What is X-Ray Fluorescence (XRF)

X-ray fluorescence, or XRF, is a technique for analysing the different elements of a broad range of materials.

They include metals, plastics and minerals.

The aim of this piece explain what XRF is, how and why people use it, its advantages and disadvantages, the equipment involved, and look at the kind of data it produces.

What is XRF?

X-ray fluorescence involves the use of specialist instruments to analyse different materials by directing x-ray beams at them.

The fluorescence part of the name refers to the radiation that certain substances produce when they receive the x-ray beams.

XRF is an analytical technique, and the x-rays give different readings depending on what elements exist in the material being analysed.

This form of analysis is spectroscopic:

  • Spectroscopy measures the interaction between matter, or material, and the electromagnetic spectrum
  • The electromagnetic spectrum consists of the different wavelengths of visible light, radio waves and x-rays
  • Electromagnetic radiation is the energy which takes these various forms and it occurs when atoms absorb energy, which changes how their electrons behave.

Everything is made of atoms, therefore all materials should give off electromagnetic radiation.

This, therefore, provides a means of analysing what is in them.

XRF can provide both a quantitative and qualitative analysis of elements, in concentrations that range from parts per million (ppm) to higher amounts.

This analysis can work with solids, liquids and powders.

There are two forms of XRF spectroscopy:

  • Energy dispersive (EDXRF), and
  • Wavelength dispersive (WDXRF)

We will examine the differences between these analytical methods later.

What is XRF Used For?

Because XRF enables the simultaneous detection of elements, it is well suited to rapid qualitative, quantitative and semi-quantitative analysis of materials.

It can detect concentrations from 100 per cent down to below parts per million.

With appropriately configured spectroscopic instruments, XRF can cover a huge range of elements, from sodium to magnesium, and even metals as light as beryllium.

Who uses XRF?

XRF is a non-destructive analytical technique, and doesn’t harm the material being analysed.

That makes it a suitable technique for a wide variety of industries and disciplines.

These include:

  • Mining
  • Food and drink
  • Continuous process industries
  • Environment
  • Forensic science
  • Pharmaceuticals
  • Medicine
  • Recycling

Research labs and quality controllers use XRF techniques.

These analytical techniques are important in teaching, in ensuring safer formulations of pharmaceuticals and the quality and safety of polymers used in various industries.

What is the difference between WDXRF and EDXRF?

The two methods of XRF are wavelength dispersive (WDXRF) and energy dispersive (EDXRF).

What makes these two methods different is how they detect and analyse fluorescent x-rays.

WDXRF is a detection system that separates the x-rays by wavelength.

It does this by directing the x-rays to a crystal, which then sends the rays in different directions according to their wavelengths.

There are two systems for using this method:

  • Sequential – this places a detector at a fixed position and the crystal rotates, allowing the detector to pick up different wavelengths one after another, building up the XRF spectrum
  • Simultaneous – this method uses several detectors or crystals at the same time, detecting a range of wavelengths

Wavelength dispersive XRF offers a higher resolution, making it suitable for analysing more complex samples.

However, the WDXRF method is less efficient because it involves more components, which it offsets with a higher powered x-ray.

In practical terms, this makes WDXRF systems more expensive and more complex to use.

EDXRF measures the different x-ray energies emitted from a sample directly.

This method generates an XRF spectrum by counting and plotting the relative x-rays at each energy point.

It is a rapid method, repeating the analysis at a high rate before sorting the results into different energy channels.

Compared to WDXRF, the energy dispersive method can acquire an entire XRF spectrum pretty much all at the same time.

This means it can detect most elements from the periodic table in just a few seconds.

What is the difference between XRF and XRD?

XRF is x-ray fluorescence, while XRD is x-ray diffraction.

The principles behind XRD are different to those of XRF.

XRF can analyse the different elements occurring in a sample material, but not how they are combined.

On the other hand, XRD will reveal details about the crystallography of materials.

Crystallography is the science of the structure of atoms in crystalline solids.

Like XRF, XRD is a non-destructive analysis method but unlike the former it identifies single crystals, rather than elements.

The process is similar, involving x-ray beams and a detector, but what XRD measures is different.

XRF and XRD are complementary methods, and there are combined XRF and XRD instruments.

What can XRF detect?

X-ray fluorescence can detect and measure most elements in the periodic table running from Uranium, the heaviest element, all the way to lighter elements such as magnesium and beryllium.

This means XRF can determine the elemental composition of any material.

For a more specific, sensitive result, WDXRF is the preferred method but EDXRF will measure a broad range of elements in each sample simultaneously.

As a versatile method of analysis, XRF is widely used for different sectors and disciplines. Here are some examples:

  • It is used for multi-element analysis of rock, soil and sediment samples in environmental geochemistry
  • XRF can provide qualitative analysis of paint samples
  • It has also been used in the clinical, biological and pharmaceutical analysis of elements occurring in herbal medicines
  • XRF is a useful research and development tool in the construction industry, where it can determine the make-up of metal alloys and cement.

