Columns | Column: Molecular Spectroscopy Workbench

In this column, I describe what I believe may be the origin of this fluorescence emission and support my conjecture with some measurements of polycyclic aromatic hydrocarbons (PAHs). Understanding the origin of these interfering backgrounds may enable you to design experiments with less interference, avoid the laser illuminations that make things worse, or both.

Because there is a different Raman signature for each of the polymorphs as well as the contaminants, Raman microscopy is an ideal tool for analyzing the structure of these materials as well as identifying possible contaminants that would also interfere with performance.

In this article, we show a cross-sectional map of a film compared to a depth profile to evaluate the quality of depth profile measurements.

Here, we will show the chemical and spectral changes that occur during the cure of a commercial epoxy.

The use of polarized Raman microscopy to complement the polarized light micrographs of spherulites in polymers reveals much about the orientation behavior of the lamellae in spherulites.

Raman spectroscopy has the capability of accurately determining the physical and chemical properties of bioplastics (polyhydroxybutyrate hexanoate [PHBHx]), which helps determine the maximum crystallinity a polymer can experience.

Studying the crystallinity of biodegradable polymers can be done without the complexity of larger instruments. Here’s how.

Extractables and leachable are typically studied using chromatographic techniques. This study explores what we can learn by using from using Raman microscopy for such studies.

Here, we explain how Raman spectroscopy is valuable for characterizing industrially important carbon materials.

Raman spectra can help determine protein structure. Here’s how.

We explain how to characterize barium fluorohalides with Raman spectroscopy.

Innovative database search technology can help Raman spectroscopists identify molecular vibrations; here, we show how to use these tools more effectively.

To better understand Raman spectral profiles, we briefly examine spectral line shapes, while discussing spectral shape, bandwidth, and broadening as percentage of Gaussian and Lorentzian components.

We illustrate how matching the optical properties of the microscope objective with the sample properties improves the spatial resolution and chemical speciation in depth profile measurements using confocal Raman microscopy.

Raman and photoluminescence spectroscopy were combined with imaging to examine the spatial variation of solid-state structure and electronic character of two-dimensional (2-D) tungsten disulfide (WS2) crystals, which represent a family of new inorganic 2-D materials.

Raman spectra were measured in combination with 2D-COS analysis to understand how the addition of propyl side groups to a biopolymer backbone influences the structure of the polymer at the atomic level.

Raman spectroscopy is extremely useful for characterizing crystalline materials.

Low frequency Raman scattering measurements can be used to predict physical properties of polymers and the crystalline polymorphic form of active pharmaceutical ingredients (APIs). These measurements are made by recording the Stokes and anti-Stokes side of the laser line with the laser centered on the detector. Spectra of polyethylene and linear alkanes were recorded down to 4 cm-1.

Raman imaging provides detailed crystal orientation information for two-dimensional MoS2 prepared by chemical vapor deposition on silicon substrates. These two-dimensional crystals consist of individual atomic layers of sulfur, molybdenum, and sulfur atoms.

Raman microscopy, combined with X-ray fluorescence, can provide detailed information about extractables and leachables from medically implantable devices.

Raman measurements can be used to infer the temperature of a material based on the Stokes and anti-Stokes band signal strengths, and by applying a predefined mathematical relationship. In reality, the temperature, Raman shift, and laser excitation wavelength all interact, and should be considered to better understand these measurements.

Raman 2D-COS spectral data provide information on conformational changes of polymers. Here, Raman spectra of ethylene vinyl acetate and vinyl acetate copolymer are measured and interpreted, enabling a description of morphological changes related to the vinyl acetate group.

When stress is applied to an object, it can produce strain. Strain can be detected through changes in peak position and bandwidth in Raman spectra. Here, we show examples of how strain in technologically important materials appears in the Raman spectra.

