Device for Raman Difference Spectroscopy

Comprehensive multi-gas study by means of fiber-enhanced Raman spectroscopy for the investigation of nitrogen cycle processes

The extensive use of synthetic fertilizers has led to a considerable increase in reactive nitrogen input into agricultural and natural systems, resulting in negative effects in multiple ecosystems, the so-called nitrogen cascade. Since the global population relies on fertilization for food production, synthetic fertilizer use needs to be optimized by balancing crop yield and reactive nitrogen losses. Fiber-enhanced Raman spectroscopy (FERS) is introduced as a unique method for the simultaneous quantification of multiple gases to the study processes related to the nitrogen cycle. By monitoring changes in the headspace gas concentrations, processes such as denitrification, nitrification, respiration, and nitrogen fixation, as well as fertilizer addition were studied. The differences in concentration between the ambient and prepared process samples were evident in the Raman spectra, allowing for differentiation of process-specific spectra. Gas mixture concentrations were quantified within a range of low ppm to 100% for the gases N2, O2, CO2, N2O, and NH3. Compositional changes were attributed to processes of the nitrogen cycle. With help of multivariate curve resolution, it was possible to quantify N2O and CO2 simultaneously. The impact of fertilizers on N-cycle processes in soil was simulated and analyzed for identifying active processes. Thus, FERS was proven to be a suitable technique to optimize fertilizer composition and to quantify N2O and NH3 emissions, all with a single device and without further sample preparation.

Spectrally Resolved and Highly Parallelized Raman Difference Spectroscopy for the Analysis of Drug-Target Interactions between the Antimalarial Drug Chloroquine and Hematin

Malaria is a severe disease caused by cytozoic parasites of the genus Plasmodium, which infiltrate and infect red blood cells. Several drugs have been developed to combat the devastating effects of malaria. Antimalarials based on quinolines inhibit the crystallization of hematin into hemozoin within the parasite, ultimately leading to its demise. Despite the frequent use of these agents, there are unanswered questions about their mechanisms of action. In the present study, the quinoline chloroquine and its interaction with the target structure hematin was investigated using an advanced, highly parallelized Raman difference spectroscopy (RDS) setup. Simultaneous recording of the spectra of hematin and chloroquine mixtures with varying compositions enabled the observation of changes in peak heights and positions based on the altered molecular structure resulting from their interaction. A shift of (-1.12 ± 0.05) cm-1 was observed in the core-size marker band ν(CαCm)asym peak position of the 1:1 chloroquine-hematin mixture compared to pure hematin. The oxidation-state marker band ν(pyrrole half-ring)sym exhibited a shift by (+0.93 ± 0.13) cm-1. These results were supported by density functional theory (DFT) calculations, indicating a hydrogen bond between the quinolinyl moiety of chloroquine and the oxygen atom of ferric protoporphyrin IX hydroxide (Fe(III)PPIX-OH). The consequence is a reduced electron density within the porphyrin moiety and an increase in its core size. This hypothesis provided further insights into the mechanism of hemozoin inhibition, suggesting chloroquine binding to the monomeric form of hematin, thereby preventing its further crystallization to hemozoin.

Molecular Interactions Identified by Two-Dimensional Analysis-Detailed Insight into the Molecular Interactions of the Antimalarial Artesunate with the Target Structure β-Hematin by Means of 2D Raman Correlation Spectroscopy

A thorough understanding of the interaction of endoperoxide antimalarial agents with their biological target structures is of utmost importance for the tailored design of future efficient antimalarials. Detailed insights into molecular interactions between artesunate and β-hematin were derived with a combination of resonance Raman spectroscopy, two-dimensional correlation analysis, and density functional theory calculations. Resonance Raman spectroscopy with three distinct laser wavelengths enabled the specific excitation of different chromophore parts of β-hematin. The resonance Raman spectra of the artesunate-β-hematin complexes were thoroughly analyzed with the help of high-resolution and highly sensitive two-dimensional correlation spectroscopy. Spectral changes in the peak properties were found with increasing artesunate concentration. Changes in the low-frequency, morphology-sensitive Raman bands indicated a loss in crystallinity of the drug-target complexes. Differences in the high-wavenumber region were assigned to increased distortions of the planarity of the structure of the target molecule due to the appearance of various coexisting alkylation species. Evidence for the appearance of high-valent ferryl-oxo species could be observed with the help of differences in the peak properties of oxidation-state sensitive Raman modes. To support those findings, the relaxed ground-state structures of ten possible covalent mono- and di-meso(Cm)-alkylated hematin-dihydroartemisinyl complexes were calculated using density functional theory. A very good agreement with the experimental peak properties was achieved, and the out-of-plane displacements along the lowest-frequency normal coordinates were investigated by normal coordinate structural decomposition analysis. The strongest changes in all data were observed in vibrations with a high participation of Cm-parts of β-hematin.

