Laser-Induced Breakdown Spectroscopy for Qualitative Analysis of

Oct 2, 2017 - Laser-induced breakdown spectroscopy (LIBS) has developed into an important analytical tool with a broad range of applications including...
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Laboratory Experiment pubs.acs.org/jchemeduc

Laser-Induced Breakdown Spectroscopy for Qualitative Analysis of Metals in Simulated Martian Soils Curtis Mowry,*,† Rob Milofsky,*,‡ William Collins,‡ and Adam S. Pimentel† †

Materials Reliability Department, Sandia National Laboratories, Albuquerque, New Mexico 87123, United States Department of Chemistry, Fort Lewis College, Durango, Colorado 81301, United States

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S Supporting Information *

ABSTRACT: This laboratory introduces students to laser-induced breakdown spectroscopy (LIBS) for the analysis of metals in soil and rock samples. LIBS employs a laser-initiated spark to induce electronic excitation of metal atoms. Ensuing atomic emission allows for qualitative and semiquantitative analysis. The students use LIBS to analyze a series of standard samples that contain various elements and construct a table of emission line wavelengths for each element analyzed. Students then identify metals in various soil and rock samples. Students gain valuable experience in qualitative analysis using an important spectroscopic tool, while gaining hands-on experience with a spectrometer employing a high energy laser pulse as an excitation source. The LIBS spectrometer is applicable to upper and lower division chemistry courses and can be used as an effective demonstration tool for students in 5th to 12th grade. KEYWORDS: First-Year Undergraduate/General, Analytical Chemistry, Atomic Spectroscopy, Lasers, Hands-On Learning/Manipulatives, Laboratory Instruction, Metals

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walls), and Mg (for chlorophyll for photosynthesis).17−19 While several LIBS experiments have been described in this Journal,20−22 this represents the first example of solid phase analysis of soils and rocks in general and analytical chemistry laboratory settings. Herein, we describe a LIBS-based experiment designed to give first-semester general chemistry and upper division analytical chemistry students hands-on experience in real-time chemical analysis. In this experiment, we challenge the students to address the following question: “If you had to grow potatoes on Mars, could you use the LIBS on the Rover Curiosity to predict whether potatoes could be grown on Mars?” To this end, students are given several Earth soil types (obtained locally) and encouraged to bring in their own rock sample for LIBS analysis. Qualitative analysis of the emission spectra allows for a deeper appreciation of the presence (or absence) of essential elements for growing plants. Subsequent identification of elements including Ca, Mg, Mn, Si, Ti, and Zn is possible through the generation of an emission line wavelength table with known standards. Finally, variation in the elemental composition of the terrestrial soil samples, potato skins, as well as known spectra from Martian soil (obtained by the Rover Curiosity’s LIBS13−15,23) is determined, and the students are able to postulate whether LIBS technology could indeed be helpful to grow potatoes on Mars. When reduced to practice, this experiment succeeds in achieving several important learning objectives. Students are able to relate the emission spectra of various metals to their composition within a mixture. They are also able to verify the presence of these metals by relating emission intensities at

tudents in undergraduate general chemistry and analytical chemistry laboratories are typically introduced to metal analysis using either a simple flame test or a flame atomic absorption spectrometer (AAS). Both techniques are generally limited to single-element analysis, making the determination of metals in complex, real-world samples cumbersome. Additional pitfalls associated with flame tests include poor selectivity (e.g., K in the presence of Na), limited sensitivity, and challenges with data interpretation.1 The use of highly flammable solvents (e.g., methanol) for flame tests, which carry a high “wow” factor, have resulted in accidents in general chemistry laboratories.2,3 This lab introduces students to laser-induced breakdown spectroscopy (LIBS), a mature method for the rapid multielement analysis of solid samples, without significant sample preparation. Laser-induced breakdown spectroscopy (LIBS) has developed into an important analytical tool with a broad range of applications including anthropological specimens, art restoration/authentication, environmental chemistry, biomedical analysis, and space exploration.4−6 In fact, hand-held/field portable LIBS spectrometers are now commercially available and have been employed for geochemical analysis.7 Numerous reviews concerning the use of LIBS for geochemical analysis8,9 and a paper describing the use of LIBS for analysis of nutrients in soils10 have been published as have books devoted to the subject of LIBS.11,12 The LIBS spectrometer deployed on the NASA Mars rover Curiosity allowed for the elemental analysis of Martian rocks, soil, and outer rock layers.13−15 Andy Weir’s recent publication of The Martian16 makes the analysis of Martian soils a timely topic. LIBS allows students to rapidly determine whether soil conditions would be amenable to growing certain crops (e.g., potatoes like those depicted in Weir’s novel, or bananas) by allowing real-time measurements of key elements including K (used in enzymes), Ca (in cell © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: February 16, 2017 Revised: September 19, 2017

