24www.microscopy-today.com • 2012 Januarydoi:10.1017/S1551929511001398Standards-Based Quantification in DTSA-II— Part IINicholas W.M. Ritchie National Institute of Standards and Technology, Gaithersburg, MD [email protected] Introduction This article is the fourth in a series describing the setup and use of NIST DTSA-II. The previous articles [1–3] may be viewed in the Microscopy Today archives at www.microscopy-today.com. Part I of this article [3] described how to use DTSA-II to quantify an energy dispersive X-ray spectrum from a material of unknown composition. That presentation covered each step and provided background material but lacked a concrete example. This follow-up article provides two such examples. Setup and PreliminariesFollow along on your computer. You are encouraged to follow along using DTSA-II on Windows, OS X, or Linux [4]. I have made the necessary files available on the NIST web site [5]. The files are compressed into a single ZIP archive file. Expand the archive file into a convenient location using one of the many commercial or free ZIP file tools (WinZip, 7-Zip, PKZIP, or others). The archive contains two examples and a pair of script files. The script, config.py, is provided as a convenient mechanism to define a detector like the one on which the spectra were collected. You only need to run this script once. Scripts are executed from the DTSA-II “Command” tab (right-most tab in the middle of the main window). Select the “Open” button on the “Command” tab to locate and open the file config.py. The script will take a fraction of a second to execute and will report success. Restart DTSA-II after executing the script. The script creates a new detector definition. Make this definition the default by selecting it from the “Default Detector” selection panel on the “Spectrum” tab. Select the instrument “FPS” and the detector “Detector 0 – Medium.” This step ensures that the calibration and efficiency of the detector are appropriate for the example spectra. You may now use the “File → Open” menu item to load the tutorial spectra into memory. Preparing spectra. The archive contains two examples—both taken from NIST’s extensive collection of engineered glasses. Example 1 is more straightforward because it uses pure element and simple compound standards to quantify a Si, Al, Mg, Na, Fe, and O bearing glass. Example 2 is more sophisticated. This example uses standards similar to the unknown to improve accuracy but requires additional reference spectra. The example spectra are divided into three types: unknowns, standards, and references. The labels “unk,” “std,” and “ref” are appended to the file name for each type respectively. The labels are a useful shortcut to identify the type of information that is recorded in each spectrum file. The least information is available for “unk” (unknown) spectra. “Unk” files must contain the beam energy, the probe current, the live time, and basic energy calibration data. “Std” (standard) spectra contain this information plus a record of the composition of the material from which the spectrum was collected. “Ref” spectra fall between “std” and “unk.” Strictly speaking, you don’t need to know the probe current or composition for “ref” spectra, however it is important that the spectrum contained with “ref” represent a clean example of one or more line families of the element(s) for which this spectrum is to act as a reference. By clean, we mean that the shape of the relevant characteristic lines are not modified by the presence of another element’s characteristic lines. Adding information. As collected, spectra often do not contain the requisite information. DTSA-II provides tools to add information and save the resulting data to a new file. DTSA-II does not overwrite existing spectra because it respects the sanctity of the raw data. The “Tools → Assign material” menu item allows you to associate a composition (the “Standard Composition” property) with one or more selected spectra. The “Tools → Edit spectrum properties” menu item allows you to modify the beam energy, probe current, live time, and working distance associated with one or more selected spectra. If you are following along with the example data set, open one or more of the “std” spectra and review the spectrum properties and composition in the tables and via the menu items. Collecting standard spectra. A recent addition to DTSA-II is the “create standard” menu item, which is available from the spectrum list’s context menu. This command verifies that all information necessary for a standard is present in the spectrum data and allows you to update this information. It will also sum together multiple spectra into a single spectrum. To ensure robust, reliable standards, it is best to collect 3 or more spectra from different points on the standard material and then check the shape and intensity of the spectra against each other for consistency. Inconsistent spectra can be discarded and the remaining spectra summed into a single high-quality standard spectrum. Highlighting the spectra, one-by-one, in the spectrum list allows you to review the spectrum properties, including a standard’s composition. The properties are displayed in the table to the right of the spectrum list and the composition in the table in the lower-right corner.When you are done, clear the “std” and “ref” spectra from memory by selecting these spectra in the spectrum list (use the “Ctrl” key to select multiple files). Click the right mouse button over the spectrum list to bring up a menu and select the “Clear selected” menu item.Example 1: Quantifying an Unknown Glass Use “File → Open” to load the three spectra labeled “Glass[?] unk.msa” from the “Example 1” directory. To start the quantification process, select the unknown spectra in the spectrum list—they will display in the spectrum plot window—and then invoke the “Tools → Quantification alien.” A dialog will present itself. The dialog consists of a sequence of panels through which you progress by providing the requested information and selecting the “Next” button. If26www.microscopy-today.com • 2012 JanuaryFourth panel. The fourth panel asks if there are any additional elements that should be considered. These are elements that are not measured directly but are