Quantitative analysis of very light elements by electron microanalysis. Application to carbon

Quantitative electron probe microanalysis of ultra-light elements (i.e. with atomic numbers below 9) raises a number of practical and theoretical problems. This paper examines the main experimental difficulties associated with X-ray emission, detection and specimen preparation and proposes possible improvements whenever possible. In particular, it also presents the performance characteristics and limitations of multi layer analyzers as well as what should be modified in the measurement channels of the Microbeam-type Cameca microanalyzers. As regards the quantitative analysis, the inadequacies of the conventional ZAF matrix correction program are examined and an improved version of this model designed for the analysis of ultra-light elements is described. This quantitative analysis method is used to analyze low carbon contents in steels. This type of analysis poses several specific problems among which one is linked to the carbon being present in the steel both in an homogeneous form (solid solution) and in a heterogeneous form (carbide). Moreover, due to the specimen contamination by a superficial carbon deposit during the analysis, the background X-ray emission cannot be measured in a straightforward manner. Solutions to these different problems are suggested. The different difficulties are illustrated with some examples of analysis performed on carbon in several matrices (steel, segregated veins, welded joint)