Fission fragments or other energetic plasma ions that strike a condensed-phase sample, desorbing and ionizing intact involatile molecules, constitute plasma desorption mass spectrometry (PDMS). In the classical 252Cf implementation, nuclear fission fragments pass through a thin foil bearing the sample; the resulting ion spike desorbs molecular ions that are mass-analyzed by time-of-flight, allowing molecular-weight determination of proteins and complex lipids without a thermal vaporization step ^[1]. The ionization event is therefore a plasma-induced desorption, distinct from a continuous inductively coupled plasma and from glow-discharge sputtering of elemental ions ^[2]. Low-temperature plasma desorption at atmospheric pressure is a later, related source that uses a helium or other plasma to desorb surface organics; it shares the plasma-desorption name but not the fission-fragment mechanism ^[3]. Conceptual boundaries exclude MALDI, electrospray, and chemical ionization. Essential features are an energetic plasma or fission-fragment flux, a condensed sample, and a mass analyzer that records the desorbed molecular ions .