Standards-Based Quantification in DTSA-II
them. Tus, 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. Te 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 Spectra Before 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 insulator
• A set of standard samples representing all the elements in the unknown with the possible exception of oxygen. Oxygen can be measured directly but is oſten calculated based on an assumed stoichiometry
• A picoammeter and Faraday cup (see Figure 2) or an in-lens probe current meter to measure the probe current. We need to know the number of electrons per second (usually expressed in nA or pA) striking the sample. Tis is not the same as the emission current, the number of electrons per second leaving the electron gun cathode (usually 10s or 100s of μA).
You will also need: • Reference spectra for all measured elements. In many cases the standard spectrum can also serve as the reference spectrum. Te “Quantification alien” will tell you when additional references are required.
High-accuracy quantitative analysis can only be performed using spectra collected from flat-polished bulk samples mounted perpendicular to the electron beam. Te definition of bulk is a little loose, but it generally means at least as large as the electron beam interaction volume (on the order of a few micrometers) but potentially as large as the interaction volume of the charac- teristic X-rays (oſten tens to thousands of micrometers.)
Using the “Quantification Alien” Once you have collected the requisite information, you can start the quantification process. Follow along on Figure 3
Figure 2: A Faraday cup serves as an electron trap. Electrons enter the small hole, but electrons cannot escape as low-energy secondaries or backscattered electrons.
or 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. Te 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. Tis 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.) Te “File” button
Table 1: There are three different types of spectra involved in the quantification process: standards, references, and unknowns. Standards provide intensity data (characteristic X-ray events per nanoamp per second.) References provide characteristic line shape information. References are required because sometimes the characteristic lines present in a standard spectrum are located at similar energies. The difference between standards and references is subtle. Often standards can act as references for one or more characteristic line families, in which case an additional reference is not required. Unknowns represent the spectra we wish to interpret.
Spectrum type
Standard Reference Unknown
32
Same beam energy
3 *
3
Known probe
current 3
3
Known live
time 3
3 Same
element / elements
3 3 3
Same
detector 3 3 3
Same
detector position
3 3 3
Same detector
resolution 3 3 3
Same detector
calibration 3 3 3
Unobstructed views of the
characteristic lines
3 *It is usually best to collect references at the same beam energy and close to the same probe current, although in many cases it is not strictly necessary.
www.microscopy-today.com • 2011 September
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