In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris
Cost-Effective Spectroscopic Instrumentation for the Physical Chemistry Laboratory
University of Missouri—St. Louis St. Louis, MO 63121
W
Gary A. Lorigan,* Brian M. Patterson, Andre J. Sommer, and Neil D. Danielson Department of Chemistry and Biochemistry, Miami University, Oxford, OH 45056; *
[email protected] Developing experiments for a junior/senior undergraduate physical chemistry laboratory class can be very difficult and time consuming. Several textbooks provide excellent laboratory experiments that demonstrate important physical chemistry principles (1–3). However, the spectroscopic instrumentation needed to carry out the individual experiments may not be readily available within your department. Furthermore, the instrument may be too expensive to purchase for just one particular undergraduate laboratory experiment. To address these problems, we purchased a series of inexpensive modular miniature UV–vis fiber-optic spectrometers from Ocean Optics Inc. to carry out several physical chemistry undergraduate laboratory experiments. One such instrument is the S2000 spectrometer, which can easily be configured with a personal computer (PC) to carry out absorption, emission, and Raman scattering experiments. The instruments can be purchased to cover a variety of wavelengths at various resolutions. Recently, these instruments were shown to be useful for an undergraduate instrumental analysis laboratory course to conduct absorption experiments (4 ). This work focuses on the versatility of the S2000 instruments to carry out absorption, emission, and Raman scattering experiments in an undergraduate physical chemistry laboratory. Experimental Procedure Experimental Apparatus and Material The miniature fiber-optic S2000 spectrometers, LS-I tungsten halogen lamp, and fiber-optic cables were purchased from Ocean Optics Inc. Gas discharge tubes and a voltage power supply were purchased from Edmund Scientific. The spectrographs employed in this experiment were optimized for the spectral region of interest by selecting the appropriate grating which is installed and set at the factory. Table 1 provides the specific details for each instrument. For each experimental setup, an A/D board was placed into a Dell Pentium II computer to collect the corresponding spectra. The data can be easily processed on a PC with Microsoft Excel. However, the experimental data in this manuscript were processed on a Power Macintosh G3 computer utilizing the data analysis software programs Igor Pro (Wavemeterics Inc.) and Microsoft Excel. Table 1. Spectrograph Characteristics Spectral Characteristic Grating Groove Density/ (lines/mm)
Blaze Wavelength/ nm
Spectral Bandwidth/ nm
Iodine absorption
2400
Holographic/UV
467–575
Nitrogen emission
600
300
200–850
1200
750
510–775
Experiment
Raman
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Setting Up the Instrumentation We have modified several undergraduate physical chemistry laboratory experiments that are available in textbooks through the implementation of these Ocean Optics S2000 spectrometers (1–3). The theory and data analysis of the individual experiments are beyond the scope of this report. However, details are shown in the supplemental materials.W As observed in Figure 1, the Ocean Optics S2000 spectrometer is very versatile and can be easily configured with fiber-optic cables to conduct absorption, emission, and Raman scattering experiments. Figure 1A is a schematic of the laboratory setup for collecting experimental data in the absorption mode. The light source used in the absorption mode is a LS-I tungsten halogen lamp obtained from Ocean Optics Inc. Fiber-optic cables (Ocean Optics Inc.) are used to transfer the light from the lamp to a 10-cm path-length sample cell containing I2 crystals and from the sample cell to the S2000 spectrometer, which is directly interfaced with an A/D board on a PC. The experimental data are stored on the PC with the Ocean Optics software. Students can save their data to disk and export it to an Excel spreadsheet for further analysis. Similarly, Figure 1B shows the instrumental setup for an emission laboratory experiment. Gas discharge tubes containing either hydrogen, mercury, or nitrogen are placed into a standard 115-V SP200 power supply. A fiber-optic cable is