A Simple Laser Induced Breakdown Spectroscopy (LIBS) System for

Mar 13, 2013 - A LIBS (laser induced breakdown spectroscopy) spectrometer constructed by the instructor is reported for use in undergraduate analytica...
0 downloads 4 Views 429KB Size
Download PDF
Article pubs.acs.org/jchemeduc

A Simple Laser Induced Breakdown Spectroscopy (LIBS) System for Use at Multiple Levels in the Undergraduate Chemistry Curriculum David W. Randall,* Ryan T. Hayes, and Peter A. Wong Department of Chemistry and Biochemistry, Andrews University, Berrien Springs, Michigan 49104-0430, United States S Supporting Information *

ABSTRACT: A LIBS (laser induced breakdown spectroscopy) spectrometer constructed by the instructor is reported for use in undergraduate analytical chemistry experiments. The modular spectrometer described here is based on commonly available components including a commercial Nd:YAG laser and a compact UV−vis spectrometer. The modular approach provides a flexible arrangement that allows the use of the components in other experimental techniques, such as Raman spectroscopy and measurement of lifetimes of excited states. Integrating LIBS into the undergraduate analytical chemistry curriculum gives students experience with this important, emerging analytical method as well as hands-on experience with this common type of laser. Finally, experiments in which the LIBS spectrometer is used in both upper- and lower-division chemistry courses as well as a use for forensic chemistry are outlined. KEYWORDS: Upper-Division Undergraduate, First-Year Undergraduate/General, Analytical Chemistry, Environmental Chemistry, Laboratory Instruction, Atomic Spectroscopy, Lasers, Instrumental Methods, Forensic Chemistry, Laboratory Equipment/Apparatus

L

very respectable detection levels to parts per million (ppm) levels (depending on element),1,34,35 making LIBS a bit less sensitive than ICP-OES (inductively coupled plasma-optical emission spectroscopy) or graphite furnace AA or AE (atomic absorption or atomic emission). The slightly lower sensitivity is offset by that facts that LIBS requires very little sample preparation or digestion and works for all elements without element-specific configuration (AA lamps). Furthermore, a quantitative LIBS experiment for undergraduates characterizing silicate samples has been published recently in this Journal.19 The simple LIBS spectrometer described here can be constructed for use in a primarily undergraduate institution. Further refinements to the instrument described here are certainly possible (vida infra). Also described is a LIBS-based experiment to give upper-division undergraduate students more experience with LIBS concomitant with the use of a nanosecond pulsed laser. Finally, an experiment to give firstyear college students hands-on, safe experience with a laser and an optical spectrometer for a chemical analysis is described.

IBS (laser induced breakdown spectroscopy) continues to emerge as an important atomic analytical technique1−4 finding application in a broad range of fields including cultural heritage samples,5,6 environmental science,7 biomedical samples,8−10 analysis of molten inorganic salts,11 space exploration,12,13 forensic science,14,15 mineral analysis in plants,16 aerosol samples,17,18 undergraduate chemical education19 among many others as described in recent monographs,20−23 reviews,24−27 and a dedicated issue of Spectrochemica Acta B.28 Of special interest29 is the LIBS spectrometer in the “ChemCam” instrument on the NASA Mars rover Curiosity that is designed (along with other instruments) to assess the chemical composition of Martian rocks30,31 from distances up to 7 m. LIBS is based on atomic emission and is appealing analytically because one can ascertain in a matter of seconds the elemental composition of a sample with minimal sample preparation.1,2,27,32 Further, LIBS can analyze all elements including light, difficult-to-analyze carbon, nitrogen, and halogens. The physical basis of LIBS is essentially that of atomic emission, but uses a unique excitation method.1,2 Energy to populate atomic excited electronic states is supplied by a focused, high-powered laser pulse upon a sample that causes a small portion to be ablated. A plasma forms with temperatures reaching 10,000 K1,33 that places ablated atoms and ions in excited electronic states. A fraction of these atoms and ions return to the ground state by atomic emission.2 By matching the observed emission peaks to characteristic emission peaks for an element (or by comparing the spectrum to that of a known standard), one can identify the material present. Recent work demonstrates that LIBS can be used quantitatively with © 2013 American Chemical Society and Division of Chemical Education, Inc.

