Selectivity
Portable XRF spectrometers come equipped
Selectivity of XRF analyzers for individual
with silicon (drifted with lithium) [Si(Li)] or
metals depends on both the energy emitted by
mercuric iodide crystal detectors that have reso-
the primary source and the resolution of the
lutions from 170 to 2500 eV, or gas-proportional
detector. Portable XRF systems capable of in-situ
detectors with resolution anywhere from 700 to
analysis rely on one or more radioactive sources
3000 eV. Table 1 lists common primary sources and
for primary incident radiation. For a metal to be
the analytes that they can excite, along with the K
detected, the incident energy emitted during the
and L energy lines for Pb and Hg.
radioactive decay of a primary source must be
greater than the excitation energy of the inner-
Area of metal detection
shell electrons of the element (or elements) to be
The effective sample volume measured dur-
analyzed. Briefly, the electrons most often excit-
ing XRF analysis is a function of the positioning
ed during XRF analysis are located in the K and L
of the source and detector and the energy used to
shells. Once excited, these inner-shell electrons
excite or assess the analytes. With regard to in-
are lost, and electrons from an outer shell fill the
strumental geometry, maximum analysis depth
voids created. In the process of the electrons go-
(penetration) is achieved when the source and
ing from an outer to an inner shell, element-spe-
detector are positioned in a parallel configura-
cific energies (photons, i.e., X-ray fluorescence)
tion. In this orientation the angle between the in-
are emitted. This fluorescent energy is measured
cident radiation going into the sample matrix
in kilo electron volts (keV; 1 keV = 1000 eV). Sub-
and the fluorescent energy returning to the de-
scripts α, β, and γ can accompany the K and L
tector is minimized. Therefore, the least amount
notations, indicating which outer shell the elec-
of substrate has to be transversed.
trons fell from, thus further specifying the dis-
The penetration depth with regard to the inci-
crete spectral energies measured.
dent and fluorescent radiation is inversely
proportional to wavelength and directly propor-
tional to energy. The radiation energy necessary
Table 1. XRF primary sources and emission
for exciting electrons, and the energy lost by the
energy of metals that have fluorescent ener-
electrons that fill these vacancies, is greater for
gies close to Pb and Hg.
the K shell than the L shell for a given element.
Primary energy
Useful for the analysis of
Source
(keV)
K energies
L energies
Portability, user friendliness, cost
Conceptually, a small, light XRF system would
Fe-55
--
Si-V
Nb-Ce
be best suited for either human-portable or robotic
(1423)*
(4158)
implementation. Likewise, instrument perfor-
Cm-244
--
Ti-Se
La-Pb
mance verification and the number of steps to
(2234)
(5782)
acquire measurements or view spectra should be
Cd-109
87.9 & 22.1
Ba-W
Cu-Mo
minimal. Because these instruments contain
(5674)
(2942)
radioactive sources that can have short half-lives
Hf-U†
Am-241
59.6
Zn-Nb
(< 2 years), rental and upgrade cost should also
(3041)
(7292)
be evaluated.
Hg-U†
W-U†
Co-57
121.9 & 136
(8092)
(7492)
INSTRUMENT SELECTION JUSTIFICATION
Emission energy (keV)
Metals
K
L
Atomic weight
The SCITEC MAP-3, an XRF analyzer manu-
factured for the analysis of lead in paint,
Ir
64.99
9.173
77
equipped with a Co-57 radioactive source, was
Pt
66.82
9.44
78
Au
68.79
9.71
79
selected for the following reasons. This XRF sys-
Hg
70.82
10.27
80
tem allows for simultaneous analysis of both K-
Tl
72.86
10.27
81
and L-shell lines of Pb and Hg and has an ambi-
Pb
74.96
10.55
82
ent-temperature Si(Li) detector with a spectral
Bi
77.1
10.84
83
resolution of about 2.5 keV. In addition, the
Po
79.3
11.1
84
source and detector are positioned next to each
* Range of atomic weights.
other within the scanner, so they face in the same
† Useful range of emission energies includes Pb and
direction (almost parallel geometry). All other
Hg.
2
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