XRF Analysis: What is X-Ray Fluorescence?

Material Analysis in Seconds/Scan with a Bruker Gun!

 

XRF Analysis by Bruker XRF Analyzer

A Bruker XRF Analyzer

XRF analysis is a powerful form of inorganic elemental analysis: it can tell us what chemical elements a material is composed of. X-ray fluorescence (XRF) technology has come a long distance over the past six decades: what was originally a manual analytical method (used in mostly academic science and, at the time, by just a handful of daring industrial ventures) has shaped up into a familiar, small automatic instrument – a handheld “gun” – capable of comprehensive reliable analysis of materials. Contact Bruker Inc., a world leading maker of handheld XRF spectrometers to find out how best to apply XRF analysis for the benefit of your own industry and business, to schedule a free demo, or for a quote.

This article provides a general introduction to XRF analysis. (You may also browse any articles of a variety of XRF applications in our site library.) Perhaps the most striking recent change concerns the size of XRF machinery. Although large laboratory installations are still required today for extra-high levels of analytical precision, most bread-and-butter XRF analysis tasks are performed in situ (i.e. on location, not in a lab) by commercially manufactured portable and handheld XRF analyzers, a best-in-class example of which is the Bruker S1 TITAN (weight 1.5 kg or 3.3 lbs, with battery!). The analysis sample no longer needs to travel to be tested: the XRF analyzer travels to it instead. Standing at the forefront of XRF innovation and not limited to the S1 series, Bruker XRF guns are used in metal and alloy analysis, precious metal analysis (including gold, silver etc. purity testing), Positive Material Identification (PMI), various types of Non-Destructive Testing (NDT), in geochemical prospecting, mining, consumer product and electronic scrap testing, HAZMAT screening, contaminated soil assessment, archaeology (archaeometry), museum artwork analysis, as well as numerous other fields. Get in touch with Bruker to find out whether handheld XRF is the right option for the type of material analysis you have in mind.

Advantages of Handheld XRF

Although limited to the Mg-U element range, Bruker’s S1 TITAN handheld instruments solve a gigantic glut of XRF analysis demand in science and industry. The great benefits of modern handheld XRF analysis recommend it for wide industrial use:

  • Simultaneous multielement testing (all elements in the Mg to U range);
  • Near lab level accuracy;
  • Immediacy (test results in just seconds);
  • Completely nondestructive and noninvasive: the material remains intact and unaltered by analysis;
  • Beyond portability, true in situ capability (meaning that you can point the instrument at the test item without moving the latter);
  • Ease of use (with present-day automation, only minimal training is expected);
  • Safety of use (and no embedded radioactive source);
  • Minimal sample preparation is needed in the general case, and few sample restrictions apply. Old-style analytical machinery usually comes fitted with tricky sample exchange mechanisms and vacuum chambers, and correct sample positioning is a constant concern;
  • Hey, no denying, it also just looks much cooler than the old style! 😊

One important limitation of XRF is that XRF “ignores” carbon, a major “blind spot”. Other equipment is more suitable to organic analysis situation.  As regards inorganic analysis, although there exist diverse other forms of elemental of elemental analysis, some with functions similar to XRF, very few of them can rival handheld XRF’s bouquet of advantages. Bruker Inc. would like to hear about your interest XRF analysis. Click here to tell us today! An analytical expert will provide you with prompt and competent guidance.

XRF Analysis Mechanism

X-rays are a short wavelength (high energy, high frequency) electromagnetic radiation. Spectrally speaking, they are located between the gamma rays and UV. X-Rays are the mechanism used in scanning and analyzing major and trace elements in metals, ores, soil, and other materials. The primary, high energy X-ray photons emitted by the source can excite secondary, lower energy, “fluorescent” X-ray photons from of the sample’s atomic structure. The emergent X-ray fluorescent spectrum, scanned with an appropriate XRF detector, is a composite of the individual “diagnostic” or “characteristic” radiation quanta of the elements in the sample, the specific energy of each being proportional to the corresponding concentration.

In a bit more detail, the excitation of an X-ray flooded XRF analysis item causes the material to become ionized. If the radiation energy is high enough to eject a firmly lodged inner shell electron from an atom, the atom itself turns unstable as an outer shell electron is substituted in place of the inner shell electron that has “gone missing.” When this happens, energy escapes, due to the fact that an inner shell electron is tighter-bound than an outer shell one. The resultant radiation – X-Ray Fluorescence (XRF) – is of a weaker energy level.

Differences in energy among electron shells are unchanging, so the emitted radiation always has characteristic energy, and the resulting fluorescent X-rays can be used to detect the abundances of elements that are present in the sample.

Albeit theoretically this method can measure nearly all the chemical elements, the range is more limited in practice, since real-life handheld XRF analyses take place in atmospheric air, which tends (in the absence of special measures) to weaken the low-energy fluorescent radiation, so that the elements below about Mg become hard or impossible to detect. XRF analysis is most perceptive to the elements measured from the K-line series whose absorption edges fall just below the energy of the excitation source’s characteristic emission lines. These are the elements stacked up to molybdenum in the Periodic Table. The K-series lines of the elements above Mo are harder to excite with sufficient sensitivity. Nonetheless, they can be verified at a lower sensitivity.

Penetration Depth of Analysis

How much of a sample is analyzed in a single scan? That is a function of the energy of the characteristic XRF ray and of its critical penetration depth (also referred to as “XRF saturation depth”), defined as the depth below the irradiated surface beyond which 99% of an element’s X-Ray line emission is undetectable due to being absorbed in the sample. In general, the lower atomic number corresponds to a low energy of the XRF lines emitted. For the lower atomic numbers, critical penetration depth is in the μm range, while for the higher atomic numbers analyzed based on the K-line, they fall in the 1–10 mm range. As a rule of thumb, the analyzed mass is typically between 10s of mg and a gram. This should especially be understood when analyzing materials that are nonhomogeneous. Highly homogeneous materials such as alloys pose less concern. A Bruker gun can perform efficient averaging when analyzing a sample.

