Apparatus and methods are disclosed for an ion source that comprises a flow-through ultraviolet lamp for photoionization. One embodiment of the apparatus comprises a tubular outer element and a tubular inner element. The inner element is disposed within the outer element to provide a space between the inner and outer elements. The inner element is open at its ends to provide a pathway through that element. A gas discharge between the elements produces ultraviolet radiation, which transmits through the wall of the inner element into the region within the inner element and ionizes molecules that flow through the region. The ion source can be employed in conjunction with a mass analyzer for mass spectrometry. In the method for ionizing molecules in an ion source, vaporized molecules are flowed through the region within the inner element, the region being surrounded by UV radiation. The radiation ionizes vaporized molecules in the surrounded region.