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German physicist Wilhelm Conrad Roentgen is noted for his accidental discovery of X-rays in 1895 during his analysis of cathode rays in high voltage, gaseous discharge tubes, but a way to use X-Rays for analysis was not established until 1913 by Henry Moseley1.
Moseley discovered the mathematical relationship between the wavelength of a characteristic X-ray photon and the atomic number of the exciting element, and in 1925 Dirk Coster and Yoshio Nishina were the first scientists to use primary X-rays, rather than electrons, to excite a sample2. Following these discoveries and many others, the potential of X-rays as an analytical technique rapidly increased, allowing for practical and commercial applications to be brought to market during the 1940s-1950s.
X-Ray Fluorescence (XRF)
The principle of X-ray fluorescence (XRF) originates from the ability of an electron to be ejected from its atomic orbital by a photon, or lightwave, of sufficient energy. The energy of this photon must be greater than the energy of the electron bound to the nucleus of an atom, in which its emissivity is based on the difference in energies between two electron orbitals3.
This difference in energies is produced when an electron from a higher energy level orbital is transferred to the lower energy level orbital upon electron ejection. Within any element, the energy difference between two specific orbitals is always the same; therefore the emission of a photo during this electron transition will have the same energy as well. In order to determine the identity of an element, the particular wavelength of fluorescent X-ray light emitted by an element is directly related to the amount of the analyte of interest within the sample3.
With various applications - pharmaceuticals, cosmetics, agriculture, plastics, rubbers, textiles, fuels, chemicals, metal finishing and refining, microelectronics, and environmental analysis - X-ray fluorescence offers these industries a powerful quantitative and qualitative analytical tool for the elemental analysis of materials4. The basic structure of any XRF spectrometer includes a source, a sample, and a detection system, where the source irradiates the sample, and the detector measures the fluorescence radiation emitted from the sample4.
XRF and Mining
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Several handheld XRF analyzers exist for mining and exploring purposes, in order to ensure a fast and accurate measurement for all aspects of mining. Examples of mining applications for handheld XRF devices include; core analyzers for
exploration drilling, mineral exploration, geochemical testing and mapping, waste processing and metal recovery, ore grade control, silver ore mining, and mineral lab alternatives.
Handheld XRF devices allow mining experts to make an on-site geochemical analysis of the mine face, drill core, or prepared samples, reducing the turnaround times compared to laboratory analysis of the samples. With the same capability and precision as other XRF instruments, handheld XRF allows operators to bring the analyzer to the sample rather than having to take the sample into the lab, which is especially useful when the test specimen is large or heavy.
The United Nuclear XRF Probe
The earliest handheld energy-dispersive XRF probe was built by United Nuclear in the early 1980s, which was originally used to investigate highly radioactive holding takes, as well as the presence of uranium in the soil. Weighing over 70 pounds, this tool comprised of a measurement head connected to a cart, where the electronics displayed the received data.
Modified between 1982-1983, United Nuclear commercialized the MAP-1 device, which had the capability to detect uranium, as well as other elements in the soil through the creation of a “front pack,” while also reducing the weight of the instrument to 50 pounds. The evolution of these instruments continued through the 1980s as United Nuclear developed MAP-2 and MAP-3 analyzers for enhanced lead detection purposes.
XRF For Commercial Use
In 1994 electric contracting company Amptek developed the XR-100 X-ray detector for commercial use. This thermoelectrically cooled and simple to use detector replaced the need for liquid nitrogen to cool detectors in many applications. The XR-100 device was selected for the Pathfinder Mission to Mars, where it successfully analyzed rocks and soil in a cost-effective and precise manner.
The first fully-handheld XRF detector was created in 1994 by Niton Inc., a Massachusetts-based company, in which this Niton XL-309 instrument offered intensified analytical performance at a lower price than the previous instruments on the market.
As interest in these analytical systems began to grow, the National Aeronautics and Space Administration (NASA) in conjunction with KeyMaster Technologies designed the first TRACER II unit, which included an argon transmission target. This aspect of the instrument allowed NASA and KeyMaster to create a portable vacuum XRF analyzer that had the ability to perform on-the-spot chemical analysis, which was a task previously only possible in a chemical laboratory.
The first applications of the TRACER II unit allowed NASA to quickly and accurately determine elemental composition on large objects, such as a rocket motor, which was a major breakthrough for this organization. Modifications to the TRACER II unit allowed increased sensitivity to specific metals, including a new ability to measure magnesium and aluminum content in aluminum alloys.
As demands for increased accurate and efficacious handheld XRF tools began to rise, competition between industries to produce grew as well. In 2008, Brucker Elemental produced Bruker XFLASHTM silicon drift detector (SDD) technology that was integrated into the first-ever SDD-based handheld XRF unit, also known as the S1 TRACER. Still a unit with one of the best resolutions, Bruker’s S1 TRACER allows the analysis of light elements, including magnesium, aluminum, and silicon in air, while also providing an increased concentration range for these elements of interest.
References and Further Reading
- X-Ray Fluorescence Analytical Techniques
- Overview of X-Ray Fluorescence
- Basic Theories
- Mining & Exploration Using Handheld XRF Analyzers