How XRF Works – X-Ray Fluorescence Spectroscopy

X-ray fluorescence (XRF) is a non-destructive analytical method used to determine elemental concentrations in various materials.

XRF works by striking a sample with an x-ray beam from an x-ray tube, causing characteristic x-rays to fluoresce from each element in the sample. A detector measures the energy and intensity (number of x-rays per second at a specific energy) of each X-ray, which is transformed into an elemental concentration using either a  non-standard technique such as fundamental parameters or user-generated calibration curves.

The presence of an element is identified by the element’s characteristic X-ray emission wavelength or energy. The amount of an element present is quantified by measuring the intensity of that element’s characteristic X-ray emission.

XRF – Step by Step


The Atomic Level

atom-basic-configuration
ONE – All atoms have a fixed number of electrons. These electrons are arranged in orbitals around the nucleus. Energy Dispersive XRF (EDXRF) typically captures activity in the first three electron orbitals, the K, L, and M lines.
atom-basic configuration showing-shells
TWO – These electrons are arranged in orbitals around the nucleus. Energy Dispersive XRF (EDXRF) typically captures activity in the first three electron orbitals, the K, L, and M lines.
atom_irradiated_electrons
THREE – The primary photons from the X-ray tube have high enough energy that it knocks electrons out of the innermost orbitals, creating a vacancy (1). An electron from an outer orbital will move into the newly vacant space at the inner orbital to regain stability within the atom (2).
atom-electron-fluorescence-closeup
FOUR – As the electron from the outer orbital moves into the inner orbital, it releases energy in the form of a secondary X-ray photon. This energy release is known as fluorescence. All elements produce fluorescence “characteristic” to themselves. Each element’s fluorescence is unique to itself.

The XRF Instrument Level

xrf-on-a-sample-1
FIVE – High-energy primary X-ray photons are emitted from an X-ray tube and strike the sample.
xrf-on-a-sample-2

SIX – The fluorescent energy is transferred to a detector, where it is absorbed and transferred into an electrical signal and then into a number (digitized).

xrf results screens

Results can be viewed in the form of percentages, or as spectrum. The XRF will process (digitize, count) about 200,000 or more x-rays every second. These detected x-rays form a spectrum. Each peak in the spectrum is from a characteristic x-ray that was emitted by a specific element, like Cr, or Ni, etc.

The height of the peak is proportional to concentration of the element. The peak height is converted to a percentage or ppm of that element via a calibration method – either fundamental parameters or factory or user-derived empirical calibrations (see below).


Interference

Elemental analysis techniques experience interferences that must be corrected or compensated for in order to achieve adequate analytical results.

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.

Quantitative elemental analysis

XRF Spectrometry uses Empirical Methods (calibration curves using standards similar in property to the unknown) or Fundamental Parameters (FP) to arrive at quantitative elemental analysis.

FP is 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.

FP, accompanied by stored libraries of known materials, determines not only the elemental composition of an unknown material quickly and easily, but can identify unknown material as well.

Spectrometers

SciAps uses the EDXRF Spectrometer technique because of 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.

Handheld EDXRF units contain all three, in a rugged, easy-to-use form factor.

Handheld, field-portable EDXRF units are taken directly to the sample, regardless of where the sample is–in a cave, up a mountain, in a lab, on a wall, in a manufacturing/processing plant. These units offer ease of use, rapid analysis time, lower initial purchase price, and substantially lower long-term maintenance costs.

SciAps X – the Premier Handheld X-ray Technology

1
The x-ray tube irradiates a solid or a liquid sample.

2
Atoms in the sample are struck with X-rays of sufficient energy, i.e. greater than the atom’s K or L shell binding energy, causing an electron to be ejected from the K or L shell level of the atom.

3
An electron in a higher shell fills the K or L level vacancy by emitting energy and “jumping down” to that lower energy level.

4
When the electron drops to the lower K or L shell level it emits a photon at a specific wavelength to the atom’s structure (a characteristic x-ray).

5
The emitted photons (X-rays) are measured by an energy-dispersive detector on the XRF analyzer. The detector and associated electronics measure the energy of each X-ray, and count the number of X-rays per second at that energy. An X-ray spectrum consists of energy along the horizontal axis and the intensity (#/s) along the vertical axis.

6
On-board processors use either standardless methods such as fundamental parameters or user-generated (empirical) calibration curves to relate the X-ray spectrum to elemental concentrations.