A vacuum of a MS is mostly determined by the flow rate of the carrier gas that is flowing into the MS from the GC. Other sources of gas are from desorption of adsorbed gas (water, air) and contamination. The simplest evacuating system is a single pumping system that evacuates the chamber with the one main pump. The vacuum pressure is roughly proportional to the carrier gas flow rate and inverse proportional to the pumping speed of the main pump.