Laser-Induced Breakdown Spectroscopy

Laser-induced Breakdown Spectroscopy

Introduction Laser-induced breakdown spectroscopy (LIBS) is an atomic emission spectroscopic technique that allows for rapid elemental analysis of a wide variety of solid, liquid or gaseous samples. It utilizes a pulsed laser to generate a microplasma on the surface of a target and a spectrometer to collect the emitted light for simultaneous identification and quantification of all elements (H through U.) Its application in an undergraduate setting is essentially unknown compared to other AES methods such as AA, ICP or XRF. The recent introduction of affordable broadband LIBS systems makes this an attractive technique for undergraduate programs and offers several important advantages. LIBS is straight-forward, inherently sensitive (low ppm range), requires no sample preparation, consumes only small amounts of sample (ng), very fast (essentially a “point and shoot” system which lends itself to analysis of a wide variety of objects), and is amenable to use in the field for direct or stand-off analysis. The approach gives qualitative as well as quantitative information. It has been successfully applied to the analysis of artworks, biomaterials, cultural heritage objects, environmental samples, explosives, industrial alloys, and pharmaceuticals to name only a few examples. Given the nature of the method and the robust character of the components LIBS is currently being developed for use in unusually harsh environments such as on the ocean floor or in outer space. Using funds provided by The Special Grant Program of the Chemical Sciences of the Camille and Henry Dreyfus Foundation Juniata College purchased an Ocean Optics LIBS 2000+ spectrometer and a Nd:YAG laser in the summer of 2003. This paper briefly summarizes how we are using it in the undergraduate curriculum, both for coursework and for student research projects.

LIBS Instrumentation Our system uses a Big Sky Q-switched Nd:YAG laser (1064 nm, 7 ns pulse, 50 mJ max, with a 75 mm focal length lens) which is focused onto the sample surface. The emission is observed at a 45º angle to the laser pulse using seven spectrometer modules to provide high resolution (FWHM 0.5 nm) spectra from 200 nm to 980 nm. The instrument is housed in a chamber equipped with a laser-interlock system to prevent the laser from firing while the door is open. The door of the chamber is fitted with a 25 x 25 cm high O.D. viewing window to provide appropriate protection for the class 4 laser. Students are also required to wear laser glasses when working with the LIBS system.

LIBS in Undergraduate Courses The LIBS system has already been used in two courses at Juniata and will be introduced in another this coming spring. Analytical Chemistry– Students work in pairs and use a suite of ten analytical methods to analyze zinc pyrithione, the active ingredient found in anti-dandruff shampoos. Using the method of known addition, students directly analyze the shampoo to determine the zinc content. The viscous liquid splatters on the detector head and it is difficult to construct the calibration curve but reasonable data can be obtained that match the expected value and no additional sample preparation is required. Forensic Science Laboratory– In this upper level chemistry course students were given the task of developing a LIBS method for the analysis of laboratory glassware that has been used to prepare methamphetamine. Attempts to simulate the phosphorous triiodide reaction frequently used in the manufacture of the actual drug were unsuccessful so the idea could not be tested. On another occasion, students used LIBS to identify a small fragment of wire which had been found at a mock crime scene.

Chemistry of Art– Students will synthesize pigments, prepare their own paint (egg tempera and oil), and then analyze the dried paint to determine what elements are present in the sample. They will then use the correlation library they create as a class to analyze unknown samples using LIBS.

LIBS for Undergraduate Research Undergraduate students can make important contributions to the LIBS field if given the opportunity. The pace of such research is necessarily slower given the time they can usually devote solely to laboratory work but progress can be made on relevant questions, especially relating to the application of LIBS for analysis of a variety of materials. An advantage of LIBS in this regard is that it is easy to begin to collect data for real-world applications. This tends to generate excitement which, in turn, draws the student into the project and motivates them to learn more about the complex processes and challenges associated which are involved in plasma spectroscopy.

Since acquiring our instrument it has been easy to interest students in LIBS research projects. So far students have presented a total of four posters and given one talk at regional and national conferences on their LIBS work (see titles below). This type of experience is invaluable in their training as scientists. A listing of several project titles followed by brief descriptions and preliminary data for several more will serve to illustrate opportunities for involving undergraduates in LIBS research.

Use of Gels as Solid Matrices for Analysis of Aqueous Solutions by LIBS (Christopher Spiese) Detection of Heavy Metals in Acid Mine Drainage by LIBS after Deposition onto Silica (Bob Grimminger Marsha Loth, Christopher Spiese) Analysis of NIST Steel and Bronze Samples Using LIBS (Luci Condon & Megan Dieckman) Comparison of SEM/EDS and LIBS for Depth Profiling of Automobile Paint Samples for Potential Use in Forensic Investigations (Ashley Heckman & Elise Zimmerman)

Analysis of Pigment Layers in Paintings As part of the preparation for the planned Chemistry of Art course exercise in which students will analyze paints, preliminary work has been done to verify the feasibility of the analysis using our LIBS system. A series of acrylic, oil, water-soluble oil (and eventually alkyd and egg tempera) paints were brushed onto a canvas board or watercolor paper. An additional set of samples were prepared using acrylic paints in which multiple layers (3 to 5) were added over the course of several days.