The spectral behavior of water is studied using Raman with an NIR spectrograph and dual wavelength lasers for measurements of both the fingerprint and the OH stretching regions. Raman spectra are recorded between 5 ⁰C and 80 ⁰C, and treated with both band-fitting and the 2D-COS algorithm revealing improved insights into the spectral behavior of water.

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.

Two-dimensional correlation spectroscopy (2D-COS) can reveal subtle chemical interactions that are difficult to discern when analyzing individual spectra. A demonstration of the subtleties of this technique can be seen in the analysis of ethanol–water and ethanol–methanol mixtures.

Resonance Raman spectroscopy demonstrates up to 106 signal enhancement. In this overview a detailed description of how resonance Raman spectroscopy differs from normal Raman spectroscopy is given with some specific examples explained.

Spectral changes revealed by two-dimensional correlation spectroscopy can be used to interpret structural changes in polymers determined by processing conditions, so that materials can be rationally engineered for particular applications with known mechanical requirements.

Infrared microspectroscopy has led to important advances in a wide range of fields, as biologists, chemists, geologists, materials scientists, microscopists, and spectroscopists around the world have awakened to the values of nanotechnology. The small world is getting larger.

Because of the structural use of polymers, it is important to understand the origin of their strength, elongation to break, elasticity, flexibility, and so on. It is in the very low frequency region (the so called “terahertz” range) where it is believed that the real indicators for crystallinity appear.

This introduction to two-dimensional correlation spectroscopy (2D COS) illustrates how to understand the structural changes occurring in polyester molecules in the fibers as they are annealed under tension.

Raman spectroscopy is a convenient method for probing the chemical bonding and solid state structure of crystals, and it is sensitive to the presence of dopants, impurities, and crystal defects in the host-crystal lattice. Here is what you need to know about how a hostcrystal Raman spectrum is affected by dopants and impurities.

We show Raman spectra of polymeric fibers acquired as a function of increasing stress and temperature. With knowledge of Raman band assignments, it becomes possible to understand, in detail, the molecular changes that are responsible for polymer orientation and crystallization.

The Raman spectra of a particular face of a single crystal can be significantly different if acquired with different microscope objectives. This article explains the underlying physics of changes in relative intensity and even peak position of certain Raman bands depending on the microscope objective used to acquire the spectrum.

The resonance Raman spectra of carotenoids vary with subtle changes on the functional side groups, making these spectra useful for identifying and characterizing carotenoids.

The Raman spectra of crystalline and amorphous solids of the same chemical composition can be significantly different primarily because of the presence or absence of spatial order and long range translational symmetry, respectively. The purpose or goal of this installment of Molecular Spectroscopy Workbench is to help readers understand the underlying physics that affect the Raman spectra of crystalline and amorphous solids. Wave vector, reciprocal space, and the Brillouin zone are explained with respect to Raman spectroscopy of solids.

Controlling the orientation of the laser and Raman polarization relative to the sample orientation can provide a wealth of information in the Raman spectrum that would be difficult to achieve by other methods. Presentation of the spectra makes these effects appear straightforward, but because of multiple instrumental factors, and experimental design, it is easy to produce erroneous results. This column is going to explain what instrumental components affect the polarization effects that are observed, and how the sample setup itself affects the Raman signal.

Raman spectroscopy can be used to determine the temperature of a material when other more direct means are either impractical or not possible. The material’s temperature can affect the peak position of Raman bands. When a Raman band shifts significantly with temperature, the monitoring of the peak position can be the most straightforward manner of determining temperature provided the Raman spectrometer has sufficient spectral resolution. Another method is to determine the temperature from the ratio of the Stokes and anti-Stokes signal strengths of a given Raman band. The latter method requires the ability to detect light at wavelengths longer and shorter than that of the laser and an accurate measurement of the wavelength dependent instrument response function.

Raman spectra of celluloses modified for use in the pharmaceutical, food, and materials industries will be compared and analyzed, with the goal of determining spectroscopic features that can be of use in aiding in the determination of physical and chemical properties.