Investigations on the Novel Antimalarial Ferroquine in Biomimetic Solutions Using Deep UV Resonance Raman Spectroscopy and Density Functional Theory

Deep ultraviolet (DUV) resonance Raman experiments are performed, investigating the novel, promising antimalarial ferroquine (FQ). Two buffered aqueous solutions with pH values of 5.13 and 7.00 are used, simulating the acidic and neutral conditions inside a parasite's digestive vacuole and cytosol, respectively. To imitate the different polarities of the membranes and interior, the buffer's 1,4-dioxane content was increased. These experimental conditions should mimic the transport of the drug inside malaria-infected erythrocytes through parasitophorous membranes. Supporting density functional theory (DFT) calculations on the drug's micro-speciation were performed, which could be nicely assigned to shifts in the peak positions of resonantly enhanced high-wavenumber Raman signals at λ_exc = 257 nm. FQ is fully protonated in polar mixtures like the host interior and the parasite's cytoplasm or digestive vacuole (DV) and is only present as a free base in nonpolar ones, such as the host's and parasitophorous membranes. Additionally, the limit of detection (LoD) of FQ at vacuolic pH values was determined using DUV excitation wavelengths at 244 and 257 nm. By applying the resonant laser line at λ_exc = 257 nm, a minimal FQ concentration of 3.1 μM was detected, whereas the pre-resonant excitation wavelength 244 nm provides an LoD of 6.9 μM. These values were all up to one order of magnitude lower than the concentration found for the food vacuole of a parasitized erythrocyte.

Parallelized Raman Difference Spectroscopy for the Investigation of Chemical Interactions

Raman spectroscopy provides an extremely high chemical selectivity. Raman difference spectroscopy is a technique to reveal even the smallest differences that occur due to weak interactions between substances and changes in the molecular structure. To enable parallelized and highly sensitive Raman difference spectroscopy in a microtiter-array, a diffractive optical element, a lens array, and a fiber bundle were integrated into a Raman spectroscopy setup in a unique fashion. The setup was evaluated with a microtiter-array containing pyridine-water complexes, and subwavenumber changes below the spectrometer's resolution could be resolved. The spectral changes were emphasized with two-dimensional correlation analysis. Density functional theory calculation and "atoms in molecule" analysis were performed to simulate the intermolecular long-range interactions between water and pyridine molecules and to get insight into the involved noncovalent interactions, respectively. It was found that by the addition of pyridine, the energy portion of hydrogen bonds to the total complexation energy between pyridine and water reduces. These results demonstrate the unique abilities of the new setup to investigate subtle changes due to biochemically important molecular interactions and opens new avenues to perform drug binding assays and to monitor highly parallelized chemical reactions.

Fiber-Enhanced Raman Gas Spectroscopy for the Study of Microbial Methanogenesis

Microbial methanogenesis is a key biogeochemical process in the carbon cycle that is responsible for 70% of global emissions of the potent greenhouse gas methane (CH₄). Further knowledge about microbial methanogenesis is crucial to mitigate emissions, increase climate model accuracy, or advance methanogenic biogas production. The current understanding of the substrate use of methanogenic microbes is limited, especially regarding the methylotrophic pathway. Here, we present fiber-enhanced Raman spectroscopy (FERS) of headspace gases as an alternate tool to study methanogenesis and substrate use in particular. The optical technique is nondestructive and sensitive to CH₄, hydrogen (H₂), and carbon dioxide with a large dynamic range from trace levels (demonstrated LoDs: CH₄, 3 ppm; H₂, 49 ppm) to pure gases. In addition, the portable FERS system can provide quantitative information about methanol concentration in the liquid phase of microbial cultures through headspace gas sampling (LoD 25 ppm). We demonstrate how FERS gas sensing could enable us to track substrate and product levels of microbial methanogenesis with just one instrument. The versatility of Raman gas spectroscopy could moreover help us to elucidate links between nitrogen and carbon cycle in microbial communities in the near future.

