A lens assembly for an outer source of a mass spectrometer, the assembly comprising: a housing; a plurality of lens elements, each lens element comprising a radially extending electrically conductive protrusion, the lens elements being linearly arranged in the housing such that the protrusions are aligned with one another, and an interface connector having a plurality of sockets to receive the protrusions therein and create an electrical connection between the protrusions and sockets. A housing for a resistance temperature detector is also disclosed.