Photoluminescence can provide information about the composition and solid state structure of a material. The high spectral resolution of a Raman spectrometer can be useful in performing photoluminescence spectroscopy of solid state materials, particularly when the emission spectra consist of narrow bands or even lines. Having the capability to perform photoluminescence and Raman spectroscopies simultaneously with the same instrument is advantageous, especially when studying 2D crystals. When used to perform photoluminescence spectroscopy, the Raman spectrometer becomes two instruments in one.

In the “Molecular Spectroscopy Workshop” column, we have been trying to provide hands-on advice and easy-to-implement tips to start using spectroscopic tools for the analyst who has the responsibility to derive answers to questions as quickly as possible. Very often the identity of an unknown is of ultimate importance, and very few analytical chemists coming out of graduate programs have been taught to systematically analyze spectra to infer identity of the source. In addition, it is rare that an industrial environment will provide resources for the analyst to be educated in this field. Thus, the availability of any means to provide spectral identification will make the difference between success and failure; that is, acquiring a spectrum is useless if one cannot identify it. In this column, I discuss some of the fundamentals of spectral interpretation, illustrate the use of searching software, including mixture analysis, and show how sometimes the software can provide spectral interpretation.

Were it not for the problem of photoluminescence, only one laser excitation wavelength would be necessary to perform Raman spectroscopy. Here, we examine the problem of photoluminescence from the material being analyzed and the substrate on which it is supported. Selecting an excitation wavelength that does not generate photoluminescence reduces the noise level and yields a Raman spectrum with a superior signal-to-noise ratio. Furthermore, we discuss the phenomenon of resonance Raman spectroscopy and the effect that laser excitation wavelength has on the Raman spectrum.

The use of Raman spectroscopy to produce material images whose contrast is derived from chemical or crystallographic species has been seen as quite useful since the introduction of the Raman microscope in 1976, but particularly, more recently, with the development of more sensitive and easier-to-use instruments. When the various species in the field of view have spectra with non-overlapping analytical bands, simple univariate analysis can provide good images. When overlapping bands are present, multivariate techniques, especially MCR (Multivariate Curve Resolution), have been successfully applied. However, there are cases where even MCR results may be problematic. We will look at some maps of a ceramic composite containing SiC, Si, B4C, and Carbon, where each of these species has non-unique spectra to see what type of results flexible software can produce. What is the goal in this type of exercise? For some of us, creating images is like a teenager’s computer game. But really what we are trying to do is to extract information about a sample from its Raman image. A beautiful rendition is nice, but it must yield information. The following will show how Raman maps can provide useful information on a sample.

The application of vibrational selection rules is usually taught with respect to the vibrational motions of individual molecules. However, many of the materials studied by infrared or Raman spectroscopy are solids and even single crystals. Furthermore, some materials such as covalent or ionic solids have no molecular species associated with them. Consequently, there is a need to understand the application of group theory for the determination of spectroscopically active vibrational modes of crystals. The correlation method is ideal for that purpose.

This column is a mini survey of progress that has been made over the last few years in surface enhanced Raman scattering (SERS). The potential of SERS to provide signals of analytes at very low concentrations continues to beckon the analytical chemist. What the last years have produced is a body of work showing the role of the physical properties of metals, based on their geometrical and electronic properties, in enhancing the signals. As this field matures, we foresee production of surface enhancing films and particles that will provide reproducible Raman signals for applications in areas such as environmental and biomedical studies.

Raman bands in the low energy region of the spectrum of crystals are attributed to so-called external lattice vibrational modes. The Raman bands from these external vibrational modes (low energy phonons) are very sensitive to crystal structure and orientation with respect to the incident laser polarization and to molecular interactions within the crystal. The low energy vibrational modes of many organic molecular crystals have very high Raman scattering cross-sections. Raman spectra and images of low energy phonons in so-called two dimensional (2D) crystals such as few-layer MoS2 reveal spatial variations in the solid state structure that are not evident in the higher energy bands.

