Patent ID: 7095015

Claim:
A mass spectrometer comprising: an ion source; an orthogonal acceleration Time of Flight mass analyser comprising an electrode for orthogonally accelerating ions, an ion detector and a drift region therebetween; an ion gate upstream of said electrode; and control means for switching said ion gate between a first mode and a second mode, said second mode having a lower ion transmission efficiency than said first mode, wherein in a mode of operation said control means: (i) switches said ion gate from said first mode to said second mode at a time T 1 ; and (ii) causes said electrode to inject or orthogonally accelerate ions into said drift region at a later time T 1 +ΔT 1 ; wherein ΔT 1 is set such that ions having a mass to charge ratio ≦ a value M1 are not substantially injected or orthogonally accelerated into said drift region by said electrode.