The use of FT-IR imaging as an analytical tool for the characterization of drug delivery systems

Advances in ATR-FTIR Spectroscopic Imaging for the Analysis of Tablet Dissolution and Drug Release

One of the major challenges in the development of effective pharmaceutical formulations for oral administration is the poor solubility of active pharmaceutical ingredients. For this reason, the dissolution process and drug release from solid oral dosage forms, such as tablets, is usually thoroughly studied in order to understand the dissolution behaviour under various conditions and optimize the formulation accordingly. Standard dissolution tests used in the pharmaceutical industry provide information on the amount of drug released over time; however, these do not allow for a detailed analysis of the underlying chemical and physical mechanisms of tablet dissolution. FTIR spectroscopic imaging, by contrast, does offer the ability to study these processes with high spatial and chemical specificity. As such, the method allows us to see the chemical and physical processes which occur inside the tablet as it dissolves. In this review, the power of ATR-FTIR spectroscopic imaging is demonstrated by presenting a number of successful applications of this chemical imaging technique to dissolution and drug release studies for a range of different pharmaceutical formulations and study conditions. Understanding these processes is essential for the development of effective oral dosage forms and optimization of pharmaceutical formulations.

Physicochemical properties and drug-release mechanisms of dual-release bilayer tablet containing mirabegron and fesoterodine fumarate

Introduction: In this study, a dual release bi-layer tablet containing Fesoterodine fumarate (Fst) 5 mg and Mirabegron (Mrb) 50 mg was prepared to investigate the different release behavior of each drug in bilayer tablet. The bilayer tablet was prepared based on monolayer-tablet formulation of each drug.

Methods: The optimized bi-layer tablet showed an in vitro dissolution profile similar to commercial reference tablets Toviaz and Betmiga, based on a satisfactory similarity factor. Drug-release kinetics of each drug in the bilayer tablet were evaluated based on dissolution profiles. Drug-release behavior was evaluated by observing the surface of each layer by scanning electron microscopy and measuring the changes in weight and volume of the tablet during dissolution. Drug transfer between each layer was also investigated by Fourier -transform infrared spectroscopic imaging by observing the cross-section of the bilayer tablet cut vertically during dissolution.

Results: The release of Fst was well suited for the Higuchi model, and the release of Mrb was well suited for the Hixson-crowell model. Compared with dissolution rate of each monolayer tablet, that of Fst in the bilayer tablet was slightly reduced (5%), but the dissolution rate of Mrb in bilayer tablet was dramatically decreased (20%). Also, a drug-release study confirmed that polymer swelling was dominant in Fst layer compared with polymer erosion, and degradation was dominant in MRB layer. Fourier-transform infrared imaging and 3-D image reconstruction showed that drug transfer in the bilayer tablet correlates with the results of drug-release behavior.

Conclusion: These findings are expected to provide scientific insights in the development of a dual-release bilayer drug-delivery system for Fst and Mrb.

Hot Melt Extrusion: Highlighting Physicochemical Factors to Be Investigated While Designing and Optimizing a Hot Melt Extrusion Process

Hot-melt extrusion (HME) is a well-accepted and extensively studied method for preparing numerous types of drug delivery systems and dosage forms. It offers several advantages: no solvents are required, it is easy to scale up and employ on the industrial level, and, in particular, it offers the possibility of improving drug bioavailability. HME involves the mixing of a drug with one or more excipients, in general polymers and even plasticizers, which can melt, often forming a solid dispersion of the drug in the polymer. The molten mass is extruded and cooled, giving rise to a solid material with designed properties. This process, which can be realized using different kinds of special equipment, may involve modifications in the drug physicochemical properties, such as chemical, thermal and mechanical characteristics thus affecting the drug physicochemical stability and bioavailability. During process optimization, the evaluation of the drug solid state and stability is thus of paramount importance to guarantee stable drug properties for the duration of the drug product shelf life. This manuscript reviews the most important physicochemical factors that should be investigated while designing and optimizing a hot melt extrusion process, and by extension, during the different pre-formulation, formulation and process, and post-formulation phases. It offers a comprehensive evaluation of the chemical and thermal stability of extrudates, the solid physical state of extrudates, possible drug-polymer interactions, the miscibility/solubility of the drug-polymer system, the rheological properties of extrudates, the physicomechanical properties of films produced by hot melt extrusion, and drug particle dissolution from extrudates. It draws upon the last ten years of research, extending inquiry as broadly as possible.

