Laser-Induced Breakdown Spectroscopy: A Closer Look at the Capabilities of LIBS

Laser-induced breakdown spectroscopy (LIBS) is a technique that works well for a wide range of applications. In this interview, Dr. Richard R. Hark, a professor in the Department of Chemistry at Juniata College in Huntingdon, Pennsylvania, discusses his work with LIBS in applications such as forensic science, conflict minerals, and geochemical fingerprinting. Part II of this interview will focus on Hark's work using LIBS for emergency response to hazardous materials along with an interview with a first-responder who has been involved in that research.

Early in your career you synthesized a large number of novel ninhydrin analogs as reagents for visualizing latent fingerprints on porous surfaces. Did that research naturally lead to your interest in laser-induced breakdown spectroscopy (LIBS)?

My background in forensic science did lead directly to my involvement with LIBS. I became familiar with the technique in 2002 and immediately saw the advantages of applying LIBS to the analysis of trace evidence.

TOM FULLUM/GETTY IMAGES

I came to Raman spectroscopy in a more roundabout manner. Since 1999 I have taught a class called "The Chemistry of Art," which is an interdisciplinary course that explores the intersection of chemistry with the visual arts. In this laboratory-based class we learn about artists' materials, issues facing conservation scientists, and many basic chemistry concepts as we explore the chemistry and history of art media such as paints, dyes, metals, alloys, ceramics, glass, plastics, paper and fibers, and photographic materials. Because of the desire I had to get involved in research related to cultural heritage objects, I was able to spend a sabbatical leave at University College London and the Victoria and Albert Museum in 2007–2008. While there, I was privileged to work on a number of projects involving the analysis of medieval manuscripts and miniatures using Raman spectroscopy.

How have you incorporated LIBS into your undergraduate teaching and research efforts?

Our undergraduates have been using the technique in course work and research since I acquired our first LIBS instrument in 2003. LIBS has been a topic in our sophomore analytical chemistry course as well as the subject of upper-level special topics courses over the past decade. More than 20 undergraduate students have been involved in LIBS research projects over the past 11 years, resulting in numerous conference presentations and several papers. Last fall, Juniata was the site for the first Laser-induced Breakdown Spectroscopy for Undergraduate Research and Teaching (LIBS-URT) conference. The second LIBS-URT will be held again at Juniata October 10–11, 2014. Our goal is to promote awareness of the technique among faculty who teach and do research with undergraduate students.

LIBS can be used for multiple applications in forensic science, including glass, paint, ink and paper, counterfeit currency, ammunition and gunshot residue, fingerprints, wood, fibers, and biological materials. Are there other materials encountered in forensic studies that might be suitable for analysis by LIBS? What are the limitations for its use in forensic science?

In addition to the types of evidence mentioned, LIBS has application for the analysis of drugs and their synthetic precursors, hair, soil, explosives, and nuclear material. Every analytical technique suffers from disadvantages that must be understood and taken into account so that the method can be appropriately applied to forensic evidence. While there are some inherent limitations of the LIBS technique, principally related to matrix effects, the limit of detection, and level of precision, the fact that LIBS has been utilized in very few court cases probably represents the greatest hurdle to more general use of the method. The admissibility in court of a particular type of scientific evidence must be established using the Daubert and Frye standards (1,2) before a new analytical technology gains widespread acceptance. It also takes time for forensic practitioners to develop familiarity with, and then confidence in, new methodology.

Has LIBS reached its full capacity in forensic science yet?

Since LIBS offers the ability to carry out rapid, minimally destructive elemental analysis directly on virtually every type of material of importance to criminal investigations without the need for sample preparation, I believe that LIBS has great potential in the field of forensic science. As more portable, ruggedized LIBS instruments are developed and made commercially available, the use of the technique as a high-throughput screening tool at crime scenes and to collect data of evidentiary value will likely increase. This is an evolving field that is moving from an initial phase, in which preliminary studies demonstrated a proof-of-concept, toward a more mature stage where best practices and standard operating procedures (SOPs) are being developed, known rates of false positives and false negatives are being established, and LIBS is gaining greater visibility among forensic scientists.

You have also used LIBS to analyze paint samples, artworks, and archeological artifacts. What challenges did you encounter in using LIBS for that research? What is the most interesting result you've found so far in that work?

I used LIBS on some Neolithic skull fragments from Jordan as part of a project to learn if the discoloration of bones was because of natural causes or the application of an iron oxide pigment, but the results were inconclusive. We have not utilized LIBS on oil paintings or other more sensitive artworks, but the technique would find good application with analysis of frescoes. For my work with cultural heritage objects I tend to use only nondestructive techniques such as Raman and X-ray fluorescence (XRF). We did carry out a preliminary study that compared LIBS to scanning electron microscopy–energy-dispersive X-ray spectroscopy for depth profiling of automobile paint. We created cross-sectional graphs that reflected the ablation of successive layers of the paint finish by plotting the LIBS emission line intensities versus the number of laser pulses.

Your recent research has involved analyzing conflict minerals using LIBS to determine their origin. What are conflict minerals and what spurred your interest in this research area?

So-called "conflict minerals" are natural resources that are mined in areas associated with armed conflict and human rights abuses and include materials such as "blood diamonds" or columbite-tantalite (known as coltan). While working on a project to analyze pegmatite minerals, we recognized that our work had an obvious application to the conflict mineral problem. Because the LIBS emission spectrum provides a unique chemical signature of a material that can be used to discriminate geological specimens originating in one place from samples of the same kind from other locations, we felt that it would be possible to use this technique to verify if a particular sample came from a source controlled by a legitimate mining organization or a militant group.