Rapid Raman Spectroscopic Analysis of Stress Induced Degradation of the Pharmaceutical Drug Tetracycline

Stress factors caused by inadequate storage can induce the unwanted degradation of active compounds in pharmaceutical formulations. Resonance Raman spectroscopy is presented as an analytical tool for rapid monitoring of small concentration changes of tetracycline and the metabolite 4˗epianhydrotetracycline. These degradation processes were experimentally induced by changes in temperature, humidity, and irradiation with visible light over a time period of up to 23 days. The excitation wavelength λ_exc = 413 nm was proven to provide short acquisition times for the simultaneous Raman spectroscopic detection of the degradation of tetracycline and production of its impurity in small sample volumes. Small concentration changes could be detected (down to 1.4% for tetracycline and 0.3% for 4-epianhydrotetracycline), which shows the potential of resonance Raman spectroscopy for analyzing the decomposition of pharmaceutical products.

Highly Sensitive Detection of the Antibiotic Ciprofloxacin by Means of Fiber Enhanced Raman Spectroscopy

Sepsis and septic shock exhibit a rapid course and a high fatality rate. Antibiotic treatment is time-critical and precise knowledge of the antibiotic concentration during the patients’ treatment would allow individual dose adaption. Over- and underdosing will increase the antimicrobial efficacy and reduce toxicity. We demonstrated that fiber enhanced Raman spectroscopy (FERS) can be used to detect very low concentrations of ciprofloxacin in clinically relevant doses, down to 1.5 µM. Fiber enhancement was achieved in bandgap shifted photonic crystal fibers. The high linearity between the Raman signals and the drug concentrations allows a robust calibration for drug quantification. The needed sample volume was very low (0.58 µL) and an acquisition time of 30 s allowed the rapid monitoring of ciprofloxacin levels in a less invasive way than conventional techniques. These results demonstrate that FERS has a high potential for clinical in-situ monitoring of ciprofloxacin levels.

Fiber-Enhanced Raman Sensing of Cefuroxime in Human Urine

Fiber-enhanced Raman spectroscopy was developed for the chemically selective and sensitive quantification of the important antibiotic cefuroxime in human urine. A novel optical sensor fiber was drawn and precisely prepared. In this fiber structure, light is strongly confined in the selectively filled liquid core, and the Raman scattered signal is collected with unprecedented efficiency over an extended interaction length. The filling, emptying, and robustness are highly improved due to the large core size (>30 μm). Broadband step-index guidance allows the free choice of the most suitable excitation wavelength in complex body fluids. The limit of detection of cefuroxime in human urine was improved by 2 orders of magnitude (to μM level). The quantification of cefuroxime was achieved in urine after oral administration. This method has great potential for the point-of-care monitoring of antibiotics concentrations and is an important step forward to enable clinicians to rapidly adjust doses.

Fiber enhanced Raman spectroscopic analysis as a novel method for diagnosis and monitoring of diseases related to hyperbilirubinemia and hyperbiliverdinemia

