Handheld XRFs have been a mainstay in the Positive Metals Identification (PMI) sector for a long time. In recent years, increases in performance have extended the reach of these instruments while handheld LIBS analysers have also raised their heads and demonstrated they have a place in the industry. Handheld instruments have become very popular due to their small form factor, portability and ability to rapidly identify materials non-destructively without dragging them back to the lab.
PMI is a critical aspect of asset management in an array of industries such as oil and gas refining, chemical processing, power generation, aerospace and marine. PMI is also used in manufacturing to ensure the right alloy stock is being used for the application in question.
The question is often asked, which technique is better? In this article we will look at both technologies/instruments to see how they best fit your application.
The first commercially available handheld XRF’s became available in 1994, freeing lab technicians and geologists from the confines of their labs which housed larger benchtop and floor standing systems. Since then, the systems have become smaller, lighter, faster, with longer battery life and new capabilities adding to their usability and there are now a vast number of manufacturers of these instruments. The newest system on the market is the SciAps X-550, which continues to raise the bar in terms of performance and features as well as having a slim form factor so it can access tighter spaces.
Handheld XRFs work in a similar way to their larger brethren, exposing samples to a high energy X-ray beam which ionises the sample by dislodging electrons from low energy orbitals, (typically K and L orbitals). These vacancies are filled by electrons from higher energy outer orbitals. This movement results in the release of X-rays of a given wavelength, which are characteristic of the element/atom affected. Thus, analysis of the resultant spectrum by an energy dispersive X-ray spectrometer enables the identification of the elements present. The intensity of the resultant peaks is proportional to their abundance, making quantitative analyses possible and hence identification of the alloy in question.
These instruments can effectively measure all elements from magnesium to uranium. They have deservedly grown a reputation for being able to accurately analyse most metals and alloys, especially those containing high levels of alloying transition metals or refractory metals e.g. stainless steel, titanium, nickel, cobalt based alloys as well as special alloys based on zirconium, tungsten or tantalum. More recent systems, like the X-550, are now able to quickly and accurately analyse aluminium and magnesium alloys where older systems struggled to analyse such alloys in reasonable timeframes.
Light elements have been traditionally difficult to measure using XRF, and moreso portable XRF. This is because the fluorescent X-rays generated by said light elements have such low energy that they struggle to travel through the air and reach the detector. The development of the silicon drift detector (SDD) has alleviated this limitation. However, elements lighter than magnesium are still beyond the capabilities of handheld XRFs.
While the LIBS (Laser Induced Breakdown Spectroscopy) phenomena was discovered in the 1960’s shortly after lasers were invented, the first (bulky) commercial system came into being in the 1990’s. Handheld devices however, have only entered the market in recent years.
The physics of LIBS analysis is similar to more traditional spark OES. In both cases a tiny amount of material is vapourised from the surface using a high energy source (electrical spark or pulsed laser). In the case of LIBS, a tiny plasma plume is generated. As the resultant ions decay back to their ground state, they emit light of a characteristic wavelength. This light interacts with a diffraction grating that splits it into its component wavelengths, which are then analysed by a spectrometer. Given that each element has its own characteristic wavelength, the composition of the sample can be quantified.
Using a laser was key to the miniaturisation of the technology due to its energy efficient nature which can be powered by a battery. Generating an electrical spark on the other hand requires a lot of energy. The laser spot size is also much smaller than the spark site, requiring far less (approx. 1000x less) argon. So, where a spark OES might require an argon gas bottle, handheld LIBS can be served by a tiny cannister, akin to those used in soda siphons. Another contributing factor was the miniaturisation of the detector. Despite their diminutive size, side-by-side tests have shown that both techniques produce the same results.
The argon gas used in systems like the SciAps Z200 C+ purges the measurement region ensuring any extraneous atmospheric species are excluded from the analysis. These systems also typically analyse multiple spots in a single measurement to guarantee accuracy.
This type of instrument has really come to the fore in the last few years as it has the ability to accurately measure carbon content, something which XRF is not able to do. The most sensitive instruments are able to distinguish L, H and standard grades of stainless steels (e.g. 316L with less than 0.03% carbon, 316 and 316H) and other carbon steels. Furthermore, inbuilt algorithms allow technicians to determine carbon equivalents (CE) which is important for in-field asset management where weld repairs are required e.g. pipeline maintenance.
Handheld LIBS instruments also excel at measuring alloying elements present in low concentrations. This includes elements such as nickel, chromium and copper which are used in small quantities in carbon steels for petrochemical and nuclear power plants.
LIBS has the advantage over XRF in that it is able to measure light elements as well as carbon, so the analysis of alloys containing lithium, beryllium and boron is now possible.
LIBS is also the ideal solution for analysing sulfidic corrosion in refineries where it can measure silicon contents below 0.02% in less than 3 seconds. It is also perfect for determining Flow Accelerated Corrosion (FAC) requiring measurement of chromium at levels less than 0.03% in just seconds with no need for X-ray radiation.
Although a relative newcomer to the market, the handheld LIBS technique has been recognised as an accepted method by the American Petroleum Institute for measuring carbon and other alloying elements in steels and stainless steels in accordance with API Recommended Practice 578 (3rd Edition). In this application, carbon content and carbon equivalents are measured in preparation for weld repairs. Closer to home, First Gas (NZ) have also developed their own workflow using handheld LIBS for the same purpose prior to affecting weld repairs and hot taps to their gas pipeline, negating the time-consuming need to send samples back to the lab.
In both cases, the instruments have in-built libraries/databases allowing instantaneous metal/alloy identifications to be made. Some instruments also feature cameras so specific regions can be targeted for analysis and the ability to record images, as well as GPS so measurements can be matched back to specific locations. For convenience, some also include WiFi and Bluetooth communication so measurements can be transferred in real-time back to another PC or central lab. Other features that may be of use are customised automated reports, automated backup of data to the cloud and merging of data from multiple instruments, including XRF or LIBS, even from different manufacturers.
Both techniques have the relative strengths and weaknesses making them more complimentary than competitive for positive metals identification. So it comes down to what you are needing to analyse. In short, if you need to measure alloys containing carbon and low alloys steels, in particular if you need to differentiate L and H grade stainless steels, LIBS is for you. For pretty much everything else, XRF is the better choice. If you don’t know what you are dealing with, you are best off taking both instruments into the field with you using something like the “one box solution”.