■

CONSTRUCTING THE LIBS SPECTROMETER As reviewed by Pasquini et al.,27 commercial LIBS instruments are available for purchase from several vendors.36−39 List prices for a complete systems can approach $90,000 (which includes a Nd:YAG laser). However, for a limited number of upperdivision laboratory experiments, purchasing a dedicated, though very capable, instrument may not be fiscally justifiable. A highpowered laser (most often a Q-switched Nd:YAG) is a Published: March 13, 2013 456

dx.doi.org/10.1021/ed300350a | J. Chem. Educ. 2013, 90, 456−462

Journal of Chemical Education

Article

with a 5.0 cm focal length lens (diameter = 4.0 cm) on a sample mounted in a sample holder. As with all spectrometers, sample placement is an important consideration. Simple sample holders are constructed of strips of cardstock paper (∼8 mm × ∼70 mm) painted matte black. The solid sample to be measured is attached to a sample holder with rubber cement, which dries in ∼2 min. Although more sophisticated sample holders might be envisioned, care must be exercised in selecting a material that does not reflect light and does not melt or cause a fire hazard. As shown in Figure S2 (in the Supporting Information), the cardstock sample holder is attached with a thumbscrew to a vertically mounted compact translation stage that gives fine-adjustment capability in the z direction (height).52 It is necessary to adjust z after each sample is loaded to ensure that the laser strikes the sample. As Figure 2

necessary (and expensive) element of the system that can be adapted to other experiments and student research.40 The approach of constructing modular spectrometers for educational use based on commonly available components is appealing pedagogically and economically41 and has been reported by several other groups.42−46 The simple LIBS instrument described here adopts such a modular approach that allows for the (expensive) laser to be repurposed so that students have the opportunity to learn other experimental techniques such as Raman spectroscopy and measurement of excited state lifetimes. Further refinement of instrument design is certainly possible with additional complexity47 and a recent example of a sophisticated LIBS instrument used for quantitative undergraduate labs has been recently reported in this Journal.19 The point in this article is to emphasize that a simple spectrometer design and construction by the instructor gives acceptable LIBS results and exposes students to this powerful methodology. A logical extension of the design described here would be to perform simultaneous Raman and LIBS (as on the Mars rover) and to improve instrument design to make quantitative measurements possible, which likely requires a more sophisticated data acquisition configuration (vida infra).14,19,47−49 The central feature of a LIBS system is a high-power laser needed for sample ablation and plasma formation (an unavoidable expense in a LIBS instrument). The system described here (Figure 1) uses a Continuum Minilite II

Figure 2. Detail of the arrangement of the sample holder and detector for the lab-built LIBS spectrometer.

indicates, the sample is positioned in the xy plane so that laser light is incident upon the sample at approximately 45°. The sample holder is positioned within a 1 cm cuvette holder that is fixed to the optics table. The cuvette holder helps to position the sample holder more reproducibly. For a given laboratory period, it is usually necessary to adjust the position of the sample holder in the xy plane once to optimize the LIBS signal intensity. The optimal sample position may not be precisely aligned with the spectrometer input. The optical configuration and alignment in xy plane is performed by the instructor for reasons of time efficiency. However, if a learning objective for the lab included optics configuration or instrument design, the optical alignment could be performed by students.53 The spectrum of emitted photons is acquired with a USB2000 Ocean Optics compact UV−vis spectrometer54 connected to a computer. The embedded, miniaturized f/4 Czerny− Turner spectrometer is coupled to a 2048-element linear CCD array. The light source, cuvette holder, and fiber-optic cable, used for typical UV−vis spectrometry applications, are removed from the spectrometer unit. The emitted photons are collected directly at the spectrometer input with no collection optics. As shown in Figure 2, the spectrometer is placed approximately 90° to the incoming laser light and approximately 45° to the plane where the sample is mounted on the sample holder. The spectrometer entrance slits are adjusted to the height of the beam. After the beam hits the sample, the emitted photons from the plasma will be sensed by the spectrometer’s CCD detector. The Ocean Optics software is used to collect and

Figure 1. Schematic overview of the LIBS spectrometer.

Nd:YAG nanosecond pulsed laser configurable for 1064 nm (fundamental), 532 nm (second harmonic), 355 nm (third harmonic), or 266 nm (fourth harmonic) laser radiation. For LIBS experiments, the 1064 nm fundamental is used, consistent with commercial vendor design.27 The fundamental emits the highest laser power (50 mJ in 7 ns pulse gives an average power density of ∼10 W cm−2 50), which produces the necessary intense plasma. Further, scattered 1064 nm radiation is outside the optical detection window of the spectrometer. Scattered photons from the visible (and UV Nd:YAG harmonic lines) could be of sufficiently high intensity to damage permanently the detector, were they to be used for plasma formation.51 The laser can be operated to 10 Hz, but a single ∼7 ns pulse of ∼50 mJ is usually sufficient to produce a reasonable emission spectrum. Multiple pulses may be helpful if the sample is difficult to ablate or form a plasma. The laser light is focused 457