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DOI: 10.1021/acs.jchemed.7b00133 J. Chem. Educ. XXXX, XXX, XXX−XXX

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EXPERIMENTAL PROCEDURE In our first demonstration of LIBS, a total of 336 students ranging from 5th to 12th grade were divided into groups ranging in size from 2 to 12 students for 15 min sessions. Each student group probed a set of 1 or 2 soil samples. During the LIBS analysis, the software was set to perform 10 shots at 0.5 s intervals with a 2.1 ms integration time. During this time, the focus was adjusted as the soil was ablated (changing the focus level), or the sample stage was moved. For the soil samples, all were sieved, and the fraction between 0.250 and 0.125 mm was used for the LIBS analysis in most cases. The black soil was first crushed via mortar and pestle to obtain smaller particles, and the 0.5−0.25 and 0.25−0.15 mm fractions were combined. While sieving was not necessary for the experiment, it enabled easier focusing of the laser to the surface and made it simpler to see the laser ablation “crater”. Following the demonstration, 25 general chemistry students and 30 analytical chemistry students were divided into groups of 2−4 students for 30 min sessions. For the analysis of Russet potatoes, LIBS spectra were collected using a single pulse, with the laser focused on the skin of the sample. The flesh from a ripe banana was analyzed without further preparation, using a single pulse from the laser. For student rock samples, LIBS spectra were collected using a single pulse. Each student group collected and plotted spectra for one metal standard, one soil sample, one soil sample spiked with 1− 5% by mass of their metal standard (note that, after adding 1− 5% by mass of metal standard to their soil sample, the soil sample was again crushed and mixed via a mortar and pestle), and either a potato or a banana (as another K-rich sample) or their own rock sample. Using data from ChemCam23 (and Figure 8 in the Supporting Information), each group identified the wavelengths corresponding to the largest peak(s) associated with the metal standard they were assigned. Next, elemental emission line wavelength tables were constructed by combining data from each group. Finally, the raw spectral data from each group were combined, by the instructor, into one common spreadsheet for further analysis. Depending on the number of students, the data can either be collected and analyzed in one 3 h lab period, or collected during one period and analyzed during a subsequent lab.

specific wavelengths to changes in emission intensities following spiking experiments (addition of the target metal to the sample matrix). And, perhaps most importantly, utilizing LIBS, students can understand how analytical techniques developed on Earth can help us perform complex chemical analysis in places that may, as of yet, be unreachable by humans.

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Laboratory Experiment

MATERIALS AND METHODS

Chemicals

Solid metal standards (Al, Cu, Fe, Mn, Ti, and Zn) were obtained from International Advanced Materials (North Charleston, SC) as evaporation crucibles or slugs. All metal standards were >99.9% pure. Silicon wafers (10.2 and 12.7 cm diameter and 25 μm thick) used for determining Si-emission line wavelengths were obtained from Monsanto and SigmaAldrich. Nitrate salt solutions, for deposition on wafers, of Ca, Li, Mg, K, and Na (ICP-MS calibration standards) were obtained from Spex Certiprep (Metuchen, NJ). Chloride or nitrate salt solids, for spiking soils, were obtained from Millipore Sigma (St. Louis, MO). Soils used were collected by the side of the road of New Mexico State Highway 550, primarily between the cities of Cuba and Bloomfield, New Mexico. The red soil was collected a few miles south of Cuba, New Mexico. Soils were chosen on the basis of their ease of collection and diversity of appearance. They were stored in sealed, ziplock plastic bags. Russet potatoes were obtained at a local grocery store (Durango, CO), sliced in half, and analyzed (through the skin) by LIBS within 3 h. Bananas, also obtained from a local grocery store (Durango, CO), were peeled immediately prior to analysis of the flesh. Equipment

