Chemical imaging with variable angles of incidence using a diamond attenuated total reflection accessory

Temperature-Dependent Infrared Refractive Index of Polymers from a Calibrated Attenuated Total Reflection Infrared Measurement

We demonstrate a straightforward method by which a commonly available reference sample such as water can be used to calibrate an attenuated total internal reflection infrared absorbance measurement in order to account for the polarization of the beam incident on the internal reflecting element, and the spread of angles about the nominal angle of incidence. This enables quantitative comparison of attenuated total reflection-derived absorbance data with spectra calculated from optical constants. We then apply this calibration to the measurement of temperature-dependent absorption spectra of a polydimethylsiloxane sample. We illustrate that the extracted optical constants scale with the temperature-dependent changes in the polymer density better than the raw absorbance values on vibrational resonance.

Model-Based Optimization of Laser Excitation and Detection Improves Spectral Contrast in Noninvasive Diffuse Raman Spectroscopy

Spatially offset Raman spectroscopy (SORS) is a powerful technique for subsurface molecular analysis of optically turbid samples. Numerical modeling of light propagation has been used to investigate opportunities for improving spectral contrast and signal to noise ratio when imaging regions of interest located 0-4.5 mm below the surface in polymer bulk material. Two- and three-dimensional modeling results demonstrate that when analyzing a certain region of interest (ROI) of finite lateral dimensions below the sample surface, offsetting both the laser source and detector in opposite directions from the central point of the ROI can increase the spectral contrast as compared to conventional SORS approach where the detector or the laser source is maintained at the central point (centered SORS). The outlined modeling results have been validated experimentally using a bulk polymer sample with a trans-stilbene ROI (cylinder) below the sample surface. The results show that modeling of the spatial configurations of laser excitation and detection points can be used to optimize the instrument configuration to achieve significant improvements (up to 2.25-fold) in performance over the conventional centered SORS. Such optimal solutions can then be implemented, for example, using robust fiber optic probes, moveable optics, or flexible spatial light modulator instruments for specific applications.

Recent advances and applications to cultural heritage using ATR-FTIR spectroscopy and ATR-FTIR spectroscopic imaging

Scientific investigation of cultural heritage objects plays a vital role in a responsible modern approach to conservation and archaeology. Recent advances in spectroscopy, such as the development of Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy and ATR-FTIR spectroscopic imaging, have opened up a window of opportunities for characterisation of materials in artefacts and collections from museums. This review summarises some of the recent advances and applications of these ATR-FTIR spectroscopic analytical techniques in the area of cultural heritage studies, including examples of cross-sections of oil paintings, paper, textiles, plastic objects, potteries, glasses and mineral artefacts. Two of the major advantages of ATR mode measurements are minimal or no requirements for sample preparation and its provision for high lateral spatial resolution. In addition to conventional single point detection, two-dimensional mapping and imaging is especially beneficial for chemical visualisation of multi-layered structure cultural objects. This review also explores the implications of these advantages as well as some limitations and provides a brief outlook for the possible future developments in this area.

Apparatus for High-Precision Angle-Resolved Reflection Spectroscopy in the Mid-Infrared Region

Fourier transform (FT) spectroscopy is a versatile technique for studying the infrared (IR) optical response of solid-, liquid-, and gas-phase samples. In standard Fourier transform infrared (FT-IR) spectrometers, a light beam passing through a Michelson interferometer is focused onto a sample with condenser optics. This design enables us to examine relatively small samples, but the large solid angle of the focused infrared beam makes it difficult to analyze angle-dependent characteristics. Here, we design and construct a high-precision angle-resolved reflection setup compatible with a commercial FT-IR spectrometer. Our setup converts the focused beam into an achromatically collimated beam with an angle dispersion as high as 0.25°. The setup also permits us to scan the incident angle over ∼8° across zero (normal incidence). The beam diameter can be reduced to ∼1 mm, which is limited by the sensitivity of an HgCdTe detector. The small-footprint apparatus is easily installed in an FT-IR sample compartment. As a demonstration of the capability of our reflection setup, we measure the angle-dependent mid-infrared reflectance of two-dimensional photonic crystal slabs and determine the in-plane dispersion relation in the vicinity of the Γ point in momentum space. We observe the formation of photonic Dirac cones, i.e., linear dispersions with an accidental degeneracy at Γ, in an ideally designed sample. Our apparatus is useful for characterizing various systems that have a strong in-plane anisotropy, including photonic crystal waveguides, plasmonic metasurfaces, and molecular crystalline films.

Infrared spectroscopy and spectroscopic imaging in forensic science

Infrared spectroscopy and spectroscopic imaging, are robust, label free and inherently non-destructive methods with a high chemical specificity and sensitivity that are frequently employed in forensic science research and practices. This review aims to discuss the applications and recent developments of these methodologies in this field. Furthermore, the use of recently emerged Fourier transform infrared (FT-IR) spectroscopic imaging in transmission, external reflection and Attenuated Total Reflection (ATR) modes are summarised with relevance and potential for forensic science applications. This spectroscopic imaging approach provides the opportunity to obtain the chemical composition of fingermarks and information about possible contaminants deposited at a crime scene. Research that demonstrates the great potential of these techniques for analysis of fingerprint residues, explosive materials and counterfeit drugs will be reviewed. The implications of this research for the examination of different materials are considered, along with an outlook of possible future research avenues for the application of vibrational spectroscopic methods to the analysis of forensic samples.

