A mass spectrometer comprising: an RF ion guide (6a;6b;7) having an axial direction (z) comprising a plurality of electrodes, said RF ion guide comprising a first ion guide section, a third ion guide section, an intermediate second ion guide section provided between the first and third ion guide sections (6a, 6b), and a fourth ion guide section arranged adjacent the intermediate second ion guide section; a device arranged and adapted to apply an AC or RF voltage to said electrodes in order to confine ions within said first, second and third ion guide sections (6a, 7, 6b) in a first direction (y) orthogonal to said axial direction (z); and a control system, wherein said control system is arranged and adapted in a first mode of operation to apply a DC voltage additionally to said electrodes forming the first, second and third ion guide sections (6a, 7, 6b) in order to confine ions within said first, second and third ion guide sections (6a, 7, 6b) in a second, different direction (x) orthogonal to said axial direction (z); wherein said ions are not confined axially within said first, second and third ion guide sections (6a, 7, 6b) during said first mode of operation; and said mass spectrometer is characterised in that: said control system is arranged and adapted to switch said second ion guide section to operate in a second mode of operation wherein said DC voltage applied to said electrodes of the second ion guide section is varied so that a portion of said ions that were confined in the second direction (x) but not the axial direction during the first mode of operation experiences an electric field in a direction which is substantially orthogonal to the initial axial direction (z) of said ions so that the portion of ions is orthogonally sampled from said second ion guide section into said fourth ion guide section, wherein said ions are pre-separated according to a physico-chemical property prior to arriving at said first ion guide section and/or whilst being transmitted through said first ion guide section, and orthogonal sampling of said ions is synchronised to the introduction of said ions into said first ion guide section, such that orthogonally sampling the portion of said ions has the effect of selecting ions having physico-chemical property within a particular range. an RF ion guide (6a;6b;7) having an axial direction (z) comprising a plurality of electrodes, said RF ion guide comprising a first ion guide section, a third ion guide section, an intermediate second ion guide section provided between the first and third ion guide sections (6a, 6b), and a fourth ion guide section arranged adjacent the intermediate second ion guide section; a device arranged and adapted to apply an AC or RF voltage to said electrodes in order to confine ions within said first, second and third ion guide sections (6a, 7, 6b) in a first direction (y) orthogonal to said axial direction (z); and a control system, wherein said control system is arranged and adapted in a first mode of operation to apply a DC voltage additionally to said electrodes forming the first, second and third ion guide sections (6a, 7, 6b) in order to confine ions within said first, second and third ion guide sections (6a, 7, 6b) in a second, different direction (x) orthogonal to said axial direction (z); wherein said ions are not confined axially within said first, second and third ion guide sections (6a, 7, 6b) during said first mode of operation; and said mass spectrometer is characterised in that: said control system is arranged and adapted to switch said second ion guide section to operate in a second mode of operation wherein said DC voltage applied to said electrodes of the second ion guide section is varied so that a portion of said ions that were confined in the second direction (x) but not the axial direction during the first mode of operation experiences an electric field in a direction which is substantially orthogonal to the initial axial direction (z) of said ions so that the portion of ions is orthogonally sampled from said second ion guide section into said fourth ion guide section, wherein said ions are pre-separated according to a physico-chemical property prior to arriving at said first ion guide section and/or whilst being transmitted through said first ion guide section, and orthogonal sampling of said ions is synchronised to the introduction of said ions into said first ion guide section, such that orthogonally sampling the portion of said ions has the effect of selecting ions having physico-chemical property within a particular range. said control system is arranged and adapted to switch said second ion guide section to operate in a second mode of operation wherein said DC voltage applied to said electrodes of the second ion guide section is varied so that a portion of said ions that were confined in the second direction (x) but not the axial direction during the first mode of operation experiences an electric field in a direction which is substantially orthogonal to the initial axial direction (z) of said ions so that the portion of ions is orthogonally sampled from said second ion guide section into said fourth ion guide section, wherein said ions are pre-separated according to a physico-chemical property prior to arriving at said first ion guide section and/or whilst being transmitted through said first ion guide section, and orthogonal sampling of said ions is synchronised to the introduction of said ions into said first ion guide section, such that orthogonally sampling the portion of said ions has the effect of selecting ions having physico-chemical property within a particular range.