Handheld LIBS – The Latest Success Story in Portable Analytical Instrumentation

Handheld LIBS – The Latest Success Story in Portable Analytical Instrumentation

One analyzer, every element

The Z-903 accomplishes what no other portable analyzer has done. It’s a handheld analyzer that measures every element in the periodic table – from H to U, in seconds. It comes with Profile Builder software for desktop or tablet, allowing users to define laser parameters like raster patterns and number of shots per location, choose argon or air- based analysis, build spectral analysis models and develop calibration curves.

If you have applications targeting specific elements, SciAps offers single-spectrometer units optimal for elements such as lithium, fluorine, carbon, beryllium and others.

LIBS technology has migrated from an obscure laboratory analysis method to a mainstream field analytical technique in recent years. LIBS—laser induced breakdown spectroscopy—utilizes a pulsed laser that vaporizes a small region of a sample approximately 100 um in diameter and 5-10 um in depth. The resulting plasma cools and emits light in the UV/Vis/IR range that is captured by one or more onboard spectrometers, depending upon the spectrometer range required.

The advancement in laser technology was the key to recent commercial field successes. A viable field LIBS system had to offer speed, be lightweight, operate with batteries and perform over a wide thermal range, while providing necessary high pulse energies and high pulse repetition rates. Developing an instrument that met all these challenges proved to be a time consuming process, but was ultimately successful. For example, the laser used in a SciAps device today delivers 6-8 millijoules/pulse at a 50 Hz repetition rate with a 2 ns pulse width over a temperature range of 25 – 45C. This type of laser performance has proven critical to achieve the field analytical performance of mobile spark optical emission (spark OES) or handheld X-ray fluorescence (HHXRF).

Handheld LIBS has met commercial acceptance in several critical areas. Perhaps the best example is in the field of carbon analysis in steel and stainless alloys. Many industries require the carbon equivalent value for steel for correct welding procedures, and low-carbon grade stainless steel is often specified for many chemical and energy processes. LIBS is the only handheld technique capable of making this demanding carbon measurement. In fact, the SciAps handheld LIBS is now accepted globally by most industries that are using, fabricating, or making steels and stainless, where it’s become a disruptive technology to traditional mobile spark OES systems.

 

 

Handheld LIBS first found commercial success in alloy analysis industries. It’s now being applied to electric vehicles, strategic metals, and environmental clean-up.

 

 

 

The green economy/electric vehicle sector offers an emerging success story. Handheld LIBS is the only field technique that measures lithium content in soils, ores and brines. As such, it’s rapidly becoming a vital tool in efficient exploration as the world seeks to expand Li production to meet the battery metals demand from the EV industry, and in recycling/recovery of used batteries.

Handheld LIBS uniquely offers flexibility not found with other field analytical techniques. Because LIBS technology can create emissions from every stable element in the periodic table, a well- conceived platform offers cost effective configurations for specific measurements. Rather than the expense of a full-range system that spans emissions from 190 nm – 950 nm, users can obtain custom- configured systems optimized for a specific emission range. Recent examples include a single spectrometer analyzer optimized in the 580 – 780 nm range to measure fluorine, as a tool for in-field screening for PFAS (per- and polyfluoroalkyl substances) in various packaging and consumer products. Other examples include optimized spectrometer ranges for beryllium measurements in soil and dust as part of site dean-ups, particularly at some United States aging weapons-making facilities, and suites of light elements such as fluorine, sodium and boron in a range of mineral samples.

Handheld LIBS has found a home in the academic world as well, driven largely by the dual-purpose capability of quantitative field measurement plus its value as a classroom teaching tool in many chemistry, physics, geoscience and metallurgy departments. A litany of interesting applications have evolved over the past 5-10 years by university researchers and government laboratories.

In summary, the past 5-10 years have witnessed the birth of handheld LIBS as a viable new portable analytical technology. The technology has achieved commercial success in traditional industries like chemical, petrochemical and oil/gas, as well as providing crucial field analysis for new industries such as electric vehicles, strategic metals and emerging environmental threats such as fluorine- containing organic compounds.

FIGURE 1: Approximate (conservative) detection limits obtainable with LIBS, highly dependent on the material and laser(s) and spectrometer used for analysis.

Thermo Fisher and Scantech are the leading suppliers of PGNAA instruments. PGNAA is particularly popular in the ce­ment industry. However, the relatively large size and radiation protection requirements render it a limited technique.

For traditional thin-section analysis, laser ablation ICP-MS (LA-ICP-MS) has become the standard. A femtosecond laser, short wavelength excimer laser, or wavelength-converted Nd:YAG laser is used to ablate material in micrometer-sized spots from the sample. The ablated material is transported to an ICP-MS instrument for sensitive elemental analysis. When used in conjunction with automated systems from suppliers like Teledyne CETAC, Elemental Scientific Lasers, and Applied Spectra, LA-ICP-MS, as a technique, provides the user with detailed micrometer-level analysis of geosamples, including full elemental composition. Moderately-sized samples can take in excess of 12 h to run because of the scanning speeds, and the cost of the entire system usually exceeds $400,000.

