Simultaneous Application of Raman and Laser-Induced Breakdown Spectroscopy in the Gas Phase with a Single Laser and Detector.

Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) are powerful tools for molecular and elemental analysis, respectively. Their combined application, however, is challenging due to the differences in the signal generation and detection characteristics. This note proposes three experimental schemes for the simultaneous application of Raman and LIBS for gas-phase diagnostics. Ring-cavity optical pulse stretchers facilitate shaping suitable pulse pairs from a Q-switched laser that enables the quasi-simultaneous detection of the Raman and LIBS signals on a single detector.

Laser applications to chemical, security, and environmental analysis: introduction to the feature issue.

This Applied Optics feature issue on laser applications to chemical, security, and environmental analysis (LACSEA) highlights papers presented at the LACSEA 2020 Seventeenth Topical Meeting sponsored by The Optical Society (OSA).

The gas-phase formation of tin dioxide nanoparticles in single droplet combustion and flame spray pyrolysis.

Tin dioxide (SnO2) nanoparticles synthesized via flame spray pyrolysis (FSP) have promising applications for gas sensors. The formation of SnO2 nanoparticles in the gas-phase has been investigated using single droplet combustion and FSP. Precursor solutions of Tin (II) 2-ethylhexanoate dissolved in Xylene with varying Sn concentrations were selected as the precursor-solvent system. The selected precursor-solvent system has its stability and ability to synthesize homogeneous nanoparticles, compared to metal nitrate based precursor solutions. The precursor-solvent system was studied using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and thermogravimetric analysis (TGA). The SnO2 nanoparticles were characterized using X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), and transmission electron microscopy (TEM). Droplet surface micro-explosions were observed during the single droplet combustion of the precursor solutions. It is because of the heterogeneous vapor-phase nucleation, which is beneath the liquid droplet surface and caused by precursor thermal decomposition. The results show that the size of nanoparticles obtained both from FSP and single droplet combustion increases with increasing metal-precursor concentration. The TEM images of the particles from such droplet combustion reveal two types of nanoparticles with different sizes and morphologies. The current work provides fundamental understanding of precursor decomposition and particle formation during single droplet combustion, which help in-depth understanding of the flame spray pyrolysis.

The effect of introducing an ether group into an imidazolium-based ionic liquid in binary mixtures with DMSO.

Ether-functionalized ionic liquids (ILs) have successfully been employed in diverse applications, but their interactions with other solvents are not understood well. In this work, mixtures of 1-methoxyethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EOMIMFSI) and dimethylsulfoxide (DMSO) are studied in terms of their solution structure and hydrogen bonding interactions. The corresponding alkyl-substituted IL 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIMFSI) is analyzed for comparison. A combination of FTIR spectroscopy, excess spectroscopy, and quantum chemical calculations is employed for this purpose. The datasets allow drawing a number of conclusions as follows: (1) the ether group forms intramolecular hydrogen bonds that compete with anions and DMSO; hence, introducing ether groups into the imidazolium-based IL leads to the weakening of hydrogen bonds in the mixtures. (2) With the help of excess spectra and quantum chemical calculations, some complexes such as ion clusters, ion pairs, and individual ions were identified and assigned in the two systems. The solution structures at different concentrations were examined by analyzing the excess spectra of ν(C2-H) and ν(C-D) in the two IL-DMSO-d6 systems. (3) The introduced ether groups result in changes of the main interaction sites, which were found to be concentration-dependent. In the EOMIMFSI-DMSO system, when isolated ions are the main existing form, the C2-Hs are still the main sites interacting with DMSO. However, when ion pairs or larger ion clusters are the main existing species, the C4-Hs are the main sites interacting with DMSO.

Revisiting the Liquid-Liquid Phase Behavior of n-Alkanes and Ethanol.

