| CPC H01J 49/068 (2013.01) [H01J 49/067 (2013.01)] | 13 Claims |

|
1. A lens assembly for an outer source of a mass spectrometer, the assembly comprising:
a plurality of lens elements; and
a plurality of spacers, interspersed between the lens elements to form a lens stack with a spacer between each neighbouring pair of lens elements,
wherein each of the lens elements and spacers comprise opposing first and second axial faces and,
wherein at least one axial face of a lens element or spacer comprises one of a pair of complementary keying features and the opposing axial face of a neighbouring spacer or lens element comprises the other of the pair of complementary keying features, wherein the pair of complementary keying features are uniquely positioned in each lens element and/or spacer.
|