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v 34 n 3 CONTENTS m 2019 ® COLUMNS March 2019 Volume 34 Number 3 Molecular Spectroscopy Workbench ...... 10 Raman Spectroscopy and Polymorphism David Tuschel The differentiation of polymorphs is important, particularly in the pharmaceutical industry. We demonstrate the practicality of using Raman spectroscopy to differentiate crystal forms for polymorph characterization and screening, and explain aspects of chemical bonding and solid state structure that affect the Raman spectra of crystal lattice vibrational modes.
IR Spectral Interpretation Workshop ...... 22 Organic Nitrogen Compounds II: Primary Amines Brian C. Smith Amines, which contain carbon, hydrogen, and nitrogen, come in six varieties, but using the N-H stretching peak positions by themselves will distinguish between all the different amine types.
Spectroscopy Spotlight ...... 45 The LIBS Advantage in Mining and Energy Applications Laura Bush Cover image courtesy of Laser-induced breakdown spectroscopy (LIBS) has seen significant expanded adoption in 3D generator/AdobeStock. recent years, particularly in industrial applications where it can provide important advantages over other techniques. Mohamad Sabsabi, of the National Research Council of Canada, has been leading programs to research and implement the use of LIBS in mining and energy applications.
ON THE WEB ARTICLES
WEB SEMINARS 2019 Salary Survey: Some Unusual Trends ...... 26 Jerome Workman, Jr. Ensuring Integrity of Drug Formula- Our annual salary and employment survey looks at salaries, workload, job satisfaction, and tion from Development to QC the overall job market for spectroscopists. This year, we had some surprising results. Katherine Paulsen and Mike Garry, Sr., Thermo Fisher Scientific Trends in Spectroscopy: A Snapshot of Notable Advances and Applications . . . . 36 ICP-OES Technology Advancements Michael MacRae Addresses the Axial/Radial Dilemma Spectroscopic measurement factors into every facet of modern life. Here, we survey on Plasma Viewing While Offering noteworthy recent advances in and applications of atomic and molecular spectroscopy, Improvements in Overall Performance touching on their uses in fields such as biomedicine, materials science, environmental Olaf Schulz, Spectro Analytical Instruments monitoring, agriculture, pharmaceutical research, public safety, and more.
Re-think Quantitative Analysis: Solu- tions for Challenging Measurements DEPARTMENTS Adam J. Hopkins and Elena Hageman, News Spectrum ...... 9 Metrohm USA Calendar ...... 49
The Role of Raman Spectroscopy in Short Courses ...... the QA/QC Laboratory 50 Tom Tague, Bruker Corporation Products and Resources ...... 52
spectroscopyonline.com/spec/webcasts Ad Index ...... 54
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8 Spectroscopy 34(3) March 2019 www.spectroscopyonline.com Editorial Advisory Board
Fran Adar Horiba Scientific Rachael R. Ogorzalek Loo University of California Los Angeles, David Geffen School of Medicine Russ Algar University of British Columbia Howard Mark Mark Electronics Matthew J. Baker University of Strathclyde R.D. McDowall McDowall Consulting Ramon M. Barnes University of Massachusetts Gary McGeorge Bristol-Myers Squibb Matthieu Baudelet University of Central Florida Linda Baine McGown Rensselaer Polytechnic Institute Rohit Bhargava University of Illinois at Urbana-Champaign Francis M. Mirabella Jr. Mirabella Practical Consulting Solutions, Inc. Paul N. Bourassa Blue Moon Inc. Ellen V. Miseo Illuminate Michael S. Bradley Thermo Fisher Scientific Michael L. Myrick University of South Carolina Deborah Bradshaw Consultant John W. Olesik The Ohio State University Lora L. Brehm The Dow Chemical Company Steven Ray State University of New York at Buffalo George Chan Lawrence Berkeley National Laboratory Jim Rydzak Specere Consulting John Cottle University of California Santa Barbara Jerome Workman Jr. Biotechnology Business Associates David Lankin University of Illinois at Chicago, Lu Yang National Research Council Canada College of Pharmacy
Barbara S. Larsen DuPont Central Research and Development Spectroscopy’s Editorial Advisory Board is a group of distinguished individuals assembled to help the publication fulfill its editorial mission to promote the effec- tive use of spectroscopic technology as a practical research and measurement tool. Bernhard Lendl Vienna University of Technology (TU Wien) With recognized expertise in a wide range of technique and application areas, board members perform a range of functions, such as reviewing manuscripts, suggesting Ian R. Lewis Kaiser Optical Systems authors and topics for coverage, and providing the editor with general direction and feedback. We are indebted to these scientists for their contributions to the publica- tion and to the spectroscopy community as a whole.