dx.doi.org/10.1021/ed300350a | J. Chem. Educ. 2013, 90, 456−462

Journal of Chemical Education

■

POSSIBLE EXTENSIONS TO THE SPECTROMETER The purpose of this article is to emphasize the relative ease with which a LIBS spectrometer can be created and used in undergraduate courses at the introductory and upper-division level. This approach makes necessary compromises in design that ultimately sacrifice some aspects of performance. The instrument described here is well adapted to qualitative elemental identification and appears less so for quantitative applications. There are other publications that describe how to build a high-end LIBS spectrometer.14,19,47−49 Such a higherend system was used to good effect in a recently published experiment in this Journal where LIBS was used in a quantitative way by normalizing analytical peaks to an internal potassium standard of known concentration.19 Some elements of the current system to consider when developing an instrument for quantitative use would include: • Precisely controlled triggering for laser pulse and data acquisition • Repeatable and precise sample positioning • Delivery, choice, and analysis of internal standard for quantitation • High spectrometer resolution and high overall bandwidth • High sensitivity of spectrometer The experiments and apparatus described here have been used to analyze solid materials. With the sample holder described here, it could be possible to utilize pelletized samples as described by Chinni19 among others.

subtract a dark spectrum, adjust integration time, and record spectra for further analysis. To record LIBS spectra, students initiate spectral acquisition and “simultaneously” press the laser trigger button. The integration time in the software is typically set sufficiently long to include the single laser pulse (and subsequent emission) allowing for uncertainty in the precise timing between the laser pulse and acquisition gate, but not so long that noise collected when no signal is present becomes problematic. Integration times of ∼350 ms seem to be an adequate balance.55 Again, more sophisticated (and complex) triggering arrangements are possible.47,49 Figure S3 (in the Supporting Information) shows a timing diagram of the instrument. If the laser is triggered to generate multiple pulses, longer spectral integration times allow for the possibility to integrate more signal (and more noise). The manual synchronization between manual laser trigger and spectral acquisition described above complicates pulse counting. However, by controlling the laser repetition rate and spectral acquisition time it is possible to know the pulse count to within one. This is acceptable for the qualitative work described here, but would pose a difficulty for quantitative work. The entire apparatus described is mounted on an optics bench plate.56 While care in optical alignment including reproducibility of sample position is important for optimal performance and quantitation, extreme care in alignment is not needed for qualitative work. After samples have been loaded, the entire sample area can be covered with black cloth to minimize laser scattering. A sketch of the sample and detector positioning is in Figure 2. Chemical samples were prepared as described in the Supporting Information. If the number of students in a laboratory session is large, the requirement of laser goggles57 for each student could be financially discouraging. This issue is addressed in large, lowerdivision laboratory sections by setting up the LIBS spectrometer (laser, sample, and spectrometer) in a windowless room. All lights are turned off to eliminate background signals from Hg emission in fluorescent lights. A USB cable connects the spectrometer to its data acquisition computer in an adjacent room. This cable and a remote laser triggering cable pass between the two rooms through a hole in the wall. After the sample has been placed and the laser room darkened, students move to the lighted room where the data acquisition computer is located. A push-button remote control, activated from the lighted room, triggers a laser pulse. The user “simultaneously” manually initiates spectral data acquisition from the Ocean Optics software (vida supra). If necessary, the laser pulse can be retriggered remotely to obtain a satisfactory emission spectrum. In this arrangement, the students can acquire and examine spectra in the laser-free room, greatly reducing the risk of exposure to high-powered laser radiation. Collecting data remotely in the lit room also minimizes the hazard presented by of having a group of students in a darkened room containing optically aligned components on the optics table.

■

Article

■

USING THE LIBS APPARATUS IN UPPER-DIVISION INSTRUMENTAL ANALYSIS A LIBS experiment is included in the atomic spectroscopy module of a one-semester upper-division instrumental analysis course. The objectives for the students for the LIBS experiment are to learn about the LIBS method, use atomic emission lines to identify the presence of elements in samples, to manage the collection of a large amount of data, and to get practical experience using a commonly encountered laser. As mentioned previously, it could be possible to augment the learning objectives to include assembly and alignment of the modular LIBS spectrometer and to perform more characterization of the spectrometer. A four-hour lab activity is built around qualitative assessment of elements present in various unknowns. To illustrate the applicability of LIBS to answer quickly a relevant environmental question, students are presented with a paint chip (see the Supporting Information to make the samples) and asked to determine whether it contains lead using LIBS (Figure 3). Tums and Pepto-Bismol samples are provided to groups of 1−3 students, along with relevant standards, who are to determine some of the elements present in these common materials (Figure 4). Finally, students are given 2−3 unknowns and relevant standards used to identify (some of) the elements present. Students mount the samples on the holder, position the sample holder in the apparatus, and initiate data acquisition. Students then compare their data with atomic emission line tables, either printed as found in the CRC Handbook of Chemistry61 (or similar) or online at the NIST Atomic Spectra Database.62 This definitively assigns peaks and therefore confirms the specific atomic origin of the emission spectra.63 In post lab work, students are asked to characterize the bandwidth of spectrometer by examining an isolated peak from