Reflectometric monitoring of the dissolution process of thin polymeric films

Pharmaceutical thin films are versatile drug-delivery platforms i.e. allowing transdermal, oral, sublingual and buccal administration. However, dissolution testing of thin films is challenging since the commonly used dissolution tests for conventional dosage forms correspond rather poorly to the physiological conditions at the site of administration. Here we introduce a traditional optical reflection method for monitoring the dissolution behavior of thin polymeric films. The substances, e.g. drug molecules, released from the film generate an increase in the refractive index in the liquid medium which can be detected by reflectance monitoring. Thin EUDRAGIT^® RL PO poly(ethyl acrylate-co-methyl methacrylate-co trimethylammonioethyl methacrylate chloride) (RLPO) films containing the model drug perphenazine (PPZ) were prepared by spraying on a glass substrate. The glass substrates were placed inside the flow cell in the reflectometer which was then filled with phosphate buffer solution. Dissolution was monitored by measuring the reflectance of the buffer liquid. The method was able to detect the distinctive dissolution characteristics of different film formulations and measured relatively small drug concentrations. In conclusion, it was demonstrated that a traditional optical reflection method can provide valuable information about the dissolution characteristics of thin polymeric films in low liquid volume surroundings.

Advances in mechanistic understanding of release rate control mechanisms of extended-release hydrophilic matrix tablets

Approaches to characterizing and developing understanding around the mechanisms that control the release of drugs from hydrophilic matrix tablets are reviewed. While historical context is provided and direct physical characterization methods are described, recent advances including the role of percolation thresholds, the application on magnetic resonance and other spectroscopic imaging techniques are considered. The influence of polymer and dosage form characteristics are reviewed. The utility of mathematical modeling is described. Finally, how all the information derived from applying the developed mechanistic understanding from all of these tools can be brought together to develop a robust and reliable hydrophilic matrix extended-release tablet formulation is proposed.

Solid-state interactions at the core-coat interface: physicochemical characterization of enteric-coated omeprazole pellets without a protective sub-coat

Conventionally, scanning electron or transmission microscopy, Raman and near infrared (NIR) spectroscopy, terahertz, florescence, and nuclear magnetic resonance imaging have been used to characterize functional coating structure. This study highlights the use of fluorescence microscopy to investigate the physicochemical stability and coating integrity of the commercially available enteric-coated omeprazole pellets containing a basic excipient and prepared by extrusion and spheronization or drug layering on the nonpareil seed, immediately followed by enteric coating (i.e., absence of protective sub-coat). The nature of coating interface and the likely development of an in situ interfacial layer after the application of enteric coating solution was examined using HPLC, NMR, differential scanning calorimetry (DSC), and fluorescent imaging methods. Likewise for the characterization of the solid pellet structure via fluorescence microscopy, a new approach based on fracturing technique (to avoid surface contamination) rather than microtome sectioning was used and validated. Analytical data showed that the pellets containing omeprazole remained chemically stable (>99.5% recovered). Control of the microenvironmental pH by the addition of alkalinizing excipient within a core formulation or as part of drug layering on top of nonpareil seed appears to efficiently neutralize the acidic effect of enteric coating dispersion. Fluorescence images further illustrate the absence of any discernable in situ layer formation at the coat-core interface.

Analytical technologies for real-time drug dissolution and precipitation testing on a small scale

Objectives

This review focuses on real-time analytics of drug dissolution and precipitation testing on a comparatively small scale.