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Renishaw Inc. 1001 Wesemann Drive, West Dundee, Illinois, 60118, United States T +1 847 286 9953 F +1 847 286 9974 E [email protected] www.renishaw.com www.spectroscopyonline.com March 2019 Spectroscopy 34(3) 9 News Spectrum Spectral Imaging Improves Plastics Recycling Steve Buckley Joins Ocean Optics To reduce the quantity Steve Buckley has joined Ocean Optics as the vice of plastics that are president of product development and engineering. dumped into the He will lead the Ocean Optics engineering and product environment, efficient development teams globally with a focus on the recycling is essential. delivery of next-generation optical sensing systems But many plastics and solutions. cannot be economically Buckley’s career in photonics engineering and sorted for recycling, development started in academia. Ten years of and sorting plastics technology leadership in entrepreneurial ventures and that contain flame Near infrared hyperspectral imaging of businesses followed. Today, Buckley is focused on retardants is particularly samples of plastics and their classifi cation spectroscopy coupled with intelligent analysis systems as a function of type of fl ame-retardant: important. Plastics 1,2,5,6,9,10-hexabromo-cyclododecane, to gain solutions in areas such as biochemistry and that contain flame HBCD (red samples); 3,5-tetrabromo- industrial materials analysis. Most recently, Buckley was retardants to increase bisphenol A, TBBPA (yellow samples); the CEO of Flash Photonics, of Redmond, Washington, their resistance to pentabromophenyl ether, deca-BDE and the CEO of Lightspeed Microscopy, a start-up in (green samples); and reference, (blue ignition, reduce samples). Seattle, Washington. He currently writes the “Lasers & flames spreading, Optics” column for Spectroscopy magazine. minimize smoke formation, and prevent the plastic Buckley studied mechanical and aerospace from dripping can vary considerably in the amount engineering at Princeton University (Princeton, New and type of flame retardant that is added to them, Jersey), and earned his PhD in mechanical engineering because of the need to tailor the plastic to its from the University of California Berkeley. particular application and to meet safety standards. Buckley will be based in Redmond, Washington. ◾ Before recycling can occur, plastics require sorting by type and by flame retardant added. Without sorting, recycling cannot take place. In a paper published in the Journal of Spectral SPECIAL ISSUE HIGHLIGHTS Imaging, José Amigo at the University of Copenhagen, Denmark, and colleagues describe a method that ADVANCES IN PORTABLE AND uses near-infrared (NIR) hyperspectral imaging and ® HANDHELD SPECTROSCOPY chemometrics to sort plastics that contain different This special supplement from November 2018 Volume 33 Number s11 www.spectroscopyonline.com Spectroscopy types of plastic and plastics that contain different presents articles with expert advice on the following topics: additions of flame retardants (1). By using an imaging technique, the approach • Handheld Raman, Mid-Infrared and Near Infrared Spectrometers: developed by Amigo can be used during the plastics State-of-the-Art Instrumenta- sorting on a conveyor belt in a recycling plant. ADVANCES IN tion and Useful Applications PORTABLE AND HANDHELD The method discussed in the paper uses a decision SPECTROSCOPY Handheld Raman, mid-infrared, and tree chemometrics approach combined with spectral near infrared spectrometers have launched vibrational spectroscopy data obtained from NIR hyperspectral imaging and into a new era of in-the-field and on-site analysis. We assess is able to distinguish between different plastics and the technological developments that have led to this progress. flame retardants within them with 100% accuracy. Although sorting of recycling plastics has been • Portable, High-Efficiency Transmission Raman Spectroscopy for At-Line Content Uniformity Testing of studied for many years, Amigo wanted to take that Pharmaceutical Tablets process a step further and make it possible to separate Portable transmission Raman spectroscopy, combined with plastics by flame retardant. They also tested the chemometric modeling, is quickly emerging as a valued technique to be practical. “Moreover,” said Amigo, “the technique for content-uniformity testing. proposed methodology was tested with real samples • Portable Spatially Offset Raman Spectroscopy for Rapid that can be found in current recycling lines.” Hazardous Materials Detection Within Sealed Containers This technique has been successfully used by the military, fi rst responders, and customs and law enforcement operators to detect explosives, chemical agents, precursors Reference and hazardous narcotics. (1) D. Caballero, M. Bevilacqua, and J.M. Amigo, J. Spectral Imaging 8, a1 (2019). https://doi. http://www.spectroscopyonline.com/ org/10.1255/jsi.2019.a1 special-issues-11-02-2018 10 Spectroscopy 34(3) March 2019 www.spectroscopyonline.com
