Instrumental analysis

5 10 3

c. Pb 0.5 g

a. Soil Background

Figure 2. Spectra obtained with the MAP-3 XRF analyzer: a) a background soil, b) the 0.5-g Pb/Hg target on a soil

surface, c) the Pb target on a soil surface, and d) the Hg target on a soil surface.

Pb and/or Hg K-shell peak intensities are of

nel 1 to 50, a tungsten (W) K-shell peak in the

equal or greater magnitude than that of the

channel region 101 due to the composition of the

Compton peak, these metals are present at ex-

shutter for the source, and, in the region of chan-

tremely high concentrations (percent levels).

nel 176 (85 keV), the Compton backscatter peak.

In addition to these peaks, the three remaining

spectra show either or both of the L- and K-shell

Instrumental analysis

The SCITEC MAP-3 came preprogrammed

peaks of Pb and Hg. The L-shell peaks for these

two metals appear in the region of channel 26

with calibrations for Pb in both paint and soil.

(10.5 and 10.3 keV) superimposed over an elec-

The lead-in-paint calibration, which quantitively

tronic noise signal, and the K-shell peaks appear

assesses both the L and K lines of Pb, was used to

at channels 155 1 (75.0 keV) and 148 1 (70.8

check the instrument's performance on a daily

keV) for Pb and Hg, respectively.

basis. This sensitivity check, which took less than

Comparing all four spectra in Figure 2 also

5 minutes to perform, involved measuring a

shows how the intensity of the Compton peak

block of wood evenly coated with leaded paint.

changes inversely to the amount of heavy metals

Standards prepared for the analysis of metals in

present in the sample. In the spectra of the back-

soil matrices are homogeneous (finely ground),

ground soil (no target), the Compton peak has

but metallic primary explosives most likely exist

as salts. As a salt or salt aggregate, large discrep-

the highest intensity (400 counts); conversely, the

ancies in particle size are likely between primary

spectra of a sample with 0.5 g of both Pb and Hg

explosives and soil matrices. For this reason, the

on the soil surface has the lowest (250 counts).

analysis of the targets mentioned above, based

The Compton peak intensity changes because

the incoherent radiation backscatter that com-

on the factory calibration for Pb in soil (mg/kg),

prises it is caused mainly by light elements (i.e.,

served only as a relative measurement of the

oxygen, silica, etc.) in the sample matrix. As the

amount of this metal. In addition, instrumental

amount of heavier elements increases in the

measurements of many of the spectra were man-

matrix analyzed, the fluorescent energy from

ually interpreted by measuring the intensity of

light elements decreases. For instance, when the

the respective K lines, and sometimes the L lines

4