Fiber enhanced resonance Raman spectroscopy (FERS) is introduced for chemically selective and ultrasensitive analysis of the biomolecules hematin, hemoglobin, biliverdin, and bilirubin. The abilities for analyzing whole intact, oxygenated erythrocytes are proven, demonstrating the potential for the diagnosis of red blood cell related diseases, such as different types of anemia and hemolytic disorders. The optical fiber enables an efficient light-guiding within a miniaturized sample volume of only a few micro-liters and provides a tremendously improved analytical sensitivity (LODs of 0.5 μM for bilirubin and 0.13 μM for biliverdin with proposed improvements down to the pico-molar range). FERS is a less invasive method than the standard ones and could be a new analytical method for monitoring neonatal jaundice, allowing a precise control of the unconjugated serum bilirubin levels, and therefore, providing a better prognosis for newborns. The potential for sensing very low concentrations of the bile pigments may also open up new opportunities for cancer research. The abilities of FERS as a diagnostic tool are explored for the elucidation of jaundice with different etiologies including the rare, not yet well understood diseases manifested in green jaundice. This is demonstrated by quantifying clinically relevant concentrations of bilirubin and biliverdin simultaneously in the micro-molar range: for the case of hyperbilirubinemia due to malignancy, infectious hepatitis, cirrhosis or stenosis of the common bile duct (1 μM biliverdin together with 50 μM bilirubin) and for hyperbiliverdinemia (25 μM biliverdin and 75 μM bilirubin). FERS has high potential as an ultrasensitive analytical technique for a wide range of biomolecules and in various life-science applications.

Simultaneous Acquisition of the Polarized and Depolarized Raman Signal with a Single Detector

Polarization-resolved Raman spectroscopy provides much more information than its conventional counterpart. However, it usually either requires a complicated setup with two spectrographs and detectors or two measurements must be performed sequentially. This study presents a simple and straightforward approach to recording both polarization components simultaneously with a single spectrograph and detector. The vertically and a horizontally polarized laser beam exiting a Wollaston prism are focused into the sample with a small spatial separation. The scattered light from both beams is imaged onto the slit of an imaging spectrograph as two spatially separated signals, i.e., the polarized and the depolarized Raman signal. Eventually, both spectra are acquired on a single CCD chip simultaneously. Experimental data of ethanol and dimethyl sulfoxide are shown as proof-of-concept. The new method has a number of advantages, for example, laser intensity fluctuations and the polarization dependence of the diffraction grating do not play a role. The proposed approach will be useful for an improved structural analysis and it will be the enabling technology for temporally resolved enantioselective Raman (esR) spectroscopy.

Vibrational spectroscopic characterization of arylisoquinolines by means of Raman spectroscopy and density functional theory calculations

Seven new AIQ antimalarial agents were investigated using FT-NIR and deep-UV resonance Raman spectroscopy.

Rough surface Au@Ag core-shell nanoparticles to fabricating high sensitivity SERS immunochromatographic sensors

Immunochromatographic sensors (ICSs) are inexpensive, simple, portable, and robust, thus making ICSs commonplace in clinical diagnoses, food testing, and environmental monitoring. However, commonly used gold nanoparticles (AuNPs) ICSs have low sensitivity. Therefore, we developed highly sensitive surface enhanced Raman scattering (SERS) ICSs. To enhance the sensitivity of SERS ICSs, rough surface core-shell Au@Ag nanoparticles (RSAu@AgNPs) were prepared by coating silver on the surface of gold nanoflowers (AuNFs). Then these nanoparticles were used as SERS substrate in the SERS ICSs, after which the SERS ICSs were implemented to detect haemoglobin and heavy metal cadmium ion (Cd(2+)). The limit of detection (LOD) of the SERS ICSs for detecting haemoglobin was 8 ng/mL, and the linear range of the SERS ICSs was from 31.3 to 2000 ng/mL. The LOD of the SERS ICSs for detecting Cd(2+) was 0.05 ng/mL and the linear analysis range was from 0.05 to 25 ng/mL. The cross reactivity of the SERS ICSs was studied and results showed that the SERS ICSs exhibited highly specific for detection of haemoglobin and Cd(2+), respectively. The SERS ICSs were then used to detect haemoglobin (spiked in serum and in stool) and Cd(2+) (spiked in tap water, river water, and soil leaching water), and the results showed high recovery. These characteristics indicated that SERS ICSs were ideal tools for clinical diagnosis and environmental pollution monitoring.