Molecular Spectroscopy Workbench Raman Spectroscopy and Polymorphism
Single crystals can be formed that have different unit cells. Crystal polymorphism is the formation of a compound in various crystallographic forms. The differentiation of polymorphs is important, particu- larly in the pharmaceutical industry. Raman spectroscopy has emerged as a complementary method to X-ray diffraction, the “gold standard” for characterization of crystal structure. Different crystal forms of ionic or covalent solids and molecular crystals can be differentiated through Raman spectroscopy. The Raman spectra of molecular crystals consist of bands attributable to external and internal crystal lattice vibrational modes. We discuss these aspects of chemical bonding and solid state structure that affect the Raman spectra of crystal lattice vibrational modes, and demonstrate the practicality of using Raman spectroscopy to differentiate crystal forms for polymorph characterization and screening.
David Tuschel
olymorphism is a term used in various scientific Raman spectra of that compound’s specific crystal forms. disciplines with different meanings. Here, we use Raman spectroscopy is often found to be experimentally P it to describe the multiplicity of phases or crystal more convenient to use than X-ray diffraction for poly- structures of a single compound. Crystal polymorphism is morph characterization or screening. Reference Raman the formation of a compound in various crystallographic spectra of a compound’s polymorphs can be reliably used forms; that is, single crystals can be formed that have dif- to confirm crystal forms, or even to identify new ones in ferent unit cells. The differentiation of polymorphs is im- a research setting or in polymorph screening. The most portant, particularly in the pharmaceutical industry. The important aspect of Raman spectroscopy when applying it crystal form of an active pharmaceutical ingredient can to polymorph analysis is spectral resolution. That is con- affect its chemical or physical stability, and its dissolution sistent with the fact that high spectral resolution is also rate. Consequently, the crystal structure of a pharmaceuti- required to differentiate crystal forms by X-ray diffraction. cal compound can have a profound effect on its efficacy The Raman spectrometer must have spectral resolution and potency. Of course, to analyze crystal structure, X-ray sufficient to resolve the smallest differences in energies of diffraction is considered the “gold standard” for charac- the crystal lattice vibrational modes, which are affected by terizing and differentiating crystal forms of compounds. changes in molecular interactions arising from different However, once different phases of a chemical compound unit cell structures or configurations of the molecules, or have been confirmed or newly identified by X-ray diffrac- formula units within the unit cell. Just as small differences tion, those same samples can be used to generate reference in a compound’s crystal structure lead to small differences www.spectroscopyonline.com March 2019 Spectroscopy 34(3) 11 in the X-ray diffraction pattern, the ing the different types of crystal lattice defined as a crystal lattice vibrational Raman spectra of crystals with small vibrational modes from which they wave propagating through the crystal differences in bond lengths or crys- originate and comparing their suitabil- arising from repetitive and systematic tal spacing will manifest small differ- ity for distinguishing crystal forms. To atomic displacements. It should also be ences in the Raman peak positions. appreciate why Raman spectroscopy is understood that these displacements are Hence, just as high resolution X-ray sensitive to crystal structure, we need to quantized vibrations of the atoms in