LIBS Implementation for Geomaterials

The variety of needs for measuring geomaterials, and the consid­erations of existing techniques noted above, allows ample room for using laser-induced breakdown spectroscopy (LIBS) for geoma­terial analysis. LIBS forms a plasma directly from material ablated from the surface of a sample. LIBS also excites elements across the periodic table, but preferentially excites lighter elements (atomic numbers 1-20) as well as first-row transition elements, which together make up the bulk of crustal materials. Detection limits are typically in the tens of parts per million (ppm) range, and are highly dependent on the matrix and the type of laser used in the application. Figure 1 shows a conservative estimation of detection limits over many of the elements in the periodic table.

A recent review by Harmon and Senesi in March 2021 identified 640 publications in the scientific literature on geomaterials analysis using LIBS (1). The reader is referred to this excellent source for details on particular configurations; the article focuses primarily on laboratory applications but has some suggestions on how LIBS can be applied outside of the laboratory as well.

Basic implementation choices include the choice of laser source for excitation and the choice of spectrometer for emission detec­tion. For nanosecond-duration laser pulses, shorter-wavelength lasers provide more effective ablation, whereas longer-wavelength lasers couple into the evolving plasma more effectively. For this reason, 193-nm excimer lasers and 213-nm Nd:YAG lasers are preferred among nanosecond-duration lasers for LIBS on glassy materials (including geomaterials) that are partially transparent. These lasers form weaker plasmas, but this is often more than compensated for by the improved material ablation.

The ablation process with a nanosecond laser is largely a ther­mal process, forming a heat-affected zone (HAZ) in the material. The thermal nature of the process means that the ablation is not stoichiometric, and the range of particulate sizes formed in the ablation and subsequent nucleation process have varying chem­istries. The formed particles are then subject to the analytical plasma, which has an effective temperature and electron den­sity that is a function of the laser pulse energy and wavelength. For these reasons the LIBS plasma emission must be calibrated independently for each material with a given system.

The ablation process with a femtosecond laser does not rely on thermal processes. Because the femtosecond laser pulse duration is much shorter than the characteristic vibration (phonon time) in the material, there is no time for heat transfer during the laser pulse. The result is much more stoichiometric ablation, with craters that look quite symmetric and smooth, with no HAZ. The plasma from a femtosecond laser forms following the laser-material inter­action and is typically weaker than the plasma from a nanosecond laser, exhibiting lower temperatures and electron densities. Thus, the plasma formed from the femtosecond laser also exhibits higher absolute detection limits. Because of the additional cost of a femtosecond laser, the cons of using a femtosecond laser typically outweigh the advantage of using them for stoichiometric ablation. As a result, femtosecond lasers are usually used only in the laboratory.

Spectrometers are the other key choice of hardware. Many applications can use the miniature crossed Czerny-Turner type spectrometer pioneered by Ocean Optics (now Ocean Insight) in the 1990s. Properly synchronized, spectrometers that cover the range of interest (as broad as 190-800 nm is typical) with at least 0.1 nm resolution are sufficient for most applications. The CCD or CMOS detectors typically have a minimum gate time of roughly 1 millisecond, so the spectrometer acquisition is triggered with a specified delay immediately following the laser pulse and the entire LIBS emission is collected. This results in a non-negligible background, which must be subtracted, and sacrifices some signal-to-noise (S/N).

In most cases, the alternative to banks of miniature spectrom­eters is an echelle spectrometer mated to an intensified CCD (ICCD) camera or an electron-multiplying CCD (EMCCD) camera, such as those from Andor Technology, Roper Scientific, or Raptor Photonics. Several companies offer echelle spectrometers that are widely used with LIBS, notably Catalina Scientific and LTB Lasertechnik. The echelles produce higher resolution than the miniature spectrometers, and the ICCD and EMCCD detectors are much more sensitive than the nonintensified CCD and CMOS detectors. This combination yields improvement in S/N and thus results in better detection limits. Downsides include a higher price and higher susceptibility to temperature fluctuations and vibrations than the miniature spectrometer solution.

Innovative Applications

One of the fastest-evolving applications of LIBS in geosciences surrounds elemental mapping of samples. A recent review paper summarizes a great deal of relevant literature (2). LIBS has some notable features compared with laser ablation (LA)-ICP-MS, as well as a few drawbacks. Positives are solid detection limits across the periodic table, particularly for several nonmetals and light elements that are not detected as efficiently by LA-ICP-MS. Simultaneous spectral acquisition means that transport-related issues in LA-ICP-MS and parameterization associated with se­quential (sometimes known as scanning) mass spectrometers are avoided. High-speed spectral acquisition means that full spectra can be acquired at up to kilohertz (kHz) rates. However, LA-ICP-MS still has superior detection limits for some elements (particularly beyond rowfourofthe periodictable) and for isotopes, which LIBS cannot detect without either very high-resolution spectrometers or molecular emission measurements.

FIGURE 2: Mineral map of a 20 x 20 mm sample acquired at 1000 Hz and with 50 µm resolution, from reference (2), shared under the terms and conditions of the Creative Commons Attribution (CC BY 4.0) license.