HAZARDS

There are substantial safety considerations for using this labbuilt spectrometer with students.58−60 Safety precautions are indicated in the Supporting Information. 458

dx.doi.org/10.1021/ed300350a | J. Chem. Educ. 2013, 90, 456−462

Journal of Chemical Education

Article

In contrast to other experiments, the result of an analysis can be obtained without a time-consuming preparation of the sample. In this experiment, groups of 2−3 students rotate between stations where NMR, FT-IR, Raman, and LIBS data are collected by teaching assistants or the instructor for a particular simulated crime scene sample. At a fifth data analysis station, a teaching assistant helps students interpret their spectral data.67 In preparation for the LIBS analysis, the students mount a paint chip sample as given on a holder and place it in the cuvette holder. They trigger the laser by a remote switch and initiate the spectrometer at the same time to collect the emission spectrum. The spectrum of the sample is then compared with emission spectra for reference samples containing the elements of interest, which are obtained with the same equipment before the experiment. The presence or absence of the element in the sample is ascertained from comparison to these spectra. From the responses of the students, this experiment has been rated to be the most memorable one. They have been generally excited by the experience of firing a high power laser at a target sample and impressed by the result produced.

Figure 3. Student collected LIBS data to determine if lead is present in paint chips. The sets of peaks from 220−280 nm, 360−380 nm, and 406 nm (marked with ↓) are unique for Pb. The peaks at 460, 500, 566 nm (marked with *) are due to N from air.

■

LIBS IN AN AUTHENTIC FORENSIC CHEMISTRY APPLICATION Finally, the LIBS spectrometer described here was used in a forensic investigation in which a local county forensic analyst needed to ascertain whether a metal shaving collected from a suspect’s residence was aluminum or lithium. The sample was excited by laser; the emission spectrum of the sample, when compared with those of aluminum and lithium (Figure 5), confirmed definitively that it was aluminum. The entire analysis was completed within a minute, reinforcing the time advantage for the LIBS method. Figure 4. Student collected LIBS data to determine if Pepto-Bismol contains bismuth. The set of peaks from 285−310 nm (marked with ↓) are unique for Bi. The peaks at 460, 500, 566 nm (marked with *) are due to N from air.

one of the samples to comment on ways in which the experiment could be quantitative.64 Numerous other possibilities in the qualitative or quantitative analysis could be developed for biological materials, industrial applications, and environmental samples as suggested in the literature.20,21,25,26,65 At our college, upper-division lab sections are typically fewer than 5 students. For larger groups of upperdivision analytical chemistry students, one could envision 2−3 groups of students sharing the instrument: one group could mount a sample while the other acquires data on their sample. It is anticipated that sharing a LIBS instrument among student groups should be as feasible as sharing an atomic absorption spectrometer (or other instrument).

Figure 5. LIBS answers the forensic chemistry problem of whether a metallic sample is Li or Al. Nitrogen plasma line is indicated with an asterisk (*).

■

■

USING THE LIBS SPECTROMETER IN GENERAL CHEMISTRY A crime scene investigation (CSI) forensic-themed experiment was developed and incorporated into the general chemistry laboratory course to generate interest as well as to teach modern chemical techniques.66 The intent of developing this lab was to develop experiments with technique-oriented objectives and to provide hands-on experience with advanced instrumentation to beginning students.

