feedback from the output of the sensor.