Laboratory Experiment pubs.acs.org/jchemeduc
A Simple LIBS (Laser-Induced Breakdown Spectroscopy) Laboratory Experiment To Introduce Undergraduates to Calibration Functions and Atomic Spectroscopy Rosemarie C. Chinni* Department of Math and Sciences, Alvernia University, Reading, Pennsylvania 19607, United States S Supporting Information *
ABSTRACT: This laboratory experiment introduces students to a different type of atomic spectroscopy: laser-induced breakdown spectroscopy (LIBS). LIBS uses a laser-generated spark to excite the sample; once excited, the elemental emission is spectrally resolved and detected. The students use LIBS to analyze a series of standard synthetic silicate samples that contained various elements at known concentrations. The students gain valuable experience developing calibration curves to determine sensitivity and detection limits for six elements in these samples. This experiment is applicable to an analytical or instrumental analysis course.
KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Atomic Spectroscopy, Calibration, Lasers, Materials Science, Metals
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atomic technique and learn about detection capabilities pertinent to any analytical or instrumental chemistry course.
ost undergraduate laboratories introduce students to traditional atomic absorption (AA) techniques either using flame or graphite furnace for sample introduction. The detection capabilities of traditional AA are widely known and many laboratories and methods are available in the literature.1−4 The main drawback of traditional AA is that it can only analyze one element at a time. This lab is not intended to replace a typical AA laboratory; it is intended to introduce students to another type of atomic spectroscopy. Laser-induced breakdown spectroscopy (LIBS) is a simple technique that uses a laser-generated spark to excite a sample. In LIBS, a focused laser pulse is directed onto a surface; this heats, ablates, atomizes, and ionizes the surface material and results in the formation of a plasma. The plasma light is spectrally resolved and detected. Unlike AA, LIBS is a multielement technique; thus, by acquiring one spectrum, the range of elements found in the sample can be analyzed. The LIBS spectrum can provide both qualitative and quantitative information about a sample. Overall, LIBS makes rapid measurements (less than 1 min); needs little or no sample preparation; can analyze solids, liquids, and gases; does not require extensive operator training; and is considered nondestructive. Over the past couple decades, LIBS has started to gain momentum in the industrial, governmental, and educational areas. LIBS has even been incorporated into at least one undergraduate instrumental analysis book and there are now textbooks dedicated solely to this technique.5−9 Universities are also starting to discuss LIBS and undergraduate students are being involved in LIBS research.10−13 In this lab, students develop an understanding of a different type of © 2012 American Chemical Society and Division of Chemical Education, Inc.
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LIBS SYSTEMS LIBS systems consist of a pulsed or Q-switched laser, a spectral resolution device, a detector, and computer for readout. Typical lasers include the Nd:YAG laser, eximer lasers, CO2 lasers, and microchip lasers. Possible spectral resolution devices consist of a grating (in form of spectrometer or monochromator), a filter, and echelle. Potential detectors are the charged coupled device (CCD), intensified CCD (ICCD), photodiode array, intensified photodiode arrays, photomultipler tubes, and avalanche photodiodes.7 The combined cost of the Nd:YAG laser and echelle spectrograph used at this institution was approximately $80,000; there are less expensive laser systems and spectrographs available that can achieve similar results. There is also a commercial LIBS system available. Ocean Optics offers a commercial LIBS system (LIBS2500plus) that ranges in pricing from $7,500 up to $36,000 depending on the number of channels needed for the application.14
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MATERIALS AND METHODS
Chemicals
Five synthetic silicate samples (GBW 07704-06, 08, 09, Brammer Standard Company, Houston, TX) were pressed into pellets using a hydraulic press (Model-C, Carver, Inc., Wabash, IN) to create a Published: March 15, 2012 678
dx.doi.org/10.1021/ed2003862 | J. Chem. Educ. 2012, 89, 678−680
Journal of Chemical Education smooth surface for LIBS analysis. Aluminum discs (catalog #040080-001, SCP Science, Champlain, NY) were filled with the synthetic silicate samples and a 31 mm die cast set (catalog #3902, Carver, Wabash, IN) was placed in the hydraulic press with each sample. Methanol (any grade, Sigma-Aldrich Corp., St. Louis, MO) was used the clean the die cast set between samples. These synthetic silicate samples contain trace elements ranging in concentrations from 1 to 10,000 ppm; these trace elements include Ba, Be, Cd, Cr, Li, Mn, Ni, Pb, Sr, Ti, and Zn.
