Effective surface area of analysis

Instrumental analysis

Figure 3. Illustrations of spectra obtained with the MAP-3 XRF analyzer

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Table 3. Measurements of 0.5 g of Pb on soil sur-

and Compton peak, along with the adjacent

face and through soil ranging from 0.25 mm to 2.0

baseline noise. Intensities of K- and L-line peaks

mm thick.

were assessed by taking an average of the three

highest intensities (counts) in the appropriate

MAP-3

Metorex 920

channel region, while that of the baseline noise

Target

Co-57 source

Cd-109 source

Instr.†

was taken as the average intensity in the three

cover

Manual*

(mm)

PbL

PbK Comp.** (ppm)

PbLα PbLβ (ppm)

lowest channels, in the valley either just to the

right or left of the peak. This manual method of

data interpretation was performed because there

was no calibration for Hg, and it provides insight

1.0

500

510

280

6,900

2.1 9.3

2,000

as to the peak intensities (spectral peak heights)

1.5

250

410

290

5,900

0.18 1.0

270

relative to the background noise and that of the

2.0

400

280

5,600

ND 0.54

113

Compton peak.

* Manually measured fluorescent energies, baseline-corrected.

† Instrumental measurements based on Pb (mg/kg) in soil

Effective surface area of analysis

standards.

The effective area of analysis for a discrete

** Compton peak.

object on a surface was evaluated using a Pb shot

pellet and a large shallow plastic tray (100 60

two sheets of 0.2-mil polypropylene. These 6-

cm) filled 5.0 cm deep with the Lebanon Landfill

mm-high (rim) soil disks were placed between

(LLF) soil. The Pb pellet (2-mm diameter, 62 mg)

the XRF spectrometer's sensor window and a

was placed on the soil surface and the scanner

target consisting of a 0.5-g pillow of Pb. Table 3

was positioned directly over the pellet to obtain a

shows the amount of Pb measured by both the

maximum instrumental measurement of Pb in

MAP-3 (Co-57 source) and an X-Met 920

soil, using a 24-s analysis period. When this scan-

(Metorex, Inc.; Cd-109 source) XRF spectrometer.

ner is placed on the soil, the window for the

The results of this experiment show that while

source and detector remains some 0.6 cm above

the intensity of the L lines rapidly decreases as

the surface. The position of the scanner (i.e., the

the soil barrier increases, the intensity of the K

source and detector window) was fixed on the

line is not as severely affected. Indeed, the K-line

x/y plane (surface of the soil) with two rulers.

intensity remains greater than the Compton peak

The scanner was then picked up and reposi-

(Fig. 3). As will be shown in subsequent tests, the

tioned by moving it in 0.5-cm increments on the

energy emitted by the Co-57 source and the fluo-

x/y axis away from the location where the high-

rescent energy from electrons filling the voids

est measurement was obtained. The maximum

created in the K shell of Pb and Hg can easily

values obtained directly over the pellet were 4 to

penetrate this depth of soil and more.

5 times greater than the background (no pellet

under the scanner) and were at least 2 times

Tube tests

greater than the background after moving 0.5

Analysis of Pb and Hg through soil of various

cm. Positioning the scanner on the soil surface

depths was also performed using a Lucite tube 3

1.0 cm away from the point where a maximum

cm high and 2 cm in diameter. The tube was cov-

reading had been obtained often resulted in a

ered on one end with a sheet of 0.2-mil polypro-

reading similar to that of the background. This

pylene held in place with a rubber band. The

experiment indicates that the scanner excites and

covered end of the tube was positioned 2.8 cm

receives enough radiation to detect a metal

below the soil surface, in a large plastic jar (11.2

aggregate lying on the surface over an effective

cm diameter 9.5 cm high). The level of the soil

analysis area of about 1.0 cm2.

in the jar came to within 2 cm of its rim, and the

mouth of the jar was just large enough to allow

Depth of soil penetration

the sensor end of the MAP-3 scanner to fit inside

Soil disks

and rest on the flat soil surface. The tube was po-

Soil barriers of approximately 0.25, 0.50, 1.0,

sitioned directly below the sensor window

1.5, and 2.0 mm were prepared by using an ap-

(source and detector window), and the jar was

propriate amount of dry LLF soil. Using only

marked to maintain the alignment between the

snap rings and collars from 31-mm polyethylene

scanner and the target from one measurement to

X-ray cells, the soil was actually spread over a 22-

another. For this experiment, a pillow of 0.5 g Pb

mm-diam. cylindrical area and held between

was placed in the Lucite tube at 1 or 2 cm below

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