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Laser induced breakdown spectroscopy (LIBS) and time-resolved two-dimensional LIBS have been applied as quantitative and qualitative analytical methods for multiple materials and matrices, from animal tissue, to archeological ceramics, to dairy products, to zeolite composition. We were recently able to interview Professor Jorge O. Cáceres, the Director of the Laser Chemistry Group in the Department of Analytical Chemistry at the Universidad Complutense de Madrid, in Spain, who is working with LIBS for new method development. He recently spoke to us about his most recent LIBS research.
Laser induced breakdown spectroscopy (LIBS) and time-resolved two-dimensional LIBS have been applied as quantitative and qualitative analytical methods for multiple materials and matrices, from animal tissue (1), to archeological ceramics (2), to dairy products (3), to zeolite composition (4). For the analysis of animal tissue, potassium (K), Magnesium (Mg), Magnesium ion (Mg+) hydrogen (H), and oxygen (O) as well as molecular CN bands such CN (B2Σ+–X2Σ+), C2 (d3Πg–a3Πu), NH (A3Π–X3Σ−) and N2 (c3Πu–B3Πg) are determined. For the archeological study of ancient Roman pottery, Fe, Ca, and Mg are measured. For analysis of dairy products, CN emission bands are measured and multivariate discriminant analysis using principal component analysis (PCA) and neural networks (NN) is applied. For analysis of zeolites, LIBS is used to determine electronically excited neutral K, Ga, Si and O atoms, as well as ionized K+, Ga+, Ga2+, Si+, Si2+, Si3+, O+, O2+, and O3+. We were recently able to interview Professor Jorge O. Cáceres, the Director of the Laser Chemistry Group in the Department of Analytical Chemistry at the Universidad Complutense de Madrid, in Spain, who is working with LIBS for new method development. He recently spoke to us about his most recent LIBS research.
You have published multiple papers on the use of LIBS for a variety of applications and using different types of lasers. What has prompted you to investigate such a wide variety of problems using this approach? What is unique or novel about your approach to the uses and applications of LIBS?
I have always tried to make our work (that of our group) relevant to society either with outstanding results or through specific applications that are useful in real-world situations. The projects developed or under development have not only an industrial interest, but also provide answers to questions of social concern such as the analysis of the environment or the detection of contaminants and adulteration in food samples. These projects help to develop both aspects for basic and applied science. Basic research constitutes a fundamental pillar of our research activity, since it provides dynamic means to increase the understanding about underlying concepts that help to develop a diverse range of applications based on the LIBS technique. Furthermore, our group provides a platform for enthusiastic young and experienced researchers providing them technological resources to solve problems in the areas of chemical sciences and technology transfer.
Regarding our approach, I believe that we have significantly contributed to the use of artificial intelligence (AI) and in particular, the application and development of neural networks (NNs), to have a prominent place in LIBS research and I hope that this approach will grow even more in the future.
Would you explain for our readers the differences between LIBS and time-resolved two-dimensional LIBS? What are the advantages of time-resolved two-dimensional LIBS?
LIBS analysis by direct measurement of the optical emission from laser-induced plasma has been the subject of research for many years. This technique provides a quick and versatile method for analyzing different types of samples that can be inaccessible or tedious using conventional analytical techniques, and is particularly useful for analyzing samples having a complex matrix. LIBS technique provides several advantages over conventional methods for elemental analysis: (a) LIBS reduces or even eliminates the sample preparation step; (b) allows the analysis of sample in any state of matter (solid, liquid, gas); (c) the analysis is performed in a few seconds; (d) requires a very small amount of sample, few micrograms per sampling position and (e) the analysis detects all elements without bias, including those present in organic molecules (which are atomized during the process).
In addition, the short-lived features of the plasma plume, with time and space resolution, which is particularly useful to obtain information about the behavior of the species present in plasma can be investigated using LIBS for monitoring and controlling different processes. Although only partial information is inferred from LIBS plasma, this procedure enabled us to diagnose the emitting, species to evaluate the mechanisms of excitation and formation and to study the role of gas-phase reactions in the expansion and cooling process. This information provides not only a full understanding of the basic physics underlying the plasma formation and evolution but also help to develop and exploit applications.