coupled to a collimating lens and placed approximately 2 cm from the discharge tube. The fiber-optic cable is then connected to the S2000 spectrometer (which is directly connected to the PC) to collect the data. The emission spectra are gathered in the dark to minimize the effects of stray or scattered light. Finally, Figure 1C displays the configuration for collecting Raman scattering data with the S2000 spectrometer. A 35-mW He–Ne laser (Uniphase) serves as the excitation source and is directed into a holographic notch filter (Kaiser Electro– Optics), which serves as a beam splitter. Light reflected by the beam splitter (BS) is focused onto the sample using a 40× (0.65 numerical aperture) objective (Edmund Scientific). The sample, either a liquid or a solid, is placed inside a capillary tube and mounted into a sample holder (S). The Raman-scattered radiation is collected 180° to the incident light using the same objective and transmitted through the beam splitter and a second holographic notch filter (RF). The beam splitter and the second holographic notch filter serve to reject the elastically (Rayleigh) scattered light. Light passing through the second holographic notch filter is then focused on the entrance slit of the spectrograph using a plano-convex lens (15-mm diameter, 40-mm focal length) (Edmund Scientific). The entire setup is mounted on an optical bread board (TMC Manufacturing) using micro-positioners (Newport Corporation). The open
Journal of Chemical Education • Vol. 79 No. 10 October 2002 • JChemEd.chem.wisc.edu
In the Laboratory
architecture of the setup allows the students to see vital components of the system and permits them to make adjustments for optimal performance. Spectral Data The absorption spectrum of I2 gives rise to well-resolved vibronic bands between 500 and 620 nm. The spectrum consists of a series of electronic transitions from the ground X electronic state to the second excited B state. The theory behind the experiment and detailed instructions on how to interpret the data has been discussed previously (5–9). Figure 2A displays an I2 spectrum collected from 510 to 580 nm on a S2000 spectrometer configured in the absorption mode as shown in Figure 1A. Analysis of the spectrum indicates that
the v′′ = 0 to v′ = x transitions are clearly resolved as well as several v′′ = 1 to v′ = x transitions at higher wavelengths (starting at 545 nm as a low-intensity shoulder). The resolution of this particular spectrometer is not good enough to resolve the v′′ = 2 to v′ = x transitions. However, it is sufficient to carry out the desired calculations described in the literature. In this experiment, students analyze the I2 spectrum to reveal the corresponding vibrational frequencies, anharmoncities, bond energies, and potential energy diagrams for the ground X electronic state and the excited B state of I2. The S2000 spectrometers can easily be configured to conduct emission experiments. First, students calibrate the instrument with known literature emission values for Hg and/or Ar. Figure 2B displays the N2 band spectrum collected
A Absorption S2000
A Absorption 10 cm cell
I2
tungsten halogen lamp
580
560
540
520
Wavelength / nm
B Emission S2000
B Emission
fiber coupled to lens
gas discharge tube
N2
440
400
360
320
Wavelength / nm
C Raman HeNe laser
C Raman
P
CCl4 RF S2000
50x FL
BS/RF
OBJ
S BS
Figure 1. Drawings that illustrate the versatility for collecting UV–vis spectra with the Ocean Optics Inc. S2000 spectrometer. (A) Configuration for analyzing absorption spectra with the S2000 instrument. (B) Instrumental setup for collecting gas-phase emission spectra with an S2000 spectrometer. (C) Configuration for collecting Raman spectra with the S2000 spectrometer. The Raman instrument was set up with the following additional pieces of equipment: plasma filter (P), beam splitter/Rayleigh filter (BS/RF), focusing lens (FL), beam stop (BS), objective lens (OBJ), and sample holder (S).
200
400
600
800
1000
Shift / cm᎑1 Figure 2. Sample spectra collected from an Ocean Optics S2000 spectrometer in various configurations for a physical chemistry laboratory class. (A) UV–vis absorption spectrum of I2 collected from 500 to 580 nm in a 10-cm cell. (B) Emission spectrum of N2 gas collected from 300 to 460 nm. (C) Raman spectrum of carbon tetrachloride shown from 100 to 1000 cm᎑1.