Key findings

Miniaturisation of test equipment is an important trend in pharmaceutics, and several small-scale experiments have been reported for drug dissolution and precipitation testing. Such tests typically employ analytics in real-time. Fibre optic ultraviolet (UV) analytics has become a well-established method in this field. Novel imaging techniques are emerging that use visible or UV light; also promising is Fourier transform infrared imaging based on attenuated total reflection. More information than just a rate constant is obtained from these methods. The early phase of a dissolution process can be assessed and drug precipitation may eventually be observed. Some real-time techniques are particularly well suited to studying drug precipitation during formulation dispersion; for example, turbidity, focused beam reflectance measurement and Raman spectroscopy.

Summary

Small-scale dissolution tests equipped with real-time analytics have become important to screen drug candidates as well as to study prototype formulations in early development. Future approaches are likely to combine different analytical techniques including imaging. Miniaturisation started with mini-vessels or small vials and future assays of dissolution research will probably more often reach the level of parallel well plates and microfluidic channels.

Biorelevant dissolution of poorly soluble weak acids studied by UV imaging reveals ranges of fractal-like kinetics

Much pharmaceutical research has been invested into drug dissolution testing and its mathematical modeling. Even today, there is no complete understanding of the dissolution process but novel imaging tools have been introduced into pharmaceutics that may spur further scientific advancement. We used UV imaging to study the intrinsic dissolution of various poorly soluble acidic model drugs to understand the effects of heterogeneity on early intrinsic drug dissolution using a biorelevant medium: celecoxib, ketoprofen, naproxen, and sulfathiazole. All compounds were characterized using X-ray powder diffraction and thermal analysis. Raman spectroscopy and scanning electron microscopy were employed before and after the initial dissolution phase. As a result, ranges of fractal-like dissolution behavior were found with all model compounds. Intrinsic dissolution rate exhibited a power law mainly at early time points. Subsequently, after several minutes, pseudo-equilibrium was reached with a nearly constant dissolution rate. Further research should investigate whether compounds other than acids demonstrate similar early dissolution kinetics in biorelevant media. The observed fractal-like intrinsic dissolution behavior has several pharmaceutical implications. This study primarily helps us to better understand in vitro dissolution testing, particularly on a miniaturized scale. This improved understanding of early dissolution events may advance future correlations with in vivo data. Therefore, fractal-like dissolution should be considered during biopharmaceutical modeling.

Insights into the early dissolution events of amlodipine using UV imaging and Raman spectroscopy

Traditional dissolution testing determines drug release to the bulk, but does not enable an understanding of the events happening close to the surface of a solid or a tablet. UV imaging is a new imaging approach that can be used to study the dissolution behavior of chemical compounds. The UV imaging instrumentation offers recording of absorbance maps with a high spatial and temporal resolution which facilitates the abundant collection of information regarding the evolving solution concentrations. In this study, UV imaging was used to visualize the dissolution behavior of amlodipine besylate (amorphous and dihydrate forms) and amlodipine free base. The dissolution of amlodipine besylate was faster from the amorphous form than from the crystalline forms. The UV imaging investigations suggested that a solvent mediated phase transformation occurred for the amorphous amlodipine besylate and the amlodipine free base samples. Raman spectroscopy was used to confirm and probe the changes at the solid surface occurring upon contact with the dissolution media and verified the recrystallization of the amorphous form to the monohydrate. The combination of UV imaging and Raman spectroscopy is an efficient tool to obtain a deeper insight into the early events of the dissolution process.

Controlling drug nanoparticle formation by rapid precipitation

Nanoparticles are a drug delivery platform that can enhance the efficacy of active pharmaceutical ingredients, including poorly-water soluble compounds, ionic drugs, proteins, peptides, siRNA and DNA therapeutics. To realize the potential of these nano-sized carriers, manufacturing processes must be capable of providing reproducible, scalable and stable formulations. Antisolvent precipitation to form drug nanoparticles has been demonstrated as one such robust and scalable process. This review discusses the nucleation and growth of organic nanoparticles at high supersaturation. We present process considerations for controlling supersaturations as well as physical and chemical routes for modifying API solubility to optimize supersaturation and control particle size. We conclude with a discussion of post-precipitation factors which influence nanoparticle stability and efficacy in vivo and techniques for stabilization.

