Infrared (IR) Spectroscopy

Aldehydes feature a unique “lone hydrogen” atom, giving rise to unique C-H stretching and bending peaks, making them easy to spot. In this installment, a new feature is also presented, “IR Spectral Interpretation Review,” where key concepts from past columns are presented for those new to the column and for readers who need a refresher.

An introduction to the IR spectroscopy of the carbonyl group, exploring why the peak is intense and showing how to apply that knowledge to the analysis of the spectra of ketones

In forensic science, the detection of blood on fabric is a very useful tool. Therefore, it is important that the methods used for detecting blood be as accurate as possible. Michael L. Myrick and Stephen L. Morgan, both professors in the Department of Chemistry and Biochemistry at the University of South Carolina, have been investigating the use of infrared (IR) spectroscopy for this purpose, including comparing the effectiveness of infrared diffuse reflectance versus attenuated total reflectance Fourier-transform IR (ATR FT-IR). They recently spoke to Spectroscopy about their recent studies and the critical questions they have been addressing in how IR spectroscopy is used in forensic science.

This work shows that methods based on miniaturized near- and mid-infrared spectroscopy can be used effectively for the quality control of herbal medicines.

Microplastics from clothing, abrasive action on plastics, or engineered microbeads as found in some exfoliating cosmetics are showing up in many environmental systems. FT-IR microscopy is a useful tool in the analysis of microplastics, providing visual information, particle counts, and particle identification.

Naoto Nagai focuses on solving problems for industry. In this interview, he explains his research to determine the cause of resin cracks in polyoxymethylene mold plates using IR spectroscopy.

Two unrelated discussions are presented: carbohydrates and alkynes.

Coherent two-dimensional infrared spectroscopy (2D IR) uses a series of IR femtosecond laser pulses to pump and then probe the response of a system, making it possible to learn much more about the structure and dynamics of molecules than can be seen with one-dimensional IR spectroscopy. The technique’s inventor, Martin T. Zanni of the University of Wisconsin-Madison, discussed 2D IR in a 2013 interview in Spectroscopy (1). Since 2013, Zanni has applied 2D IR spectroscopy to new systems and has started a company, PhaseTech Spectroscopy, Inc., to commercialize the technique.

We will discuss three different types of ether, which are characterized by the type of carbons attached to the central oxygen.

In addition to primary alcohols there exist secondary and tertiary alcohols.

We now turn our attention to the C-O bond, how to detect its presence in a sample from an infrared (IR) spectrum, and a study of the functional groups that contain this bond. In this first installment on the topic, we study the spectra of alcohols and learn to distinguish primary, secondary, and tertiary alcohols from each other based on their infrared spectra.

Mid-infrared (MIR, 3-20 µm) sensor platforms are increasingly adopted in chem/bio analytics, and applied in areas ranging from process monitoring to medical diagnostics. Due to the inherent access to molecule-specific fingerprints via well-pronounced fundamental vibrational, rotational, and roto-vibrational transitions, quantitative information at ppm to ppb concentration levels and beyond is achievable in solids, liquids, and gases. In particular, the combination of quantum cascade lasers (QCLs) with correspondingly tailored waveguide technologies serving as optical transducers – thin-film waveguides for liquid/solid phase analysis, and substrate-integrated hollow waveguides for gaseous samples – facilitates miniaturizable and integrated optical chem/bio sensors and diagnostics applicable in, e.g., exhaled breath analysis, food safety, and environmental monitoring.

Heinz W. Siesler, Emeritus Professor at University of Duisburg-Essen, reviews Jerry Workman’s new book, The Concise Handbook of Analytical Spectroscopy Theory, Applications, and Reference Materials

Advances in spatial resolution for Fourier transform infrared (FT-IR) imaging historically have involved the use of a synchrotron source, but new optics have been developed that yield better spectral quality and spatial resolution than are provided by existing synchrotron sources. Kathleen Gough, Professor in the Department of Chemistry at the University of Manitoba, has been working with her group to conduct diagnostic tissue imaging with the new thermal source FT-IR system. She recently spoke to us about these efforts.

Now that we have completed our discussion of benzene rings and the infamous “benzene fingers,” the next topic on our hydrocarbon hit parade are carbon-carbon double and triple bonds. C=C bonds, otherwise known as alkenes, come in six different structural isomer types, while triple bonds, known as alkynes, come in two varieties. This column provides you with all the tools you need to distinguish all of these different types of molecules from each other.

