Biological microanalysis by secondary ion mass spectrometry: status and prospects

Secondary ion mass spectrometric (SIMS) analysis of biological problems is an evolving technique. Lateral resolution of currently available commercial instrumentation estimated from actual samples is 0.5 micron, and subcellular organelles can be distinguished. The interrelationship of lateral resolution, elemental concentration and ionizability are, however, important in controlling the actual lateral resolution achievable. Although depth resolutions of 5 nm have been measured in other systems, no test of depth resolution in biological systems has been done, and this parameter is also concentration and ionization dependent. The development of liquid metal ion sources in combination with scanning ion microprobes has a potential lateral resolution of as little as 20 nm, but initial studies with this instrumentation show that tissue preservation at the submicron level becomes an important issue. The current development of a cold-transfer stage for SIMS instruments may obviate the problem of submicron localization of diffusible elements, and initial studies indicate that much more needs to be understood about the ionization process in hydrated samples. Quantitation of diffusible elements using external standards has been achieved over a 30 micron diameter analyzed area. Strategies for analysis of areas limited to 1 micron or less has been suggested using image processing techniques, which take advantage of the lateral resolution inherent in the ion optical system. Matrix effects in biological tissues have been reported and constitute a serious problem for analysis of biologicals which must be addressed for each question. However, development of laser ionization of sputtered particles may both increase the sensitivity of analysis and decrease the importance of ionizability of elements. Chemical analysis of organic molecules is another use of SIMS, but, at present, at the cost of losing localized information. SIMS analysis of biological samples is being systematically evaluated and requires increased accessibility of this instrumentation to the end-user for full development of its role in physiological problems.