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The capacity to assess the properties of metal is valuable for all kinds of applications in research and industry.
In heavy industry, processes such as machining, welding, and fabrication are performed according to the properties of metal raw materials. In research, scientists determine the properties of a metal sample to do anything form fighting heavy metal pollution to developing next-generation electronics.
Traditional metal tests include methods such as the appearance test, spark test, and Rockwell hardness test. These tests are done with physical means and direct examination. More modern methods of metal testing rely on sophisticated sensors. Methods such as Optical Emission Spectroscopy (OES) and x-ray fluorescence (XRF) not only allow for a more comprehensive examination than traditional methods but can also perform examinations at higher throughput rates.
Below is a brief list of sensor-based methods that are used to analyze metals.
Optical Emission Spectroscopy (OES)
Also referred to as atomic emission spectroscopy (AES), OES is a method that uses high-energy light at an explicit wavelength to push atoms into excited electronic states, so that they emit light upon coming back down to their ground state. Every element emits light at a signature wavelength and a spectrometer is used to detect these spectral 'fingerprints'. The wavelengths of the emitted light can be used to determine the chemical nature of a sample, while the light's intensity is related to the number of atoms at each wavelength.
While this test may call for some grinding to create a necessary flat test surface, the test leaves a sample intact for further investigation.
Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
ICP-MS is a quantitative analytical technique that can analyze a broad array range of elements. The test involves using plasma to break down a sample into its basic atoms or ions. Sample ions are released from the plasma and pass to a mass spectrometer, where they are measured. Unlike with OES, ICP-MS involves the direct detection of metal atoms, as opposed to measuring the light they emit.
X-Ray Fluorescence (XRF)
Rather than forcing sample atoms into an excited state, an XRF spectrometer uses x-rays to excite sample atoms; with enough energy, they actually remove an inner-shell electron to produce a positive ion. Higher-energy electrons from the outer shell populate created gaps in the inner shell.
The surplus energy from these electrons is released as secondary x-ray photons, producing a signature fluorescence signal. The photons released by fluorescence have less energy than the original x-ray photons and, since the energy of the fluorescence is dependent upon differences in energy, it is chemically representative of the sample but not of its chemical form.
Sensors within an XRF device measure the intensity and amount of energy of secondary x-ray photons. These measurements are used to describe the properties of a metal sample.
Backscattering of the source radiation does take place and is considered a type of interference that should be mitigated or eliminated, possibly through the use of filters. Having the same wavelength as the source radiation, backscattered radiation does not interface with the sample, but it is spread from the test sample in every direction.
XRF systems are able to effectively test a wide range of metals, simply and in a non-destructive manner. An XRF can produce quantitative results, but this requires special preparations.
Scanning Electron Microscopy (SEM)
SEM involves using a beam of high-energy electrons to obtain live, high-resolution, three-dimensional imagery of a wide range of materials, including metals, at magnifications of up to 500,000 times. SEM can be used to study the topography and morphology of a metal sample.
SEM is another analytical method that is based on direct observation of a sample, not the radiation it emits. SEM is especially useful for very small sample amounts, such as a small amount of metallic dust or shavings.
References and Further Reading