A reflectron ( 109 ) is provided with at least two sets of field resistances ( 131 a - 131 n, 137 a - 137 n ) which may be connected together or separately to the reflectron electrodes ( 123 a - 123 n ) in order to be able to provide a plurality of different shaped electrical fields inside the reflectron ( 109 ).
1. A reflectron ( 109 ) for use in a mass spectrometer comprising:
a plurality of reflectron electrodes ( 123 a - 123 n ) connectable to one or more high voltage power supplies ( 127 ) wherein means for changing the electrical potentials of at least some of said reflectron electrodes ( 123 a - 123 n ) is provided in order to change the shape of the electrical field inside said reflectron ( 109 ); and
at least two sets of field resistances ( 131 a - 131 n , 137 a - 137 n ) which can be connected one set at a time, or in parallel, or in series, between the reflectron electrodes ( 123 a - 123 n ).
2. The reflectron of claim 1 , wherein one of said sets of field resistances includes field resistances ( 131 a - 131 n ) arranged to produce a linear electrical field inside said reflectron ( 109 ).
3. The reflectron of claim 1 , wherein one of said sets of field resistances includes field resistances ( 137 a - 137 n ) arranged to produce an essentially quadratic electrical field inside said reflectron ( 109 ).
4. The reflectron of claim 1 , wherein at least one of said sets of different resistances includes fewer resistances than there are reflectron electrodes.
5. The reflectron of claim 1 , wherein the at least two sets of field resistances ( 131 a - 131 n ; 137 a - 137 n ) are mounted on movable rods ( 133 ; 139 ), wherein each rod ( 133 ; 139 ) is movable between a first position where said set of field resistances ( 131 a - 131 n ; 137 a - 137 n ) are in electrical contact with said reflectron electrodes ( 123 a - 123 n ) and a second position where said set of field resistances ( 131 a - 131 n ; 137 a - 137 n ) is not in contact with said reflectron electrodes ( 123 a - 123 n ).