Qualitative X-Ray Analysis

Abstract

The first stage in the analysis of an unknown is the identification of the elements present, a process referred to as qualitative analysis. Qualitative analysis is itself a powerful tool in microanalysis. When a simple understanding of the nature of a chemical microstructure is sought, often the elemental identification accompanied by a broad classification of concentration into categories (such as “major,” “minor,” or “trace”) is sufficient to solve many problems. Note that the terms “major,” “minor,” and “trace” as applied to the concentration of constituents of a sample are not strictly defined and are therefore somewhat subjective. Each analytical method tends to define “trace” differently based upon its degree of fractional sensitivity. Because the energy dispersive x-ray spectrometer (EDS) limit of detection in bulk materials is about 0.1 wt%, the following arbitrary working definitions will be used in this text:

• • Major: more than 10 wt% (C > 0.1 mass fraction)

• • Minor: 1–10 wt% (0.01 ≤ C ≤ 0.1)

• • Trace: less than 1 wt% (C < 0.01)

Note that the definition of trace has no bottom limit. Generally, wavelength-dispersive x-ray spectrometry (WDS) can achieve limits of detection of 100 ppm in favorable cases, with 10 ppm in ideal situations where there are no peak interferences and negligible matrix absorption (see Chapter 9, Section 9.8.4).