Application of Parallel-Detection Electron Energy Loss Spectroscopy in Biology

Abstract

For many years it has been appreciated that electron energy loss spectroscopy (EELS) is the most sensitive microanalytical technique for detecting the biologically ubiquitous light elements, carbon, nitrogen and oxygen (Isaacson and Johnson, 1975). In particular, elemental ratios such as nitrogen-to-sulfur or nitrogen-to-phosphorus can provide information about relative concentrations of proteins (containing sulfur-rich amino-acids) and nucleotides (containing phosphate groups) in subcellular organelles (Leapman and Ornberg, 1988). With conventional EELS instrumentation it has been relatively easy for example to measure nitrogen present typically at levels of 5 to 10 atomic percent but phosphorus and sulfur occurring at levels below about 1 to 3 atomic percent are often only weakly visible in the core loss spectrum. Other interesting elements like calcium and iron have not generally been detectable at physiological concentrations using EELS until the recent advent of highly efficient parallel detection systems. Now it is becoming apparent that EELS may in fact compete with and even surpass the performance of energy-dispersive x-ray spectroscopy (EDXS) for such elements (Shuman and Somlyo, 1987). The aim of this paper is to compare directly the sensitivity of EELS and EDXS, to illustrate new methods of spectral processing, and to consider the state-of-the-art and future prospects for biological applications.