Fiber array based hyperspectral Raman imaging for chemical selective analysis of malaria-infected red blood cells

A new setup for Raman spectroscopic wide-field imaging is presented. It combines the advantages of a fiber array based spectral translator with a tailor-made laser illumination system for high-quality Raman chemical imaging of sensitive biological samples. The Gaussian-like intensity distribution of the illuminating laser beam is shaped by a square-core optical multimode fiber to a top-hat profile with very homogeneous intensity distribution to fulfill the conditions of Koehler. The 30 m long optical fiber and an additional vibrator efficiently destroy the polarization and coherence of the illuminating light. This homogeneous, incoherent illumination is an essential prerequisite for stable quantitative imaging of complex biological samples. The fiber array translates the two-dimensional lateral information of the Raman stray light into separated spectral channels with very high contrast. The Raman image can be correlated with a corresponding white light microscopic image of the sample. The new setup enables simultaneous quantification of all Raman spectra across the whole spatial area with very good spectral resolution and thus outperforms other Raman imaging approaches based on scanning and tunable filters. The unique capabilities of the setup for fast, gentle, sensitive, and selective chemical imaging of biological samples were applied for automated hemozoin analysis. A special algorithm was developed to generate Raman images based on the hemozoin distribution in red blood cells without any influence from other Raman scattering. The new imaging setup in combination with the robust algorithm provides a novel, elegant way for chemical selective analysis of the malaria pigment hemozoin in early ring stages of Plasmodium falciparum infected erythrocytes.

Enhanced Raman multigas sensing - a novel tool for control and analysis of (13)CO(2) labeling experiments in environmental research

Cavity-enhanced Raman multigas spectrometry is introduced as a versatile technique for monitoring of (13)CO2 isotope labeling experiments, while simultaneously quantifying the fluxes of O2 and other relevant gases across a wide range of concentrations. The multigas analysis was performed in a closed cycle; no gas was consumed, and the gas composition was not altered by the measurement. Isotope labeling of plant metabolites via photosynthetic uptake of (13)CO2 enables the investigation of resource flows in plants and is now an important tool in ecophysiological studies. In this experiment the (13)C labeling of monoclonal cuttings of Populus trichocarpa was undertaken. The high time resolution of the online multigas analysis allowed precise control of the pulse labeling and was exploited to calculate the kinetics of photosynthetic (13)CO2 uptake and to extrapolate the exact value of the (13)CO2 peak concentration in the labeling chamber. Further, the leaf dark respiration of immature and mature leaves was analyzed. The quantification of the photosynthetic O2 production rate as a byproduct of the (13)CO2 uptake correlated with the amount of available light and the leaf area of the plants in the labeling chamber. The ability to acquire CO2 and O2 respiration rates simultaneously also simplifies the determination of respiratory quotients (rate of O2 uptake compared to CO2 release) and thus indicates the type of combusted substrate. By combining quantification of respiration quotients with the tracing of (13)C in plants, cavity enhanced Raman spectroscopy adds a valuable new tool for studies of metabolism at the organismal to ecosystem scale.

Microbial respiration and natural attenuation of benzene contaminated soils investigated by cavity enhanced Raman multi-gas spectroscopy

Soil and groundwater contamination with benzene can cause serious environmental damage. However, many soil microorganisms are capable to adapt and are known to strongly control the fate of organic contamination. Innovative cavity enhanced Raman multi-gas spectroscopy (CERS) was applied to investigate the short-term response of the soil micro-flora to sudden surface contamination with benzene regarding the temporal variations of gas products and their exchange rates with the adjacent atmosphere. (13)C-labeled benzene was spiked on a silty-loamy soil column in order to track and separate the changes in heterotrophic soil respiration - involving (12)CO2 and O2- from the natural attenuation process of benzene degradation to ultimately form (13)CO2. The respiratory quotient (RQ) decreased from a value 0.98 to 0.46 directly after the spiking and increased again within 33 hours to a value of 0.72. This coincided with the maximum (13)CO2 concentration rate (0.63 μmol m(-2) s(-1)), indicating the highest benzene degradation at 33 hours after the spiking event. The diffusion of benzene in the headspace and the biodegradation into (13)CO2 were simultaneously monitored and 12 days after the benzene spiking no measurable degradation was detected anymore. The RQ finally returned to a value of 0.96 demonstrating the reestablished aerobic respiration.

