A single pumping system would require the main pump with the 5 times larger pumping speed if the flow rate of carrier gas is 5 times larger. But only adding a small main pump provides a suitable vacuum for GC/MS analysis with even a high flow rate if a differential pumping system is applied to the MS. In this system, a wall with a small hall for ion passage divides the vacuum chamber into the ion source cell and the QP rod-detector cell. Each cell is evacuated with the different pump. This hall limits the inflow of carrier gas so that the higher vacuum is achievable in the QP rod-detector cell.