A mass spectrometer is disclosed comprising a RF ion guide wherein in a mode of operation a continuous, quasi-continuous or pulsed beam of ions is orthogonally sampled from the ion guide and wherein the continuous, quasi-continuous or pulsed beam of ions is not axially trapped or otherwise axially confined within the RF ion guide. The ion guide is maintained, in use, at a pressure selected from the group consisting of: (i) 0.0001-0.001 mbar; (ii) 0.001-0.01 mbar; (iii) 0.01-0.1 mbar; (iv) 0.1-1 mbar; (v) 1-10 mbar; (vi) 10-100 mbar; and (vii) >100 mbar.