Application of fiber-optic attenuated total reflection-FT-IR methods for in situ characterization of protein delivery systems in real time

A fiber-optic coupled attenuated total reflection (ATR)-FT-IR spectroscopy technique was applied to the study of two different therapeutic delivery systems, acid degradable hydrogels and nanoparticles. Real time exponential release of a model protein, human serum albumin (HSA), was observed from two different polymeric hydrogels formulated with a pH sensitive cross-linker. Spectroscopic examination of nanoparticles formulated with an acid degradable polymer shell and encapsulated HSA exhibited vibrational signatures characteristic of both particle and payload when exposed to lowered pH conditions, demonstrating the ability of this methodology to simultaneously measure phenomena arising from a system with a mixture of components. In addition, thorough characterization of these pH sensitive delivery vehicles without encapsulated protein was also accomplished in order to separate the effects of the payload during degradation. When in situ, real time detection in combination with the ability to specifically identify different components in a mixture without involved sample preparation and minimal sample disturbance is provided, the versatility and suitability of this type of experiment for research in the pharmaceutical field is demonstrated.

An in vitro release study of indomethacin from nanoparticles based on methyl methacrylate/glycidyl methacrylate copolymers

Indomethacin was coupled onto some macromolecular nanostructures based on methyl methacrylate copolymers with glycidyl methacrylate and tested as a model drug. The polymeric matrices were synthesized by radical emulsion copolymerization with and without the presence of a continuous external magnetic field of 1500 Gs intensity. Mathematical analysis of the release data was performed using Higuchi, Peppas-Korsmeyer equations. NIR chemical imaging (NIR-CI) was used to provide information about the spatial distribution of the components in the studied nanostructures. This opportunity was used to visualize the spatial distribution of bioactive substances (indomethacin) into the polymeric matrix, as well as to evaluate the degree of chemical and/or physical heterogeneity of the bioactive samples. The release rate dependence on the synthesis conditions as well as on the chemical compositions of the tested polymeric systems, it was also evidenced.

Pharmaceutical applications of vibrational chemical imaging and chemometrics: a review

The emergence of chemical imaging (CI) has gifted spectroscopy an additional dimension. Chemical imaging systems complement chemical identification by acquiring spatially located spectra that enable visualization of chemical compound distributions. Such techniques are highly relevant to pharmaceutics in that the distribution of excipients and active pharmaceutical ingredient informs not only a product's behavior during manufacture but also its physical attributes (dissolution properties, stability, etc.). The rapid image acquisition made possible by the emergence of focal plane array detectors, combined with publication of the Food and Drug Administration guidelines for process analytical technology in 2001, has heightened interest in the pharmaceutical applications of CI, notably as a tool for enhancing drug quality and understanding process. Papers on the pharmaceutical applications of CI have been appearing in steadily increasing numbers since 2000. The aim of the present paper is to give an overview of infrared, near-infrared and Raman imaging in pharmaceutics. Sections 2 and 3 deal with the theory, device set-ups, mode of acquisition and processing techniques used to extract information of interest. Section 4 addresses the pharmaceutical applications.

Time-resolved step-scan infrared imaging system utilizing a linear array detector

A time-resolved infrared (IR) imaging system combined with a multichannel IR microscope, which utilizes a 16 channel linear array (LA) detector, and step-scan Fourier transform infrared (FT-IR) microscope was developed. The LA detector integrates a readout circuit on each detector element, so the detected signals can be read simultaneously. Thus, this system can perform high speed imaging using the step-scan method, similar to a single channel detector. To verify the capabilities of this system, a reflective sample was examined whose position was altered using a piezo actuator activated by an alternating voltage. In addition, the localization of relaxation dynamics for the liquid crystal (LC) molecules in an LC cell under oscillating electric field conditions was detected by this system.

