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Laser-Induced Breakdown Spectroscopy (LIBS) in Cultural Heritage Cite This: Anal

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Laser-Induced Breakdown Spectroscopy (LIBS) in Cultural Heritage Cite This: Anal Analytical Methods View Article Online AMC TECHNICAL BRIEFS View Journal | View Issue Laser-induced breakdown spectroscopy (LIBS) in cultural heritage Cite this: Anal. Methods,2019,11,5833 Analytical Methods Committee AMCTB No. 91 Received 18th September 2019 DOI: 10.1039/c9ay90147g www.rsc.org/methods Laser-induced breakdown spectroscopy This is primarily because of the lack, to obtain clean spectra, it is essential to (LIBS) is a versatile technique that provides date, of commercial instruments dedi- start measuring the plume emission aer nearly instant elemental analysis of materials, cated to heritage applications, but also in a short delay (typically 0.5–1 ms) with both in the laboratory and in the field. This is some instances, of its micro-destructive respect to the laser pulse. done by focusing a short laser pulse on the character. surface of the sample, or object, studied and analysing the resulting spectrum from the Instrumentation laser-induced plasma. LIBS has been How does LIBS work? employed in the analysis of archaeological In recent years, a variety of LIBS instru- and historical objects, monuments and works The fundamental principle underlying ments, both bench-top and portable, – of art for assessing the qualitative, semi- LIBS stems from the brief interaction have been introduced by manufacturers – quantitative and quantitative elemental just a few nanoseconds between for speci c applications such as content of materials such as pigments, a focused laser pulse and a target object. screening of on-site materials or indus- pottery, glass, stone, metals, minerals and This concentration of light both in space trial process monitoring. The use of these fossils. It is also used in robotic planetary and time (leading to irradiance values of instruments for the investigation of 2 rovers. This Technical Brief describes the the order of a few GW/cm ) is key to the heritage objects would require several Published on 08 November 2019. Downloaded 11/30/2019 8:19:11 PM. ‘ basic concepts of LIBS, presents relevant initiation of a process (known as laser adaptations so as to ensure, for example: ’ aspects of instrumentation and discusses how ablation ) that triggers the rapid forma- ne energy adjustment of the laser pulse; the technique is applied in the context of tion of a microplasma plume just above accurate aiming and proper focusing of cultural heritage studies. the sample surface. Following the laser the beam; single pulse operation; and, pulse, this microplasma persists for a few the ability to handle samples and objects microseconds, emitting radiation that of varied sizes. The small size of the arises from the relaxation of its constitu- heritage market has not encouraged ents (electrons, excited atoms and ions). manufacturers to perform such instru- Introduction Recording this emission with a spec- mental modications. As an outcome, trometer produces a typical LIBS spec- the relatively small number of research Laser-induced breakdown spectroscopy trum (Fig. 1) with emission peaks at groups using LIBS for heritage applica- (LIBS) was introduced as a potential characteristic wavelengths, reecting the tions have been employing custom- analytical tool in the context of cultural elemental composition of the sample designed instruments optimized for use heritage studies during the mid-1990s. It (qualitative analysis). The peak intensi- in heritage studies. facilitates the rapid elemental analysis of ties or areas can also be associated with As a typical example of LIBS instru- samples or objects examined, in situ, with the concentrations of specic elements in mentation used for the analysis of no need for any sample or surface prep- the substrate (quantitative analysis). It is cultural heritage objects, a custom-built aration. Despite these attractive features, noted that up to around 0.5 msaer the mobile LIBS spectrometer (Fig. 2) the use of LIBS is still not widespread interaction of the laser pulse with the developed at IESL-FORTH for research among conservators or archaeologists. sample, the emission of the plasma is purposes, is briey described. The strong and mainly in the form of an system consists of an optical probe head unstructured spectral continuum. As this (1), a spectrometer (2), and a power continuum decays, sharp atomic emis- supply unit (3). It weighs less than 9 kg sion lines emerge. Therefore, in order to and ts in a compact case. It is operated This journal is © The Royal Society of Chemistry 2019 Anal. Methods,2019,11,5833–5836 | 5833 View Article Online Analytical Methods AMC Technical Briefs recognised that there is an act of sampling in LIBS but it is made by the laser under the control of the analyst (see also ref. 7). A single laser pulse is suffi- cient for obtaining a spectrum with a high signal-to-noise ratio. In routine measurements, a few pulses, typically 