Raman Spectroscopy in the Undergraduate Curriculum Downloaded from pubs.acs.org by COLUMBIA UNIV on 12/09/18. For personal use only.
Chapter 12
Integration of Raman Spectroscopy in Undergraduate Instruction and Research at Pace University Zhaohua Dai, Alexis Javornik, Claudia Sobolewski, Tabitha Batte, John Viola, JamieLee Rizzo, Demosthenes Athanasopolous, and Elmer-Rico E. Mojica* Department of Chemistry and Physical Sciences, Pace University, New York, New York 10038, United States *E-mail:
[email protected].
The integration of Raman spectroscopy into undergraduate instruction and research at Pace University is discussed in this chapter. This includes the introduction of the principle of Raman instrumentation, first by theory through the use of Gaussian calculations and interpretation of readily available spectra and then using the instrument after the acquisition of a B&W Tek Miniram in 2012. The availability of the instrument allows students to expand their research in sol-gel, ionic liquid, pharmaceuticals, and biodiesel. The development of simple experiments to incorporate into laboratory classes and side-by-side comparisons of experimental results and theoretical calculations are also presented. The challenges and successes of having this instrument in our teaching facility are also described.
© 2018 American Chemical Society
Introduction Vibrational spectroscopy is a classical technique and one of the oldest spectroscopic methods. It is a collective term that is used to describe two analytical techniques—infrared and Raman spectroscopy. Although both are nondestructive, noninvasive tools that provide information about the molecular composition, structure, and interactions within a sample, they can be simply differentiated: IR is based on periodic changes of dipole moments, whereas Raman is based on polarizabilities. Raman spectroscopy is not utilized as much as IR in undergraduate education. However, recent advances resulted in the development of relatively cheap and portable Raman devices that are now used for on-site analysis. Pace University is a private institution founded in 1906. A metropolitan, private university that enrolled nearly 13,300 students in Fall 2017, Pace University has two campuses, one in Westchester County and the other in New York City. The Department of Chemistry and Physical Sciences in New York City has undergraduate majors in chemistry, biochemistry, and forensic science. The BS degree in chemistry is accredited by the American Chemical Society (ACS) and prepares students for graduate school, medical school, or employment in the chemical or pharmaceutical industries. The Forensic Science program under the Department of Chemistry and Physical Sciences offers both BS and MS degrees. The department is composed of 11 full-time faculty, with more than 120 undergraduate students. Some of the graduates enter professional or graduate school programs. The department offers General Chemistry I and II to first-year students, and Organic Chemistry I and II are offered to second-year students. Students in the department usually take Physical Chemistry I and II, Analytical Chemistry, and Instrumental Analysis during the third year. Biochemistry majors have courses in biochemistry during their third year instead of Physical Chemistry. The Department of Chemistry and Physical Sciences is located on the third floor of One Pace Plaza. The building is Pace University’s main academic facility in New York City, where the bulk of its classroom and research activities are held. The instruments are placed in two rooms that were renovated in 2016 and 2017 that serve as instrumental rooms for instruction and research purposes, respectively. Among the instruments are the required NMR spectrometer, chromatographic instruments such as GC-MS (gas chromatography mass spectrometry), GC-FID (gas chromatography flame ionization detector), HPLC (high performance liquid chromatography), and capillary electrophoresis. An electrochemical system (voltammetric analyzer) is also present, as well as some spectroscopic instruments such as UV-Vis (ultraviolet-visible), fluorescence, AAS (atomic absorption spectroscopy), and FT-IR (Fourier transform-infrared). A portable Raman spectrometer (BW Tek) that serves as a workhorse for all Raman studies was purchased in 2012 with a Verizon Thinkfinity grant. In addition, a computer room with personal computers for theoretical calculations and simulations was recently renovated.
