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.
We wrap up our introduction to the theory of infrared spectral interpretation with a discussion of the correct process to follow when interpreting spectra. The author has developed this 12-step system over many years of interpreting spectra, and finds it gives him the best results. The process includes knowing how a spectrum was measured, systematically identifying peaks, and the proper use of infrared spectral interpretation aids. The answer to last column’s quiz is also disclosed.
Infrared spectroscopy is an appealing technique for application to forensic samples because it offers the benefits of being non-destructive and non-hazardous, fast, reasonably sensitive, and resistant to some of the interferences of many commonly used techniques. Our research team has been focusing on detecting biological fluids on fabrics, which are inherently anisotropic substrates for spectroscopy. The work presented here investigates the effect of azimuthal angle of the sample on the infrared diffuse reflection spectra of fabrics with a goal of removing sampling differences as a source of analytic variation.
Infrared and Raman bands are typically described in terms of peak location and peak height. The shape of the peak-its form, width, and asymmetry-is less well understood. Bandshapes largely depend upon interactions between vibrating molecules and their environment. An understanding of this relationship may enhance spectral interpretation and can explain unexpected behaviors, in both qualitative and quantitative analyses. This paper presents basic concepts that influence line shapes and shows the complex lineshape changes in an apparently simple system of acetone and water.
Although near-infrared (NIR) spectroscopy is not a particularly sensitive technique, it can be implemented with little or no sample preparation and thus is well suited to applications such as process monitoring, materials science, and medical uses. We asked a panel of experts to comment on important current applications of NIR, as well as emerging new areas of application and the challenges involved in those newer applications.
Identity testing is used in the pharmaceutical, food, and dietary supplement industries (amongst others) to ensure raw materials and final products have the correct chemical composition by answering the spectral question: Are these two samples the same? The first part of this installment instructs readers on the correct way to perform identity testing. The interpretation portion of the installment wraps up our discussion of straight chain alkanes by discussing how to determine chain length from infrared spectra. We also go over the answer to the problem from the last installment.
Multiple angle incidence resolution spectroscopy (MAIRS) has proven useful for characterization of the in-plane (IP) and out of plane (OP) vibrations of thin films on solid substrates. The MAIRS technique computes the IP and OP spectra by performing a regression analysis on a series of oblique-incidence transmission spectra collected over a range of angles of a single thin film sample mounted on a transparent substrate. MAIRS replaces the more traditional technique of the collection of a transmission spectrum of a thin film on a transparent substrate, followed by collection of a reflection absorption spectrum of the same film on a metallic substrate. Often times, preparation of the same thin film on different substrates with different chemical and physical properties can be problematic. This paper will discuss details of the electromagnetic theory of MAIRS, and demonstrate its use in producing the IP and OP spectra of several thin film samples.
In recent years there has been increased use of silicones in medicine, especially for medicinal implants. Quality control of intracorporeal-used silicones is an important task for ensuring patients’ health, but it is also a challenging one. The traditional mechanical methods used for the quality control of these silicone products, like rheometric measurements, tend to waste a lot of raw material. In this study, near-infrared spectroscopy (NIRS) has been used to replace the traditional method (rheometric measurements of control samples) using rheometry only as reference method to generate different calibration models. The applicability of NIRS as non-invasive analysis method is proven and the developed calibration models for curing processes of a silicone-adhesive at different temperatures are shown.
A huge amount of information is contained in the FTIR spectra of soils in the mid infrared (MIR) region (4000 to 400 cm-1). The spectra provide an overall chemical profile of the soil, encompassing fundamental vibrations of both the organic and mineral components. Interpretation of the spectrum of individual soils can provide a powerful means of differentiating between samples and therefore has considerable potential for use in forensic applications, and indeed we have successfully used laboratory-based FTIR analysis of soil to provide evidence in forensic casework. In recent years handheld FTIR spectrometers have become available and this makes it possible for in situ or field-based FTIR analysis of soils at a crime scene. However, reliable and tested protocols are not yet available for field-based FTIR analysis of soil. This paper discusses the sampling options for field-based FTIR of soil and describes tests of the methodology we are developing, for a handheld FTIR, on soil samples tested in the context of a mock crime scene.
In honor of Spectroscopy’s celebration of 30 years covering the latest developments in materials analysis, we asked experts to assess the current state of the art of six key spectroscopic techniques. Here, the experts weigh in on what they considered the most important recent advances in infrared (IR) spectroscopy.
Continuing the theory and practice themes from previous columns, the theory portion of this column will be a discussion of the proper way of handling the infrared spectral interpretation of mixtures. In my opinion, mixtures are the biggest obstacle to interpreting infrared spectra, and I will share with readers five tried-and-true techniques for dealing with them. The practice portion of the column will give the answer to the last installment’s problem, and complete the spectral analysis of straight chain alkanes.
In honor of Spectroscopy's celebration of 30 years covering the latest developments in materials analysis, we asked a panel of experts to assess the current state of the art of infrared (IR) spectroscopy and to try to predict how the technology will develop in the future.
Interpreting infrared spectra is fun, but to do it properly one must be grounded in theory, which might be not so enjoyable for some. To cover theory and interpretation judiciously, this installment (and the next several installments) will begin with a section on theory and end with coverage of interpreting spectra. Here, we introduce the theory behind light and spectral units and the interpretation of methyl and methylene groups contained in straight alkane chains.
There is a continuing need for Fourier transform infrared (FT-IR) users to receive training in how to interpret the infrared spectra they measure. This new column will provide practical advice about how to do this. This first installment will present why this type of column is important, discuss some basic IR theory, and lay out a blueprint for future installments.
Macro attenuated total reflection FT-IR spectroscopic imaging is a powerful and underutilized tool. This article presents an overview of approaches and opportunities for using this method to study dynamic processes such as diffusion, sorption, crystallization, and dissolution.