the diffraction is needed to resolve poly- understand the origins of Raman scat- crystal lattice and are travelling waves. morphs, so too is high spectral reso- tering from solid state materials. Raman The phonon has the characteristics of a lution required to differentiate crystal scattered photons are generated in solid travelling wave insofar as it has a propa- forms by Raman spectroscopy. state materials through the creation or gation velocity, wavelength, wave vector, Many spectroscopists are chem- annihilation of phonons which corre- and frequency. It is important to note that ists, and therefore most likely learned spond to Stokes and anti-Stokes Raman all of the peaks in a Raman spectrum of vibrational spectroscopy as molecular scattering, respectively. A phonon is a crystalline solid are attributed to pho- spectroscopy. If you are a chemist, you learned about the normal vibrational modes of discrete molecules, as op- posed to solid state materials, and their Raman or infrared activity based upon molecular spectroscopic selection rules. Raman spectra of compounds in the liquid or vapor phase consist of narrow bands whose widths depend upon the degree of chemical interaction between the molecules. The weaker the chemical interaction, the narrower the band will be. In fact, the bands of a compound in the vapor phase will be narrower than those of the same compound in the liquid phase for the very reason that the molecu- lar interactions are weaker (1). The broad infrared absorption and Raman bands of water demonstrate the effect of hydrogen bonding and the result of different de- grees of chemical interaction between the molecules in a neat liquid. The breadth of the O-H stretching modes in particular is a manifestation of the distribution of vi- brational energy states as a result of these many and different chemical interactions. 3D Raman image of a pharmaceutical ointment. Those same molecular interactions can affect the vibrational spectrum of a com- pound in the solid state, and are dependent upon how the molecules are configured or 3D Ram oriented in the unit cell. gg Interpreting the Raman spectra of compounds or materials in the solid Turn ideas into discoveries state requires the knowledge of concepts and mathematical treatments other than Let your discoveries lead the scientifi c future. Like no other those of molecular spectroscopy. The system, WITec’s confocal 3D Raman microscopes allow for purpose of this installment of “Molecu- cutting-edge chemical imaging and correlative microscopy lar Spectroscopy Workbench” is to dem- with AFM, SNOM, SEM or Profi lometry. Discuss your ideas with us at [email protected]. onstrate the use of Raman spectroscopy for differentiating polymorphs, and to explain the underlying bases for that capability. In particular, we examine Raman AFM SNOM RISE www.witec.de and discuss the low and high frequency regions of a Raman spectrum, explain- MADE IN GERMANY 12 Spectroscopy 34(3) March 2019 www.spectroscopyonline.com
nons, and not only those at low energy or Raman shift. P.M.A. Sherwood makes that point clear in his fine book Vibra- tional Spectroscopy of Solids: “All crystal vibrations involve the entire lattice and are thus lattice vibrations (a term some- times unfortunately only applied to exter- nal vibrations) and such vibrations can be considered as a wave propagating through the crystal lattice” (2). Unlike the normal vi- brational mode of an individual molecule, a phonon is a crystal lattice vibrational wave travelling through the crystal. This is why a mathematical treatment of the vibrational motions of the crystal’s atoms must take into account the medium through which Figure 1: Raman spectra of the rutile (red) and anatase (blue) crystalline forms of titanium the phonon is propagating, and why the dioxide (TiO ). 