Mixtures of alkanes and ethanol are important in many areas, for example, as fuel blends. This paper describes new experimental data obtained for the liquid-liquid equilibrium phase behavior of normal alkanes (n-alkanes; CnH2n+2; 9 ≤ n ≤ 24) with ethanol. The results were obtained by applying the cloud point method in a temperature range of T = 230-423 K at ambient pressure. All systems are partially miscible with an upper critical solution point. The two phase regions of the phase diagrams show no indication of any obvious optical irregularities, like birefringence, coloring, formation of schlieren, or remarkable turbidity, except critical opalescence. With increasing length of the molecular chain of the n-alkanes, the (liquid-liquid) critical point is shifted to higher temperatures and higher ethanol content. The data are analyzed numerically implying Ising criticality. The nonsymmetric shape of the phase body is considered in different approaches for describing the diameter by presuming (a) the validity of the rectilinear diameter rule, (b) a nonlinear diameter predicted in the theory of complete scaling, and (c) combining both concepts. The numerical analysis yields the critical temperature, the critical composition, the width, and the diameter of the phase diagrams. The results are compared with literature data sets from similar mixtures; these data are also evaluated in terms of the models applied here. Phase diagrams of 13 different sets of mixtures are measured and analyzed to extract general aspects of the behavior of the normal alkane-ethanol mixtures. A simple Flory-Huggins-like approach allows a semiquantitative description of the experimental results of the critical temperatures. Therefore, it confirms the picture of molecular ordering within the solutions.

Grape Seeds: Chromatographic Profile of Fatty Acids and Phenolic Compounds and Qualitative Analysis by FTIR-ATR Spectroscopy.

The primary product of the oenological sector is wine. Nonetheless, the grape processing produces large amounts of by-products and wastes, e.g., the grape seeds. In the context of a sustainable production, there is a strong push towards reutilizing these by-products and waste for making useful derivatives since they are rich of bioactive substances with high additional value. As it is true for the wine itself, bringing these by-products derivatives to the market calls for quality measures and analytical tools to assess quality itself. One of the main objectives is to collect analytical data regarding bioactive compounds using potentially green techniques. In the present work, the profile of fatty acids and the main phenolic compounds were investigated by conventional methods. The qualitative analysis of the main functional groups was carried out by Fourier Transform Infrared (FTIR) spectroscopy. Moreover, the successful use of FTIR technique in combination with chemometric data analysis is shown to be a suitable analytical tool for discriminating the grape seeds. Grape seeds of different origin have different content of bioactive substances, making this technique useful when planning to recover a certain substance with specific potential application in health area as food supplement or nutraceutical. For example, Cesanese d'Affile seeds were found to have a rather high fat content with a significant fraction of unsaturated fatty acids. On the other hand, the seeds of Nero d'Avola exhibit the highest amount of phenolic compounds.

Chemistry of iron nitrate-based precursor solutions for spray-flame synthesis.

Understanding the chemistry of iron-based metal-organic precursor solutions for spray-flame synthesis is a key step to developing inexpensive and large scale applications for gas-phase synthesized, nano-sized iron oxide particles. Owing to the large variety of available organic solvents and iron compounds, the choice of a suitable precursor-solvent pair is challenging. Systematic investigations of the precursor chemistry of iron-based systems are currently not available. This work aims at filling this gap by providing a detailed spectroscopic analysis of mixtures containing iron(iii) nitrate nonahydrate and alkyl alcohols (C2-4). Moreover, the impact of adding 2-ethylhexanoic acid is explored. The FTIR spectra reveal the formation of carboxylates and allow deriving information about the coordination of the metal-carboxylate complexes. The stability of the precursor solutions is investigated by monitoring precipitation phenomena and turbidity. Furthermore, gas chromatography is employed to provide additional information on oxidation products and esters as well as to aid the interpretation of the FTIR data. It is found that the formation of esters has an enhancing effect on iron sorption and, thus, it promotes precursor stability.

Identification of Passion Fruit Oil Adulteration by Chemometric Analysis of FTIR Spectra.

