Qualitative Analysis
In simplest terms,
qualitative analysis involves the determination of what elements are present in
the portion of the sample irradiated by the electron beam. This is accomplished
by resolving and identifying the (elements giving rise to) fluorescent characteristic
x-rays emitted from the sample. Qualitative analysis has three principal
applications: phase identification, identification of elements present for
quantitative analysis, and determining proper background offsets for
quantitative analysis by WDS. Phase identification is by far the dominant use
of qualitative methods, and is an especially valuable tool for characterizing
complex, fine-grained materials. This is demonstrated in the following example.
This sample in the
image at the left was suspected of being a metallic meteorite, for which the
metallic phase characteristically should be an Fe-Ni alloy. This backscattered
electron image shows morphologies consistent with quenching (very rapid
cooling) of molten material rather than the repetitive exsolution textures
(Widmanstätten patterns) that are characteristic of (hexahedral) Ni-Fe
meteorites. Qualitative analysis by EDXA shows the individual components to be:
(1) Pb-Cu-Sb alloys; (2) Fe-As alloy; (3) Fe metal; and (4) Fe-sulfide. Hence, the
textures and compositions of the component phases identify this material as
more likely being a metallurgical slag than a metallic Ni-Fe meteorite.
Methods and Analysis Time
As in quantitative analysis,
there are two techniques for the qualitative resolution of fluorescent x-rays:
Energy-Dispersive X-ray Analysis (EDXA) and Wavelength-Dispersive Spectrometry
(WDS) (for more information on these methods, see the "Analyzer"
section of What is an electron microprobe?).
EDXA is the more commonly used method for simple phase
identification, because it is extremely rapid: an entire x-ray spectrum of an
unknown can be acquired and displayed in a matter of seconds. Because the
relative intensities of x-ray lines displayed in the spectrum are proportional
to the abundances of elements in the sample (especially for x-rays arising from
similar types of electron transitions), EDXA spectra allow users to estimate
the chemical stoichiometry of the sampled compound. This application can be
enhanced by semi-quantitative approaches involving rapid standardless
analytical methods, which take only a few additional seconds and can provide a
reasonable estimate of elemental weight fractions, oxide weight fractions, or
atomic proportions. EDXA is especially useful on rough samples, because the
detection of x-rays is not as dependent on beam-sample-detector geometry as is
the WDS method. Therefore, EDXA can be used very effectively with electron
imaging methods to characterize loose grains or unpolished materials. The principal
limitation of EDXA as a qualitative tool lies in detecting elements present at
trace levels, which produce weak intensities that are commonly very difficult
to resolve from the background x-radiation (the "Bremmstrählung",
which is white x-radiation produced by the deceleration of electrons of the
primary beam).
Qualitative analysis by WDS involves scanning one or more
spectrometers along a specified geometric interval, to record the intensities
of all x-ray wavelengths reaching the detector. By specifying the range of
spectrometer movement(s) and the diffraction element(s) (with particular
interplanar spacing), the analyst can accumulate either a complete record of
the x-ray spectrum or concentrate on wavelength regions for specified elements.
Although a bit more time consuming than EDXA, rapid scans of the spectrometers
can acquire a complete x-ray spectrum in about 1 minute. The principal
advantages of qualitative analysis by WDS are superior resolution of x-ray
lines and higher peak/background intensity ratios compared to EDXA. In
conjunction with options for extended counting times and controlling the
wavelength regions scanned, these advantages are particularly important in
identifying the presence of elements at trace to minor level abundance.
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