HAZARDS
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RESULTS AND DISCUSSION
The Nd:YAG laser is a class IV laser that produces both visible and invisible laser radiation. It is necessary to avoid eye and skin exposure to the radiation. Proper eyewear should be used at all times when the laser is in use. The laser interlocks should be set up in conjunction with the room door to protect others entering the room. Gloves should be used to handle the synthetic silicate samples. Methanol is extremely flammable and toxic by inhalation. Methanol may be fatal or cause blindness if swallowed.
Equipment
A Nd:YAG laser (1064 nm) (Surelite II, Continuum, Santa Clara, CA) was operated at 10 Hz and 90 mJ per pulse. A 75 mm focal length lens was used to focus the laser pulse onto the sample. The spot size diameter on the sample was approximately 0.085 cm; this produced an approximate power density if 2.6 GW/cm2. The light was collected with an optical fiber (QP1000-2-UV-VIS, Ocean Optics, Dunedin, FL) placed near the sample and spectrally dispersed and detected with an echelle spectrograph with an ICCD (echelle, SE200, Catalina Scientific, Tucson, AZ; and ICCD, DH734-18F-03, IStar, Andor Technology, Belfast, Ireland). The echelle−ICCD used has a resolving power of approximately 1600. The data were taken using a 1 μs time delay and a 20 μs gate width, during which time the detector integrated the signal. A total of 30 shots were averaged per spectrum, which, as the laser was operated at 10 Hz, resulted in a 3 s collection window for each measurement. A digital delay generator (BNC Model 575-4C digital delay and pulse generator, Berkeley Nucleonic Corp., San Rafael, CA) was used to control the timing between the laser and the ICCD. A typical LIBS setup is shown in Figure 1. A list of other possible lasers, wavelength selection devices, and detectors is available in the Supporting Information.
Calibration curves were generated for six elements; the elements and pertinent information are listed in Table 1. LIBS produces Table 1. Pertinent Data for Elements Analyzed in the Synthetic Silicate Samples Element
Line Type
Wavelengtha/nm
Concentration Rangesb/ ppm
Ba Be Pb Li Mn Kc Sr Ti
II II I I I I II II
493.41 313.04, 313.11 405.78 670.78, 670.79 403.08, 403.31, 403.45 766.49 407.77 334.94
204−10,000 2.1−100 20.5−1000 33−1010 207−10,000 NAc 23−1000 204−10,000
a
For some elements (Be, Li, and Mn), a group of lines was not resolved. The area was measured under the whole group of lines. The wavelengths listed in this table are the literature values.18−20 bThe concentrations are listed in the Brammer Standard Online Catalog for Industrial Materials. c K was at constant concentration in the synthetic silicate samples; this element was used to ratio the other analyte peak areas.
both neutral (I) and singly ionized (II) atomic lines. The neutral lines are longer lived in the plasma whereas the singly ionized lines decay faster. Whether a neutral or singly ionized atomic line was used for this analysis depended on the specific element to be analyzed; the atomic lines chosen here are based on previous work with LIBS. These lines were chosen due to their emission intensity and if there were any interfering species at the specific line. There are many books and databases of atomic lines that list line types and relative intensities among other characteristics.18−20 When the laser plasma forms, there is a continuum of light that produces a raised background in the LIBS spectrum. To reduce this background and enhance the atomic lines, it is useful (but not necessary) to delay the detector. Typically, in LIBS, a 1 μs time delay is used; the time delay represents the time that the detector is delayed before viewing the plasma. Here, a 20 μs gate width was used; the gate width represents the time that the detector views the signal. The calibration curves were constructed two ways for each element. The first way plotted the analyte peak area versus the concentration; the next way plotted the ratioed peak areas versus the concentration. These synthetic silicate samples contain constant concentrations of certain elements. Sometimes, with LIBS, there is shot-to-shot variation in the spectra that are due to laser power fluctuations; thus, by taking a ratio of the analyte peak area to the peak area of an element with constant concentration, this shot-to-shot variation can be corrected and can sometimes produce better calibration curves. The students were required to plot the data these two ways to determine if there was a significant difference between the calculated detection limits.
