While simpler in terms of the positioning of the detector versus the sample, EDXRF spectrometers require sophisticated electronics and computer software in order to interpret the detector output. Nowadays this is less complicated, though, due to important technological advances in multichannel analyzers and faster computers, and EDXRF is often the technique of choice for fast multielement analyses. Although germanium detectors are utilized, the most common type in service is the Si Li , or lithium-drifted silicon, detector.
A semiconductor detector operates based on the principle that an X-ray photon incident upon the diode material will give up its energy to form electron-hole pairs, the number of which is proportional to the energy of the photon. The high voltage applied across the diode quickly collects the released charge on a feedback capacitor, and the resulting proportional voltage pulse amplified by a charge-sensitive preamplifier.
The output of the preamp is fed to a main amplifier system. The pileup rejector, part of this system, deals with the probable event that two pulses will arrive very close together in time. From this point, the pulse is converted to a digital signal and processed in the multichannel analyzer MCA Jenkins In the MCA, dead time , caused by high counting rates, must be corrected. Peaks in the energy spectrum, once acquired, are subject to a large degree of massaging by the software in the connected computer. Sophisticated algorithms sense and quantitatively correct for high backgrounds due to Compton scattering from low atomic number matrices Metz Spectrometers that use secondary targets may acquire several energy spectra for each sample, one from each target.
Since each target yields better sensitivity in one part of the spectrum, the information from the energy spectra is combined to quantitate each element being analyzed.
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Accurate quantitative data on the entire mass spectrum may be obtained in a matter of minutes using EDXRF. For both of the Bruker Tracer instruments we use we have incorporated a secondary target made of thin sheets of copper, aluminum and titanium to optimize the spectra for the analysis of obsidian and any other analyses focusing on elements with with fluorescent energies between about 10 and 20 kV.
We have developed a world-renowned set of obsidian calibration standards that we have used to calibrate our own instruments and Bruker now runs this calibration on all portable XRF instruments heading out to museums and archaeologists. With this calibrations it is possible to acquire quantitative concentrations for many elements that are comparable to data acquired by mosre costly and destructive neutron activation analysis NAA. The tube voltage can be varied up to 45 kV, although we generally analyze the obsidian with a setting of 40kV. The secondary target, or filter, primarily used includes a 6 mil thick sheet of copper used to block X-rays below about 20kV a 2 mil sheet of titanium added to remove the secondary copper X-rays and a 12 mil sheet of aluminum to absorb the titanium X-rays.
Sample preparation is highly variable depending on the matrix and goals of the analysis. Most of the materials we analyze obsidian, metals, and ceramic paints do not require any sample preparation. The choice of sample preparation depends on the nature of the X-ray beam relative to the sample. For example, a piece of obsidian that is 1 cm thick and has a clean, flat surface will provide ideal results.
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As sample sget smaller, thinner, or less homogenous it is necessary to understand the nature of the X-ray beam and how it interacts with the sample. This small beam is fine for homogenous materials, but heterogenous material such as crystalline rocks and tempered pottery may need to be analyzed multiple times in numerous areas to generate a representative average composition.
The small beam size is ideal for isolating specific painted elements on the surface of ceramics and also aids in the analysis of very small obsidian artifacts. Perhaps even more important than the area of the beam is the depth of analysis. As a general rule, the higher up the energy spectrum, the greater the depth of X-ray penetration in the sample. For example, the analysis of iron 6. In thick homogenous samples this depth of analysis makes little difference, but if samples are thinner, it effects to resulting spectrum in different ways depending on the specific sample thickness and particular element of interest.
Ferguson in press addresses a number of approaches to quantitative analysis of thin samples. The ability to analyze samples without destructive sample preparation procedures has been a great advancement for archaeologists. We can now analyze large and valuable artifact assemblages that would have been off-limits to destructive proceedures. However, for non-archaeological applications of XRFthe most common method of sample prep is pelletizing, which can be made to work for most matrices that can be ground into an homogeneous powder, including soil, minerals, and dried organic matrices such as tissues or leaves.