SUMMARY The assembly of a simple LIBS spectrometer using commonly available components including a compact UV−vis spectrometer and Nd:YAG laser was described. This spectrometer can be integrated into the chemistry curriculum at multiple levels to answer authentic problems in analytical chemistry. Providing access to this lab-assembled instrument opens the possibility of adding another instrument to students’ arsenal. Students find 459

dx.doi.org/10.1021/ed300350a | J. Chem. Educ. 2013, 90, 456−462

Journal of Chemical Education

Article

(13) Cremers, D. A., Remote Analysis by LIBS: Application to Space Exploration. In Laser-Induced Breakdown Spectroscopy, 1st ed.; Thakur, S. N., Singh, J. P., Eds.; Elsevier: Amsterdam; Boston, 2007; pp 353− 380. (14) Moros, J.; Lorenzo, J. A.; Lucena, P.; Tobaria, L. M.; Laserna, J. J. Anal. Chem. 2010, 82 (4), 1389−1400. (15) Silva, M. J.; Cortez, J.; Pasquini, C.; Honorato, R. S.; Paim, A. P. S.; Pimentel, M. F. J. Braz. Chem. Soc. 2009, 20 (10), 1887−1894. (16) Trevizan, L. C.; Santos, D.; Samad, R. E.; Vieira, N. D.; Nomura, C. S.; Nunes, L. C.; Rufini, I. A.; Krug, F. J. Spectrochim. Acta, Part B 2008, 63 (10), 1151−1158. (17) Hahn, D. W. Spectroscopy 2010, 24, 23−28. (18) Panne, U.; Hahn, D. Analysis of Aerosols by LIBS. In LaserInduced Breakdown Spectroscopy (LIBS): Fundamentals and Applications; Miziolek, A. W., Palleschi, V., Schechter, I., Eds.; Cambridge University Press: Cambridge, U.K.; New York, 2006; pp 194−253. (19) Chinni, R. C. J. Chem. Educ. 2012, 89 (5), 678−680. (20) Miziolek, A. W.; Palleschi, V.; Schechter, I. Laser-Induced Breakdown Spectroscopy (LIBS): Fundamentals and Applications; Cambridge University Press: Cambridge, U.K.; New York, 2006. (21) Thakur, S. N.; Singh, J. P. Laser-Induced Breakdown Spectroscopy. 1st ed.; Elsevier: Amsterdam; Boston, 2007. (22) Noll, R. Laser-Induced Breakdown Spectroscopy; Springer: New York, 2011. (23) Cremers, D. A.; Radziemski, L. J. Handbook of Laser-Induced Breakdown Spectroscopy; John Wiley & Sons: Chichester, West Sussex, England; Hoboken, NJ, 2006. (24) Evans, E. H.; Day, J. A.; Palmer, C. D.; Smith, C. M. M. J. Anal. At. Spectrom. 2009, 24 (6), 711−733. (25) Taylor, A.; Day, M. P.; Marshall, J.; Patriarca, M.; White, M. J. Anal. At. Spectrom. 2012, 27 (4), 537−576. (26) Carter, S.; Fisher, A. S.; Goodall, P. S.; Hinds, M. W.; Lancaster, S.; Shore, S. J. Anal. At. Spectrom. 2011, 26 (12), 2319−2372. (27) Pasquini, C.; Cortez, J.; Silva, L. M. C.; Gonzaga, F. B. J. Braz. Chem. Soc. 2007, 18 (3), 463−512. (28) Yalcin, Ş.; Fantoni, R. Spectrochim. Acta, Part B 2012, 74−75 (0), 1−2. (29) Agle, G. W. D. C. Rover’s Laser Instrument Zaps First Martian Rock http://www.nasa.gov/mission_pages/msl/news/msl20120819b. html (accessed Dec 2012). (30) Wiens, R. C.; Maurice, S.; Barraclough, B.; Saccoccio, M.; Barkley, W. C.; Bell, J. F., III; Bender, S.; Bernardin, J.; Blaney, D.; Blank, J.; Bouyé, M.; Bridges, N.; Bultman, N.; Caïs, P.; Clanton, R. C.; Clark, B.; Clegg, S.; Cousin, A.; Cremers, D.; et al. Space Sci. Rev. 2012, DOI: 10.1007/s11214-012-9902-4. (31) Cousin, A.; Forni, O.; Maurice, S.; Gasnault, O.; Fabre, C.; Sautter, V.; Wiens, R. C.; Mazoyer, J. Spectrochim. Acta, Part B 2011, 66 (11−12), 805−814. (32) Gaudiuso, R.; Dell’Aglio, M.; De Pascale, O.; Senesi, G. S.; De Giacomo, A. Sensors 2010, 10 (8), 7434−7468. (33) Cremers, D. A.; Radziemski, L. J. Handbook of Laser-Induced Breakdown Spectroscopy; John Wiley & Sons: Chichester, West Sussex, England; Hoboken, NJ, 2006; pp 23−52. (34) Tognoni, E.; Palleschi, V.; Corsi, M.; Cristoforetti, G.; Omenetto, N.; Gornushkin, I.; Smith, B. W.; Winefordner, J. D. From Sample to Signal in Laser-Induced Breakdown Spectroscopy: A Complex Route to Quantitative Analysis. In Laser-Induced Breakdown Spectroscopy (LIBS): Fundamentals and Applications; Miziolek, A. W., Palleschi, V., Schechter, I., Eds.; Cambridge University Press: Cambridge, U.K.; New York, 2006; pp 122−170. (35) Cremers, D. A.; Radziemski, L. J. Handbook of Laser-Induced Breakdown Spectroscopy. John Wiley & Sons: Chichester, West Sussex, England; Hoboken, NJ, 2006; p 40. (36) AvantesLIBS AvaLIBS-50 and AvaLIBS-100 Laser Induced Breakdown Spectroscopy http://www.avantes.com/SpectrometerSystems/AvaLIBS-50-and-AvaLIBS-100-Laser-Induced-BreakdownSpectroscopy-/Detailed-product-flyer.html (accessed Dec 2012).