Simultaneous FTIR spectroscopic imaging and visible photography to monitor tablet dissolution and drug release

PURPOSE

Previous studies of hydroxypropyl methylcellulose (HPMC)-based tablet during exposure to water showed a number of 'fronts' moving into the tablet but led to contradictory interpretations. These fronts are related to water penetration into and dissolution of the tablet, but the exact nature can not be derived from visible photographic evidence. A method to study tablet dissolution simultaneously by Fourier transform infrared-attenuated total reflection (FTIR-ATR) imaging and macro-photography can assist in providing correct interpretation of the observed fronts.

METHODS

Therefore, the combination of macro-photography and FTIR-ATR spectroscopic imaging was developed and used to interpret the physical changes leading to the observed fronts. Buflomedyl pyridoxal phosphate (BPP), a coloured drug, was used as a model drug.

RESULTS

The quantitative results obtained by FTIR-ATR imaging enabled the attribution of the three observed fronts (inside to outside) to: (1) true water penetration, possibly combined with (partial) dissolution of buflomedyl pyridoxal phosphate (BPP); (2) total gellification of HPMC; (3) erosion front.

CONCLUSIONS

The method to study dissolution of a tablet simultaneously by FTIR-ATR imaging and macro-photography has been developed and used to obtain reliable interpretation of the fronts observed during tablet dissolution.

Predicting the drug concentration in starch acetate matrix tablets from ATR-FTIR spectra using multi-way methods

The amounts of drug and excipient were predicted from ATR-FTIR spectra using two multi-way modelling techniques, parallel factor analysis (PARAFAC) and multi-linear partial least squares (N-PLS). Data matrices consisted of dissolved and undissolved parallel samples having different drug content and spectra, which were collected at axially cut surface of the flat-faced matrix tablets. Spectra were recorded comprehensively at different points on the axially cut surface of the tablet. The sample drug concentrations varied between 2 and 16% v/v. The multi-way methods together with ATR-FTIR spectra seemed to represent an applicable method for the determination of drug and excipient distribution in a tablet during the release process. The N-PLS calibration method was more robust for accurate quantification of the amount of components in the sample whereas the PARAFAC model provided approximate relative amounts of components.

Paclitaxel distribution in poly(ethylene glycol)/poly(lactide-co-glycolic acid) blends and its release visualized by coherent anti-Stokes Raman scattering microscopy

Mechanisms underlying the release of paclitaxel (PTX) from poly(ethylene glycol)/poly(lactic-co-glycolic acid) (PEG/PLGA) blends were investigated by coherent anti-Stokes Raman scattering (CARS) microscopy. PLGA, PEG, and PTX were selectively imaged by using the resonant CARS signal from the CH3, CH2, and aromatic CH stretch vibrations, respectively. Phase segregation was observed in PLGA films containing 10 to 40 wt.% PEG in the absence of PTX loading. The PEG phase existed in the form of crystalline fibers in the (8:2, weight ratio) and (7:3) PLGA/PEG films. CARS observation indicated that PTX preferentially partitioned into the PEG domains in the (9:1) and (8:2) PLGA/PTX films, but exhibited a uniform mixing with both PLGA and PEG in the (7:3) PLGA/PEG film. The solid dispersion of PTX into PEG domains was attributed to a strong interaction between PTX and PEG, supported by the disappearance of PEG crystallization in the PTX-loaded PLGA/PEG film evidenced through X-ray diffraction analysis. PTX release was induced by exposing the film to an aqueous solution and monitored in real time by CARS and two-photon fluorescence microscopy. Fast dissolution of both PEG and PTX was observed at the film surface. Upon infiltration of water into the film, the PEG domains were rearranged into ring structures enriched by both PTX and PEG. The CARS data provided visual evidence explaining the accelerated burst release followed by more sustained release of PTX from the PLGA/PEG films as measured by HPLC.

