[¦rez·ən·ənt ‚ī·ə·nə‚zā·shən ¦mas spek′träm·ə·trē] (spectroscopy) An instrumental technique for quantitative identification of trace impurities (at or below the part-per-billion level), it begins with laser-induced or ion-induced desorption, followed by resonant laser ionization (usually from two or three lasers), and then analysis by time-of-flight mass spectrometry. Abbreviated RIMS.