Standards-Based Quantification in DTSA-II—Part I
Nicholas W.M. Ritchie National Institute of Standards and Technology, Gaithersburg, MD 20899-8371
nicholas.ritchie@nist.gov
Introduction Quantifying an X-ray spectrum is the process of converting
a measured spectrum into an estimate of the composition of the material from which the spectrum was collected. In a certain sense, interpreting X-ray spectra is very simple. A spectrum from even the most complex material can be thought of as a sum of spectra from the constituent elements (see Figure 1). To first approximation, if you know the spectrum for the constituent pure elements, you can estimate the spectrum for the complex material. Regardless of the soſtware vendor or the name of the algorithm, this is the basis for quantification of all X-ray spectra. Typically, the result from a quantification is a composition:
a measure of the relative amount of the elemental constituents. A common way to report the composition is in terms of mass fraction: the fraction by mass of each element in the material. Sometimes, compositions are reported in atomic fraction: the fraction by number of atoms per volume of each element. Tis article is part of a series describing DTSA-II, a new
soſtware application that simulates and quantifies X-ray spec- tra. Tere have been two previous articles in the series [1, 2].
Three Types of Spectra Spectra from materials of known composition are called
standard spectra. Te quantification scheme implemented by DTSA-II, standards-based quantification, compares the unknown spectrum to spectra measured from known standards materials. For accurate quantification, the standard spectra should be collected under very similar conditions to the unknown spectrum. In practical terms, this means the standard should be collected with the same beam energy (keV), at the same working distance, and with the same detector configured in exactly same way (distance, take-off angle, and
pulse process time). At a minimum, you must know the probe current, the number of electrons striking the sample per unit time. It is generally a good idea for the standard’s probe current to be similar to that of the unknown. Standard spectra serve to provide characteristic X-ray
intensity information. Characteristic X rays are the peaks in the spectrum and are associated with specific elements. On an element-by-element basis, the number of X-ray counts in the associated characteristic peak is compared to the number of X-ray counts in the equivalent peak in a standard spectrum. Tis ratio (called the k-ratio), when appropriately scaled through the ZAF correction factors [3] to account for effects from other elements, equals the mass fraction. You must know the following information about the
conditions under which the standard spectrum was collected: the beam energy, the probe current, the live time, and the composition of the standard material. Tis information is best recorded in the file representing the standard spectrum (see the first Helpful Suggestion at the end). Sometimes a second kind of spectrum, called a reference
spectrum, is also required. Standard spectra are oſten collected from materials with many elements. Te characteristic X-ray lines from one element may be similar in energy to characteristic lines from another element resulting in an overlap or interference. Interferences make it difficult or impossible to determine the peak intensities on an element-by- element basis. An interference can be resolved using spectra in which the element X-ray line shape is clearly resolved. Tese spectra in which the peak shapes are clearly resolved are called reference spectra. Typically reference spectra are collected from pure elements or carefully chosen simple compounds. Because reference spectra only provide shape information, it is not necessary to know the probe current when collecting
Figure 1: An example showing how the spectrum from a compound (K411 glass) can be expressed as the sum of scaled spectra from the constituent elements. The multipliers have historically been called k-ratios and represent the first approximation to the composition.
30 doi:10.1017/S155192951100085X
www.microscopy-today.com • 2011 September
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