Rapid monitoring of intermediate states and mass balance of nitrogen during denitrification by means of cavity enhanced Raman multi-gas sensing

The comprehensive investigation of changes in N cycling has been challenging so far due to difficulties with measuring gases such as N2 and N2O simultaneously. In this study we introduce cavity enhanced Raman gas spectroscopy as a new analytical methodology for tracing the stepwise reduction of (15)N-labelled nitrate by the denitrifying bacteria Pseudomonas stutzeri. The unique capabilities of Raman multi-gas analysis enabled real-time, continuous, and non-consumptive quantification of the relevant gases ((14)N2, (14)N2O, O2, and CO2) and to trace the fate of (15)N-labeled nitrate substrate ((15)N2, (15)N2O) added to a P. stutzeri culture with one single measurement. Using this new methodology, we could quantify the kinetics of the formation and degradation for all gaseous compounds (educts and products) and thus study the reaction orders. The gas quantification was complemented with the analysis of nitrate and nitrite concentrations for the online monitoring of the total nitrogen element budget. The simultaneous quantification of all gases also enabled the contactless and sterile online acquisition of the pH changes in the P. stutzeri culture by the stoichiometry of the redox reactions during denitrification and the CO2-bicarbonate equilibrium. Continuous pH monitoring - without the need to insert an electrode into solution - elucidated e.g. an increase in the slope of the pH value coinciding with an accumulation of nitrite, which in turn led to a temporary accumulation of N2O, due to an inhibition of nitrous oxide reductase. Cavity enhanced Raman gas spectroscopy has a high potential for the assessment of denitrification processes and can contribute substantially to our understanding of nitrogen cycling in both natural and agricultural systems.

Fiber-enhanced Raman multigas spectroscopy: a versatile tool for environmental gas sensing and breath analysis

Versatile multigas analysis bears high potential for environmental sensing of climate relevant gases and noninvasive early stage diagnosis of disease states in human breath. In this contribution, a fiber-enhanced Raman spectroscopic (FERS) analysis of a suite of climate relevant atmospheric gases is presented, which allowed for reliable quantification of CH4, CO2, and N2O alongside N2 and O2 with just one single measurement. A highly improved analytical sensitivity was achieved, down to a sub-parts per million limit of detection with a high dynamic range of 6 orders of magnitude and within a second measurement time. The high potential of FERS for the detection of disease markers was demonstrated with the analysis of 27 nL of exhaled human breath. The natural isotopes (13)CO2 and (14)N(15)N were quantified at low levels, simultaneously with the major breath components N2, O2, and (12)CO2. The natural abundances of (13)CO2 and (14)N(15)N were experimentally quantified in very good agreement to theoretical values. A fiber adapter assembly and gas filling setup was designed for rapid and automated analysis of multigas compositions and their fluctuations within seconds and without the need for optical readjustment of the sensor arrangement. On the basis of the abilities of such miniaturized FERS system, we expect high potential for the diagnosis of clinically administered (13)C-labeled CO2 in human breath and also foresee high impact for disease detection via biologically vital nitrogen compounds.

Structural differences between native Hen egg white lysozyme and its fibrils under different environmental conditions

The difference in molecular structure of native HEWL and its fibrils, grown at a pH value near physiological pH 7.4 and at a pH value just above the pI, 10.7 in presence and absence of Cu(II) ions, is discussed. We focus on differences between the molecular structure of the native protein and fibrils using principal component analysis of their Raman spectra. The overlap areas of the scores of each species are used to quantify the difference in the structure of the native HEWL and fibrils in different environments. The overall molecular structures are significantly different for fibrils grown at two pH values. However, in presence of Cu(II) ions, the fibrils have similarities in their molecular structures at these pH environments. Spectral variation within each species, as obtained from the standard deviations of the scores in PCA plots, reveals the variability in the structure within a particular species.