Historical Background and Technological Progress

X-rays were first announced to the world by Wilhelm K. Röntgen, a discovery that won the great German physicist the Nobel Prize in 1901. X-Ray spectroscopy began in 1909, when the Englishman Charles Glover Barkla discovered the correlation between X-rays radiating emitted by sample and the elemental sample’s atomic weight. In 1913, another Englishman, Henry G. J. Moseley, found a way to number the chemical elements with the help of an X-ray instrument, by noticing that within an X-ray spectrum the K-line transitions, are displaced equally every time the atomic number increased by one. So important was this realization that it allowed Moseley to renumber the chemical elements, revising the Mendeleev Periodic Table.

The original X-ray source Röntgen introduced was an ion tube (not yet a vacuum tube, but filled with air at low pressure), where electrons were generated by bombing a cold cathode with ions in a high voltage gas discharge, and were electrically accelerated upon the anode, producing a flash of X-rays. Those old school ion tubes had a brief lifetime and were unstable. It was the US physicist and inventor William D. Coolidge who took the X-ray generator tube to the next level, coming up with a construction still used today. The Coolidge tube utilizes is filled with high vacuum and utilizes a hot wolfram filament as its electron generator. A great many types of X-ray tubes are used today, including those used in handheld XRF analysis. 

X-ray fluorescence radiation was first harnessed by industrial elemental testing labs in the 1950s. The evolution of XRF has proceeded through the development of the technological components required by XRF analysis, namely:

  • X-Ray Radiation sources
  • XRF detectors
  • Microprocessors

WDXRF vs. EDXRF Analysis

Original XRF was wavelength-dispersive (WDXRF; the terms WDX and WDS are also used, with “s” for “spectrometry”), i.e. based on counting the number of X-rays of a specific wavelength diffracted by a crystal lattice. WDXRF spectrometers are usually calibrated for XRF analysis strictly at the wavelengths of the emission lines of those elements for which a material needs to be tested, rather than over a broad spectrum simultaneously. The more flexible method of energy-dispersive XRF analysis (EDXRF, also called EDX or EDS) developed later. Under EDXRF, a silicon drift detector (SDD for short) using lithium-drifted silicon crystals can scan of the energy of a photon upon detection. EDXRF analyzers are capable of analyzing most chemical elements with conveniently low limits of detection (LOD). Bruker’s portable and handheld XRF analyzers use EDXRF. Originally, “portable XRF” was developed for situations where moving the test object was not an option, but today handheld testing has become the default method because of its trusty accuracy and sheer convenience (test anywhere, usually with zero sample preparation). Today, it is the heavy lab equipment that is reserved for special analytical tasks requiring extra high precision, while Bruker’s handheld XRF approximates lab-level precision and is in the vast majority of cases exact enough to verify elemental composition and compliance.

The ‘Miniaturized’ X-Ray Tube

In the old days, a big and unwieldy XRF analysis instrument would contain a sealed radioactive source inside it, enabling the machine to bombard the test sample with X-ray photons. Natural or artificial radioactive isotopes originally served as handy sources X-ray radiation for elemental analysis. Their benefits include their compactness, relative cheapness, constant and continuous radiation, and independence of external power sources, and, importantly, their portability. Their main problem is their unsafety, with ensuing restrictions on handling and use. The sealed source approach eventually impeded the development of portability, because only a few source types have decay patterns compatible with use in portable applications. Today’s Bruker XRF guns use a miniature internal X-ray tube instead. There are salient benefits to using X-ray tubes, as against sealed radiation sources, namely:

  • Finite half-life requires periodic source replacement.
  • Devices containing radioactive materials are heavily regulated, must be registered, require special care and special licenses to operate. Safety restrictions also apply to X-Ray tubes, but not at times when those are turned off and inactive.
  • A statically embedded radiation source needs shielding, which also imposes limitations on brightness. An X-ray tube is brighter.
  • X-ray tube spectra are more readily optimizable, and better at exciting the lower atomic number elements.

XRF Detectors

S1 Titan XRF Detector & X-Ray TubeThe central moment in the evolution of XRF analysis techniques and of “minituarization” was the rise in the 1970s of the noncryogenic semiconductor Si(Li) detector. Now an XRF analyzer no longer required a cryostat with liquid nitrogen. (The most impactful of these new detector types – the Si(PIN), with its improved performance – still used in model 300 of the Bruker S1 TITAN™.) A further step was the emergence in 1983 of the Silicon Drift Detectors. The SDD, like other solid state XRF detectors, measures the energy of a detected X-ray photon by the ionization it produces in the detector. The SDD’s innovation is that it uses as its detector material a high purity silicon wafer with an extra low leakage current. (Single-stage Peltier cooling suffices thanks to it.) A transversal field created by a set of ring electrodes makes the charge carriers ’drift’ to a small collection electrode. This XRF analysis breakthrough significantly boosts count rates. Bruker’s S1 TITAN models 800 and 600 use our revolutionary latest-generation large area CUBE™ SDD detector, resulting in the most powerful performance and the lowest LoDs.

More Information

Please feel free to browse our extensive library of articles on various handheld XRF applications. Industries face a variety of specific elemental analysis requirements, depending on the materials and elements analyzed and the expected accuracy of quantification. Contact Bruker Inc. with any relevant questions!

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