2 energy of the phonon depends upon the configuration of the atoms and chemical bonds within the unit cell. In chemistry courses, we learn about Raman scattering through the interaction of light with molecules. A molecule, initially in the ground state, interacts with an incident photon driv- ing the molecule to a virtual energy state whereupon it drops to the first excited vibrational state and emits a photon (Stokes Raman scattering). The energy difference between the in- cident and scattered photons is equal to the energy difference between the ground and first excited vibrational states. Had the molecule initially been in the first excited vibrational Figure 2: Raman spectrum obtained from a grain of anatase titanium dioxide (TiO2) with an impurity phase of rutile present (green). The Raman spectra of reference rutile and anatase state and ended in the ground state, phases are shown in red and blue, respectively. a quantum of energy would have been transferred from the molecule to the scattered photon, and its energy would have been greater than that of the in- cident photon (anti-Stokes Raman scattering). Hence, one can observe the Raman spectrum at either longer (Stokes) or shorter (anti-Stokes) wave- lengths relative to that of the incident monochromatic laser beam. When dealing with gases or liq- uids, it is appropriate to speak of the interaction of a photon with individ- ual molecules. However, very often we deal with solid state materials for which there may be no molecular spe-
cies, such as titanium dioxide (TiO2), silicon (Si), carbon (C, graphene or Figure 3: Raman spectrum obtained from a grain of rutile titanium dioxide (TiO2) with an diamond), or calcium carbonate impurity phase of anatase present (green). The Raman spectra of reference rutile and anatase (CaCO3). Raman spectroscopy of solid phases are shown in red and blue, respectively. state materials involves the inelastic www.spectroscopyonline.com March 2019 Spectroscopy 34(3) 13 scattering of light by phonons, quanta that have the energy of crystal lattice vibrations. The Stokes and anti-Stokes Raman scattering consists of the gen- eration or annihilation of a phonon in the solid, respectively. In crystalline materials, the phonons can be un- derstood as crystal lattice vibrational modes whether the crystal is a covalent or ionic solid or a molecular crystal such as TiO2, barium fluoride (BaF2), or water (H2O, ice), respectively. When speaking of Raman spectra of Figure 4: Raman spectra obtained from grains of paracetamol form I (red) and form II (blue). solid state materials, some spectrosco- pists will describe certain bands as pho- collectively in an ice crystal. Some low en- of long range translational symmetry nons, and others as molecular vibrations. ergy Raman bands are a result of shear or that is present in a crystal. The internal Strictly speaking, this is not correct. All of interlayer breathing modes of the crystal crystal lattice vibrational modes arise the bands in a Raman spectrum of a solid layers. The shear modes can be pictured from the coupling through the crystal arise from phonons. A more appropriate as atomic or molecular layers moving of the local vibrational modes observed distinction is to speak of external and antiparallel to each other within their for the molecular species. We can think internal crystal lattice vibrational modes respective planes, whereas the breathing of these types of phonons as collective when speaking of solid state Raman band modes involve the layers moving away local modes modified through coupling assignments, particularly for molecular from and towards each other. The exter- with other molecules in the crystal and crystals (2). An external crystal lattice vi- nal crystal lattice vibrations are generally affected by their arrangement in the crys- brational mode can be thought of as the of low energy, and, of course, would be tal lattice. Their Raman shifts are often collective motion of molecules as a whole, absent from the liquid or gas spectrum similar but not identical to those of the such as whole water molecules moving of the material because of the absence molecular (liquid or gas) spectrum. 14 Spectroscopy 34(3) March 2019 www.spectroscopyonline.com
tice modes. Perhaps less well known is the sensitivity of the external modes to molecular interactions, such as hydro- gen bonding or even the weaker van der Waals forces between molecules, or formula unit layers within the crystallo- graphic unit cell. This sensitivity to the chemical interactions results in the en- ergies of the external modes, and there- fore their Raman peak positions, being strongly affected by the configurations of molecules or formula units within the unit cell. We present and discuss the
Raman spectra of TiO2, paracetamol, and carbamazepine to demonstrate the merits of using Raman spectroscopy to Figure 5: Raman spectra obtained from different grains of paracetamol form I. differentiate crystal forms for polymorph characterization and screening.
