WHAT IS XRF (X-RAYFLUORESCENCE SPECTROMETRY)?
X-ray
fluorescence spectrometry is one of the most efficient instrumental means to
detect the elemental composition of the homogeneous obsidian. It’s speed,
precision for incompatible elements, ready availability,
and consequent low cost make EDXRF particularly ideal for the
non-destructive analysis of archaeological obsidian. It is not necessarily the
best suited for the analysis of heterogeneous substances, but with recent
advances in x-ray spectrometry and new software it is becoming one of the best
instruments for the analysis of nearly any archaeological material
A BRIEF INTRODUCTION ð XRF OF
ARCHAEOLOGICAL OBSIDIAN
(HOW IT WORKS)
When the atoms in a sample material
are irradiated with high-energy primary x-ray photons, electrons are ejected in
the form of photoelectrons. This creates electron 'holes' in one or more of the
orbitals, converting the atoms into ions - which are unstable.
To restore the atoms to a more stable state, the
holes in inner orbitals are filled by electrons from outer orbitals. Such
transitions may be accompanied by an energy emission in the form of a secondary
x-ray photon - a phenomenon known as "fluorescence"
The various electron orbitals are called K, L,
M, etc., where K is closest to the nucleus. Each corresponds to a different
energy level - and the energy (E) of emitted fluorescent photons is
determined by the difference in energies between the initial and final orbitals
for the individual transitions.
The amount of x-ray fluorescence is
very sample dependent and quantitative analysis requires calibration with
standards that are similar to the sample matrix. For obsidian analyses, North
American labs use obsidian and rhyolite standards from USGS, NIST, and the
Japan Geological Survey. The technique provides an elemental, not a chemical
analysis. XRF is inapplicable to the first 11 elements of the periodic table.
Sample penetration varies from about 0.01mm to 1mm in depth depending on the
sample material. Analysis is occasionally complicated by interfering x-ray
lines and by matrix effects, which are corrected in both EDXRF and WXRF by
linear and quadratic algorithms.
Characteristic x-ray emissions result in an
energy spectrum that is a "fingerprint" of the specimen. The
intensities of the peaks in the spectrum are roughly proportional to the
concentrations of the constituent elements
X-ray fluorescence can be measured
and quantified in two ways. Wavelength dispersive XRF uses a crystal to
separate the various wavelengths: for every angle of incident radiation, the
only wavelength reflected to the detector is the one that conforms to Bragg’s
formula:
nl
= 2d sin q
where n is a whole number
1-n, l is the wavelength of the x-ray radiation used; d is
a constant characteristic of every crystalline substance (i.e. the x-ray
crystal); and q is the angle on incidence of the x-radiation on the sample.
So, by changing the angle of the
crystal, you can select for specific elements of interest. In the Philips PW
2400 at Berkeley, this is all done automatically and any combination of
elements can be analyzed. The system changes crystals for the various elements,
calculates the overlap of elements within the spectrum and yields results in
any form desired: qualitative, ratio, quantitative, graphic.
The second and more common method in
North America for analyzing obsidian is EDXRF
EDXRF systems detect elements on the
periodic table between atomic numbers 11 (Na) and 92 (U). Samples can be
analyzed non-destructively with little or no sample preparation in minutes and
in some cases seconds. Elements in concentrations from as low as a few parts
per million to 100% may be analyzed in the same sample simultaneously. Accuracy
of less than one percent relative error are attainable with comparable
reproducibility
Analysis by EDXRF, like WXRF
involves use of ionizing radiation to excite the sample, followed by detection
and measurement of X-rays leaving the sample that are characteristic of the
elements in the sample.
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