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Integration in Instruction The concept of vibrational spectroscopy is introduced as early as the first year. In the General Chemistry I (CHE 111) course (1), the IR region is introduced during the electromagnetic radiation spectrum section. Located above the visible region, IR is divided into near IR, mid IR and far IR, and during lecture, there is a short description of the IR method that is utilized in this region. In General Chemistry II (CHE 112) (1), during the discussion of the three physical states (State of Matter, Liquids, and Solids chapter), it is often said that solids are nearly incompressible and are rigid. Kinetic molecular theory explains this property by saying that particles making up a solid exist in contact, and unlike those in gas and liquid, they do not move about, but oscillate or vibrate about fixed sites. This is usually expanded, and it is mentioned that the IR and Raman spectroscopic methods are used to identify functional groups present in some solid materials. In the second semester of Organic Chemistry (CHE 223), IR spectroscopy is introduced as a technique used to determine organic molecules. IR is useful in organic structure determination and verification, and it is defined as a class of electromagnetic radiation that may use frequencies between 4000 and 400 cm-1(wavenumber) (2). The term wavenumber, which is reciprocal in wavelength, is used as a unit in IR instead of Hertz. An IR spectrum is a highly characteristic property of an organic compound and can be used both to establish the identity of a compound and to reveal the structure of a new compound. The IR spectrum helps reveal the structure of an organic compound by giving information on which functional groups are present or absent in a given molecule. A particular group of atoms or functional groups give rise to characteristic absorption bands, such as hydroxyl groups that absorb strongly at 3200–3600 cm-1 and carbonyl functionality with a strong absorption at 1650–1800 cm-1. The fingerprint region, absorption in the 625–1300 cm-1 range, is used to confirm the degree of substitution of benzene. For example, mono-substituted benzene will exhibit two peaks (700 and 760 cm-1); ortho-disubstituted benzene will exhibit one peak (735–770 cm-1); para-disubstituted benzene, one peak (790–840 cm-1); and meta-di-substituted benzene, two peaks (680–730 and 750–810 cm-1). Note, students are introduced to 1H and 13C NMR prior to IR because NMR spectroscopy provides much more detailed information as compared with IR. IR is used to substantiate what is usually predetermined from NMR—the presence or absence of key functionalities. A brief overview of electromagnetic radiation is provided because students were already taught this in General Chemistry. Following that, the components of a spectrophotometer are discussed—the light source, sample, detector, and interpreter and sample preparation (e.g., choosing the proper solvent, sample cell composition). Following this, a number of IR spectra are provided to the class via PowerPoint, and key functionalities are identified. Compounds such as acetic acid, cyclohexene, benzaldehyde, benzyl alcohol, and 3-pentanone are shown and discussed. Molecules that bear extended conjugation, as in an α,β-unsaturated carbonyl-containing compound, are discussed because the IR is affected. Those compounds introduce some single-bond character into a carbonyl group through 201
resonance. Therefore, less energy is required to stretch a C-O single bond than a C-O double bond, which leads to the reduced double-bond character and results in a lower stretching frequency, and the stretching of a C-C double bond will also be lowered. At this level, students have experience using the IR spectrometer in the lab to determine the functional group present to aid them in identifying their unknown. At the end of Organic Chemistry II, the final laboratory experiments are the qualitative lab. Students are given two unknown samples. Their task is to determine what they are and to eventually make derivatives of each. They utilize their knowledge of solubility, melting and boiling points, and IR as part of the sample determination. Since the purchase of the instrument, Raman spectroscopy has been introduced at this level with the concept that the technique is complementary to IR, and students have the option to use the instrument should there be a long line for the IR instrument. By the third year, students start to get familiar with the Gaussian software used in Physical Chemistry courses. In Physical Chemistry, we follow the modern curriculum, discussing the atomic and molecular structure and spectroscopy in the first part (CHE 301) and thermodynamics and kinetics in the second (CHE 302). The vibrational and rotational spectroscopy and their coupling is theoretically introduced in the lecture of CHE301 and further explored in the computational lab of the course. The vibrational matrix is calculated as the Hessian of the second derivatives of the total energy of the previously equilibrated molecular structure with respect to the atomic coordinates. The diagonalization of the matrix results in the normal modes. The rotational modes are determined from the diagonalization of the moment of inertia tensor, constructed relative to an optimized coordinate system with origin at the center of mass of the molecule. These calculations are performed using various computational (Gaussian09, GAMESS-US, NWChem, Psi4, MPQC) and visualization programs (Gauss View, Avogadro, Gabedit, PyVib2). The students experiment on the computational method (MM, semiempirical, HF, DFT) and the choice of basis set and compare the outcomes against experimental spectra. They identify the spectral peaks with specific vibrations in the molecule. In addition, they study optical isomers using CVD and Raman Optical Activity calculations (Figure 1).