Anatase and Rutile TiO2 There are multiple crystalline phases
of TiO2, and the ones with which you may be the most familiar are anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic). Of these three, the two most commonly en- countered in an industrial setting are anatase and rutile. Anatase belongs
to the crystal class of D4h, with four formula units per crystallographic unit cell. The correlation method for vibrational selection rules predicts six Raman active modes (A + 2 B + 3 Figure 6: Raman spectra obtained from different grains of paracetamol form II. 1g 1g Eg) for anatase TiO2 (3,4). Rutile TiO2 also belongs to the crystal class of D4h, but has two formula units per crystal- lographic unit cell. The correlation method predicts four Raman active
modes (A1g + B1g + B2g + Eg) for rutile TiO2 (3,5). Clearly, then, one should in principle be able to differentiate the
anatase and rutile phases of TiO2 by Raman spectroscopy just by applying the Raman polarization selection rules to identify the symmetry species of Raman bands and by the total number of bands observed in the Raman spec- trum. Nevertheless, the application of group theory and Raman polarization Figure 7: Raman spectra obtained from grains of paracetamol form I (red) and form II (blue). selection rules for the purpose of iden- tifying or differentiating crystal forms As you may expect, the external lattice the characterization and screening of is not always straightforward. vibrational modes or low energy phonons polymorphs. In addition, Raman spec- Group theory and the correlation are very sensitive to crystal structure. tra of low energy phonons are dependent method are beneficial for identify- That is why the low frequency region of upon the crystal orientation with respect ing Raman active modes and assign- the Raman spectrum has proven so use- to the incident laser polarization as are ing a symmetry species to a particu- ful in the pharmaceutical industry for the higher energy internal crystal lat- lar band, but they do not predict the www.spectroscopyonline.com March 2019 Spectroscopy 34(3) 15
Raman scattering strength of the individual crystal lattice vibrational modes. Furthermore, second order modes can also appear in the Raman spectrum, thereby complicating the assignment of symmetry species and the total count of bands attributed to fundamental lattice vibrational modes appearing in a spectrum. The Raman spectra of anatase and rutile TiO2 are shown in Figure 1 (These spectra and all the others shown in this publica- tion were acquired using 532 nm ex- Figure 8: Raman spectra obtained from grains of paracetamol form I (red) and form II (blue). citation and a long working distance 50X Olympus microscope objective). spectrum, there is a very weak band scattering. The lesson to be learned is The rutile spectrum consists of four at 828 cm-1 which has been assigned that the interpretation of even as sim- bands as predicted by the correlation to the B2g symmetry species. All four ple a spectrum as that of rutile TiO2 is method. The bands at 143, 445, and of the fundamental lattice vibrational not straightforward, and we are aided -1 610 cm have been assigned to the B1g, modes are now accounted for. You in our assignments and interpreta- Eg and A1g symmetry species, respec- will also notice a broad shoulder at tion by previously published work. It tively (6). However, the broad band at approximately 707 cm-1 which can is helpful to apply group theory and 241 cm-1 has been attributed to second- be attributed to two-phonon Raman the correlation method when perform- order or two-phonon Raman scatter- scattering. Although one might have ing crystallographic studies by Raman ing, and so cannot be counted among expected a simple spectrum consisting spectroscopy, but be sure to consult the four fundamental modes predicted of four bands based upon group theory previously reported results in order to by group theory (6). Where then is the analysis and the correlation method, perform a thorough study and make fourth fundamental mode? Although a total of six bands are present, two of the most accurate band assignments barely discernable in the rutile TiO2 which are due to second order Raman and interpretations of spectra.
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