Ions are created in the ion source. However, they may collide with many types of gas, such as carrier gas (He) and residual gas (air, water etc.), on the way to the detector to be unfortunately removed. This collision may happen in a short time with a low vacuum. We call the average distance of an ion's free flight without collision the "mean free path". If a high vacuum provides a long mean free path exceeding the dimension of the chamber, ions can easily reach the detector. QPMS is usually operated at around 10-3 to 10-4 Pa pressure, which corresponds to 5m to 50m of a mean free path.

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