INTRO TO X-RAY FLUORESCENCE (XRF) SPECTROMETRY
Although more popularly known for its diagnostic use in the medical
field, the use of x-rays forms the basis of many other powerful measurement
techniques, including X-
ray Fluorescence (XRF) Spectrometry.
XRF Spectrometry is used to identify elements in a substance and
quantify the amount of those elements present to ultimately determine the
elemental composition of a material. An element is identified by its
characteristic X-ray emission wavelength ( λ ) or energy (E). The amount of an
element present is quantified by measuring the intensity (I) of its
characteristic emission.
All atoms have a fixed number of electrons (negatively charged
particles) arranged in orbitals around the nucleus. Energy Dispersive (ED) XRF
and Wavelength Dispersive (WD) XRF Spectrometry typically utilize activity in
the first three electron orbitals, the K, L, and M lines, where K is closest to
the nucleus.
Illustration
In XRF Spectrometry, high-energy primary X-ray photons are emitted from
a source (X-ray tube) and strike the sample. The primary photons from the X-ray
tube have enough energy to knock electrons out of the innermost, K or L,
orbitals. When this occurs, the atoms become ions, which are unstable. An
electron from an outer orbital, L or M, will move into the newly vacant space
at the inner orbital to regain stability. As the electron from the outer
orbital moves into the inner orbital space, it emits an energy known as a
secondary X-ray photon. This phenomenon is called fluorescence. The secondary
X-ray produced is characteristic of a specific element. The energy (E) of the
emitted fluorescent X-ray photon is determined by the difference in energies
between the initial and final orbitals of the individual transitions.
This is described by the formula
E=hcλ-1
where h is Planck's constant; c is the velocity of light; and λ is the
characteristic wavelength of the photon.
Energies are inversely proportional to the wavelengths; they are
characteristic for each element. For example the Kα energy for Iron (Fe) is
about 6.4keV. Typical spectra for EDXRF Spectrometry appear as a plot of Energy
(E) versus the Intensity (I).
Elemental Analysis
XRF Spectrometry is the choice of many analysts for elemental analysis.
XRF Spectrometry easily and quickly identifies and quantifies elements over a
wide dynamic concentration range, from PPM levels up to virtually 100% by
weight. XRF Spectrometry does not destroy the sample and requires little, if
any, sample preparation. It has a very fast overall analysis turnaround time.
These factors lead to a significant reduction in the per sample analytical cost
when compared to other elemental analysis techniques.
Aqueous elemental analysis instrument techniques typically require
destructive and time-consuming specimen preparation, often using concentrated
acids or other hazardous materials. Not only is the sample destroyed, waste
streams are generated during the analysis process that need to be disposed of,
many of which are hazardous. These aqueous elemental analysis techniques often
take twenty minutes to several hours for sample preparation and analysis time.
All of these factors lead to a relatively high cost per sample. However, if PPB
and lower elemental concentrations are the primary measurement need, aqueous
instrument elemental analysis techniques are necessary.
All elemental analysis techniques experience interferences, both
chemical and physical in nature, and must be corrected or compensated for in
order to achieve adequate analytical results. Most aqueous instrument
techniques for elemental analysis suffer from interferences that are corrected
for by extensive and complex sample preparation techniques, instrumentation
modifications or enhancements, and by mathematical corrections in the system's
software. In XRF Spectrometry, the primary interference is from other specific
elements in a substance that can influence (matrix effects) the analysis of the
element(s) of interest. However, these interferences are well known and
documented; and, instrumentation advancements and mathematical corrections in
the system's software easily and quickly correct for them. In certain cases,
the geometry of the sample can affect XRF analysis, but this is easily
compensated for by selecting the optimum sampling area, grinding or polishing
the sample, or by pressing a pellet or making glass beads.
Quantitative elemental analysis for XRF Spectrometry is typically
performed using Empirical Methods (calibration curves using standards similar
in property to the unknown) or Fundamental Parameters (FP). FP is frequently
preferred because it allows elemental analysis to be performed without
standards or calibration curves. This enables the analyst to use the system
immediately, without having to spend additional time setting up individual
calibration curves for the various elements and materials of interest. The
capabilities of modern computers allow the use of this no-standard mathematical
analysis, FP, accompanied by stored libraries of known materials, to determine
not only the elemental composition of an unknown material quickly and easily,
but even to identify the unknown material itself.
Spectrometers
Innov-X Systems utilizes the EDXRF Spectrometer technique for its
mechanical simplicity and excellent adaptation to portable field use. An EDXRF
system typically has three major components: an excitation source, a
spectrometer/detector, and a data collection/processing unit. The ease of use,
rapid analysis time, lower initial purchase price and substantially lower
long-term maintenance costs of EDXRF Spectrometers have led to having more
systems in use today worldwide than WDXRF Spectrometer systems. Handheld, field
portable EDXRF units can be taken directly to the sample as opposed to bringing
the sample to the analyzer and configuring it to fit in an analysis chamber.
Innov-X Systems portable, handheld EDXRF units solve real 21 st century
application problems: solving crimes, analyzing alloys, exposing pollution,
preserving history, searching for WMD's, conserving art treasures, and a myriad
of other elemental field-oriented analyses.
Sample/Specimen
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