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Figure 1. Calculated vibrational circular dichroism (top) and Raman optical activity (bottom) of pinene isomers.
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In Instrumental Analysis (CHE 331), both IR and Raman are covered in lecture and lab. Students are required to analyze different samples by IR using both KBr pellets and ATR. Greenhouse gases and xylenes are also analyzed. Raman spectroscopy is used in Instrumental Analysis to analyze xylene mixtures and over-the-counter (OTC) drugs. At the end of the semester, each student is given an unknown xylene mixture and an OTC drug with the objective to identify the xylene isomers present in the unknown and the identity of the active ingredient in the drug. In Forensic Analytical Spectroscopy (FOR 620), a master’s-level graduate class available as an elective to graduating undergraduates, students are asked to use IR to differentiate controlled substances that are primary amines (such as amphetamine), secondary amines (such as methamphetamine), and tertiary amines (such as cocaine). They also use IR to differentiate free base drugs and their salts (such as cocaine and its HCl salt). IR-based breathalyzer for drunk-driving cases is covered in lecture. Raman is used to analyze mock explosives, duct tape, matches, ink, paint (art forensics), and lipsticks. For the mock explosives, students are given the chance to experience obtaining the Raman spectra of any nitro-containing compound such as nitrobenzene or nitrotoluene. The nitro group is considered the most common exclosophores, a functional group that gives organic compounds explosive properties. Results from the Raman spectra (Figure 2) of nitrotoluene and nitrobenzene show the vibrational mode of the nitro group, which can be found around 1310–1381 cm-1 (3). This signature or vibrational band can be correlated to any nitro-containing explosives in real life.
Research Practice and Familiarization Raman spectroscopy research started even before the purchase of the portable instrument. All students in the Mojica research group who opted to work on Raman spectroscopy were first taught how to use the Gaussian software to optimize the geometry of a given chemical and to determine the frequency. Most of these students, who are in their second year, gained experience using the Gaussian software before they formally learned to use it in the Physical Chemistry course. Students were asked to draw structures of simple molecules such as nitrogen, carbon dioxide, and water, which were then optimized and calculated for frequency in both IR and Raman. Small molecules were used so calculations could be completed in a short time (around 2 or 3 minutes). In this calculation, density functional theory calculation (B3LYP) at 631g basis set was used. Results from the calculations were compared with the values reported in the website (4) (Table 1) maintained by the Department of Chemistry of Purdue University. The molecular geometry and vibrational frequencies in this site were calculated using the Hyperchem program and semiempirical method (AM1 Hamiltonian). These calculations allow students to experience the use of different computational models of different theoretical levels and compare them with one another or with experimental outputs. Results in vibrational frequencies from B3LYP calculations were consistently lower compared with the AM1 method in 204
most calculations except for those above the 3500 cm-1 region and for methane, in which a reverse trend was observed. In addition, this website is also a very helpful tutorial for students because the vibrational modes are animated. The calculated peaks are also presented as either Raman active/inactive or IR active/inactive or both, which aids students in understanding more on how both methods complement one another. Using the IR data obtained in an experiment involving greenhouse gases (CO2, CH4, N2, and water) in the instrumental analysis course, N2 was found to have an IR spectrum with a straight line, confirming that it is IR-inactive due to its molecule symmetry. Results from Raman calculation (Figure 3) showed a peak around 2353 cm-1, lower than the 2744 cm-1 by AM1. CO2, on the other hand, showed an IR peak in the region where it was predicted by calculation (2284 cm-1), and Raman calculation displayed a peak at 1282 cm-1. Lastly, water spectra showed peaks predicted by IR calculations. It is expected that these peaks will be observed in Raman because these peaks are both IR- and Raman-active. Students were excited with the experience of using the website and Gaussian software as a springboard in their understanding of Raman spectroscopy. Animation of vibrational modes in both tools significantly enhanced their learning experience.
Figure 2. Raman spectra of nitro-containing compounds as mock explosives.