1– 5, are delivered at each spot investigated on the object/sample surface. Separate spectra are recorded when depth proling information is required. This can be achieved because of the nature of the laser ablation which results in the removal of thin layers of materials (of the Fig. 1 LIBS spectrum collected from a leaded brass object. Characteristic atomic emission lines order of a few microns per pulse) during from Cu, Zn and Pb are observed. analysis (see also ‘Advantages and limitations’). and controlled through a laptop mechanisms of laser ablation and plasma Advantages and computer. A micrometer translation formation, while double-pulse LIBS has stage is used for accurate positioning of been found to enhance sensitivity. limitations the probe head and, where necessary, Employing spectrometers based on LIBS provides multi-elemental analysis a tripod or appropriate scaffolding is echelle gratings offers superb spectral data, including low atomic number used to position the equipment in front resolution and thus accurate qualitative elements such as, for example, Li, F, Na, of the object (Fig. 3). analysis, although their use increases Mg, Al, and Si with typical limit of The probe head houses a small-sized, analysis time. The use of intensied CCD detection (LOD) values in the range of pulsed Nd:YAG laser operating at detectors provides the ability to time- 0.5% in single-pulse spectra (see further 1064 nm (10 mJ per pulse, 10 ns) and is resolve the spectra and reject the early discussion in Analysis of Spectra). equipped with a miniature charged- continuum background, also offering Spatial resolution: LIBS is effectively coupled device (CCD) camera. The enhanced sensitivity. a microprobe technique which provides camera allows the analyst to visualize the analysis of areas 0.1–0.2 mm across, sample and accurately aim the instru- which can be further reduced to 0.01 mm ment, and facilitates photographic docu- Analysis protocol if a microscope objective is employed Published on 08 November 2019. Downloaded 11/30/2019 8:19:11 PM. mentation during all stages of analysis. In a typical analysis, the object/sample is (Fig. 4). Specic instruments exploit specic placed at the focal point of the lens and Speed: the simplicity of the technique features of laser sources or spectrometers the probe head (with the laser deacti- and its speed permit the analysis of and detectors. Picosecond or femto- vated) is positioned accurately. The area a large number of objects in a short time. second pulses can be used (not available probed by the laser beam is approxi- For example, LIBS is ideal for the quick in mobile systems), leading to different mately 0.2 mm in diameter. It should be screening and characterization of Fig. 2 Main subunits of a custom-built mobile LIBS spectrometer (left) and the instrument in a carrying case (right). 5834 | Anal. Methods,2019,11,5833–5836 This journal is © The Royal Society of Chemistry 2019 View Article Online AMC Technical Briefs Analytical Methods Environmental applications: LIBS is ideal for detecting environmental contaminants on surfaces. A signicant drawback of LIBS is that by its very principle of operation – relying on laser ablation – the technique is, strictly speaking, invasive. It may be classied as minimally invasive or micro- destructive, given the very small area affected, but the fact remains that the analysis does remove a very small amount of material (typically of the order of a few nanograms) to generate plasma. The material analysed no longer remains in place and if the analyst wishes to repeat the analysis at the same spot, they will obtain data from a fresh layer of material. This is not a problem with homogeneous materials, but it could lead to confusion with materials of which the composition changes abruptly with depth. Quantitative analysis is possible but Fig. 3 Analysis of salts in the inner wall of the Venetian castle of Koules at Heraklion, Crete complex. The use of external calibration (Greece) with the mobile LIBS spectrometer shown in Fig. 2. requires the availability of matrix- matched calibrators (reference materials with compositions that match those of artefacts from archaeological excavations Depth proling: it is possible to the objects under analysis). This is (Fig. 5). perform the equivalent of depth prole straightforward in the case of archaeo- Sampling: the technique can be used analysis. For example, if a painting is logical metal alloys (typically binary to in situ, so there is no need for sampling examined, each laser pulse removes quaternary alloys) but it can be and/or sample preparation, which can be a small amount of material from the demanding with more complex matrices time consuming operations that some- surfaceandthereforethefollowing such as pottery or minerals. In some times damage the object. pulse always probes a new section of the cases, calibration-free methods can be Published on 08 November 2019. Downloaded 11/30/2019 8:19:11 PM. Versatility: almost any size of object paint, at a layer that is slightly deeper employed.
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