There are safety and quality control issues which XRF can help to address, as well as key areas of product development and testing.

What are the advantages and disadvantages of XRF?

As a common x-ray technique, XRF can measure a wide range of elements, such as the percentage of metals within inorganic materials.

Here are its main advantages:

  • EDXRF is a rapid process, making it a useful research and development tool
  • It is generally non-destructive, so you can test sample materials without posing any risk to them
  • XRF allows for simple and fast sample preparation, and it has low running costs. It does not involve applying any gasses, liquids or acids in the testing process, which means it is a highly adaptable technique for use on-site in different locations. This is especially valuable for production line testing.
  • XRF spectroscopy instruments require no daily re-calibration.

There are, however, limitations to XRF as an analytical method which could, in certain circumstances, be disadvantages:

  • The reliance of EDXRF on quantity can limit measurements, with 5ml to 10 ml typically being the typical volume required for best performance
  • It can also face limitations in measuring lighter elements
  • As an alternative, WDXRF can handle more complex samples, but it is more expensive, more time-consuming and requires more expertise to operate.

How does XRF work?

X-rays are part of the electromagnetic spectrum, with wavelengths between 0.01 and 10 nm (nanometers).

The German engineer and physicist Wilhelm Röntgen reported that x-rays could penetrate matter in 1895.

This led to their widespread medical use.

However, during the interaction between x-rays and matter, part of the x-ray is absorbed.

Absorption causes fluorescence, the radiation the matter produces.

This becomes the basis for the XRF process:

  • The x-ray beam affects the electrons surrounding an atom in the sample material
  • This causes an unstable state of high energy
  • In the process of regaining its stability, the atom displaces high energy electrons, which release excess energy as a result
  • This energy is emitted as fluorescence

This entire process occurs in fractions of a second.

The measurement of energy from the electron that is displaced, then replaced, is different depending on each specific element.

In other words, the emitted fluorescent x-ray is like an identifying signature.

XRF continues to develop, broadening the information this method can obtain from sample materials.

Portable XRF technology especially is developing and improving, with handheld HRF spectrometers enabling the testing and chemical analysis of many varied materials in different conditions and locations.

We will look at the XRF process in greater detail in the next section.

What kind of equipment does HRF require?

HRF equipment is usually consists of either benchtop lab instruments or handheld spectrometer devices.

In either case, the process is essentially the same.

  • The instrument beams an x-ray onto the sample material. This beam has enough energy to affect the electrons in the inner shells of the atoms in the sample.
  • This x-ray beam interacts with the atoms in the sample, displacing electrons from the inner orbital shell
  • There is a difference in the energy from the x-ray beam and the energy that holds the atom’s electrons in their orbital positions
  • When the x-ray beam’s energy is higher than the energy holding the electrons in place, it knocks the electrons out of orbit
  • This leaves vacant spaces, and to fix this instability the electrons from a higher orbit move down to the lower position. This in turn causes fluorescence.
  • The disparity between the higher orbiting electron’s energy before it moves down, and after, is determined by the distance it travels
  • Each element’s orbital electrons have unique distances between them, which makes the fluorescent energies they emit unique
  • The XRF instrument detects these unique energy emissions, to then determine the quantity of each element present in the sample material

The time this process takes will depend on the percentage levels of elements within the sample.

Reading higher percentages can take only seconds, while calculating parts per million (ppm) may take several minutes.

What are the Components of an XRF Analyser?

In spectroscopy, there are two fundamental components that an XRF analyser will have to function effectively and efficiently:

  • An x-ray output to emit beams onto the sample material
  • A sensitive detection mechanism to measure and analyse the fluorescent x-rays the sample emits.

XRF analysers typically include an x-ray source, a detector, a digital signal processor and a central processing unit.

This enables the operator to quickly perform the process and read the results.

How do I use an XRF analyser?

Handheld XRF analysers are a portable means of carrying out x-ray fluorescence.

The handheld XRF instrument emits x-rays from its lens, or sample window.

This is a focused beam capable of passing through many materials without losing strength.

For this reason, operating the instrument requires safety awareness.

The operator should only hold it by its handle, and only use if for analysing materials that are positioned on a surface.

The analyser should never be pointed at a sample held in someone’s hand.

The operator should keep their hands clear of the lens and the front area of the instrument.

That is because when the beam is emitted, a small amount of the x-rays will be scattered back towards the instrument.

However, modern handheld analyser designs offer the highest level of protection against back-scattered x-ray radiation.

The alternative to a handheld model is the benchtop XRF analyser.

This type of analyser is less portable, but still relatively lightweight.

It provides speedy analysis of samples, combined with precision screening.

The Rigaku Nex-CG, for example, can perform routine analysis, but also screen samples with totally unknown makeup, such as oil slurries.

This is especially useful in the analysis of incoming waste in the waste industry.

Benchtop XRF analysers are closed-source x-ray spectrometers, which means they operate with full x-ray shielding that guarantees safe operation.