Mojica Research Group The main reason students joined the group is to do undergraduate research as part of their requirement in Research in Chemistry (CHEM 480) or as a requirement for the Honors College. Aside from course credits, student research outputs include presentations (posters and talk), theses for those in Honors College, and publications for some students. A student usually works on a research topic that utilizes various instruments including those in vibrational spectroscopy (IR and Raman). 205
All students who worked in Raman-based projects in the group were first asked to obtain the Raman spectra of toluene, benzene, and acetonitrile using the Raman portable spectrometer (B&W Tek Miniram). All solvents were placed in closed container vials, and their spectra were obtained by simply focusing the laser into the samples. Before the purchase of the portable instrument, the main training for students working on Raman spectroscopic studies was the experimental data for toluene obtained using a JASCO NRS-2100 confocal dispersive Raman spectrometer (Easton, MD) based at York College. In addition, theoretical calculations were performed using DFT-B3LYP at 631g basis set to aid in the assignment of peaks observed in experimental results.
Table 1. Raman Calculations Using DFT Versus AM1 Hamiltonian Methods and Assigned Vibrational Modes Theoretical (DFT)
Webpage (AM1)
Vibrational mode
Nitrogen 2352
2744
N-N stretching
1282
1480
C-O symmetric stretching
1619
1885
H-O-H bending
3617
3506
O-H asymmetric stretching
3782
3585
O-H symmetric stretching
1404
1380
C-H bending
1604
1412
C-H bending
3044
3104
C-H asymmetric stretching
3168
3215
C-H symmetric stretching
Carbon dioxide
Water
Methane
Raman spectra of the three solvents toluene, benzene, and acetonitrile obtained experimentally and by theoretical calculations are shown in Figure 4. Functional groups, with aid from the Gaussian simulations, were assigned to peaks from experimental results. The students were trained on how to determine the vibrational mode of the peaks observed in experimental results using the three solvents, dubbed “TAB.” They become familiar with vibrational modes such as ring stretching found in ~1000 and ~1600 cm-1 for toluene and benzene and the cyano (C≡N) stretching around ~2200 cm-1. The inclusion of the Raman spectra of toluene obtained from a confocal dispersive Raman spectrometer (benchtop) is to introduce students to the concept of using standards to determine the performance of the instrument. Usually, toluene is the first one to be run every time an analysis is to be performed to make sure that the instrument is working well. Once the 206
students gained this experience, they worked on different projects that utilized the Raman instrument. The Raman projects available in the group are divided into three groups: cyano compounds, phenolic compounds, and pharmaceutical compounds. The main reason for using the TAB solvents as preliminary training was to expose the students to simple compounds that contain functional groups they will encounter when working with more complicated compounds.
Figure 3. Calculated Raman intensity for different greenhouse gases obtained using DFT-B3LYP at 631g basis set level.
Some of the finished studies done in the cyano group project included the sol-gel process and ionic liquid. The sol-gel process is a convenient and versatile method of preparing transparent materials under ambient temperature (5). It is a useful tool to create highly structured glass ceramics that can be used in different applications such as extracting materials, separation and purification of materials, and as sensors. Typical steps in the sol-gel process are hydrolysis, condensation, and polycondensation. Raman spectroscopy was used to probe on the sol-gel mechanism by looking at the solid-to-liquid transition of a xerogel containing cyanoethyltriethoxy-silane and tetraethoxysilaneil. The cyano functional group was chosen because of its unique vibrational signature. Thin films of the gels were prepared and each step was monitored by vibrational spectroscopy (IR and Raman). Results showed that the vibrational modes of the cyano functional group shifted to higher wavenumber once transformed to solid (gel). In addition, broadening of the peak was observed during the process, implying a heterogeneous environment. Computational calculations were also performed and compared with the obtained experimental data. 207
Figure 4. Raman spectra of toluene, benzene, and acetonitrile.
