What Is X-Ray Fluorescence (XRF)

What is X-Ray Fluorescence (XRF)
An X-ray fluorescence (XRF) spectrometer is an x-ray instrument used for routine,

relatively non-destructive chemical analyses of rocks, minerals, sediments and
fluids. It works on wavelength-dispersive spectroscopic principles that are similar
to an electron microprobe (EPMA). However, an XRF cannot generally make
analyses at the small spot sizes typical of EPMA work (2-5 microns), so it is
typically used for bulk analyses of larger fractions of geological materials. The
relative ease and low cost of sample preparation, and the stability and ease of use
of x-ray spectrometers make this one of the most widely used methods for
analysis of major and trace elements in rocks, minerals, and sediment.
Fundamental Principles of X-Ray

Fluorescence (XRF)
The XRF method depends on fundamental principles that are common to several
other instrumental methods involving interactions between electron beams and x-
rays with samples, including: X-ray spectroscopy (e.g.,SEM - EDS), X-ray
diffraction (XRD), and wavelength dispersive spectroscopy (microprobe WDS).
The analysis of major and trace elements in geological materials by x-ray

fluorescence is made possible by the behavior of atoms when they interact with
radiation. When materials are excited with high-energy, short wavelength radiation
(e.g., X-rays), they can become ionized. If the energy of the radiation is sufficient
to dislodge a tightly-held inner electron, the atom becomes unstable and an outer
electron replaces the missing inner electron. When this happens, energy is
released due to the decreased binding energy of the inner electron orbital
compared with an outer one. The emitted radiation is of lower energy than the
primary incident X-rays and is termed fluorescent radiation. Because the energy of
the emitted photon is characteristic of a transition between specific electron
orbitals in a particular element, the resulting fluorescent X-rays can be used to
detect the abundances of elements that are present in the sample.
X-Ray Fluorescence (XRF) Instrumentation -

How Does It Work?
The analysis of major and trace elements in geological materials by XRF is made
possible by the behavior of atoms when they interact with X-radiation. An XRF
spectrometer works because if a sample is illuminated by an intense X-ray beam,
known as the incident beam, some of the energy is scattered, but some is also
absorbed within the sample in a manner that depends on its chemistry. The
incident X-ray beam is typically produced from a Rh target, although W, Mo, Cr
and others can also be used, depending on the application.
Show Caption
When this primary X-ray beam illuminates the sample, it is said to be excited. The
excited sample in turn emits X-rays along a spectrum of wavelengths
characteristic of the types of atoms present in the sample. How does this happen?
The atoms in the sample absorb X-ray energy by ionizing, ejecting electrons from
the lower (usually K and L) energy levels. The ejected electrons are replaced by
electrons from an outer, higher energy orbital. When this happens, energy is
released due to the decreased binding energy of the inner electron orbital
compared with an outer one. This energy release is in the form of emission of
characteristic X-rays indicating the type of atom present. If a sample has many
elements present, as is typical for most minerals and rocks, the use of
aWavelength Dispersive Spectrometer much like that in an EPMA allows the
separation of a complex emitted X-ray spectrum into characteristic wavelengths
for each element present. Various types of detectors (gas flow proportional and
scintillation) are used to measure the intensity of the emitted beam. The flow
counter is commonly utilized for measuring long wavelength (>0.15 nm) X-rays
that are typical of K spectra from elements lighter than Zn. The scintillation
detector is commonly used to analyze shorter wavelengths in the X-ray spectrum
(K spectra of element from Nb to I; L spectra of Th and U). X-rays of intermediate
wavelength (K spectra produced from Zn to Zr and L spectra from Ba and the rare
earth elements) are generally measured by using both detectors in tandem. The
intensity of the energy measured by these detectors is proportional to the
abundance of the element in the sample. The exact value of this proportionality for
each element is derived by comparison to mineral or rock standards whose
composition is known from prior analyses by other techniques.
Applications
X-Ray fluorescence is used in a wide range of applications, including
 research in igneous, sedimentary, and metamorphic petrology

 soil surveys
 mining (e.g., measuring the grade of ore)
 cement production
 ceramic and glass manufacturing
 metallurgy (e.g., quality control)
 environmental studies (e.g., analyses of particulate matter on air filters)
 petroleum industry (e.g., sulfur content of crude oils and petroleum
products)
 field analysis in geological and environmental studies (using portable,
hand-held XRF spectrometers)
X-Ray fluorescence is particularly well-suited for investigations that involve
 bulk chemical analyses of major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K,
P) in rock and sediment
 bulk chemical analyses of trace elements (in abundances >1 ppm; Ba, Ce,
Co, Cr, Cu, Ga, La, Nb, Ni, Rb, Sc, Sr, Rh, U, V, Y, Zr, Zn) in rock and sediment -
detection limits for trace elements are typically on the order of a few parts per
million
X-ray fluorescence is limited to analysis of
 relatively large samples, typically > 1 gram
 materials that can be prepared in powder form and effectively
homogenized
 materials for which compositionally similar, well-characterized standards
are available
 materials containing high abundances of elements for which absorption
and fluorescence effects are reasonably well understood
In most cases for rocks, ores, sediments and minerals, the sample is ground to a
fine powder. At this point it may be analyzed directly, especially in the case of
trace element analyses. However, the very wide range in abundances of different
elements, especially iron, and the wide range of sizes of grains in a powdered
sample, makes the proportionality comparison to the standards particularly
troublesome. For this reason, it is common practice to mix the powdered sample
with a chemical flux and use a furnace or gas burner to melt the powdered sample.
Melting creates a homogenous glass that can be analyzed and the abundances of
the (now somewhat diluted) elements calculated.

What Is X-Ray Fluorescence (XRF)

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What Is X-Ray Fluorescence (XRF)

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What is X-Ray Fluorescence (XRF)

An X-ray fluorescence (XRF) spectrometer is an x-ray instrument used for routine,

Fundamental Principles of X-Ray

The analysis of major and trace elements in geological materials by x-ray

X-Ray Fluorescence (XRF) Instrumentation -

 research in igneous, sedimentary, and metamorphic petrology

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