There is growing interest in using Raman as a spectroscopic probe of biological systems based on its high information content, its compatibility with an aqueous environment, and the spatial resolution that is consistent with physical optics (as good as ~0.5 μm).

 Resonance and off-resonance Raman spectroscopy and imaging are used to examine the spatial variation of the solid-state structure and electronic character of few-layer MoS2 flakes. Simultaneous acquisition of photoluminescence spectra with the Raman scattering provides complementary ways of rendering Raman and photoluminescence spectral images of thin-film MoS2.

There are multiple circumstances where characterization of a collection of particles has value to analysts. In some environments, materials are plagued by particulate contamination that impacts product quality.

Segmented channel waveguides have been fabricated in single-crystal KTiOPO4 through a topotactic process of partial cation exchange. The ion-exchanged waveguides maintain the high nonlinear susceptibility of KTiOPO4 to function as frequency doubling laser light sources

Segmented channel waveguides have been fabricated in single-crystal KTiOPO4 through a topotactic process of partial cation exchange. The ion-exchanged waveguides maintain the high nonlinear susceptibility of KTiOPO4 to function as frequency doubling laser light sources

Metal oxides often occur in crystals where there are no molecular units. Here, we provide an introduction to the concepts that need to be understood when analyzing metal oxides in materials such as paint, ceramic pigments, corrosion films, catalysts, and minerals.

We examine vapor-phase Raman spectroscopy through the acquisition of spectra from gas molecules confined to the headspace.

Results of tablet matching measurements and characterization of packaging are presented.

Polarized Raman spectra are presented along with a discussion of the association of the symmetry species of the normal vibrational mode and the depolarization ratio of Raman scattering.

The physics that determine how gratings and spectrographs work are summarized in simple terms for new users of Raman equipment.

What exactly is a "Raman image" and how is it rendered? The authors explain those points, and demonstrate the use of Raman imaging for the characterization of thin-film and ion-implanted silicon structures. High spectral resolution makes it possible to resolve or contrast the substrate silicon and polysilicon film in Raman images and thus aids in the chemical or physical differentiation of spectrally similar materials.

The presence of electronic transitions in the visible part of the spectrum can provide enormous enhancement of Raman signals.

Characterizing oil shale, particularly black shale consisting of kerogen, with Raman spectroscopy

How the behavior of polarized Raman spectra can elucidate the microscopic structure of fibers

Here, real spectra illustrate how to be your own critic when evaluating band-fitted spectra.

Micro-Raman spectroscopy has been used to depth-profile a waveguide produced by an ion-exchange reaction in a single crystal of a ferroelectric metal oxide, and to reveal the changes in chemical bonding and atomic structure that occur in this process.

The interpretation of Raman spectra is discussed for a series of functionalized celluloses.

Polarization/orientation micro-Raman spectroscopy promises to be an important analytical tool to complement micro-X-ray diffraction.

Raman has a unique capability to characterize nanoscale materials that are between crystalline and amorphous.

How can you navigate the maze of choices for detecting molecular vibrations with mid-infrared (IR), near IR (NIR), and visible (Raman)? Understanding what is being measured, how it is measured, and the advantages and disadvantages of each technique, will help.

Raman spectroscopy can be used to measure the vibrational spectra of both organic and inorganic materials.

Graphene has potential applications ranging from computer monitors to solar cells, and Raman spectroscopy is a useful method for its characterization.

Fran Adar examines the program at ICORS 2010, providing a sense of how applications and instrumentation are driving each other in the field of Raman research.

The experimental determination of the depth resolution of a Raman microscope is described.

Cellulosic feedstocks from biological harvests (such as timber, prairie grass, and corn stover) or industrial–urban waste have been proposed as a source for the production of energy in the form of fermentation-produced ethanol biofuel.

Columnist Fran Adar discusses applications for Raman and FT-IR spectroscopy.