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On the other hand, ionic liquids (ILs) are a new class of purely ionic, saltlike materials that are liquid at unusually low temperatures. These materials manifest physiochemical behaviors quite unlike water or organic solvents. They possess high ionic conductivity, high ion concentrations, and excellent oxidative stability, making them ideal materials for demanding applications at elevated temperatures. ILs have many applications, such as powerful solvents, electrolytes (electrically conducting fluids), and in power sources (batteries, capacitors, and fuel cells). To better understand the unique properties of ionic liquids, a student measured the Raman spectra of 1-butyl-3-methylimidazolium thiocyanate or [BMIM][SCN] in different environments. Raman spectra were correlated to the results of ab initio calculations. The data obtained were discussed in terms of their implication to the function of ionic liquids. Results of these studies were presented by the students in ACS national meetings (sol-gel study during the Dallas 2014 meeting and ionic liquid study in the Boston 2015 meeting). The sol-gel study is now being validated and will be submitted for publication in a peer-reviewed journal in the near future. Part of the ionic liquid study is included in a book chapter to be published by ACS (6). The first Raman study ever done by a group member is on a pharmaceutical compound, specifically, an antibiotic. The main project is to look at the vibrational and electronic properties of chloramphenicol (CAP), an antibiotic originally derived from the bacterium Streptomyces venezuelae. This antibiotic is an inhibitor of bacterial ribosomal peptidyl transferase activity. It is also known as Chloromycetine and Paraxin and is a broad-spectrum antibiotic exhibiting activity against a variety of aerobic and anaerobic microorganisms. A student obtained the vibrational (Raman and infrared) and UV-Vis spectra of CAP and correlated them with theoretical calculations. The spectra acquired from the portable Raman spectrometer and confocal dispersive Raman spectrometer are presented side by side with theoretical simulation in Figure 5. There are more peaks observed in the portable Raman than in the benchtop, but the lower signal-to-noise ratio of the portable Raman can be the reason for this. The major peaks observed are similar in both the confocal dispersive Raman spectrometer and the portable Raman spectrometer. Although there is a small deviation, major peaks from both instruments matched one another. Based on the structure, it is expected to give a strong nitro vibrational band that can be observed near the 1350 cm-1 region. The symmetric (νNO2) mode is predicted and found to be very strong and polarized in Raman spectra at around 1310–1381, whereas the antisymmetric stretching mode exhibits only medium-to-weak intensity around 1531–1600. The symmetric (νNO2) mode observed at 1338 cm-1 using the portable Raman instrument and at 1353 cm-1 using the benchtop instrument is close to that reported in the literature, which is at 1551 cm-1 (7). This is also the highest peak observed in both instruments. Other significant peaks are the ones observed around ~1600 cm-1, which is due to ring stretching, and around 1100 cm-1, which can be assigned to NH in-plane bending and CH2 bending. The complete assignment for the peaks is listed in Table 2.
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At the same time, another student worked on his Honors thesis utilizing vibrational spectroscopy (IR and Raman) to differentiate expired medications such as metoprolol with more recently manufactured medications. Results show that there is a difference in the spectra of the new and expired medications, and it can be correlated with identifying whether the medication is expired using IR and Raman. The results were confirmed by mass spectrometric analysis, which demonstrated differences in the new and expired medications. Recent projects involve the use of Raman instruments (portable and benchtop confocal dispersive Raman spectrometers) for discriminating antibiotics and phenolic compounds from the same group or class. One study looked at differences in fluoroquinolone antibiotics wherein four samples were used, namely ciprofloxacin, enrofloxacin, norfloxacin, and sarafloxacin. Another study focused on differences in nine sulfa drugs or sulfonamides. In terms of phenolics, different compounds consisted of derivatives of benzoic acid and cinnamic acids hydroxybenzene. Distinct differences could be observed among the samples, and the results from both spectrometers are in agreement with the computational wavenumbers. In addition, unique peaks for each sample were used as markers for identification. All of these studies are part of Honors theses and their results were presented at the ACS National Meeting in San Francisco in 2017. At present, new members of the group are validating these studies for possible publication in peer-reviewed journals. The availability of the portable Raman instrument had opened new venues to expand the research capability of the group. Before, the main challenge in integrating Raman spectroscopy was that most, if not all, of the analysis was based on theoretical calculations and experimental results obtained from a visit to another institution to use a confocal Raman spectrometer. The purchase of the portable Raman spectrometer enabled students to perform Raman analysis at any given time in the department. Raman spectroscopy has been added to a plethora of analysis that can be performed by students in addition to them using other instruments to characterize, identify, and sometimes quantify a given sample. Although there are still limitations with regard to the signal-to-noise ratio and the presence of fluorescent materials in samples analyzed that can interfere with Raman signals, the experience gained by students in using the instrument and their exposure to this kind of instrumentation are very big advantages for them. All students taking Instrumental Analysis have the chance to use the instrument, and they are very happy with the opportunity to do so.
