Abstract: Apparatus and method are described for the quantitative analysis of a specific specie within a sample. The analysis has sufficient sensitivity for the detection of as little as one atom of the desired species. The method is accomplished by bombarding a sample with a highly focused charged particle beam; for example, a beam of positive argon ions having an energy from five to thirty kilovolts and a current of one milliampere or greater. This beam impinging upon the sample creates a cloud including secondary ions and neutral particles of the constituents of the sample. The cloud is irradiated with a laser beam having selected wavelengths therein for the unique ionization of the desired specie by means of resonance ionization spectroscopy (RIS). In most applications some energy and/or mass discrimination is required. The energy discrimination can be accomplished by passing the RIS ions through an energy filter, with the ions emanating therefrom having a narrow range of energy.