Passion fruit oil is a high-value product with applications in the food and cosmetic sectors. It is frequently diluted with sunflower oil. Sunflower oil is also a potential adulterant as its addition does not notably alter the appearance of the passion fruit oil. In this paper, we show that this is also true for the FTIR spectrum. However, the chemometric analysis of the data changes this situation. Principal component analysis (PCA) enables not only the straightforward discrimination of pure passion fruit oil and adulterated samples but also the unambiguous classification of passion fruit oil products from five different manufacturers. Even small amounts-significantly below 1%-of the adulterant can be detected. Furthermore, partial least-squares regression (PLSR) facilitates the quantification of the amount of sunflower oil added to the passion fruit oil. The results demonstrate that the combination of FTIR spectroscopy and chemometric data analysis is a very powerful tool to analyze passion fruit oil.

Spectroscopic and computational insights into the ion-solvent interactions in hydrated aprotic and protic ionic liquids.

Ionic liquids (ILs) and their aqueous solutions are emerging media for solving and manipulating biochemical molecules such as proteins. Unleashing the full potential however requires a detailed mechanistic understanding of how suitable protic and aprotic ILs behave in the presence of water in the first place. The present work aims at making an important step by performing a combined experimental and computational study of two selected ILs and their mixtures with water: the aprotic cholinium propionate ([Chl][Pro]) and the protic N-methyl-2-pyrrolidonium propionate ([NMP][Pro]). IR and Raman spectroscopy reveal stronger ion-solvent interactions in [Chl][Pro]-H2O systems compared to [NMP][Pro]-H2O mixtures. This can be explained by the tightly packed ion-pair associations in [NMP][Pro] comprising the protic -N+-H counterpart, which allows the establishment of highly directional and strong interionic hydrogen bonds. The spectral decomposition of the O-D stretching band into three sub-peaks showed that the protic [NMP][Pro] favors the self-association of water molecules. On the other hand, the predominant fraction of water-anion/cation aggregates exists in aprotic [Chl][Pro]. These hydrated systems can be envisaged using quantum-chemical calculations in the following way: H2O[Chl]+H2O[Pro]-H2O and H2O[NMP]+[Pro]-H2O, which implied preferable solvent-shared ion-pair (SIP) configurations for [Chl][Pro]-H2O systems, whereas the contact ion-pair (CIP) state prevails for the [NMP][Pro]-H2O systems. The latter holds even in the water-rich regime. In future work, these findings will be the basis for an understanding of the underlying principles that govern the interactions of ions with bio-molecules in aqueous solutions.

Chemometric analysis of enantioselective Raman spectroscopy data enables enantiomeric ratio determination.

In-line determination of the enantiomeric ratio is still a challenge in process analytical technology (PAT). This study combines enantioselective Raman (esR) spectroscopy with partial least-squares regression (PLSR) to determine the enantiomeric fraction of the chiral molecule (5,6)-diphenyl-morpholin-2-one diluted in dimethyl sulfoxide (DMSO) as a proof-of-concept. Morpholinone derivates are potential candidates for pharmaceutical applications. The PLS weights were carefully analyzed in order to avoid misleading regression results, e.g. caused by sample impurities. A suitable PLSR model was found with two components and it was validated by a leave-one-out cross-validation. The enantiomeric fraction ef(+) could be calculated with deviations from the prepared ef(+) in the range of -0.031 and +0.052 from the esR spectra recorded at a half-wave retarder angle of 30.0°.

Vapor Liquid Equilibria of 1-Ethyl-3-methylimidazolium Triflate (C2mimTfO) and n-Alkyl Alcohol Mixtures.

The isobaric vapor liquid equilibria (VLE) of different binary mixtures of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (C2mimTfO) with the n-alkyl alcohols, methanol, ethanol, propan-1-ol, and butan-1-ol, are studied at the pressures of p = 500, 700, and 1000 mbar, covering a composition range 0.25-0.35 ≤ x(solvent) ≤ 1.0. Complementarily, the experimental results are compared with calculations by the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EoS). For deriving suitable PC-SAFT parameters, experimental liquid densities were determined for the neat IL C2mimTfO and its longer homologues, 1-butyl-3-methylimidazolium trifluoromethanesulfonate (C4mimTfO) and 1-hexyl-3-methylimidazolium trifluoromethanesulfonate (C6mimTfO), in a temperature range of 288.15 K ≤ T ≤ 363.15 K (C2mimTfO) and 293.15 K ≤ T ≤ 363.15 K (C4mimTfO and C6mimTfO), respectively. The PC-SAFT EoS is found to be suitable for describing the VLEs under study with good accuracy (AARDVLE ≤ 0.4%).