the LIBS method appealing because it requires little sample preparation and provides results very quickly. These attributes of LIBS could be of particular benefit to analytical chemistry curricula where students develop and answer analytical chemistry questions.68−73

■

ASSOCIATED CONTENT

S Supporting Information *

Detailed information about the hazards; student handouts; instructions for the instructor; schematics of the LIBS. This material is available via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Students in our Instrumental Analysis course have helped guide our thoughts as this lab was developed. Purchase of the Nd:YAG laser at Andrews University was supported by a matching grant (SG-95-112) from the Camille and Henry Dreyfus Foundation to P.A.W., who initially developed and deployed this laboratory exercise. Dewey Murdick provided the metal shaving samples. The peer reviewers graciously called our attention to several valuable references.

■

REFERENCES

(1) Cremers, D. A.; Radziemski, L. J. History and Fundamentals of LIBS. In Laser-Induced Breakdown Spectroscopy (LIBS): Fundamentals and Applications; Miziolek, A. W., Palleschi, V., Schechter, I., Eds.; Cambridge University Press: Cambridge, U.K.; New York, 2006; pp 1−39. (2) Thakur, S. N.; Singh, J. P. Fundamentals of Laser Induced Breakdown Spectroscopy. In Laser-Induced Breakdown Spectroscopy, 1st ed.; Thakur, S. N., Singh, J. P., Eds.; Elsevier: Amsterdam; Boston, 2007; pp 3−22. (3) Sneddon, J.; Lee, Y. I. Anal. Lett. 1999, 32 (11), 2143−2162. (4) Sneddon, J., Chem. Educator 1998, 3 (6), S1430-4171(98)062608. DOI: 10.1333/s00897980260a. (5) Anzano, J.; Gutierrez, J.; Villoria, M. Anal. Lett. 2005, 38 (12), 1957−1965. (6) Anglos, D.; Miller, J. C. Cultural Heritage Applications of LIBS. In Laser-Induced Breakdown Spectroscopy (LIBS): Fundamentals and Applications; Miziolek, A. W., Palleschi, V., Schechter, I., Eds.; Cambridge University Press: Cambridge, U.K.; New York, 2006; pp 332−367. (7) Burakov, V. S.; Raikov, S. N.; Tarasenko, N. V.; Belkov, M. V.; Kiris, V. V. J. Appl. Spectrosc. 2010, 77 (5), 595−608. (8) Singh, V. K.; Rai, A. K. Laser Med. Sci. 2011, 26 (5), 673−687. (9) Telle, H. H.; Samek, O. Biomedical Applications of LIBS. In Laser-Induced Breakdown Spectroscopy (LIBS): Fundamentals and Applications; Miziolek, A. W., Palleschi, V., Schechter, I., Eds.; Cambridge University Press: Cambridge, U.K.; New York, 2006; pp 282−313. (10) Bechard, S.; Mouget, Y. LIBS for the Analysis of Pharmaceutical Materials. In Laser-Induced Breakdown Spectroscopy (LIBS): Fundamentals and Applications; Miziolek, A. W., Palleschi, V., Schechter, I., Eds.; Cambridge University Press: Cambridge, U.K.; New York, 2006; pp 314−331. (11) Panne, U.; Neuhauser, R. E.; Haisch, C.; Fink, H.; Niessner, R. Appl. Spectrosc. 2002, 56 (3), 375−380. (12) Salle, B.; Cremers, D. A.; Maurice, S.; Wiens, R. C. Spectrochim. Acta, Part B 2005, 60 (4), 479−490. 460