In this column, Fran Adar discusses what progress is being made in the application of Raman spectroscopy to studies of disease states and why it is worth the effort to pursue.

In this article, columnist Fran Adar will review the important features of the Raman spectra of these materials and indicate why the extracted information is important for material development and engineering.

Columnist Fran Adar discusses the physical determinants of spatial resolution and developing methods to improve the mapping speeds of Raman images.

Chemical analysts who use spectroscopy to extract molecular information from samples have been following the developments in Raman instrumentation. Vibrational spectroscopy provides detailed molecular information, but Fourier-transform IR has been much easier to use than Raman. Now that Raman equipment is smaller, cheaper, faster, and easier, analysts are interested. Columnist Fran Adar will discuss why.

Columnist Fran Adar discusses surfaced-enhanced Raman scattering (SERS). The phenomenon is described and the enhancement factors that make it so attractive for analytical purposes are pointed out. In particular, she reviews the state-of-the-art from the point of view of the instrumentation and the robustness of the measurements.

Raman microscopy was developed as a tool for microanalysis complementary to the electron microscope, which enabled identification of the elements in a microspot. The first realization for Raman imaging was implemented using a nonconfocal optical method. Subsequently, a confocal scheme was developed, which provided better contrast in the Raman image. A number of successful examples from pathology, pharmaceutical analysis, and geology will be shown.

Columnist Fran Adar discusses the evolution of Raman spectroscopy instrumentation and new applications for this more sensitive, easy-to-use technology.

Guest author Rebecca Bain of SYFT Technologies discusses a new technique for VOC analysis. SIFT-MS can be used successfully in a wide variety of applications, including medical research, shipping container air analysis, laboratory analysis, and air-quality monitoring.

Guest author John Coates describes handheld measurements performed using a palm-sized device that can be used either as an open-ended spectrometer or as a dedicated analyzer.

Guest author Art Springsteen discusses standards for reflection measurements and the issues surrounding this topic.

Guest author John Coates discusses a miniature spectral measurement system that operates as a full spectrum analyzer, a colorimeter, or a multichannel photometer in the form of a spectral engine.

Emil Ciurczak presents his review of the newest and most interesting products in the field of spectroscopy at the 2006 Pittcon Conference.

Tunable diode laser absorption spectroscopy (TDLAS) has evolved from a laboratory specialty technique to the foundation for highly sensitive and specific gas-phase sensors for the process environment. This article discusses two recently developed products, a portable trace gas monitor, and a water vapor flow sensor.

Guest author John Coates describes a new, compact handheld Raman instrument.

Attendees from the pharmaceutical industry and FDA discussed a range of topics, including new spectroscopic analysis techniques, chemical imaging, calibration, and current efforts at ASTM.

Maligned by early design issues, the polarization interferometer has not been a significant tool in many analysts' arsenals. New technical improvements could change that.

Recent advances in thin-film filter technology have enabled dramatic improvements in the performance of filters for laser-based analytical instrumentation.

Recent advances in thin-film filter technology have enabled dramatic improvements in the performance of filters for laser-based analytical instrumentation.

Recent advances in thin-film filter technology have enabled dramatic improvements in the performance of filters for laser-based analytical instrumentation.

A variable filter array spectrometer can help defeat the environmental restraints of traditional infrared spectrometers.

NIR-CI adds a completely new dimension to conventional spectroscopy.

This installment provides a wrap-up of the 18th International Forum for Process Analytical Chemistry.

This installment examines one of the key differences between the Near-IR and Mid-Range IR and highlights a couple of applications where Mid-IR can be effectively employed as a process analytical technology (PAT).

This second installment in the pair looks at NIR applications performed at-line and on-line, as well as some points to be considered in implementing NIR as a process analytical tool.

Rapid near-infrared spectroscopy measurement near the process line of lab applications may speed up the process.

The author describes applying validation principles originally designed for lab instruments to process instruments.

The author discusses the process analytical technologies initiative and its effect on spectroscopy.