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Figure 5. Raman spectra of CAP obtained using a portable (above) and confocal dispersive Raman (below) spectrometer with theoretical calculation.
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Table 2. Peaks Assignment for Chloramphenicol PRI
CDRS
Assignments
369
O-H out-of-plane bending
410
Ring deformation
459
Ring deformation
532
NO2 rocking
587
O=C-N out-of-plane bending
631
613
695 765
N-H bending 750
823 854
C-Cl symmetric stretching N-H bending
C-Cl asymmetric stretching Ring breathing
834
C-H2 rocking
873
C-H bending
939
C-C stretching
979
C-H2 rocking
1018
C-O stretching
1112
1095
C-H rocking
1196
O-H rocking
1249
C-H rocking
1353
1338
NO2 stretching + NH in-plane bending
1494
ring stretching
1528
NO2 stretching
1604
1587
Ring stretching
1688
C=O stretching
2910
C-H2 stretching
2971
C-H2 asymmetric stretching
3005
C-H stretching
3042
C-H stretching (aromatic)
3086
C-H stretching (aromatic)
3113
C-H stretching (aromatic)
3200
N-H stretching
3260
N-H stretching
CDRS, confocal dispersive Raman spectrometer; PRI, portable Raman instrument.
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Dai Research Group The Dai research group, which specializes in organic synthesis, instrumentation, and forensic science, used portable Raman spectroscopy to monitor the thermal degradation of biodiesel and to differentiate between car paint chips. Recent advancements in Raman have now made it a great contender for forensic field analysis. Portable Raman instruments that use shorter wavelength light sources of 780 to 840 nm (8) are now widely available. This wavelength range increases the efficiency of the Raman scattering and makes for good analysis of many samples found in forensic science such as polymers, explosives, gunshot residue, and fire debris. Additionally, Raman could be ideal for on-scene fire analysis because water used to extinguish the fire would not cause interference, as it would in IR analysis. This means Raman analysis potentially can be conducted quickly and inexpensively on-site with no sample preparation. The applicability of the portable Raman device for fire debris analysis of biofuels was tested by studying the degradation of vegetable oil (8, 9), from which biodiesel is most commonly derived, and B100 biodiesel. Small amounts of Crisco vegetable oil (~1 mL) were put into six disposable vials. The vials were simultaneously placed in an oven, heated, and removed every 30 minutes for instant analysis. The experiment was performed at temperatures of 110, 120, 160 and 200 °C with each set containing six vials removed at times of 30, 60, 90, 120, 150, and 180 minutes. Upon removal, each vial was immediately analyzed via the portable Raman spectrometer. After the Raman spectrum of the sample was collected, a small amount was scraped from the vial and pressed onto the ATR plate for FT-IR analysis. The experiment was later repeated using B100 biodiesel fuel heated at 180–200°C and analyzed by the same method. The Raman spectrum of pure vegetable oil (Figure 6) was obtained and compared with a reference value to ensure accuracy. Peaks located between 943 cm-1 and 1069 cm-1 were determined to be from asymmetrical C-O-C bonds. The small peak at 400cm-1 and peaks at 1254–1292 cm-1 were found to be from aliphatic C-C bonds, the peak at 1431 was assigned to asymmetric CH3/CH2 bonds, the peak at 1648 cm-1 was assigned to C=C bonds, and the small peak at 1735cm-1 was assigned to C=O bonds. Finally, the broad area peak ranging from approximately 2800 to 2960 cm-1 was determined to be from C-H bonds. The summary of the Raman peak assignment can be seen in Table 3. The Raman spectrum for the B100 biodiesel (Figure 7) showed peaks in locations similar to the vegetable oil spectrum. However, the B100 showed higher signal abundance, especially in the fingerprint region. This is possibly due to higher sample fluorescence than the vegetable oil samples. Peaks were able to be identified and can be seen in Table 4.
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Figure 6. Raman shift of pure vegetable oil.
Table 3. Pure Vegetable Oil Raman Shift Peak Assignment Raman shift (cm-1)
Peak assignment
400
δC-C
843–861
Asymmetrical υC-O-C
956
Asymmetrical υC-O-C
1068
Asymmetrical υC-O-C
1254–1292
υC-C aliphatic chains
1431
Asymmetrical δCH3/ δCH2
1648
υC=C
2800–2960
υC-H
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Figure 7. Raman shift of B100 biodiesel fuel.
