Quantitative Procedures in X‐Ray Fluorescence Analysis

Abstract

The intensity of a characteristic line from an element in a prepared specimen is related to concentration of the element [1,2,3]. Unfortunately, this relationship is generally not linear. There are two main factors which account for the nonlinearity, matrix effects and specimen heterogeneity effects. An expression can thus be written to equate the concentration C of a given element with these two factorswhere AT is a function of a number of instrumental factors and is generally referred to as the calibration constant. R is the measured counting rate from the element in question, M a term describing the matrix effect, and 5 a term representing specimen effects. While Equation 12.1 is not directly soluble, it does at least serve as a starting point in the development of more practical mathematical relationships.In an X-ray analytical laboratory, the quantitative method of analysis employed will be typically predicated by a number of circumstances of which probably the four most common are: 1) the complexity of the analytical problem; 2) the time allowable; 3) the computational facilities at the disposal of the analyst; and 4) the number of standards available. It is convenient to break quantitative analytical methods down into two major categories, single element methods and multiple element methods, as shown in Table 12.1. The simplest quantitative analysis situation to handle is the determination of a single element in a known matrix. A slightly more difficult case might be the determination of a single element where the matrix is unknown. As shown in the table, three basic methods are commonly employed in this situation: use of internal standards, use of standard addition, or use of a scattered line from the X-ray source. The most complex case is the analysis of all, or most, of the elements in a sample, about which little or nothing is known. In this case, a full qualitative analysis would be required before any attempt is made to quantitate the matrix elements. Once the 175 X-Ray Fluorescence Spectrometry, Second Edition