Hand-held X-Ray Fluorescence (XRF)

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Exploration Technique: Hand-held X-Ray Fluorescence (XRF)

Exploration Technique Information
Exploration Group: Field Techniques
Exploration Sub Group: Data Collection and Mapping
Parent Exploration Technique: Data Collection and Mapping
Information Provided by Technique
Lithology: Bulk and trace element analysis of rocks, minerals, and sediments.
Hand-held X-Ray Fluorescence (XRF):
Hand-held X-Ray Fluorescence is a portable analytical technique derived from the instrumentation used in traditional lab-based XRF analysis. The technique is used for bulk chemical analysis of rock, mineral, and sediment samples in the field. The technique depends on the fundamental principles of x-ray interactions with solid materials, similar to XRD analysis. XRF analysis is one of the most commonly used techniques for major and trace element analysis, due to the relative ease and low cost of sample preparation.
Other definitions:Wikipedia Reegle

X-Ray Fluorescence is a (relatively) non-destructive bulk chemical analysis technique routinely applied to rock, minerals, sediments, and fluids. Traditional lab-based XRF analysis is one of the most commonly used methods for measurement of major and trace elements. The first hand-held XRF analyzer was commercially released in 1994, allowing for these analyses to be performed rapidly in the field. Hand-held XRF devices have since seen widespread use in mining/mineral exploration for ore grade control and mapping, in the metals industry for scrap sorting, and in environmental studies for hazardous materials identification, to name a few. Devices are intuitive and easy to use, generally resembling a gun that is placed in contact with the material of interest. XRF analysis is performed by pulling a trigger, and is typically completed within a few seconds.

Photo showing the DELTA Handheld XRF analyzer (and operator). Photo from the Olympus Corporation merchant website.[1]

Related Techniques
Portable XRF analysis depends on the fundamental principles of electron beam and x-ray interactions with solid materials, similar to other analytical techniques.[2] Other techniques that operate on these principles include X-Ray Spectroscopy (through Energy-Dispersive X-ray Spectroscopy (EDX)) and Wavelength Dispersive Spectroscopy (WDS) typically performed using a SEM or EPMA, and X-Ray Diffraction (XRD) analyses.

Field Procedures
Analysis is performed by placing the analyzer in contact with the sample and pulling a trigger to activate the x-ray/gamma-ray beam. Analysis of major and trace elements typically takes only a few seconds, allowing for numerous analyses to be performed over the course of a single field session. Data can be displayed on a built-in screen or a personal field computer for most devices, and can be exported for further interpretation.

Physical Properties
During analysis, materials are excited using a high-energy incident beam of short wavelength radiation (X-rays or gamma rays) and become ionized. Inner electrons are ejected from lower energy orbitals (usually the K and L orbitals), making atoms in the sample unstable until the electron holes are filled by electrons from a higher energy outer orbital. When an electron moves from a high energy orbital to a lower energy orbital, energy is released in the form of X-rays that are characteristic of the type of atom present. Continuous ionization of the sample and absorption of energy from the incident beam allows for analysis of the complex X-ray spectrum emitted by the excited material. An Energy Dispersive X-Ray (EDX) Spectrometer is used to measure the energy intensity of the different wavelengths of the emitted X-ray spectrum as it is detected. Peak intensities measured at different wavelengths are characteristic of each element, and are proportional to the abundance of the elements present in the sample.[2] The exact quantitative abundance of each element is determined by comparing these data to mineral and rock standards of known composition.

Schematic diagram illustrating the interaction of incident X-rays and electrons with different energy levels in the material being analyzed. Diagram from the Tawada Scientific X-Ray Fluorescence website.

EDX spectrometers are used in portable XRF devices because of their smaller size and simpler design compared to the WDS spectrometers used in laboratory instruments. Miniature X-ray tubes or gamma ray sources can also be used with EDX spectrometers, making them cheaper and allowing for miniaturization that enhances portability (most commercial handheld XRF devices weigh less than 2 kg). Despite these advantages, XRF devices that utilize EDX spectrometers are less accurate than laboratory XRF analyzers, due to their lower resolution and problems with lower count rates and long dead-times. This being the case, laboratory XRF instruments should be used for high precision bulk elemental analysis.

Schematic diagram showing the instrumentation of a typical hand-held XRF analyzer. Diagram from the Tawada Scientific X-Ray Fluorescence website.

Data Access and Acquisition
Bulk elemental analysis by XRF can detect trace elements (Ba, Ce, Co, Cr, Cu, Ga, La, Nb, Ni, Rb, Sc, Sr, Rh, U, V, Y, Zr, and Zn) present in abundances >1 ppm in a sample, with typical detection limits on the order of a few ppm. Abundances of major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P) in rock and sediment samples are also detected in the same analysis. [2]

Potential Pitfalls
For the results of any single handheld XRF analysis to be considered representative of the rock, mineral, or sediment under investigation, specific site selection on the sample and an awareness of the spot size analyzed by the instrument are critical. For example, the bulk chemistry of a rock consisting of orthoclase, biotite, and quartz might be measured to contain only K, Al, and Si if a large orthoclase crystal were measured, whereas a different spot on the same sample with representative proportions of all three minerals would also contain Mg and Fe. Failure to account for this during fieldwork can lead to quality control issues throughout a dataset.

Additional limitations of handheld XRF analysis include:

  • The elements detectable by XRF analysis are typically limited to the range between Magnesium and Uranium for most commercially available instruments.
  • Portable XRF devices utilize EDX spectrometers, which are less accurate than laboratory XRF instruments that use WDS spectrometers.
  • XRF analysis cannot distinguish between isotopes of the same element, so additional analysis using other instruments is necessary if this type of data is desired.
  • XRF analysis cannot distinguish ions of an element in different valence states (e.g. Fe2+ from Fe3+).
  • Relatively large sample sizes are needed for analysis, usually in excess of 1 gram.

  1. DELTA Education & Research Handheld XRF Analyzer
  2. Cite error: Invalid <ref> tag; no text was provided for refs named X-Ray_Fluorescence_.28XRF.29

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