Table 4. B100 Raman Shift Peak Assignment Raman shift (cm-1)
Peak assignment
174–400 region
δC-C aliphatic chains
756-831
Asymmetrical υC-O-C
959
Asymmetrical. υC-O-C
1068
Asymmetrical υC-O-C
1254–1292
υC-C aliphatic chains
1428
Asymmetrical δCH3/ δCH2
1648
υC=C
2800–2960
υC-H
215
Thermal Degradation Analysis The thermal degradation procedure was performed four times total, each at increasing temperatures of 100, 120, 160, and 200 °C. No changes were observed in the sample spectra in the 100–160 °C runs. During the analysis of the 200°C run, several minor changes were observed. The growth of a broad peak at around 3500 cm-1 can be seen as well as the loss of some of the minor peaks that appeared at 3008 cm-1 and 1236.4 cm-1 on the original spectrum. Another minor peak can be seen appearing in the heated samples at approximately 970 cm-1. A stacked view of the 200°C sample IR spectra can be seen in Figure 8. The results of the vegetable oil thermal degradation can be seen in Figure 9. Because more of each sample was used in the heating experiment than the bacterial one, results were able to be obtained using the Raman device. Spectra showed almost no significant changes from the pure vegetable oil spectrum. Samples, after being heated for 120, 150, and 180 minutes, showed sudden sharp peaks at about 404, 679, and 1528 cm-1, respectively, that were not as defined in the earlier readings.
Figure 8. Stacked view of 200 °C thermal degradation of vegetable oil.
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Figure 9. Raman shift of vegetable oil heated at 200 °C.
The Raman spectra were also able to be obtained for the B100 samples, which can be seen in Figure 10. The Raman spectra showed differences in signal in the fingerprint region. There is a sharp increase in peaks in the fingerprint region after the sample was heated, similar to that observed in vegetable oil when heated. In addition, a decrease in the background was observed, which could mean the elimination of fluorescent materials. However, in terms of Raman shift, there are no significant changes when B100 samples were heated similar to the vegetable oil. There are also no significant changes seen in the IR spectra. The Dai group continuously used Raman spectroscopy side by side with IR spectroscopy in most of its research. Problems commonly encountered were the presence of fluorescent materials in the samples analyzed. For instance, in the group’s research involving paint chips, not all samples could be analyzed under identical conditions due to the heterogeneity of the vehicle paint. Fluorescence from the pigments and dyes posed major obstacles for Raman measurements. Some paint chips absorbed too much energy from the laser and got burned. These are just some of the limitations that the group commonly encountered in using Raman spectroscopy.
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Figure 10. Raman shift of B100 samples heated at 200 °C for 0–120 minutes.
Conclusions The acquisition of a portable Raman spectrometer by the Department of Chemistry and Physical Chemistry at Pace University greatly enhanced the teaching and learning experience of both the faculty and the students. With this instrument, Raman spectroscopy has been successfully integrated side by side in teaching with the more established IR spectroscopy and has opened a new opportunity in research whereby both faculty and students could have an additional method for doing chemical analysis. The training the students acquired in operating the instrument gave them hands-on experience that will prove to be beneficial once they start their professional lives or careers.
Future Plans The department is planning to utilize the portable Raman instrument in other laboratory classes. One laboratory experiment being developed in Biochemistry is identifying different amino acids. In Advanced Inorganic Chemistry, Raman analysis will be used as an additional technique to characterize inorganic compounds being synthesized in a laboratory class. There is also a plan to apply for an NSF-MRI (National Science Foundation-Major Research Instrumentation) grant to purchase a bench-top confocal dispersive Raman spectrometer in the near future. The department is also upgrading an old Raman spectrometer donated by Nanocrystals Technology L.P. in 2014, with the intention of using different laser source lines. 218
Acknowledgments We would like to acknowledge the Pace University Verizon Thinkfinity grant for enabling the purchase of the portable Raman spectrometer and Pace University Scholarly Research for defraying the cost of the maintenance of the instrument and the upgrade of the donated Raman instrument.
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