X-ray Fluorescence: Non-destructive Elemental Analysis

Point a handheld device at a painting, a gold ring, a lump of ore or a child’s toy, press a trigger, and within seconds it tells you which elements are present and roughly how much of each. No samples are cut off, nothing is dissolved, nothing is damaged. That’s X-ray fluorescence (XRF), one of the most versatile tools in analytical chemistry, used by art conservators, archaeologists, scrap-metal dealers, geologists and customs officers alike.

The principle

XRF depends on the inner-shell electrons of atoms.

1. Excitation. A beam of high-energy X-rays hits the sample. If an X-ray photon has enough energy, it can knock an electron out of an inner shell of an atom, typically the innermost K shell (n = 1) or the L shell (n = 2).
2. Relaxation. This leaves a vacancy. An electron from a higher shell immediately drops down to fill it.
3. Fluorescence. The energy difference between the two shells is released as a photon of X-ray radiation. This is fluorescence: absorbing radiation of one energy and emitting radiation of another.

The emitted X-rays have energies characteristic of each element, because the gaps between inner shells depend on the nuclear charge. By measuring their energies, you identify the elements present; by measuring their intensities, you estimate how much of each there is.

Naming the emission lines

X-ray lines are named after the shell where the vacancy was:

• Kα: an electron drops from the L shell to the K shell
• Kβ: an electron drops from the M shell to the K shell
• Lα, Lβ: vacancies in the L shell filled from higher shells

Kα lines are usually the strongest and most useful for lighter and mid-weight elements; heavier elements are often identified by their L lines.

Moseley’s law: why each element is unique

In 1913, the young British physicist Henry Moseley measured the X-ray emission lines of many elements and found a simple relationship: the square root of the X-ray frequency increases steadily with the element’s atomic number, not its atomic mass.

√f ∝ (Z − σ)

where Z is the atomic number and σ is a small screening constant.

This was a landmark discovery. It showed that atomic number is the fundamental property that orders the periodic table, confirmed the correct order of elements like cobalt and nickel (whose atomic masses are out of sequence), and predicted missing elements. It’s also the reason XRF works: each element emits X-rays at energies determined by its atomic number. Moseley was killed in the First World War in 1915, aged 27. See the history of the periodic table.

Typical Kα energies

Element	Atomic number	Kα energy (keV, approx.)
Silicon	14	1.74
Calcium	20	3.69
Titanium	22	4.51
Iron	26	6.40
Copper	29	8.05
Zinc	30	8.64
Silver	47	22.2
Lead	82	75.0 (Lα about 10.55)

Energies rise steadily with atomic number, as Moseley found. Because heavier elements need much more energetic X-rays to eject a K-shell electron, analysers often identify them by their lower-energy L lines instead.

Two types of XRF instrument

Energy-dispersive XRF (EDXRF)

A semiconductor detector (often a silicon drift detector) measures the energy of each incoming X-ray photon. The instrument builds up a spectrum of counts against energy, with a peak for each element.

• Fast, compact and relatively inexpensive.
• Used in handheld and benchtop analysers.
• Detects many elements at once.

Wavelength-dispersive XRF (WDXRF)

A crystal diffracts the X-rays, separating them by wavelength before they reach the detector.

• Better resolution and lower detection limits.
• Larger, more expensive laboratory instruments.
• Used for high-precision analysis, for example of cement, glass and geological samples.

Strengths of XRF

• Non-destructive: ideal for irreplaceable objects like paintings, manuscripts, coins and archaeological finds.
• Minimal sample preparation: solids, powders and liquids can often be analysed directly.
• Fast: a handheld reading can take seconds.
• Multi-element: detects most elements from about sodium or magnesium to uranium in one measurement.
• Portable: handheld units can be taken into the field, museums, scrapyards and customs ports.

Limitations

• Light elements are hard to detect. Elements lighter than sodium (such as carbon, nitrogen and oxygen) emit very low-energy X-rays that are absorbed by air and the detector window. Hydrogen, helium and lithium can’t be detected at all.
• It analyses the surface. X-rays only penetrate a short way (micrometres to millimetres, depending on the material), so coatings, corrosion and paint layers can give misleading results about the bulk.
• Matrix effects. Other elements in the sample can absorb or enhance the X-rays of the element of interest. Accurate results need calibration with standards of similar composition.
• Detection limits are typically in the parts-per-million range, less sensitive than ICP-MS.
• Overlapping peaks between elements can complicate interpretation.

Uses of XRF

• Art and cultural heritage: identifying pigments (lead white, vermilion containing mercury, Prussian blue containing iron), detecting later repairs and spotting forgeries that use pigments not available in a painting’s supposed era.
• Archaeology: determining the composition of bronze, glass and ceramics, and tracing where obsidian tools came from.
• Metal recycling and alloy identification: instantly sorting stainless steel grades, aluminium alloys and precious metals.
• Jewellery and precious metals: checking gold purity (carat) without damaging the item.
• Mining and geology: measuring ore grades on site.
• Consumer safety: screening toys, electronics and paints for restricted elements such as lead, cadmium and mercury.
• Environmental: screening contaminated soils for heavy metals.
• Space exploration: X-ray instruments on Mars rovers have analysed the elemental composition of Martian rocks and soils.

Safety with handheld analysers

Handheld XRF analysers contain a small X-ray tube, so they produce ionising radiation while the trigger is pressed. They’re designed with shielding and interlocks, and in normal use the dose to the operator is very low, but they must never be pointed at people, and operators are trained and often required to be licensed. In many countries, the devices are registered with radiation safety authorities.

XRF vs X-ray diffraction

Don’t confuse XRF with X-ray diffraction (XRD):

• XRF tells you which elements are present.
• XRD tells you how atoms are arranged in a crystal, identifying specific compounds or minerals by their crystal structures.

A sample of titanium dioxide gives titanium and oxygen (if detectable) in XRF, while XRD can distinguish between its different crystal forms, rutile and anatase. The two techniques are often used together.

Key takeaways

• XRF knocks out inner-shell electrons with X-rays and measures the characteristic X-rays emitted when the vacancies are filled.
• Emission energies depend on atomic number (Moseley’s law), so each element has a unique fingerprint.
• Energy-dispersive XRF is fast and portable; wavelength-dispersive XRF is more precise.
• XRF is non-destructive and multi-element but struggles with light elements and only analyses the surface.
• It’s used in art, archaeology, recycling, mining, consumer safety and planetary science.