Different models of analyser are ideal for different purposes, across a variety of industries and sectors.

What training do I need to use an XRF analyser?

Both handheld and benchtop models of spectrometer are designed with ease of use in mind.

However, some training is necessary to ensure safe operation of these instruments.

There are also some fundamental safety aspects, which you must consider, when operating an XRF analyser:

  • Proper use – you must all types of XRF instrument in the right way
  • Controlled areas – be clear where you will operate the instrument, and ensure that no one can be in the path of the x-ray beams
  • Storage – keep instruments in a safe, secure area
  • Prevent possible exposure – ensure proper training and safe working practices

We provide specialist training seminars for XRF and other forms of elemental analysis.

Check our seminars and training section for details.

Is XRF data easy to analyse?

XRF analysers can determine the chemical makeup of a sample by measuring the fluorescent, or secondary, x-ray the sample emits in response to an x-ray beam directed onto it.

Each element that is present in a sample will emit a characteristic set of fluorescent x-rays, unique to it.

This is the equivalent of a fingerprint.

The most important part of XRF analysis is being able to look critically at the data collected and displayed in a spectrum.

However to interpret this data properly, and easily, first requires that the XRF instrument has been properly calibrated.

Calibration will confirm that any measurements are accurate by measuring them against a pre-set standard.

Unlike other some forms of spectroscopy, XRF does not require regular re-calibration between operations, but it is advisable to re-calibrate instruments periodically to ensure accurate readings.

As for the data itself, this comes in three forms:

  • Quantitative
  • Semi-quantitative
  • Qualitative

XRF analysers have pre-programmed calibrations and settings to achieve these different measurements.

However, although modern XRF analysers can function at a point-and-shoot level, it is important to understand the differences between these forms of analysis, to then interpret the data properly and practically.

Are XRF results quantitative, semi-quantitative or qualitative?

What comes first is the purpose of the analysis.

It is important to obtain the data that is appropriate to your objectives.

The type of results you get will also depend on the sample itself.

Deciding what results you are looking for will be easier if you first understand what they measure and how they are different from each other.

It will also help you develop your XRF skills.

We will explore each from of data in more detail, but here is a brief outline of each:

  • Quantitative data is the absolute quantity of an element that is present in the sample you are analysing.
  • Semi-quantitative data compares data from samples to gain information about the relative quantities of elements in each
  • Qualitative data is number of counts of x-ray fluorescent energies specific to each element in a sample.

Quantitative data

In XRF quantitative data contains a number and a unit, which is usually ppm (parts per million) or the percentage weight of the element present in the sample.

There are instrument calibrations which convert raw, qualitative data into quantitative data.

These calibrations include:

  • Fundamental parameters method (FPM) – software algorithms based on a theoretical intensity of x-ray beams and other aspects of the process calculate element concentrations based on peak intensities
  • Calibration with standards – based on accurately known element concentrations, the instrument will come up with calibration curves which will then calculate concentrations of the sample’s x-ray fluorescent output. This method is only effective when analysing samples that have similar patterns.

Some calibrations are more heavily mathematics-reliant, while others are reliant on prior analysis of known samples.

Modern, advanced XRF instruments will calculate and report data quantitatively without additional input from the user, but it is important for the user to understand when this type of analysis will be most reliable.

There are various conditions necessary for successful quantitative analysis:

  • The sample must be homogeneous, with no different layers, impurities or other materials trapped in it
  • Calibration must be appropriate for the sample
  • There must be known concentrations of samples to test against
  • The sample must meet the right thickness conditions (see below)

For quantitative analysis, samples must have infinite thickness.

What this means is that they should be thick enough to ensure the main x-ray beam from the instrument will penetrate them, but not escape out of the other side.

Semi-quantitative data

Where the conditions are not right for extracting quantitative data, this provides an alternative.

Essentially, semi-quantitative data works by comparison.

By analysing data from sample to sample, the user can obtain data about relative element concentrations.

It means you can then see that sample A contains an approximate percentage more of a specific element than sample B.

Qualitative data

Qualitative data is also known as the raw spectrum in XRF.

It lets the user know which elements are present in a specific sample, but not how much of each element there is.

This data appears in a spectrum graph:

  • The graph’s x-axis represents the element-specific fluorescent energies
  • The y-axis represents the counts or pulses recorded

The graph shows peaks where the instrument has detected element-specific fluorescent energies.

When a peak is higher, it means there have been more counts of that specific fluorescence corresponding to an element.

Qualitative data answers the question: what is in the sample?

The raw spectrum is ideal for the rapid analysis of non-uniform or non-homogeneous materials.

Fundamentally, with qualitative data, what you see is what you get: if there is a peak present at a certain fluorescent energy level, then the corresponding element will be present in the sample.

More information about XRF

For more information about x-ray fluorescence and XRF techniques and instruments, and their applications, contact us.

Telephone +44 (0) 161 442 9963, email enquiries@scimed.co.uk or complete our online enquiry form.