An example of such mapping is included in Figure 2, where the Curiosity LIBS system from Elemission was used to map 20 x 20 mm and larger regions of mineral samples with 50 pm resolution at 1000 Hz (3). The LIBS system was previously calibrated with a machine learning approach on a separate sample, with ground-truth concentration data provided by the Tescan Inte­grated Mineral Analyzer (TIMA), a scanning electron microscope- based analyzer. Tests showed good agreement on mineral mass contribution between TIMA and the LIBS analyzer after calibra­tion. Of particular interest and indicated in Figure 2 was the identification of braggite, a mineral containing platinum, palladium, and nickel. Lightigo has recently introduced the Fi re Fly LIBS chemical imaging and analysis system, which is equipped for resolution down to 10 pm, depth profiling, and data acquisi­tion rates up to 100 Hz.

FIGURE 3: Example UV spectrum from the Perseverance Mars rover SuperCam (4), with igneous rock (blue), smectite clay (orange), and magnesium-rich
carbonate (gray), from reference (5); insert shows the expanded spectral
region. The figure is shared under the terms and conditions of the Creative
Commons Attribution (CC BY 4.0) license

The ChemCam LIBS instrument has been mentioned in the press for nearly a decade since touching down on the surface of Mars on the Curiosity rover on August 6, 2012. The ChemCam LIBS in­strument quantifies elements and miner­als up to 7 m from the rover using LIBS. The ChemCam has a custom diode- pumped DIVA laser developed by Thales and three custom Ocean Optics (now Ocean Insight) HR2000 spectrome­ters. Substantial effort was made to obtain calibration data here on Earth under Mars conditions before and concurrent with the flight, and the raw and calibrated results from every spectrum can be found on the Planetary Data System hosted at Washing­ton University of St. Louis. This trove of data has been a gold mine for researchers test­ing different calibration methods for LIBS on geological samples.

Earlier this year, the next-generation SuperCam touched down with the Per­severance Mars rover. With a similar LIBS system combined with a Raman spectros­copy system, an improved color camera, and an acoustic monitor, SuperCam is providing new data. The design of the SuperCam was covered in the May 2017 issue of Spec-troscopy (4). Figure 3 illustrates an example UV LIBS calibration spectrum taken during testing. The resolution on the UV channel is 0.2 full width at half maximum (fwhm) or better (5).

The detailed information available from the LIBS spectra, enabled by the science that has allowed LIBS to be so successful on the Mars rovers, is quickly emerging in field geology and mining applications. SciAps advertises their Z-300 handheld LIBS analyzer for geological applications, and there are many additional handheld LIBS systems to choose from. Having a handheld LIBS instrument with the ability to measure and type geological samples in the field, including light elements, could save geologists from having to bring back pounds of rocks in their backpacks from field expeditions for laboratory analysis.

Perhaps even more exciting are appli­cations on bulk materials for conveyors or drill core analysis. The Ecore from Elemis- sion is one of many commercial examples; LTB Lasertechnik and Ocean Insight are among the companies that provide com­prehensive solutions for geological analy­sis. We have shared before our success in measuring coal elemental composition as well as coal properties (6). In mining and resource exploration, not only can ore composition be assessed, but many other properties can be teased out of LIBS data—examples from the literature

include kerogen hydrogen-to-carbon ratios, specific surface area, and rock hardness, among other properties (1). Particular measurements require special­ized setups, so whether the sample is in the measurement or in the field, it’s a key step to configure the laser, spectrometer, and sample preparation correctly for success.

The role of analytical instruments in geosciences is a combination of better understanding our natural world and op­timizing resource recovery to efficiently and cleanly fuel our economy. It is clear that from the laboratory to the field that LIBS has an increasing role to play in geosciences. We expect that LIBS will “rock on” for some time!

References

  • S. Harmon and G.S. Senesi, J. Appl. Geochem. 128, 104929 (2021) 104929.
  • Limbeck, L. Brunnbauer, H. Lohnin- ger, P Porizka, P. Modlitbova, J. Kaiser, P Janovszky, A. Keri, and G. Galbacs, Anal. Chim. Acta 1147, 72-98 (2021).
  • Rifai, Minerals 10(10), 918 (2020).
  • C. Wiens, S. Maurice, and F.R. Perez, Spectroscopy 32(5), 50-55 (2017).
  • C. Wiens, et al., Space Sci. Rev. 217(4) (2021). doi: 10.1007/s11214-020-00777-5
  • G. Buckley, Spectroscopy 30(1), 24-31 (2015).

Steven G. Buckley, PhD, is the Gen­eral Manager of the Applied Systems business at Ocean Insight, an affili­ate associate professor at the Uni­versity of Washington, and has started and advised numerous companies in spectroscopy and in applications of machine learning. He has approximately 40 peer-re­viewed publications and 6 patents. His work in practical optical spec­troscopy, such as LIBS, Raman, and TDL spectroscopy, dovetails with the coverage in this column, which reviews methods (new and old) in laser-based spectroscopy and opti­cal sensing. Direct correspondence to: SpectroscopyEdit@mmhgroup.com.

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