20 1 An Introduction to EELS
electron detector. Even then, the measured signal will be less than for EELS because
only a fraction of the excited Auger electrons (those generated within an escape
depth of the surface) are detected.
X-ray emission spectroscopy (XES) can be performed on bulk specimens, for
example, using an electron-probe microanalyzer (EPMA) fitted with a wavelength-
dispersive (WDX) spectrometer. As a method of elemental analysis, t he EPMA
technique has been refined to give good accuracy, about 1–2% of the amount present
with appropriate standards and corrections for atomic number, absorption, and fluo-
rescence effects. The accuracy becomes 5–10% for biological specimens (Goldstein
et al., 2003) A mass fraction detection limit of 1000 ppm (0.1 wt%) is routine for
most elements, and 1 ppm is possible for certain materials and operating conditions
(Robinson and Graham, 1992), which corresponds to a detection limit of 10
−17
gin
an analyzed volume of a few cubic micrometers. The WDX spectrometer detects all
elements except H and He and has an energy resolution ≈10 eV. Alternatively, bulk
specimens can be analyzed in a scanning electron microscope fitted with an energy-
dispersive x-ray (EDX) spectrometer, offering shorter recording times but poorer
energy resolution (≈130 eV), which sometimes causes problems when analyzing
overlapping peaks.
Higher spatial resolution is available by using a thin specimen and a TEM fit-
ted with an EDX detector, particularly in STEM mode with an electron probe of
diameter below 10 nm. Characteristic x-rays are emitted isotropically from the spec-
imen, resulting in a geometrical collection efficiency of typically 1% (for 0.13-sr
collection solid angle). For a tungsten filament electron source, the detection limit
for medium-Z element in a 100-nm specimen was estimated to be about 10
−19
g
(Shuman et al., 1976; Joy and Maher, 1977). Estimates of the minimum detectable
concentration lie around 10 mmol/kg (0.04% by weight) for potassium in biological
tissue (Shuman et al., 1976). For materials science specimens, the detection limits
tend to be lower: metal catalyst particles of mass below 10
−20
g have been analyzed
using a field-emission source (Lyman et al., 1995), although radiation damage is a
potential problem (Dexpert et al., 1982). For medium-Z elements in a 100-nm-thick
Si matrix, mass fraction detection limits are in the range 0.05–3% (Joy and Maher,
1977; Williams, 1987). With a windowless or ultrathin window (UTW) detector,
elements down to boron can be detected, although the limited energy resolution of
the EDX detector can lead to peak-overlap problems at low photon energies; see
Fig. 1.11a. Quantitative analysis is usually carried out using the ratio methods of
Cliff and Lorimer (1975) or of Watanabe and Williams (2006) for thin inorganic
specimens or of Hall (1979) in the case of biological specimens. For the anal-
ysis of light elements, extensive absorption corrections are necessary (Chan and
Williams, 1985).
The ultimate spatial resolution of thin-film x-ray analysis is limited by elastic
scattering, which causes a broadening of the transmitted beam; see Fig. 1.12.For
a 100-nm-thick specimen and 100-keV incident electrons, this broadening is about
4 nm in carbon and increases with atomic number to 60 nm for a gold film of the
same thickness (Goldstein et al., 1977). Inelastic scattering also degrades the spatial
resolution, since it results in the production of fast secondary electrons that generate