Figure 1. A typical LIBS setup.
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Laboratory Experiment
EXPERIMENTAL PROCEDURE
Students worked in groups of 2−3. Each group examined a set of five pressed synthetic silicate discs that were prepared before the class.15 Each sample was analyzed five times. The students were taught how to use the LIBS system, analyze the pressed samples, and save their data. Each spectrum took less than 1 min to run and save. The data were processed using Microsoft Office Excel 2010 and a LabView program executable file written to calculate emission peak areas and peak intensities. Students plotted analyte peak area against concentration (in ppm) to obtain the calibration curves. They then calculated the detection limits (DL) based on a 3σ detection as defined by IUPAC.16 If a program is not available to calculate peak areas, the peak intensities can be used instead and these can be easily read from Microsoft Excel. The lab took 3 h.17 679
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Laboratory Experiment
Bostian from Applied Research Associates in Albuquerque, NM, for creating the Lab View executable file for peak area analysis.
Students calculated the detection limits (DL) using DL = 3σ/m; where σ is the standard deviation of the lowest readable concentration and m is the slope of the linear calibration curve. Detection limits for the various elements ranged from 8 to 300 ppm; these are shown in Table 2 with their correlations.
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Table 2. Detection Limits, Sensitivities, and Linear Correlations for Various Elements in the Synthetic Silicate Samples Ratioeda
Unratioed
Element
Detection Limit (ppm)
Sensitivity (ppm−1) [correlation]
Detection Limit (ppm)
Sensitivity (ppm−1) [correlation]
Ba Be Li Mn Pb Sr Ti
230 0.71 190 280 38 9.3 130
270 [0.96] 12000 [0.99] 2000 [0.98] 240 [0.99] 380 [0.99] 1400 [0.99] 170 [0.97]
200 0.45 150 260 56 14 190
1.9 × 10−4 [0.87] 8.4 × 10−3 [0.99] 1.3 × 10−3 [0.91] 1.7 × 10−4 [0.97] 2.8 × 10−4 [0.99] 1.0 × 10−3 [0.95] 1.3 × 10−4 [0.89]
a
The data were ratioed to K.
These detection limits are in the normal ranges for the specific elements.7 Also, a comparison of the detection limits using the ratioed and unratioed data did not show any improvement from one method to another. In general, the unratioed calibration curves produced only slightly better linear correlations than the ratioed ones. Therefore, the students learned that there was no benefit of ratioing this data, which meant that the laser power was not fluctuating during their experiment. The sensitivities are measured directly from the slope of the linear calibration curves (Table 2). Generally, there was not a correlation between sensitivity and detection limit. This is expected because the detection limit depends not only on the sensitivity, but also the signal reproducibility. Furthermore, the concentrations of each element in the synthetic silicate samples varied among them. For example, beryllium ranged in concentration from 2.1 to 100 ppm whereas barium ranged in concentration from 204 to 10,000 ppm; these concentration ranges are shown in Table 1.
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SUMMARY Students gained valuable experiences using LIBS, performing data analysis using Microsoft Excel, constructing calibration curves, and calculating detection limits for the various elements analyzed. All of these results are highly applicable for any instrumental and analytical chemistry course. This is a very simple laboratory that introduces students to a different type of atomic spectroscopy and can be incorporated very easily into undergraduate education.
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ASSOCIATED CONTENT
S Supporting Information *
Student handout and notes for the instructor. This material is available via the Internet at http://pubs.acs.org.