Effect of Moisture and Pressure on Tablet Compaction Studied With FTIR Spectroscopic Imaging

FTIR spectroscopic imaging using a diamond ATR accessory has been applied to examine the influence of moisture and compression pressure on the density and components distribution of compacted pharmaceutical tablets. The model drug and excipient used within this study are ibuprofen and hydroxypropylmethylcellulose (HPMC). Chemical images of these compacted tablets were captured in situ without removing the tablet between measurements. A powder mixture of both, drug and excipient, prior to compaction, were subjected to a controlled environment, using a controlled humidity cell. Histograms were plotted to assess the density distribution quantitatively. This FTIR spectroscopic imaging approach enabled both measurement of water sorption and enhanced visualization of the density distribution of the compacted tablets.

Applications of ATR-FTIR spectroscopic imaging to biomedical samples

FTIR spectroscopic imaging in ATR (Attenuated Total Reflection) mode is a powerful tool for studying biomedical samples. This paper summarises recent advances in the applications of ATR-FTIR imaging to dissolution of pharmaceutical formulations and drug release. The use of two different ATR accessories to obtain chemical images of formulations in contact with water as a function of time is demonstrated. The innovative use of the diamond ATR accessory allowed in situ imaging of tablet compaction and dissolution. ATR-FTIR imaging was also applied to obtain images of the surface of skin and the spatial distribution of protein and lipid rich domains was obtained. Chemical images of cross-section of rabbit aorta were obtained using a diamond ATR accessory and the possibility of in situ imaging of arterial samples in contact with aqueous solution was demonstrated for the first time. This experiment opens an opportunity to image arterial samples in contact with solutions containing drug molecules. This approach may help in understanding the mechanisms of treatment of atherosclerosis.

Bis(trimethoxysilylpropyl)amine and tetraethoxysilane derived gels as effective controlled release carriers for water-soluble drugs of small molecules

The controlled release of sodium salicylate (SS) from the gels derived from bis(trimethoxysilylpropyl)amine (TSPA) or the mixture of TSPA with tetraethoxysilane (TEOS) was investigated. The experimental results suggest that the release of SS can be easily controlled by adjusting the ratio between TSPA and TEOS. Increasing the ratio between TSPA and TEOS lowers the surface area and pore volume of the gels, while enhances the interactions between the drug and the gel matrix. Therefore, reduces the release rate of the drug effectively. The overall release process is found to be diffusion controlled, and the release behavior can be well explained by considering the effects of the textual properties of the gels and the interactions between the drug and the gel matrix. TSPA is found to be a very convenient and effective precursor for the preparation of gels for controlled release of both hydrophilic and hydrophobic drugs, especially water-soluble drugs of small molecules.

Electron spin resonance microscopy applied to the study of controlled drug release

We describe our recent developments towards 3D micron-scale imaging capability, based on electron spin resonance (ESR), and its application to the study of controlled release. The method, termed ESR microscopy (ESRM), is an extension of the conventional "millimeter-scale" ESR imaging technique. It employs paramagnetic molecules (such as stable radicals or spin-labeled drugs) and may enable one to obtain accurate 3D spatially resolved information about the drug concentration, its self-diffusion tensor, rotational correlation time and the pH in the release matrix. Theoretical calculations, along with initial experimental results, suggest that a 3D resolution of approximately 1 microm is feasible with this method. Here we were able to image successfully a high spin concentration sample with a resolution of approximately 3 x 3 x 8 microm and subsequently study a single approximately 120 microm biodegradable microsphere, internalized with a dilute solution of trityl radical, with a resolution of approximately 12.7 x 13.2 x 26 microm. Analysis of the microsphere ESR imaging data revealed a likely increase in the viscosity inside the sphere and/or binding of the radical molecule to the sphere matrix. Future directions for progress are also discussed.