In-column ATR-FTIR spectroscopy to monitor affinity chromatography purification of monoclonal antibodies

In recent years many monoclonal antibodies (mAb) have entered the biotherapeutics market, offering new treatments for chronic and life-threatening diseases. Protein A resin captures monoclonal antibody (mAb) effectively, but the binding capacity decays over repeated purification cycles. On an industrial scale, replacing fouled Protein A affinity chromatography resin accounts for a large proportion of the raw material cost. Cleaning-in-place (CIP) procedures were developed to extend Protein A resin lifespan, but chromatograms cannot reliably quantify any remaining contaminants over repeated cycles. To study resin fouling in situ, we coupled affinity chromatography and Fourier transform infrared (FTIR) spectroscopy for the first time, by embedding an attenuated total reflection (ATR) sensor inside a micro-scale column while measuring the UV 280 nm and conductivity. Our approach quantified the in-column protein concentration in the resin bed and determined protein conformation. Our results show that Protein A ligand leached during CIP. We also found that host cell proteins bound to the Protein A resin even more strongly than mAbs and that typical CIP conditions do not remove all fouling contaminants. The insights derived from in-column ATR-FTIR spectroscopic monitoring could contribute to mAb purification quality assurance as well as guide the development of more effective CIP conditions to optimise resin lifespan.

Micro-Attenuated Total Reflection Fourier Transform Infrared (Micro ATR FT-IR) Spectroscopic Imaging with Variable Angles of Incidence

The control of the angle of incidence in attenuated total reflection (ATR) Fourier transform infrared (FT-IR) spectroscopy allows for the probing of the sample at different depths of penetration of the evanescent wave. This approach has been recently coupled with macro-imaging capability using a diamond ATR accessory. In this paper, the design of optical apertures for the micro-germanium (Ge) ATR objective is presented for an FT-IR spectroscopic imaging microscope, allowing measurements with different angles of incidence. This approach provides the possibility of three-dimensional (3D) profiling in micro-ATR FT-IR imaging mode. The proof of principle results for measurements of polymer laminate samples at different angles of incidence confirm that controlling the depth of penetration is possible using a Ge ATR objective with added apertures.

Cleaning-in-place of immunoaffinity resins monitored by in situ ATR-FTIR spectroscopy

In the next 10 years, the pharmaceutical industry anticipates that revenue from biotherapeutics will overtake those generated from small drug molecules. Despite effectively treating a range of chronic and life-threatening diseases, the high cost of biotherapeutics limits their use. For biotherapeutic monoclonal antibodies (mAbs), an important production cost is the affinity resin used for protein capture. Cleaning-in-place (CIP) protocols aim to optimise the lifespan of the resin by slowing binding capacity decay. Binding assays can determine resin capacity from the mobile phase, but do not reveal the underlying causes of Protein A ligand degradation. The focus needs to be on the stationary phase to examine the effect of CIP on the resin. To directly determine both the local Protein A ligand concentration and conformation on two Protein A resins, we developed a method based on attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy. ATR-FTIR spectroscopic imaging revealed that applying a carefully controlled load to agarose beads produces an even and reproducible contact with the internal reflection element. This allowed detection and quantification of the binding capacity of the stationary phase. ATR-FTIR spectroscopy also showed that Protein A proteolysis does not seem to occur under typical CIP conditions (below 1 M NaOH). However, our data revealed that concentrations of NaOH above 0.1 M cause significant changes in Protein A conformation. The addition of >0.4 M trehalose during CIP significantly reduced NaOH-induced ligand unfolding observed for one of the two Protein A resins tested. Such insights could help to optimise CIP protocols in order to extend resin lifetime and reduce mAb production costs.

Fast drying and film formation of latex dispersions studied with FTIR spectroscopic imaging

Drying of thin latex films (∼20 μm) at high drying speeds (of the order of seconds) has been studied by fast chemical imaging. ATR-FTIR spectroscopic imaging combined with a fast "kinetic" mode was used to acquire spectral images without coaddition, enabling the amount of water and homogeneity of the drying film to be studied over time. Drying profiles, constructed from analyzing the water content in each image, show two stages of drying, a fast and a slow region. The formulation of latex dispersions affects the onset of slow drying and the volume fraction of water remaining at the onset of slow drying. In this work, the effect of physical properties, film thickness and glass transition temperature (Tg), were investigated, as well as the effect of monomer composition where two monomoers, 2-ethylhexyl acrylate and n-butyl acrylate, and the amount of hydrophilic comonomer, methyl methacrylate (MMA), were varied. It was found that thicker films produced slower overall drying and that the formulation with a Tg above the minimum film formation temperature did not dry evenly, exhibiting cracking. However, the drying kinetics of high and low Tg films were similar, highlighting the advantage of using a spatially-resolved spectroscopic approach. Formulations containing more MMA dried faster than those with less. This was due to the hydrophilicity of MMA and the increase in Tg of the dispersion from the addition of MMA. Overall, FTIR spectroscopic imaging was shown to be a suitable approach in measuring film drying at high speeds as both chemical changes and chemical distribution could be analyzed over time.

