30www.microscopy-today.com • 2011 Septemberdoi:10.1017/S155192951100085XStandards-Based Quantification in DTSA-II—Part INicholas W.M. RitchieNational Institute of Standards and Technology, Gaithersburg, MD [email protected] 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 software 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.This article is part of a series describing DTSA-II, a new software application that simulates and quantifies X-ray spec- tra. There have been two previous articles in the series [1, 2].Three Types of SpectraSpectra from materials of known composition are called standard spectra. The 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. This 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. This 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 often collected from materials with many elements. The 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. These 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.32www.microscopy-today.com • 2011 Septemberor on your own computer using DTSA-II, which you can download from http://www.cstl.nist.gov/div837/837.02/epq/ dtsa2/index.html. Example spectra are available on the DTSA-II “Documentation” page.1. Load the spectrum you wish to quantify into DTSA-II and select that spectrum in the spectrum list. It will then be displayed in the spectrum display.2. Identify all the elements present in the spectrum. It is helpful to use the KLM line selector to label all the characteristic peaks in the spectrum. You can use the “copy → marked elements” action on the default spectrum display menu to create a list of elements.3. Select the “Quantification alien” item from the “Tools” menu. The Quantification alien will lead you step-by-step through the quantification process.4. From page A, select the radio button labeled “Deter- mine the composition of an ‘unknown’ spectrum by MLLSQ fitting to standards” and press “Next” to proceed to the next step.5. On page B, verify that the instrument, detector, calibra-tion, and beam energy are correct. This information is read from the spectrum selected in Step (1) and should be correct. Press “Next” to proceed to the next step.6. Use the “File” button on page C to select a standard spectrum for each element identified in step (2). (You may decide to omit oxygen, which can be calculated from assumed stoichiometry.) The “File” button Standards-Based Quantification in DTSA-IIthem. Thus, standard spectra provide characteristic X-ray line intensity information, and reference spectra provide characteristic X-ray line shape information. A more thorough comparison of the differences between standards and references is provided in Table 1.The final type of spectrum is the unknown specrum: the spectrum you wish to interpret. You must know the beam energy, the probe current, and the live time for the unknown. It is easiest if this information is recorded in the file representing the unknown.Collecting the SpectraBefore starting the measurement process, you will need:• Yourunknownmaterial • Mounted with a flat, polished surface normal to the electron beam • Coated with a conductive layer if the sample is an