A correlation library was built using the single layer samples (10 shots each). The paints with organic dyes as the colorant were difficult to distinguish from each other but the inorganic pigments (e.g., copper phthalocyanine, chromium oxide) were readily identifiable. The layered paint samples were then shot 50-70 times sequentially in the same location and the correlation value obtained for each shot. It was possible to correctly identify the paint layers in most cases with the exception noted above. Future work will include SEM profile analysis of the area that was shot with the laser.

Analysis of A Suspected Meteorite Background Dr. George Drobnock, a local scientist, brought a rock to the chemistry department for analysis that had been found in the 1920’s in a field in central Pennsylvania owned by the Watkin family and was believed to be a meteorite. In conjunction with Dr. Ryan Mathur and Dr. Larry Mutti of the Juniata Geology Department, we investigated the basic chemical and physical properties of the sample using EDS (JEOL JSM-646OLV, Oxford Inca X-sight, carbon coating) and LIBS and compared these to several known meteorite samples obtained from Dr. Andrew Secree of The Pennsylvania State University. Meteorites are relatively inhomogeneous matrices and we expected that both the EDS and LIBS elemental composition data would not match with the published values since that work was done on bulk samples of the meteorites. Notably, one paper had been previously published describing the use of LIBS to investigate the topology of the Murchison meteorite. Results of Initial Examination Physical examination of the rock showed no fusion crust (it has been previously cleaned), no evidence of “thumbprint” patterning, was shiny in appearance (no evidence of corrosion) and displayed several unusual features (channels) that were more characteristic of a “meteor-wrong.” The rock has a specific gravity of 5.84 gcm-3 and was almost completely nonmagnetic (only very small grains were attracted to a neodymium magnet). All of these factors suggested that the sample was not a meteorite.

Sample Type of Meteorite Literature Analysis EDS Data (weight %) Fe 54.32% Al 1.59% M1 Watkin Mg 0.18% Ca 1.55% (suspected ??? NA Ni none Na 0.68% meteorite) O 10.02% S none Si 27.76% Ti 2.55% Fe 15.42% Al 1.61% Stone, Mg 14.49% Ca 1.29% M2 Guenie Olivine-bronzite Not reported Ni 0.91% Na 0.62% chondrite (H4) O 40.63% S 1.86% Si 21.25% Ti none Fe 17.00% Al 0.99% Stone, Mg 16.56% Ca 0.64% M3 Abee Enstatite chondrite32.52% Fe O 35.09% Na 0.80% (E4) Ni 0.79% S 6.14% Si 21.49% Ti 0.05% Fe 11.32% Al 1.19% 10.02% Ni Mg 16.01% Ca 1.24% Iron, Octahedrite 70.0 ppm Ga M4 Carbo O 39.34% Na 1.01% (IID) 87.2 ppm Ge Ni 0.30% Si S 6.14% 13 ppm Ir 22.99% Ti none Fe 78.17% Al none 8.8% Ni Stony iron, Mg none Ca none 9.6 ppm Ga M5 Vaca Muerta Mesosiderite Ni 16.73% O Na none 42.8 ppm Ge (MES) 5.10% S none 2.2 ppm Ir Si none Ti none

Comparison of Watkin and Guenie with a single shot after 5 cleaning shots at 50 mJ shows substantial differences in composition (no control for air entrainment.) All elements found using EDS were also observed in the LIBS spectra, as well as many trace elements that could not be measured using our EDS technique, but which are important for the study of meteorite origins. These data are in the process of being quantified for direct comparison. A correlation library was constructed for M1 through M5 using 5 cleaning shots followed by 5 individual shots in 3 separate areas (total of 15 shots recorded). Even given the profound inhomogeneity of all of these samples (verified by an EDS 10 x 9 grid analysis) on same scale as the crater formed by the laser (~500mm circle), the library was able to provide 100% correct sample identification when new sites were interrogated is a similar fashion. XRD analysis subsequently verified that the sample was synthetic iron silicide.

LIBS Spectra

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M2 Guenie . 800 u . ) a (

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n 400 e t n

M1 Watkin I 200

0 -200200 300 400 500 600 700 800 900 1000 Wavelength (nm)

SEM images of M1-M5 (~650 x 550 m, 200x, entire image area used for EDS analysis)

M1 Watkin M2 Guenie M3 Abee

M4 Carbo M5 Vaca Muerta Funding We gratefully acknowledge funding for this project from the following organizations • Henry and Camille Dreyfus Foundation • Ocean Optics Inc. • United States Secret Service • William J. von Liebig Foundation