Micro- and macro-attenuated total reflection Fourier transform infrared spectroscopic imaging. Plenary Lecture at the 5th International Conference on Advanced Vibrational Spectroscopy, 2009, Melbourne, Australia

Fourier transform infrared (FT-IR) spectroscopic imaging has become a very powerful method in chemical analysis. In this review paper we describe a variety of opportunities for obtaining FT-IR images using the attenuated total reflection (ATR) approach and provide an overview of fundamental aspects, accessories, and applications in both micro- and macro-ATR imaging modes. The advantages and versatility of both ATR imaging modes are discussed and the spatial resolution of micro-ATR imaging is demonstrated. Micro-ATR imaging has opened up many new areas of study that were previously precluded by inadequate spatial resolution (polymer blends, pharmaceutical tablets, cross-sections of blood vessels or hair, surface of skin, single live cells, cancerous tissues). Recent applications of ATR imaging in polymer research, biomedical and forensic sciences, objects of cultural heritage, and other complex materials are outlined. The latest advances include obtaining spatially resolved chemical images from different depths within a sample, and surface-enhanced images for macro-ATR imaging have also been presented. Macro-ATR imaging is a valuable approach for high-throughput analysis of materials under controlled environments. Opportunities exist for chemical imaging of dynamic aqueous systems, such as dissolution, diffusion, microfluidics, or imaging of dynamic processes in live cells.

Chemical imaging of microfluidic flows using ATR-FTIR spectroscopy

Elucidating the chemical composition of microfluidic flows is crucial in both understanding and optimising reactive processes within small-volume environments. Herein we report the implementation of a novel detection methodology based on Attenuated Total Reflection (ATR)-Fourier Transform Infra-Red (FTIR) spectroscopic imaging using an infrared focal plane array detector for microfluidic applications. The method is based on the combination of an inverted prism-shape ATR crystal with a poly(dimethylsiloxane)-based microfluidic mixing device. To demonstrate the efficacy of this approach, we report the direct measurement and imaging of the mixing of two liquids of different viscosities and the imaging and mixing of H2O and D2O with consecutive H/D isotope exchange. This chemically specific imaging approach allows direct analysis of fluid composition as a function of spatial position without the use of added labels or dyes, and can be used to study many processes in microfluidics ranging from reactions to separations.

Measurement of drug and macromolecule diffusion across atherosclerotic rabbit aorta ex vivo by attenuated total reflection-Fourier transform infrared imaging

Diffusion of two model drugs-benzyl nicotinate and ibuprofen-and the plasma macromolecule albumin across atherosclerotic rabbit aorta was studied ex vivo by attenuated total reflection-Fourier transform infrared (ATR-FTIR) imaging. Solutions of these molecules were applied to the endothelial surface of histological sections of the aortic wall that were sandwiched between two impermeable surfaces. An array of spectra, each corresponding to a specific location in the section, was obtained at various times during solute diffusion into the wall and revealed the distribution of the solutes within the tissue. Benzyl nicotinate in Ringer's solution showed higher affinity for atherosclerotic plaque than for apparently healthy tissue. Transmural concentration profiles for albumin demonstrated its permeation across the section and were consistent with a relatively low distribution volume for the macromolecule in the middle of the wall. The ability of albumin to act as a drug carrier for ibuprofen, otherwise undetected within the tissue, was demonstrated by multivariate subtraction image analysis. In conclusion, ATR-FTIR imaging can be used to study transport processes in tissue samples with high spatial and temporal resolution and without the need to label the solutes under study.

Chemical imaging of live cancer cells in the natural aqueous environment

Chemical imaging with Fourier transform infrared (FT-IR) spectroscopy allows the visualization of the distribution of chemical components in cells without the need for labels or added dyes. However, obtaining such images of living cells is difficult because of the strong absorption of water in the mid-infrared region. We report the use of attenuated total reflection (ATR) FT-IR spectroscopic imaging to study live human cancer cells in an aqueous environment, on a single cell level. Two complementary approaches have been used, providing flexibility with field of view and spatial resolution: (1) micro-ATR FT-IR imaging using a microscope objective with a Ge crystal, and (2) single-reflection diamond ATR-FT-IR imaging. Using both approaches, the ATR-FT-IR spectroscopic signatures allow the differentiation between several cellular organelles, e.g., the nucleus and the endoplasmic reticulum (ER). The overall cell shape can be defined by the distribution of the amide II band in the measured image, while the DNA-rich nucleus and glycogen-rich ER could be imaged using the spectral bands at 1084 cm(-1) and 1023 cm(-1), respectively. We also demonstrate the potential of ATR-FT-IR spectroscopic imaging for unraveling the details of the dynamics of biological processes, which are not accessible from cell ensemble studies, with high molecular specificity and satisfactory spatial resolution.