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Sorting metals with a hand-held XRF analyser

Excite X-ray fluorescence from stainless steel, brass, solder, a gold ring and a handful of contaminated soil, assign every peak to an element from NIST line energies — and meet the notorious overlap of lead and arsenic.

Difficulty ●○○ ≈ 20 min

Aim Identify every element in each sample — no more and no fewer — from the positions of its Kα, Kβ, Lα and Lβ lines.
Line positions from the NIST database · every peak drawn the same height

X-ray fluorescence: naming elements by their lines

Fluorescence spectrum
45678910photon energy / keVcounts (schematic)

Which elements do you think are in the sample?

Peaks

  • 5.41 keV: not yet explained
  • 5.95 keV: not yet explained
  • 6.40 keV: not yet explained
  • 7.06 keV: not yet explained
  • 7.48 keV: not yet explained
  • 8.26 keV: not yet explained

0 of 6 peaks explained.

Line energies come from the NIST X-Ray Transition Energies Database (SRD 128). The X-ray tube is assumed to run at 40 kV and the detector to see 1–40 keV: elements whose Kβ₁ lies below 40 keV show Kα₁ and Kβ₁, and those whose Lα₁ lies above 1 keV also show Lα₁ and Lβ₁; the detector resolution is taken as 150 eV. Every peak is drawn the same height — real intensities depend on concentration, fluorescence yield and matrix absorption, and quantitative work needs calibration with standards.

Procedure

  1. Choose “stainless steel”. Start with the element you are surest of and see which peaks its lines land on.
  2. Keep adding elements until every peak is accounted for and nothing is left over.
  3. Analyse the brass, the solder and the gold ring in turn. Notice that heavy elements show only L lines: their K lines lie beyond what a 40 kV tube can excite.
  4. Finally analyse the contaminated soil. Choose lead alone, then arsenic alone, and see which peak each leaves unexplained.

What to notice

  • The stainless steel has six peaks: chromium at 5.41 and 5.95 keV, iron at 6.40 and 7.06 keV, nickel at 7.48 and 8.26 keV — a Kα–Kβ pair for each element.
  • The tin in the solder shows K lines at 25.3 and 28.5 keV as well as L lines near 3.4 keV; the lead shows only its L lines at 10.55 and 12.61 keV.
  • In the soil the peak at 10.55 keV could be lead Lα₁ or arsenic Kα₁: only the peaks at 12.61 keV (lead Lβ₁) and 11.73 keV (arsenic Kβ₁) show that both are present.
  • Add manganese by mistake: its Kα (5.90 keV) lands next to chromium Kβ (5.95 keV) but 48 eV away, and its Kβ has no peak — the analyser reports that there is no manganese.

In a real lab

  • A hand-held XRF analyser emits X-rays: keep the window pressed against the sample, never point it at anyone, and follow the instrument’s interlocks and dose rules.
  • XRF sees only the surface layer: plating, paint and scale make the reading describe the surface rather than the metal, so clean the spot first.
  • In air the low-energy lines of light elements such as magnesium, aluminium and silicon are lost; quantitative results need calibration with certified alloys.

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