Chemistry Reference
In-Depth Information
Figure4.23.
Intensity profile for a flat substrate contaminated by an element that is as well
embedded throughout the substrate, as located in small grains above the substrate, as deposited like
an ultrathin layer on top of or just below the substrate. The critical angle
α
crit
is determined only by
the substrate itself.
α
1
and
α
2
are significant angles of operation. Figure from Ref. [4], reproduced
with permission. Copyright1996, John Wiley and Sons.
two or three types of contamination can be analyzed and the different fractions
can be determined [60]. For that purpose, actually an angle scan should be
performed by GI-XRF and intensity profiles should be plotted for all elements
under consideration. Such a profile is demonstrated in Figure 4.23 as an
example. It can be concluded directly from this profile that all three types
of contaminations are present.
In order to obtain quantitative results, the intensity sum is derived from
Equations 4.25, 4.26, and 4.27, and fitted to the measured curve. The concen-
tration values
c
VB
for the bulk-type,
c
AP
for the particulate-type, and
c
AL
for
the layer-type contaminations are varied until a best fit is reached via a
χ
-square
test. The ultimate values are treated as the results.
The method, however, must first be calibrated. This task consists of deter-
mining the calibration factors
N
0B
,
N
0P
,or
N
0L
. The three quantities essentially
are a product of the relative sensitivity of the analyte and the intensity of the
primary beam. They are further determined by the total mass-attenuation
coefficient of the matrices in accord with Equation 4.2. Calibration by internal
standardization is not permissible because any additional contamination must
be avoided here. Consequently, an external calibration becomes necessary, to
be carried out by means of an external standard. An overview of different
methods is given by Hockett [65]. Some users applied a particulate-type
Search WWH ::
Custom Search