From your perspective, what are the most exciting developments in LIBS over the past five years in terms of both chemometric data analysis and instrumentation development?
Nowadays, it is possible to take several spectra based on LIBS and generate high-quality elemental imaging data with micrometric spatial resolution, a ppm-scale limit of detection, and high scanning speed (up to 1000 pixels/s). Undoubtedly, the work related to the formation of megapixel LIBS images and the combination with chemometric methods is likely the most challenging and interesting development and on which, our group and collaborators continue working on. This multidisciplinary approach has been recently applied to climate research, focusing on Antarctic region and the results open a new perspective into the use of LIBS in applications related to environmental contamination and monitoring.
In your opinion, what are some of the papers you would highlight as the most influential in advancing LIBS measurements over the past five years?
I sincerely believe that there are some research papers that have marked a clear trend towards the progress of LIBS technique. We can mention here “Evaluation of supervised chemometric methods for sample classification by laser induced breakdown spectroscopy”(7), “Megapixel multi-elemental imaging by laser induced breakdown spectroscopy” (5), a technology with considerable potential for paleoclimate studies, (6). Another work that I find interesting is “Nanoparticle enhanced laser- induced breakdown spectroscopy for microdrop analysis at sub ppm levels” (8). “ChemCam activities and discoveries during the nominal mission of the Mars Science Laboratory in Gale crater, Mars” (9) and “Development of a deep-sea laser-induced breakdown spectrometer for in situ multi-element chemical analysis” (10). All these works have pushed LIBS to its technological limits where the use of any other technique is unavailable.
What recent advances in LIBS measurements do you have the most interest in pursuing?
Our advances in experimental systems, obtaining megapixel images of particles collected from atmospheric aerosols and adulteration of food, are where we are focusing our efforts. In the later, we are achieving very promising results applying artificial intelligence algorithms.
What are some major gaps in knowledge in the use of LIBS that you would like to see more research and development time devoted to?
I think the main difficulty in LIBS is the standardization of the technique. Many types of lasers, spectrometers, lines, and so on. Reproducibility is limited and prevents its implementation as a standard technique. It would be necessary to work on standardizing protocols and procedures, above all the reproducibility is precisely where we need to work.
What have been your greatest challenges in scientific discovery over your career? What is your general approach to problem solving in your scientific work?
The main challenges that I have faced are more related to economic issues. Funding agencies should give more opportunities to hire new people and extend contracts for postdocs to improve the quality of the research, however, this is a constant concern of any research group worldwide. Regarding research itself, the correct application of chemometric methods and their adaptation to LIBS spectra has been a great challenge, this approach has led us to solve complex problems not in days, but rather over the years. Understanding the behavior of these mathematical algorithms and how to get as much information as possible from data is not straight forward.
What will be your next major area of research?
Today our main research project concerns the study of atmospheric aerosols in Antarctica where, in a pioneering way, we will try to obtain measurements in situ by using portable LIBS equipment. We have recently published a paper aimed at analyzing the elemental composition of such aerosol and what we found was really surprising in terms of pollution in the Antarctic region. Our challenge now is to find the source of this contamination. Thanks to the Spanish polar committee and the security forces present in the research bases we have overcame all the logistical obstacles, and we are now enable to move in this direction.
Jorge Caceres is the Director of the Laser Chemistry Group in the Department of Analytical Chemistry at the Universidad Complutense de Madrid, in Madrid, Spain. He received his PhD in 1997 at the National University of Córdoba in Argentina. He has received several pre-doctoral and doctoral scholarships under programs such as CONICET in Argentina and –Ramón y Cajal in Spain. He has worked on developing techniques for dynamic imaging studies and molecular beam spectroscopy. In the field of lasers, he has contributed to the development of laser techniques for the application of laser induced breakdown spectroscopy (LIBS) in environmental analysis and in development of mathematical methods for data processing applied to quantification and identification. In 2008 he created the Laser Chemistry Group to explore analytical applications using different spectroscopic techniques, primarily LIBS, in a variety of application areas, such as in the food industry, forensic medicine, pathogen research, environmental analysis, and geological element analysis.