Infrared reflectance and absorption spectroscopy have been practiced for decades. New capabilities in detectors and light sources are quickly changing the landscape in the near- and mid-infrared, where fundamental vibrations and overtone bands allow sensitive measurements in applications related to food safety, precision agriculture, energy, and smart manufacturing, to name a few.  This article outlines some of the most recent innovations and how they might be applied in real-world systems.

Adhesives are a group of materials that are found extensively in manufacturing and production industries and are of great interest for quality control and failure analysis. This paper discusses the use of an array detector in conjunction with ultrafast mapping to produce kinetic chemical imaging to monitor the curing process in a two-part epoxy resin. This technique allows for simultaneous analysis of both the kinetics of the epoxy reaction along with the spatial information of the reaction. This kinetico-spatial information gives insight about localized domains that form when the epoxy is mixed and how the reaction progresses.

With the theoretical background of benzene analysis laid out in part 1 of this series, we now know what fundamental, overtone, and combination bands look like. Here, I show that the benzene fingers are a series of overtone and combination bands that can be used to distinguish substituted benzene rings from each other when other methods do not work. I review the benzene finger patterns for mono-, ortho-, meta-, and para- substituted benzene rings, and describe an easy mnemonic in which you use your fingers to help you remember the patterns.

The noninvasive discrimination of powdered root material belonging to the Polygala genus and an adulterant is presented. The quality of the approach is assessed for attenuated total reflectance mid-infrared spectroscopy and diffuse reflectance near-infrared spectroscopy. Due to the pharmaceutical importance of Polygala related plant material, conclusions are drawn towards a laboratory independent discrimination of the samples.

This study reported a combined use of ordinary Fourier transform-infrared spectroscopy (FT-IR) in conjunction with partial-least-square (PLS) multivariate regression for accurate determination of the percent compositions of four essential oils (EOs) (wintergreen, tea tree, rosemary, and lemon eucalyptus oils) that were adulterated either with lemongrass essential oil (LO) or peppermint essential oil (PO). The FT-IR spectra of the calibration sample sets of known compositions of adulterated EOs with LO or PO were measured and subjected to PLS multivariate regression analysis. The simplicity, low-cost, and high accuracy of the protocol makes it appealing for routine industrial quality assurance of consumable goods.

Polymer laminates typically make complex samples for infrared analysis, comprising multiple layers with defined thicknesses, in some cases less than 10 µm. When measuring extremely narrow laminate layers, the use of attenuated total reflectance (ATR) may provide improved spectra of the laminate cross-section, because ATR microscope objectives offer a greater spatial resolution than transmission due to additional magnification. This paper details the preparation of polymer laminate sample cross-sections and the collection of transmission and ATR spectra of various layers. Further analysis of the laminate spectra will also be explored utilizing a multivariate curve resolution (MCR) algorithm. An example laminate sample is examined utilizing all the tools available on a standard FT-IR microscope.

This installment begins with a needed discussion on the theory behind the three different types of infrared bands, how to recognize them, and how to use them to help you interpret spectra. Continuing on from the last column, this knowledge is used to help better distinguish mono- and di-substituted benzene rings from each other.

Following up on the last installment, we examine the infrared spectra of mono- and di-substituted benzene rings. We will examine numerous example spectra and learn how the position of C-H wagging peaks, and the presence or absence of a ring-bending peak, allow one to distinguish between mono-, ortho-, meta-, and para-substituted rings most of the time.

Matthew Baker, a senior lecturer in chemistry at the University of Strathclyde, in Glasgow, has won the inaugural Emerging Leader in Molecular Spectroscopy Award, which is sponsored by Spectroscopy magazine. This new annual award recognizes the achievements and aspirations of a talented young molecular spectroscopist, selected by an independent scientific committee. The award will be presented to Baker at the SciX 2016 conference in September, where he will give a plenary lecture and be honored in an award symposium.

Continuing our theme of investigating the infrared spectra of hydrocarbons, we look at the nature of aromatic bonding and why aromatic rings have unique structures, bonding, and infrared spectra. Then we examine, in detail, the spectra of mono- and di-substituted benzene rings, and learn that infrared spectroscopy easily distinguishes between ortho-, meta-, and para- structural isomers.