Regions of interest in FTIR imaging applications: Diffusion of nicotine into ethylene-co-vinyl acetate films

Effective image analysis of dynamic processes, such as diffusion and dissolution, requires precise reporting of component locations in space and time. An improved method for analyzing FTIR images is described which employs hypothesis testing in the spatial and temporal domains. Changes in the observed absorbance (over space and time) are revealed by comparison to a reference statistic, which can be tailored by choosing the size of a region of interest. This improved analysis method was used to compare the rates of diffusion of nicotine into poly(ethylene-co-vinyl acetate) film from aqueous solutions containing anionic and nonionic surfactants. Compared to a solution without surfactant, sodium dodecyl sulfate inhibited the uptake of nicotine from aqueous solution whereas Tween 40 enhanced the uptake. The nicotine diffusion rate also showed a dependence on the length of the hydrophobic segment of nonionic surfactants. These results demonstrate the roles of solubilization, wetting, and viscosity on diffusion-controlled drug release.

Release of Poorly Soluble Drugs from HPMC Tablets Studied by FTIR Imaging and Flow-Through Dissolution Tests

Spectroscopic imaging and a flow-through dissolution test have been combined to improve the possibilities of investigating the release of a poorly soluble drug (diclofenac) from pharmaceutical tablets. The presented methods aim to overcome the limitations that impede the conventional dissolution test because of its inability to observe precipitates of poorly soluble drug during tablet dissolution. The proposed flow-through set-up allows small drug particles that are being carried along in the water-flow to be analyzed, by adding a dissolution agent to the medium after it left the tablet cell. Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopic imaging provides an insight into the processes inside the tablet and is not hindered by insoluble or recrystallising drug. The techniques have been hyphenated and used to study tablets containing diclofenac sodium and HPMC (hydroxypropyl methylcellulose) in different dissolution media that influence the solubility of the drug. The release profiles obtained by flow-through dissolution test suggest the presence of particles (or precipitates) in the dissolution medium. This is consistent with the results obtained by FTIR imaging, which confirms that both proposed techniques are superior to the ordinary dissolution test when applied to poorly soluble drugs. FTIR imaging data have been analyzed by a classical least squares analysis, corrected for the parts of the tablet outside the field of view, and used to calculate the release profile. The infrared spectra of diclofenac at varying relative humidity were acquired to study the interactions of diclofenac and water, including identification of dissociated diclofenac, thus the chemical specificity of FTIR imaging was fully utilized.

Fourier transform infrared vibrational spectroscopic imaging: integrating microscopy and molecular recognition

The recent development of Fourier transform infrared (FTIR) spectroscopic imaging has enhanced our capability to examine, on a microscopic scale, the spatial distribution of vibrational spectroscopic signatures of materials spanning the physical and biomedical disciplines. Recent activity in this emerging area has concentrated on instrumentation development, theoretical analyses to provide guidelines for imaging practice, novel data processing algorithms, and the introduction of the technique to new fields. To illustrate the impact and promise of this spectroscopic imaging methodology, we present fundamental principles of the technique in the context of FTIR spectroscopy and review new applications in various venues ranging from the physical chemistry of macromolecular systems to the detection of human disease.

Visualization strategies for infrared spectroscopic images

Although infrared imaging is becoming more commonplace and publications demonstrating its utility are on the rise, only a small portion of the literature involves visualization strategies and techniques. In order to fully realize the potential of imaging, visualization of the data in the form of images needs to be examined more thoroughly. Visualization techniques are discussed using the dynamic process of polymer dissolution. The dissolution of poly(ethylene oxide)(PEO) was captured as a series of images showing the changes of the polymer front brought about by the dissolving solvent. Using a unique infrared band for both the polymer and solvent, each species is tracked through the entire experiment. The ability to reconstruct the dissolution from the spatially resolved infrared data proves very valuable. However, the processing of the data is not trivial and steps must be taken to ensure that the images best display the specific attributes of the species. Scaling and color mapping are demonstrated to be two ways to achieve excellent imagery.