Effect of deuteration on hydrogen bonding: a comparative concentration dependent Raman and DFT study of pyridine in CH₃OH and CD₃OD and pyrimidine in H₂O and D₂O

The relative effect of hydrogen bonding of pyrimidine (Pyr) in H(2)O/D(2)O and pyridine (Py) in CH(3)OH/CD(3)OD has been analyzed using Raman Difference Spectroscopic (RDS) technique and DFT calculations. This study is focused on analyzing the concentration dependent variation of linewidth, peak position and intensity of ring breathing mode of Py and Pyr. The ring breathing mode of Pyr in H(2)O and D(2)O has three components; due to free Pyr, lighter complexes of mPyr+nH(2)O/D(2)O and heavier complexes of mPyr+nH(2)O/D(2)O. The pyridine molecules, however, show only two components in CH(3)OH and CD(3)OD. Of these two components, one corresponds to free Py and the other inhomogeneously broadened profile corresponds to all mPy+nCH(3)OH/CD(3)OD complexes. The variation of peak position and linewidth establishes the role of dipole moment of complexes and the diffusion in the mixture. In case of CD(3)OD solution splitting was observed in ∼1030 cm(-1) band of Py, where an additional band at ∼1034 cm(-1) appears at x(Py) ≤ 0.4. However, this band remains single at all concentrations in case of CH(3)OH solvent.

Attenuated total reflection infrared difference spectroscopy (ATR-IRDS) for quantitative reaction monitoring

Monitoring of chemical reactors is key to optimizing yield and efficiency of chemical transformation processes. Aside from tracking pressure and temperature, the measurement of the chemical composition is essential in this context. We present an infrared difference spectroscopy approach for determining the reactant (cyclooctene) and product (cyclooctane) concentrations during a catalytic hydrogenation reaction in the solvent cyclohexane, which is present in large excess. Subtracting the spectrum of the pure solvent from the reactor mixture spectra yields infrared (IR) spectra, which can ultimately be evaluated using a curve-fitting procedure based on spectral soft modeling. An important feature of our evaluation approach is that the calibration only requires recording the pure component spectra of the reactants, products, and solvent. Hence, no time-consuming preparation of mixtures for calibration is necessary. The IR concentration results are in good agreement with gas chromatography measurements.

Structural analysis of the antimalarial drug halofantrine by means of Raman spectroscopy and density functional theory calculations

The structure of the antimalarial drug halofantrine is analyzed by means of density functional theory (DFT) calculations, IR, and Raman spectroscopy. Strong, selective enhancements of the Raman bands of halofantrine at 1621 and 1590 cm(-1) are discovered by means of UV resonance Raman spectroscopy with excitation wavelength lambda(exc)=244 nm. These signal enhancements can be exploited for a localization of small concentrations of halofantrine in a biological environment. The Raman spectrum of halofantrine is calculated by means of DFT calculations [B3LYP/6-311+G(d,p)]. The calculation is very useful for a thorough mode assignment of the Raman bands of halofantrine. The strong bands at 1621 and 1590 cm(-1) in the UV Raman spectrum are assigned to combined C[Double Bond]C stretching vibrations in the phenanthrene ring of halofantrine. These bands are considered as putative marker bands for pipi interactions with the biological target molecules. The calculation of the electron density demonstrates a strong distribution across the phenanthrene ring of halofantrine, besides the electron withdrawing effect of the Cl and CF(3) substituents. This strong and even electron density distribution supports the hypothesis of pipi stacking as a possible mode of action of halofantrine. Complementary IR spectroscopy is performed for an investigation of vibrations of polar functional groups of the halofantrine molecule.

Subwavenumber charge-coupled device spectrometer calibration using molecular iodine laser-induced fluorescence

Spectrometers configured with charge-coupled devices (CCD) or other array-based detectors require calibration to convert from the pixel coordinate to a spectral coordinate. A CCD calibration method well suited for Raman spectroscopy has been developed based on the 514.5 nm Ar(+) laser-induced fluorescence (LIF) spectrum of room-temperature molecular iodine vapor. Over 360 primary and secondary I(2) LIF calibration lines spanning 510-645 nm were identified as calibrant peaks using an instrumental resolution of 1 cm(-1). Two instrument calibration functions were evaluated with these peaks: a second-order polynomial and a function derived from simple optomechanical considerations. The latter function provided better fitting characteristics. Calibration using I(2) LIF was tested with measurements of both laser light scattering and Raman spectra. The I(2) LIF reference spectra and the signal spectra were recorded simultaneously, with no cross talk, by separating the two signals spatially along the vertical axis of the CCD imager. In this way, every CCD image could be independently calibrated. An accuracy and a precision of +/-0.05 cm(-1) were achieved with this calibration technique.