X-ray Emission from Atoms
Difficult grinding is accomplished with a hard agate mortar and pestle but many samples can be adequately homogenized by placing into a hard plastic vial, adding a plastic mixing ball, and violently shaking in a mixer mill. A powdery binder containing cellulose, starch, polyvinyl alcohol or other organics is usually weighed in and blended thoroughly with the sample, and the resulting mixture added to a deformable aluminum cup.
Buhrke p. Here particle size and homogeneity play a big factor. This is due to the variance in X-ray penetration depths with energy Jenkins Particles may be inhomogeneous also, having a different surface composition than their bulk. For example, copper sulfides may become partially oxidized at the surface, causing the relative absorption for Cu K lines to differ from that of the L lines. The L line photons will not penetrate as deeply and will tend to be emitted more from the oxide layer. One way to get around sample grinding is to fuse the sample at high temperatures with sodium or lithium tetraborate and then to pour this glass-like mixture into a mold Buhrke: Chemical reactions occur within the melt which dissolve particles and create a homogeneous liquid that hardens upon cooling.
The disadvantages to this technique include the additional time to prepare the melt and the possibility of the sample reacting with even inert crucible materials such as platinum. Homogeneous solid samples such as metals may be machined and smoothed to form disks. Whatever type of preparation is done, the surface roughness of the sample should be taken into account. A rough surface causes the penetration layer to look heterogeneous to the spectrometer. Currently XRF spectrometry is very widely applied in many industries and scientific fields.
The steel and cement industries routinely utilize XRF devices for material development tasks and quality control. Anzelmo Part 1 NIST utilizes XRF as one technique to quantitatively analyze and acceptance-test many of its standard reference materials SRMs , from spectrometric solutions to diesel fuel to coal to metal alloys Sieber The plastics industry is looking at a modified XRF spectrometer as an on-line wear monitor, taking advantage of its ability to detect particles of worn-off metal in extruded plastic pieces Metz Polish scientists are accomplishing XRF analyses on very thin films by placing the source and detector at very low angles with respect to the sample.
This technique is being applied to trace element determinations in water samples that have been evaporated to a thin film of residue Holynska XRF has been one of the tools of choice for geologists for many years, so much so that graduating geologists usually receive practical training with these devices, whereas graduating chemists probably haven't even heard of the technique. For geologists, the ability to determine major and trace components in one quick analysis with relatively little sample preparation has been a boon Anzelmo Part 1, Part 2.
X Rays: Atomic Origins and Applications | Physics
Current basic research aimed at improving XRF analyses for geological and ecological samples focuses on methods for correcting for matrix effects, in which major components absorb some of the X-rays emitted from trace components Revenko Archaeometrists have applied XRF in order to solve their ancient mysteries. An example of this was the study of the composition of blue soda glass from York Minster, England, which distinguished three compositional groups, indicating this number of possible sources for the glass.
Trace metal signatures also can effectively differentiate genuine artifacts from modern copies Jenkins Forensic scientists utilize XRF spectrometry to match samples associated with suspects i. As for other applications, here XRF can help elucidate an elemental fingerprint, without need to analyze the evidence destructively Jenkins XRF is a versatile, rapid technique which lends itself to a wide variety of samples from powders to liquids.
Medical and Other Diagnostic Uses of X-rays
The instruments have few moving parts, tend to be low-maintenance, and on a regular basis consume only liquid nitrogen and electricity. Disadvantages include fairly high limits of detection LODs when compared to other methods, as well as the possibility of matrix effects, although these can usually be accounted for using software-based correction procedures. GFAAS is also relatively slow, with one element determined at a time, and is destructive Jenkins PXRF instruments are capable of producing results comparable in many ways to the lab-based XRF at a fraction of the cost.
However, usually samples must be in liquid form, which often requires acid digestion and laborious sample prep. One can sample solids with laser ablation ICP-MS but this is better suited to tiny spot sizes on the surface. ICP techniques also require copious amounts of expensive high-purity argon gas.
Neutron activation analysis NAA uses thermal neutrons to activate isotopes of trace elements in samples. It has ppb-level LODs and is capable of fast, multi-element determinations. Certainly, though, it is not as innocuous and convenient a technique as XRF, and requires expert supervision as well as a nuclear reactor.
