Ultratrace Solids Analyses Using Resonance Ionization Spectroscopy

Sputter initiated resonance ionization spectroscopy (SIRIS) is a new, ultrasensitive analytical technique for solids analysis, particularly in the fields of semiconductors, geophysics, biology, medicine, health physics, and basic science. SIRIS uses sputtering to atomize a solid sample and tunable dye lasers to selectively excite and ionize a selected element with the use of resonance ionization spectroscopy (RIS). The RIS process provides good sensitivity and allows interference-free measurements. The sensitivity is sufficiently high that analyses can be performed with a pulsed ion beam which does not sputter away more than an equivalent monolayer of the sample during an analysis. Thus, it is inherently a technique for surface analysis and for the analysis of very thin samples. For bulk analysis or for depth profiling, material can be removed by sputtering with a dc ion beam. The RIS technique is reviewed and the SIRIS technique is described along with the apparatus used at Atom Sciences. Applications for SIRIS are discussed and the results of measurements illustrating these applications are presented. For bulk analysis of gallium in silicon, a sensitivity of 2 ppb has been demonstrated. Depth profiles of silicon in gallium arsenide were made and recent results are presented. In a medical application, blood serum has been analyzed for trace amounts of copper and molybdenum. SIRIS allows the analysis of small, less than 0.1 ml, samples and is calibrated by the use of isotope dilution. Sensitivities of 20 pg are reported for molybdenum.