dx.doi.org/10.1021/ed300350a | J. Chem. Educ. 2013, 90, 456−462

Journal of Chemical Education

Article

(37) LLA Instruments LIBS - Atomic Emission Spectroscopy. http:// www.lla-instruments.com/analytical-equipment/echelle-spectrometer (accessed 26 Feb 2013). (38) OceanOptics_LIBS Ocean Optics: The LIBS2500plus LIBS Systems. http://www.oceanoptics.com/Products/libs.asp (accessed Dec 2012). (39) StellarNet PORTA-LIBS-2000 System: Laser Induced Breakdown Spectroscopy http://www.stellarnet-inc.com/ popularconfigurations_plasmasystems.htm (accessed Dec 2012). (40) A Nd:YAG laser is utilized in the system described here and in most LIBS systems. Other lasers could be and have been used including excimer, CO2, Ti:sapphire, and semiconductor lasers. The laser needs to generate a sufficiently high power density to match the breakdown threshold irradiance, which depends on wavelength and importantly material. Cremers and Radziemski (ref 23, p 40) tabulate breakdown threshold irradiances of solids to be on the order of 1010 W/cm2 for ∼7 ns pulses. In principle, any laser that can generate that power density could be used for LIBS. A peak power calculation suggests that for a highly focused (0.1 mm beam diameter) class 3b (PAvg ≤ 30 mW pulsed) could deliver that power level. Lasers other than the class IV Nd:YAG described here have not been evaluated in the spectrometer described here. These high peak power densities are only available with pulsed lasers. (41) List price for a Nd:YAG is ∼$U.S. 20,000, which can be used for other experiments in the curriculum. List price for an Ocean Optics USB4000 is under $U.S. 3000 (USB2000, $2000, is no longer available). The various optics, mounts stages, etc. described are likely under $U.S. 5000, for a total cost of under $US 30,000 (list price). Though less expensive, this system is less capable than commercial systems (lower resolution, less sophisticated triggering, etc.). (42) Patterson, B. M.; Danielson, N. D.; Lorigan, G. A.; Sommer, A. J. J. Chem. Educ. 2003, 80 (12), 1460−1463. (43) DeGraff, B. A.; Hennip, M.; Jones, J. M.; Salter, C.; Schaertel, S. A. Chem. Educator 2002, 7 (1), 15−18. (44) Johnson, D.; Larsen, P.; Fluellen, J.; Furton, D.; Schaertel, S. A. Chem. Educator 2008, 13 (1), 82−86. (45) Nazarenko, A. Y. Spectrosc. Lett. 2004, 37 (3), 235−243. (46) Mohr, C.; Spencer, C. L.; Hippler, M. J. Chem. Educ. 2010, 87 (3), 326−330. (47) Cremers, D. A.; Radziemski, L. J. Handbook of Laser-Induced Breakdown Spectroscopy; John Wiley & Sons: Chichester, West Sussex, England; Hoboken, NJ, 2006; pp 53−98. (48) Moros, J.; Laserna, J. J. Anal. Chem. 2011, 83 (16), 6275−6285. (49) Rai, V. N.; Thakur, S. N. Instrumentation for Laser-Induced Breakdown Spectroscopy. In Laser-Induced Breakdown Spectroscopy, 1st ed.; Thakur, S. N., Singh, J. P., Eds.; Elsevier: Amsterdam; Boston, 2007; pp 113−134. (50) Assume 1 Hz rep rate, 0.8 mm focused spot size, 50 mJ per 7 ns pulse. Peak power density ≈ 1 × 109 W cm−2. (51) When using the 1064 nm fundamental, no radiation is observed from the harmonics. The scattered visible and UV Nd:YAG harmonics could, in principle, mask emission lines used for elemental identification, because the emission wavelengths of interest are often between 200 and 700 nm. However, such masked lines could be replaced with other existent atomic emission lines if they were not also masked by air plasma lines. The LIBS instrument described here has exclusively used the 1064 nm Nd:YAG fundamental. (52) The z stage is arranged above the 10 mm cuvette holder in Figure 1 such that the sample on the sample holder hangs down from the z stage into in the approximate center of the cuvette holder (see Figure S2). The cuvette holder is convenient, but not essential, for adjusting the sample position in the LIBS apparatus because it provides an approximate location in space where the sample should be placed. From a safety perspective, the sample holder provides a clear location of where the invisible high-power laser radiation will be. The cuvette holder is used for other experiments done with the laser. (53) The alignment is not exceptionally technically demanding because of the design simplicity. The alignment can be completed by the instructor in a half hour or less. The cuvette holder and laser are