It is dangerous and possibly illegal to irradiate samples whose major components are unknown. Hopefully it is clear from this discussion what niche XRF occupies in the field of elemental analysis. No technique is the answer to every analytical problem, and the analyst must be judicious in his or her selection of methodology.
XRF certainly does hold an important position in the realm of analytical chemistry. Will it flourish in popularity in the future, or will it slowly fade into obsolescence? To help provide an answer, two new XRF-related technologies will be discussed. These advances are changing the face of XRF spectrometry.
By using this setup with a thin film of sample on a flat support, it was possible to detect secondary X-rays emitted from the sample with neglible contribution from the support. The detection limits found with this method were better than conventional XRF by five orders of magnitude, and were as low as 10—12 g.
Matrix effects were eliminated and the addition of internal standards was simplified with this thin-film analysis. The discovery of TXRF roused the interest of the semiconductor industry, which must control the amount of surface contamination on cleaned wafers. In general, any type of sample can be analyzed by TXRF, as long as it is possible to deposit it in a thin layer onto a carrier.
Other samples can be laser-ablated and the resulting vapor deposited on a quartz reflector Potts: Clearly, TXRF is still in its infancy, but the possibilities for its application seem vast.
Materials at cryogenic temperatures exhibit a very low heat capacity. Theoretically, the heat deposited by one absorbed X-ray photon could change the temperature of a small volume of material enough to be measured. This approach to photon detection differs fundamentally from that of semiconductor-type devices and has been made practical by the use of materials that become superconductive below a specific transition temperature. Very slight ohmic heating is used to balance these materials on the razor-edge of this transition, creating a very sensitive thermometer.
When a photon deposits its energy into a strip of this material, the temperature and resistivity of the strip goes up, reducing the current flow and the ohmic heating rate. The temperature then drops back to the transition-edge. The blip in the current flow is recorded by detector electronics, which relate current drop to photon energy Knoll The advantage this very interesting new microcalorimeter brings to X-ray detection is increased energy resolution. This increased performance allows closely spaced spectral peaks to be resolved, increasing the sensitivity and accuracy of some complicated analyses.
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Moreover, with this new detector, XRF can now yield information on the chemical bonding state of elements by detecting minute X-ray energy shifts on the order of a few eV Sieber Continued research into the fundamentals of XRF, stimulated partially by the new advances mentioned above, means that the technique is alive and well.
Analytical chemists would be well-advised to keep their eyes on the development of XRF and can look forward to better, faster elemental analyses because of it. Anzelmo, John A. Journal of Chemical Education, August , 64 8. Journal of Chemical Education, September , 64 9. Buhrke, Victor E. Holynska, Barbara et al.
Fresenius Journal of Analytical Chemistry , , , Jenkins, Ron; Gould, R. Quantitative X-Ray Spectrometry. Second Edition, , Marcel Dekker, Inc.
Quantitative Measurements of X-Ray Intensity
However, the separation between an electron and a nucleus will be different at each collision and for different electrons, so a continuous distribution of wavelengths is produced. However, we know that the maximum amount of energy Emax which an electron can lose is the entire kinetic energy with which it arrived at the surface of the x-ray anode, equal in Joules to e Va where e is the electronic charge and Va the accelerating voltage applied to the x-ray tube.
We therefore predict the bremsstrahlung continuum to have a short-wavelength cutoff at a value lambda min given by this equation. Measurement of the x-ray spectrum confirms this conclusion and therefore provides additional evidence besides that of the photoelectric effect of the quantum nature of electromagnetic radiation. Note that the emission of an x-ray photon as an electron passes a nucleus is a momentary interaction taking place on an atomic scale, so a quantum description of x-ray production is appropriate.
If we allow a monochromatic beam of x-rays wavelength lambda to fall on a solid specimen such as graphite see Fig. Scattering from the nucleus is elastic as we have discussed so the spectrum of the scattered x-rays shows a peak at the original wavelength lambda as in Fig.
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