AES Theory: Elemental Quantitation

Auger electron peaks are proportional to elemental concentrations. However, it is seldom possible to measure concentrations from first principles. Several instrumental factors influence Auger peak heights. These include primary beam energy, sample orientation, and the energy resolution and acceptance angle of the analyzer.

The chemical states of elements in the sample also influence the process of elemental analysis by AES. Both peak intensity and peak shape vary, especially as a function of oxidation state. Changes in peak shape are important when the quantitation proceeds from a differential data display.

Sample heterogeneity must be considered for quantitative analysis. The sample should be homogeneous in the lateral directions relative to the primary beam diameter for measurements to be accurate. The Auger signals arise from an analytical volume that depends mainly on the diameter of the primary beam. If the beam is narrower than the scale of heterogeneity, then meaningful analyses can be made on islands within a sample. The thickness of the analytical volume is small because AES is highly surface sensitive. Therefore, the analyzed surface may not be representative of the bulk material. For example, many metal samples acquire thin oxide coatings when exposed to air. In spite of the above considerations, quantification of elemental concentrations is possible in cases where relative sensitivity factors have been measured in the same sample matrix.

A typical AES analysis requires quantification of major and minor elements. This concentration range is consistent with AES analytical detection limits (1 to 0.01 %). (In contrast, SIMS usually provides trace element quantification while the major elements remain essentially constant.) Since concentrations of all elements, (including the matrix) can vary in an AES measurement, it is necessary to express concentrations as percents (CE%) normalized relative to the sum of all others.

The next part of the procedure uses the same logic as SIMS RSFís.

Substituting the right side of the RSF equation for concentrations (CE and CX) in the top equation (and eliminating the matrix current terms from the numerator and denominator) give the following equation. This format is the most common method for elemental quantification by AES. However, it should be noted that Auger RSFís are often reciprocals of those defined here. (They must be incorporated by division into the Auger signal rather than multiplication.)

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