Scanning Electron Microscopy and X-Ray Microanalysis

334

20

Counts

Photon energy (keV)

Corning Glass A

E 0 = 15 keV

100 μm square

50 μm square

20 μm square

5 μm square

2 μm square

1 μm square

spot

20 000

15 000

5 000

0

2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3

10 000

Counts

Photon energy (keV)

Corning Glass A

E 0 = 15 keV

100 μm square

50 μm square

20 μm square

5 μm square

2 μm square

1 μm square

spot

160 000

140 000

120 000

10 000

80 000

60 000

40 000

20 000

0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

CorningA_1mu_15kV15nA

CorningA_2mu_15kV15nA

CorningA_5mu_15kV15nA

CorningA_20mu_15kV15nA

CorningA_50mu_15kV15nA

CorningA_100mu_15kV15nA

CorningA_spot_15kV15nA

CorningA_1mu_15kV15nA

CorningA_2mu_15kV15nA

CorningA_5mu_15kV15nA

CorningA_20mu_15kV15nA

CorningA_50mu_15kV15nA

CorningA_100mu_15kV15nA

CorningA_spot_15kV15nA

. Fig. 20.16 Corning glass A, showing Na and K migration as a function of dose for scanning beams covering various areas (20 keV, 10 nA)

values for the glass, including the alkali elements Na and K,

whereas the point beam results show reductions in the Na and

K concentrations.. Figure 20.17 shows that the measured

sodium and potassium concentrations increase to reach the

synthesized values as the scanned area dimensions are

increased to cover areas above 20 x 20-μm (nominal magnifi-

cation 5 kX) for the particular dose utilized (15 keV, 1500 nA-s).

Thus, while scanning a large homogeneous area obviously

concedes the spatial resolution capability of electron-excited

X-ray microanalysis, this approach may be the most expedient

technique to control and minimize alkali element migration.

Materials that can serve as useful standards for sodium

include certain crystalline minerals such as albite (NaAlSi 3 O 8 )

in which the sodium is much more stable under electron

bombardment. However, even for albite the use of a station-

ary high intensity point beam may produce significant migra-

tion effects, as shown in. Fig. 20.18 for spectra collected

with a stationary point beam as a function of dose (upper)

and at the same dose with a fixed beam and two different

sizes of scanned areas (lower). Thus, the use of a scanned area

rather than a fixed beam may be necessary when collecting a

standard spectrum, even on a crystalline material.

20.6.2 Materials Subject to Mass Loss

During Electron Bombardment—the

Marshall-Hall Method

Thin Section Analysis

The X-ray microanalysis of biological and polymeric speci-

mens is made difficult, and sometimes impossible, by several

forms of radiation damage that are directly caused by the elec-

tron beam. At the beam energies used in the SEM (0.1–

30  keV), it is possible for the kinetic energy of individual

beam electrons to break and/or rearrange chemical bonds.

The radiation damage can release smaller molecules such as

Chapter 20 · Quantitative Analysis: The SEM/EDS Elemental Microanalysis k-ratio Procedure for Bulk Specimens, Step-by-Step