EELS detection limits revisited: Ruby — a case study

Theoretical and experimental EELS studies have been performed to optimize the chromium (Cr) quantification in natural ruby, featuring low concentrations of Cr in alumina (Al2O3).First, the noise performance of the detection system was determined (MTF, DQE), having been vastly underestimated in literature, as there was hardly any consideration of the dose dependence on the DQE. The superiority of the MLLS fit technique over conventional background subtraction for quantitating low Cr concentrations has been proven, and could be further supported by numerical signal-to-noise analysis of simulated noise free and Poisson-noisy EEL spectra. MLLS fitted ruby spectra by using appropriate standard specimens have also been investigated from an experimental point of view, by studying the influence of different instrumental configurations on the quantification results (monochromator/non-monochromator). The detection limits could be re-established within a novel mathematical expression based on MLLS fit parameters, avoiding the need of using factors that are hard to estimate otherwise. It is possible to detect atomic concentrations of Cr in Al2O3 down to 0.03% for a given optimal determined set of analysis parameters at a particular microscope setting.