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REFERENCES
(1) L6.12 Atomic Absorption Spectroscopy, Hands-on Laboratory Experiments. http://mikeepstein.com/uc/aasexperiments.html (accessed Feb 2012). (2) Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. M. An Introduction to Analytical Chemistry, 7th ed.; Brooks/Cole Thomson Learning: Belmont, CA, 2000. (3) EPA methods for Atomic Absorption Web site. http://www. caslab.com/EPA-Methods/?word=&page=3&methodsource= &methodnumber=&methodname= (accessed Feb 2012). (4) Perkin Elmer, Atomic Spectroscopy: A Guide to Selecting the Appropriate Technique and System. http://shop.perkinelmer.com/ content/relatedmaterials/brochures/BRO_WorldLeaderAAICPMSICPMS. pdf (accessed Feb 2012). (5) Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis, 6th ed.; Thomson, Brooks/Cole: Belmont, CA, 2007. (6) Ingle, J. D.; Crouch, S. R. Spectrochemical Analysis; Prentice Hall: Upper Saddle River, NJ, 1998. (7) Cremers, D. A.; Radziemski, L. J. Handbook of Laser-Induced Breakdown Spectroscopy; John Wiley & Sons, Ltd: West Sussex, England, 2006. (8) Miziolek, A. W.; Palleschi, V.; Schechter, I. Laser-Induced Breakdown Spectroscopy; Cambridge University Press: United Kingdom, England, 2006. (9) Singh, J. P.; Thakur, S. N. Laser-Induced Breakdown Spectroscopy; Elsevier: Amsterdam, The Netherlands, 2007. (10) Dockery, C. R.; Turner, J.; Rosenberg, M. B.; Kammerdiener, K.; Mungai, S. W. Gunshot Residue Analysis in the Undergraduate Laboratory Using Toy Cap Guns. Spectrosc. Lett. 2010, 43 (7), 534− 538. Invited Special Issue on Spectroscopy in Undergraduate Education. (11) Rosenberg, M. B.; Dockery, C. R. Determining the Lifetime of Detectable Amounts of Gunshot Residue on the Hands of a Shooter Using Laser-Induced Breakdown Spectroscopy, Appl. Spectrosc. 2008, 62 (11), 1238−1241. (12) Goode, S. R.; Dockery, C. R.; Bachmeyer, M. F.; Nieuwland, A. A.; Morgan, S. L. Detecting Gunshot Residue by Laser Induced Breakdown Spectroscopy. Trends Opt. Photonics 2002, 81, 175−177. (13) Hark, R.R., Documentary on LIBS and Juniata Student Research, Juniata University, http://services.juniata.edu/cts/dmz/ projects/documentaries/LIBSweb.html (accessed Feb 2012). (14) Ocean Optics website: http://www.oceanoptics.com/Products/ libs.asp (accessed Feb 2012). (15) Students typically do not make their own discs because the samples are expensive and one set of samples can last multiple laboratories. It would take the students 30 min in class to prepare the samples. (16) IUPAC. Compendium of Chemical Terminology, 2nd ed., IUPAC: Research Triangle Park, NC, 1997. (17) The time scale for the lab will depend on the number of groups. This semester there were 7 groups. The LIBS lab was offset with another lab; one week, 3 groups did the LIBS lab, and the following week, the other 4 groups did the LIBS lab. (18) Reader, J.; Corliss, C. H.; Wiese, W. L.; G. A. Martin. Wavelengths and Transition Probabilities for Atoms and Atomic Ions: Part I. Wavelengths, Part II. Transition and Probabilities; The US Department of Commerce/National Bureau of Standards: Washington, DC, 1980. (19) National Institute of Standards and Technology (NIST), Physics Measurements Laboratroy, NIST Atomic Spectra Database Lines Form. http://physics.nist.gov/PhysRefData/ASD/lines_form. html (accessed Feb 2012). (20) Payling, R.; Larkins, P. Optical Emission Lines of the Elements; John Wiley and Sons, LTD: West Sussex, England, 2000.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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ACKNOWLEDGMENTS This work was funded through U.S. Department of Energy, Office of Science. I would also like to acknowledge Melissa 680
dx.doi.org/10.1021/ed2003862 | J. Chem. Educ. 2012, 89, 678−680