JChemEd.chem.wisc.edu • Vol. 79 No. 10 October 2002 • Journal of Chemical Education
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In the Laboratory
from 300 to 480 nm in the emission mode. Each band arises from the closely spaced rotational transitions that are superimposed upon an electronic transition between two vibrational levels in the upper and lower electronic states. The theory and instruction for analyzing the data for this experiment have been discussed in various physical chemistry textbooks (10, 11). Students assign the band transitions, prepare Deslandres tables, construct an energy-level diagram, and calculate vibrational constants. We have also utilized this instrument in the emission mode to measure the Balmer (n = 2) emission lines of hydrogen for a physical chemistry laboratory experiment (12–14 ). In this experiment, students are able to calculate the Rydberg constant (RH) and study both quantum mechanics and the Bohr theory of the hydrogen atom. Finally, we have used the Ocean Optics S2000 instrument to conduct Raman scattering experiments based upon a slightly modified version of the Raman experiment in the physical chemistry laboratory textbook by Shoemaker et al. (15–17). In this experiment, the Raman active vibrational modes of CCl4 and CBr4 are investigated at room temperature. Students assign the ν1, ν2, ν3, and ν4 stretching and bending vibrational modes to the corresponding spectral transitions, calculate the corresponding force constants, and measure depolarization ratios of the Raman bands. Figure 2C shows a representative Raman spectrum of CCl4 collected from the S2000 spectrometer as configured in Figure 1C. The system is capable of collecting both Stokes and anti-Stokes shifted wavelengths, thereby providing a basis for several more physical chemistry experiments (18). Additionally, we have examined the Raman vibrational transitions of CBr4 and dry ice with this instrument. Conclusion The incorporation of the Ocean Optics Inc. S2000 miniature fiber-optic spectrometers into our undergraduate physical chemistry laboratory has been very successful. The instrumentation and accessory equipment needed to carry out laboratory experiments are inexpensive. The interface between the PC and the S2000 is easily achieved by installing an A/D board onto the computer. The board is obtained when you purchase the S2000. The computer software for collecting data is straightforward and easy to use. Finally, undergraduate physical chemistry instructors can easily adapt classical UV–vis experiments or develop new ones with these instruments. Each
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S2000 spectrometer costs approximately $2,000 and the assembled Raman configuration costs approximately $8,500. Acknowledgments The equipment used in this study was purchased from a grant from a special program in the chemical sciences sponsored by The Camille and Henry Dreyfus Foundation (SG-99-073) and an innovation in educational spectroscopy grant from Ocean Optics Inc. (OOI99-403). We acknowledge support from Miami University for the purchase of computer equipment. W
Supplemental Material
Detailed instructions and information for students are available in this issue of JCE Online. Literature Cited 1. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry, 6th ed.; WCB/McGraw-Hill: Boston, 1996. 2. Halpern, A. M. Experimental Physical Chemistry: A Laboratory Textbook, 2nd ed., Prentice Hall: Englewood Cliffs, NJ, 1997. 3. Sime, R. J. Physical Chemistry: Methods, Techniques, and Experiments; Saunders: Philadelphia, 1990. 4. Bernazzani, P.; Paquin, F. J. Chem. Educ. 2001, 78, 796–798. 5. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Op. cit., pp 425–434. 6. Sime, R. J. Op. cit., pp 660–668. 7. McNaught, I. J. J. Chem. Educ. 1980, 57, 101–105. 8. Lessinger, L. J. Chem. Educ. 1994, 71, 388–391. 9. Long, G.; Sauder, D.; Shalhoub, G. M.; Stout, R.; Towns, M. H.; Zielinski, T. J. J. Chem. Educ. 1999, 76, 841–847. 10. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Op. cit., pp 416–424. 11. Sime, R. J. Op. cit., pp 651–660. 12. Ibid., pp 651–660. 13. Khundkar, L. R. J. Chem. Educ. 1996, 73, 1055–1056. 14. Ramachandran, B. R.; Halpern, A. M. J. Chem. Educ. 1999, 76, 1266–1268. 15. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Op. cit., pp 425–434. 16. Fetterolf, M. L.; Goldsmith, J. G. J. Chem. Educ. 1999, 76, 1276–1277. 17. Comstock, M. G.; Gray, J. A. J. Chem. Educ. 1999, 76, 1272–1275. 18. Sommer, A. J.; Stewart, S. A. Appl. Spectrosc. 1999, 53, 483–488.
Journal of Chemical Education • Vol. 79 No. 10 October 2002 • JChemEd.chem.wisc.edu