How does this fit into the broader use of LIBS in geochemical fingerprinting?

The concept of geochemical fingerprinting is based upon the fact that the Earth's crust is compositionally heterogeneous, both horizontally and vertically, and that minerals forming within the crust will reflect that inherent geographic heterogeneity. A variety of analytical techniques can be used to obtain information about the elemental composition and isotopic ratios of a geological sample and thereby establish its chemical signature. Comparison of these chemical fingerprints with a library of samples with known provenance allows an unknown sample's source to be identified. Because LIBS is a straightforward atomic emission spectroscopic technique that delivers rapid, multielement detection in real time with minimal sample preparation, it is being increasingly applied to the analysis and provenance verification of geomaterials.

How does LIBS fit into the overall set of analytical methods that can be used for geochemical fingerprinting? What are its strengths and weaknesses compared to other methods?

Many different analytical methods, such as X-ray diffraction (XRD) and XRF spectrometry, electron microprobe analysis (EMP), instrumental neutron activation analysis (INAA), and inductively-coupled plasma analysis (ICP, ICP coupled to mass spectrometry [ICP-MS] and laser ablation with ICP-MS [LA-ICP-MS]) have been used within the Earth, environmental, and archaeological science communities for geochemical fingerprinting. In addition to the general benefits that have already been mentioned, LIBS specifically offers several important advantages that make it a useful analytical technique for geochemical materials, including the fact that LIBS is particularly sensitive to light elements (such as H, Li, Be, B, and C), that it provides high spatial resolution allowing for stratigraphic analysis and mapping of complex samples, and that it can be combined with complementary spectroscopic techniques such as Raman or laser-induced fluorescence (LIF). While the typical limits of detection (low parts-per-million) and the level of precision (5–10% RSD) for LIBS experiments are normally not as good as some of the methods used for analysis of geomaterials, they are often sufficient to provide discrimination between samples of different provenance.

What impact do you expect handheld LIBS to have on geochemical fingerprinting?

The possibility of using portable or handheld LIBS instruments in the field is perhaps the most attractive feature of this technique that has yet to be exploited, not only for geochemical fingerprinting but also for many other applications. If the cost of a handheld LIBS unit were comparable to or ideally less than that of a handheld XRF instrument, I would foresee many opportunities for such a device to have a significant impact on many fields. Compared to XRF, and assuming that a handheld unit had similar analytical figures of merit as a laboratory model would, LIBS could offer a lower level of detection, the ability to see all elements, and higher throughput than handheld XRF.

What are the next steps in your research on geochemical fingerprinting?

To more fully establish the benefits of the LIBS technique for geochemical fingerprinting, we are analyzing large sample suites of volcanic rocks and carbonate minerals with known, unambiguous provenance. The creation of spectral libraries with a substantial number of samples allows us to use robust chemometric analysis tools for verification that samples of unknown origin can be reliably matched to their geographic point of origin. This is also the next step in our work with the conflict minerals project, but it has proven challenging to obtain samples from a sufficient number of mining locations in central Africa. We are also continuing our work with obsidian studies as this has important archeological applications such as the determination of the provenance of obsidian artifacts.

What other types of research can be advanced by using LIBS to study mineral samples?

The recent development of laser ablation molecular isotopic spectrometry (LAMIS) holds the potential for LIBS to be used to measure isotope ratios in geomaterials. LAMIS is based on the fact that isotopic shifts associated with molecular spectroscopy are substantially larger than those in atomic spectra. Ultrahigh resolution spectrometers are therefore not required to measure isotopic ratios in minerals for which the ablation process creates diatomic molecular species such as metal bromides, chlorides, fluorides, iodides, and oxides. The possibility of using LIBS to quantitate some radiogenic nuclides used in geochronology would be particularly exciting.

You are applying LIBS to a diverse set of analytical questions. For those applications, is LIBS still at a stage of having to prove its value compared to other analytical methods? In what other areas do you think LIBS is likely to make an important contribution? Are there any applications for which LIBS is, or is likely to become, the primary, "go-to" analytical method?

The LIBS technique is still undergoing a maturation process for many of the research areas in which I am involved, but steady progress is being made. In some industrial applications, such as the in situ analysis of molten metals and recyclable materials, LIBS has already established itself as the ideal method. It is important to recognize that although LIBS has many advantages, it is not the best solution for every application. The key is to implement the method in situations where the cost of the instrumentation is reasonable, the sensitivity and selectivity of the LIBS measurement are sufficient, and the versatility, portability, stand-off capability, and robustness of the approach are crucial. The use of a LIBS unit on the Mars rover Curiosity demonstrates that in some cases LIBS is definitely a mission-critical technique.

Coming Soon

Part II of this two-part interview will focus on Hark's work on using LIBS for emergency response to hazardous materials. We will also feature a second interview on this area of LIBS research, with Adam L. Miller, of John R. Plumer Associates, LLC, who has worked as a first-responder. Miller and Hark have worked together on the use of handheld LIBS for first responders. Part II will be published in our November 2014 issue.

References

(1) Daubert v. Merrell Dow Pharmaceuticals, Inc., 43 F.3d 1311 (9th Cir. 1995).

(2) Frye v. United States, 293 F. 1013 (D.C. Cir. 1923).