An electrode assembly is provided in a high sub-atmospheric pressure region of an ion source, between an ionization chamber and a vacuum region of a spectrometer, such as a mass spectrometer, an ion mobility spectrometer, or an ion mobility-mass spectrometer. The electrode assembly is spaced at a distance from an outlet of an ion transfer device. A voltage source imparts a potential difference between the ion transfer device and the electrode assembly to accelerate ions emitted from the outlet to a collision energy. The collision energy is effective to cause collisional heating of ions in the high sub-atmospheric pressure region without voltage breakdown. The collision energy may be set to cause unfolding of folded biomolecular ions and/or dissociation of ions.
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