Scanning Electron Microscopy and X-Ray Microanalysis

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23

standard identical in composition to the unknown object is

generally not available. However, an estimate of the concen-

trations of elements in the unknown object is always avail-

able in the ZAF procedure, including the first step, where

Ci = ki /Σk. The value of IB,bulk can therefore be estimated

from the background measured on pure element standards:

ICB I

j

,,bulks=∑ jjB,td

(23.6)

where Ij,B,std is the pure element bremsstrahlung at the energy

of interest and Cj is the concentration of element j. An exam-

ple of an analysis of a complex IN-100 particle with conven-

tional k-ratio/matrix corrections (including normalization of

the raw values) and using the k-ratio/matrix corrections aug-

mented with the P/B method is given in. Table 23.7. The

relative deviation from the expected value (RDEV) for each

element is reduced compared with the simple normalization

procedure, especially for Al, which is highly absorbed when

measured on the backside of the particle.

The special advantage of the P/B method is that it can be

applied to spectra obtained with a focused probe directed at

a specific location on a particle. Thus, particles that have a

chemically heterogeneous sub-structure can be directly stud-

ied. To be effective, the P/B method requires spectra with

high counts. Because the ratio of background intensities is

used to scale the particle peak intensities, the statistical

uncertainty in the background ratio propagates into the error

in each concentration value in addition to the statistics of the

characteristic peak. Even more importantly, the P/B method

depends on the background radiation originating in the

excited volume of the specimen only, and not in the sur-

rounding substrate. When an irregularly shaped object such

as a particle becomes small relative to the bulk interaction

volume, the penetration of the beam into the substrate means

that the continuum continues to be produced, even if the

substrate is a low atomic number element such as carbon. As

noted above, the energy-dispersive X-ray spectroscopy

collimator has a large acceptance area at the specimen. To

minimize the extraneous background contributions, the

small particles should be mounted on a thin (approximately

10–20 nm) carbon film supported on a metal grid (typically

copper, as used in the transmission electron microscope) and

mounted over a blind hole drilled into a carbon block. The

continuum contribution from such a thin film is negligible

relative to particles as small as approximately 250  nm in

diameter.

23.7 Summary

1. Particle analysis is inevitably compromised compared to
   analysis of ideal flat samples, leading to an increased
   error budget.


2. Careful attention must be paid to optimizing particle
   sample preparation to minimize substrate contributions
   to the spectrum and to reduce contributions from
   nearby particles.


3. Quantitative analysis of particles follows the k-ratio/
   matrix correction protocol. The analytical total that
   results from this procedure is an indication of the
   magnitude of particle geometry effects (mass effect and
   absorption effect).


4. Normalization of the raw concentrations (including
   oxygen by stoichiometry, if appropriate) is necessary
   to place the calculated composition on a realistic
   basis.


5. Large relative errors, exceeding 10%, are encountered after
   normalization. The analytical errors are exacerbated when
   low and high photon peaks must be used for analysis.


6. The analytical errors generally increase as the particle
   size decreases.


7. Overscanning does not decrease the analytical errors,
   and may well increase the errors depending on the
   particular combination of elements being analyzed.
   . Table 23.7 Relative Deviation from Expected Value (RDEV) observed with peak-to-background corrections compared to the raw
   concentrations and normalized concentrations after conventional k-ratio/matrix corrections
   Cbulk Craw RDEV(%) CN RDEV(%) CP/B RDEV(%)

Al 0.0603 0.0201 −67% 0.0241 −60% 0.0552 −8%

Mo 0.0353 0.0194 −45% 0.0233 −34% 0.0437 +24%

Ti 0.0519 0.0406 −22% 0.0487 −6% 0.0480 −7%

Cr 0.0965 0.0788 −18% 0.0945 −2% 0.0996 +3%

Co 0.155 0.139 −11% 0.166 +7% 0.156 +1%

Ni 0.601 0.536 −11% 0.643 +7% 0.598 −0.5%

Spherical particle: IN-100 alloy, 88 μm diameter, with beam placed at 22 μm from the top center on the backside of particle

Chapter 23 · Analysis of Specimens with Special Geometry: Irregular Bulk Objects and Particles