A charged particle beam instrument (10) is provided, the instrument comprising a charged particle optical column (12), a voltage source, a detector (14) and a sample holder (18), the column (12) being operable to direct a beam of primary charged particles at a sample (20) on the sample holder (18) to cause secondary charged particles to be released from the sample, the voltage source being operable to establish in the vicinity of the sample an electric field that has a component that draws the secondary charged particles towards the detector (14), and the detector being operable to detect secondary charged particles, wherein the instrument further comprises a further voltage source (16) variable between a first voltage that establishes a component of the electric field that draws the secondary charged particles away from the sample, so as to prevent at least some of them from colliding with the sample (20) or sample holder (18), and a second voltage that establishes a component of the electric field that draws the secondary charged particles towards the sample, so as to prevent at least some of them from colliding with the column (12), thereby increasing the number of secondary charged particles detected by the detector (14).