Laser-induced breakdown spectroscopy (LIBS) was performed using a LIBS2000 System (Ocean Optics Corp, Dunedin, FL) consisting of an imaging module, a sample chamber, a laser, and a fiber-optic coupled spectrometer (LIBS2500 CCD consisting of 7 channels, 2048 pixels per module for a combined 14,336 pixels) which collects emission. Unless noted, the delay between the spectrometer shutter and the laser was 2 μs. The laser employed was a Nd:YAG (1064 nm) from Big Sky Laser Technologies (Bozeman, MT, model Ultra) operating at ∼50 mJ per pulse (6 ns fwhm). For LIBS experimentation, the laser was used in a single-pulse mode, and was focused to a nominally 100 μm diameter spot. The sample chamber can accommodate a sample of maximum diminsions of 6 in. × 5 in. × 3 in. The sample chamber consists of an acrylic enclosure which allows the normally Class 4 categorized laser system to be operated as a Class 1 laser system. This allows users and observers to watch the experiment without the use of expensive laser eyewear. The focusing lens and sample remain inside the chamber, which is interlocked to prevent laser operation when the chamber door is open. The beam, optics, and plasma emission are all contained inside the chamber. Due to the size of the sample chamber, a sample thickness of 1 in. or less is preferred, and a smaller size allows us to more easily test desired locations on the surface. For this reason, loose soils were placed in 2 in. diameter Petri dishes for analysis. A physical beam shutter and keyed power supply create additional engineered controls to prevent tampering or unauthorized use. See Supporting Information for additional figures detailing the experimental setup.

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HAZARDS Pulsed lasers pose a significant safety hazard unless users/ observers are protected either with laser safety eyewear or, as described here, by a physical optical barrier. The focused beam points downward, and the focal length is shorter than the chamber dimensions, thus preventing damage to the chamber and subsequent beam exposure. The acrylic is also tinted to mitigate the brightness of the plasma.

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RESULTS AND DISCUSSION Emission line wavelength tables were constructed for Al, Ca, Cu, Fe, Li, Mg, Mn, K, Si, Na, Ti, and Zn (see Table 1). In our terrestrial soil lab, we measured the elemental composition of Earth soil of different colors. LIBS data were collected on yellow, black, and red soils from northern New Mexico. Using data from Table 1, a LIBS library,24 and spiking experiments, students were able to identify metals in the 3 Earth soil samples. Figure 1 illustrates some of the more abundant elements students found in the yellow, black, and red soil samples. Since multiple emission lines from Ca, Mg, Mn, Na, and K are observed, we have labeled a select set for the B

DOI: 10.1021/acs.jchemed.7b00133 J. Chem. Educ. XXXX, XXX, XXX−XXX

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These variations in elemental emission allowed students to qualitatively compare the relative elemental composition of the soil samples. For example, as shown in Figure 1, Ca and Na were abundant in all three soils in the figure, but the yellow soil had a higher relative amount of Mn and the red soil possessed the most K. Spiking experiments were used to help support the presence of Ca, Na, Mg, and Mn by observing increased emission line intensities at the wavelengths corresponding to each metal (e.g., Table 1). The emission spectrum from a white rock found embedded within the red soil sample (Figure 10 in the Supporting Information) shows additional peaks that can be attributed to molecular emission of CaO (λ = 590−630 nm). CaO bands centered at 550 and 615 nm, similar to those observed for gray aragonite, and also a CaOH band centered at 624 nm, similar to those observed for concrete,25,26 were also observed. While we did not have the appropriate mineral standards, and therefore our spectra were not examined specifically for this purpose, these bands can be used to determine mineral content. These molecular states can be longlived with respect to the optical collection time frame, and thus can be detected in many calcium-bearing minerals.27 These broad lines are mentioned here as they typically stand out among spectra of student samples, but are not listed in elemental emission line references.

