A Gas Injection System (GIS) applies at least two fluids in the vacuum chamber of a particle-optical apparatus, the gas injection system having two or more channels. Each channel is connected to an associated reservoir holding a fluid at a first side and having an associated exit opening at the other side, the exit sides individually exiting to the outside of the GIS via a nozzle with a nozzle opening. At least two exit openings separated by less than the diameter of the channels near the exit openings, preferably concentric to each other.