Scanning Electron Microscopy and X-Ray Microanalysis

408

23

23.6.6 Peak-to-Background (P/B) Method

Specimen Geometry Severely Affects the

k-ratio, but Not the P/B

Another approach to establishing quantitative X-ray micro-

analysis for objects of an irregular shape, such as particles

and rough surfaces, is the peak-to-background (P/B) method,

which is an extension of the Marshall–Hall method for the

correction of mass loss in beam-sensitive materials (see

7 Chap. 20 ) (Marshall and Hall, 1966 ; Hall, 1968 ). The P/B

method (Small et al., 1978 , 1979 a,b; Statham and Pawley,

1978 ; Statham, 1979 ; Wendt and Schmidt, 1978 ; August and

Wernisch, 1991a, b, c) is based on the observation that

although the characteristic X-ray intensity emitted from an

irregularly shaped object is highly dependent on local geo-

metric effects, the P/B ratio measured between the character-

istic X-rays and the continuum X-rays of the same energy is

much less sensitive to specimen geometry.. Table 23.6 con-

tains measurements of the k-ratio (measured relative to bulk

K411) and the P/B from the spectra of the SRM-470 (K-411

glass) shards in Fig. 23.13. The shard spectra show significant

deviations from the spectrum of the polished bulk K411,

especially at low photon energies below 4 keV. Although the

k-ratio for Mg measured for these shards varies by a factor of

50

40

30

20

10

0

Fe

relative error (%)

-10

-20

-30

-40 -30 -20 -10 0

Mg relative error (%)

10 20

K411 Spheres E 0 = 20 keV

Point beam

Overscan

a

K411 Shards E 0 = 20 keV

100

(^80) Point beam
Overscan
60
40
20
0
-20
-40
-60 -40 -20 0
Mg relative error (%)
Fe
relative error (%)
20 40
b

. Fig. 23.34 Relative errors in
normalized analyses for Mg and
Fe in K411 glass with point beam
centered on the image center of
mass and with overscanning: a
spherical particles, b fractured
shards

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