Table 1. Student-Constructed Emission Line Wavelength Data Element Al Ca Cu Fe Li Mg Mn K Si Na Ti Zn

Wavelength (nm) 281.62, 393.37, 324.75, 273.95, 670.78 279.55, 257.61, 404.41, 288.16 328.56, 498.17, 213.86,

308.21, 309.27, 394.40, 396.15 396.85, 422.67 327.39 275.57 285.21, 518.36 403.08 (representing 3 unresolved lines) 766.49, 769.90 589.00, 589.59 499.11, 500.72 330.26, 334.56, 636.23

purpose of clarity. In addition to the labeled signals, we were able to assign the majority of the peaks to emission from Ca, Mg, Mn, Na, and K as well as emission from Ba, Fe, Li, and Si. The yellow and red colors are indicative of iron oxides, which are also abundant on Mars. Additional spectra showing the presence of Fe in the terrestrial soils and Martian soil are included in the Supporting Information (Figure 9a,b). This experiment was able to rapidly detect many elements, and it was found that elemental emission varied in each sample.

Figure 1. LIBS spectra for three terrestrial soils: (a) yellow soil, (b) black soil, and (c) red soil. A select set of peaks were labeled for purpose of clarity. C

DOI: 10.1021/acs.jchemed.7b00133 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Figure 2. LIBS spectrum of (a) a Russet potato and (b) the flesh of a banana. Note that the wavelength range is much smaller here as compared to the wavelength range in Figure 1.

groups of students and can easily be adopted for demonstration purposes. As shown in Figures 1 and 2, the technique is readily adapted to a wide range of solid samples, with little to no sample preparation.

The emission spectra from a potato skin (Figure 2a) and a banana (Figure 2b) clearly show the presence of potassium. Ultimately, students were able to learn that LIBS can be used to rapidly assess the nutrient content in soil.10 Moreover, students observed that the red soil would likely be suited for growing potatoes as it contained a relatively high amount of K, in addition to other key soil nutrients such as Ca and Mg.18 Now the question becomes could one grow bananas on Mars? In order to address this question, students examined spectra from two Martian soil samples, Ithaca and Akaitcho (Figures 7−8 in the Supporting Information).23,28 The spectra from both Martian soil samples show the presence of K, supporting the notion that bananas could be grown in Martian soils. Furthermore, the elemental emission from the Martian soils shows striking similarities to the red terrestrial soil. All three soil samples show essential plant nutrients including Mg, Ca, and K. Finally, students were able to determine that a granite rock sample (Figure 5 in the Supporting Information) contained K, while other rock samples contained Ca (e.g., Figures 4 and 6 in the Supporting Information), but not K. Analysis of student-supplied samples generated additional student interest, helped students gain a better appreciation for sample-to-sample changes in elemental composition, illustrated the ability of LIBS to rapidly assess the elemental composition of solid samples, and provided additional comparisons of terrestrial rock samples to those found in Martian samples by the ChemCam.23

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00133. Notes for the instructor, handouts for students, and data collected (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Rob Milofsky: 0000-0003-4861-5146 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded through Fort Lewis College and Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract DE-NA0003525.

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SUMMARY This experiment represents a significant improvement in the traditional flame test for qualitative metal analysis and illustrates, for the first time, the use of LIBS for real-time analysis of solid samples in a teaching laboratory setting. Application of a commercially available spectrometer allows for rapid data acquisition from multiple samples. Moreover, due to the high throughput, the instrumentation is well-suited for large

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DOI: 10.1021/acs.jchemed.7b00133 J. Chem. Educ. XXXX, XXX, XXX−XXX