left in place between uses; this also minimizes alignment time. The presence of the cuvette holder assists with rough optical path alignment. It should be noted that alignment of an invisible laser beam may not be “easy” for students who have limited familiarity with optics systems alignment. SAFETY: Alignment of an invisible laser beam represents a substantial safety risk. (54) Ocean Optics USB2000: miniature Fiber Optic Spectrometer. http://www.oceanoptics.com/Products/usb2000.asp (accessed Dec 2012). (55) Spectral acquisition occurs during plasma generation and subsequent atomic emission, but also when no signal is present; signal is generated with a low duty cycle, while noise is generated the entire time spectral acquisition occurs. (56) A ThorLabs 18 in. × 24 in. optics plate or optics breadboard (similar to ThorLabs PBH11122 list price $U.S. 581) sits atop a metal desk and is convenient for mounting components. Beyond the LIBS apparatus described, other optical instruments using the Nd:YAG laser are configured on the optics breadboard (not described here). The optics plate or breadboard is not strictly required and other methods to maintain alignment between laser, lens, sample, and USB spectrometer could be possible. (57) While not recommending a particular vendor, laser safety goggles are available from many vendors including: Laser Safety Industries, LaserVision, Glendale, and NoIR. Seek a high optical density (4+ or higher) at 1064 nm and all laser lines that might be used. (58) Occupational Safety & Health Administration (OSHA) Laser Hazards. http://www.osha.gov/SLTC/laserhazards/ (accessed Dec 2012). (59) ANSI American National Standard for Safe Use of Lasers and American National Standard for Safe Use of Lasers in Research, Development, or Testing http://webstore.ansi.org/FindStandards.aspx ?SearchString=ANSI+Z136.1+and+Z136.8+Combination+Set&Search Option=0&PageNum=0&SearchTermsArray=null|ANSI+Z136.1+and +Z136.8+Combination+Set|null#.UFoSdK5rBKh (accessed Dec 2012). (60) International Electrotechnical Commission IEC 60825-1 ed2.0 Safety of laser products - Part 1: Equipment classification and requirements http://webstore.iec.ch/webstore/webstore.nsf/ ArtNum_PK/37864?OpenDocument (accessed Dec 2012). (61) CRC Handbook of Chemistry and Physics, 68th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1988. (62) Ralchenko, Y.; Kramida, A.; Reader, J.; Team, N. A. NIST Atomic Spectra Database (version 4.1). http://physics.nist.gov/asd (accessed Dec 2012). (63) Spectroscopic notation is employed in these resources. That Mg I designates un-ionized magnesium and Mg III means Mg2+ needs to be explained to chemistry students. A useful table of elements and useful lines is tabulated by Cremers and Radziemski in Handbook of Laser-Induced Breakdown Spectroscopy (ref 23) p 263ff. (64) The latter was suggested by a reviewer. (65) Butler, O. T.; Cairns, W.; Cook, J. M.; Davidson, C. M. J. Anal. At. Spectrom. 2011, 26 (2), 250−286. (66) Slowinski, E.; Wolsey, W. C.; Masterton, W. L.; Wong, P. General Chemistry Laboratory Manual Customized Version for Andrews University; Thomson Brooks/Cole: Belmont CA, 2006; pp 159−161. (67) These five stations accommodate a 60 student general chemistry lab section. The Supporting Information contains further comments for logistically managing this lab for general chemistry students. (68) Adami, G. J. Chem. Educ. 2006, 83 (2), 253−256. (69) Breslin, V. T.; Sañudo-Wilhelmy, S. A. J. Chem. Educ. 2001, 78 (12), 1647−1651. (70) Butler, L. R.; Edwards, M. R.; Farmer, R.; Greenly, K. J.; Hensler, S.; Jenkins, S. E.; Joyce, J. M.; Mann, J. A.; Prentice, B. M.; Puckette, A. E.; Shuford, C. M.; Porter, S. E. G.; Rhoten, M. C. J. Chem. Educ. 2009, 86 (9), 1095−1098. (71) Fakayode, S. O.; King, A. G.; Yakubu, M.; Mohammed, A. K.; Pollard, D. A. J. Chem. Educ. 2011, 89 (1), 109−113. 461

dx.doi.org/10.1021/ed300350a | J. Chem. Educ. 2013, 90, 456−462

Journal of Chemical Education

Article

(72) Giancarlo, L. C.; Slunt, K. M. J. Chem. Educ. 2004, 81 (6), 868− 869. (73) MacNeil, J.; Gess, S.; Gray, M.; McGuirk, M.; McMullen, S. J. Chem. Educ. 2011, 89 (1), 114−116.

462

dx.doi.org/10.1021/ed300350a | J. Chem. Educ. 2013, 90, 456−462