Comparison of existing laser-induced breakdown thermometry techniques along with a time-resolved breakdown approach.

Temperature is an important parameter for characterizing chemical, physical, and flow processes occurring in combustion environments. Laser-induced breakdown is a process widely used to determine a material's elemental components and its composition, known as laser-induced breakdown spectroscopy (LIBS). The breakdown event, or more specifically the breakdown threshold, for a low-pressure gas strongly depends on density effects emanating in the likelihood for multiphoton and avalanche ionization. In this work, a comparison of thermometry techniques using laser-induced breakdown is made and an approach to perform simultaneous gas-phase thermometry on a shot-to-shot basis and spectroscopy is demonstrated by monitoring the moment in time the thermal plasma develops along the intensity gradient of a laser pulse. Breakdown thresholds are profiled along the height of a lean methane-air and partially combusting rich propane-air McKenna flame, and correlated to radiation and convection-corrected thermocouple readings.

Laser applications to chemical, security, and environmental analysis: introduction to the feature issue.

This Applied Optics feature issue on laser applications to chemical, security, and environmental analysis (LACSEA) highlights papers presented at the LACSEA 2018 Sixteenth Topical Meeting sponsored by the Optical Society of America.

Interplay of Different Moieties in the Binary System 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate/Water Studied by Raman Spectroscopy and Density Functional Theory Calculations.

The present work reports new insights into specific interactions in aqueous solutions of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (C2mimTfO). A systematic investigation based on a combination of Raman spectroscopy and density functional theory (DFT) calculations shows evidence of self-encapsulation of the ionic moiety. Raman spectroscopy reveals preferred interactions between water molecules and the TfO- anions. The comparison of the experimental results with dispersion-corrected DFT calculations, which yield the predictions of the possible conformers of the cation-water, anion-water, and cation-anion-water structures, strongly supports the hypotheses of site-selective IL/water interactions. The obtained results allow for a detailed discussion of the nature and strength of the molecular interactions. It is shown that the TfO- anion establishes a preferred interaction with water, whereas the vibrational band at 3118 cm-1 for C-H motion at the C(2) position, the most acidic site for cation and anion interaction, does not indicate any specific energy shift, when adding water to the IL. This finding gives evidence for a self-protective microstructure of the molecules of C2mimTfO in an aqueous environment. In contrast to other ILs reported in the literature, there is no evidence of an increasing cation-anion distance in the IL ion-pair when increasing the water content. Instead, the C2mimTfO molecules undergo a perfect rearrangement, allowing interactions at other molecular sites with higher selectivity. A direct exposure to water at the cation-anion interacting site (C(2) position) is avoided. Ultimately, we show that clusters of ion-pair dimers solvated with water exhibit a more stable geometry compared to the hydrated single ion-pairs, and our calculations correctly reproduce the experimental findings.

Molecular-Level Insights into the Microstructure of a Hydrated and Nanoconfined Deep Eutectic Solvent.

Despite the recent advancements in the field of deep eutectic solvents (DESs), their high viscosity often prevents practical applications. A versatile strategy to overcome this problem is either to add a co-solvent or to confine the DES inside a nanoscaled self-organized system. This work assesses the microstructures of a hydrated and nanoconfined DES comprising benzyltripropylammonium chloride [BTPA]Cl and ethylene glycol (EG). They act as a hydrogen-bond acceptor and a donor, respectively. The hydrogen bonding between [BTPA]Cl and EG in the DES (i.e., BTEG) and the molecular states of water in the hydrated BTEG were studied by Raman spectroscopy. The results show different hydrogen-bonding associations between water-water and water-BTEG or EG molecules. In addition, we investigated the confinement effects of BTEG in a Polysorbate 80 (Tween-80)/cyclohexane reverse micellar (RM) system. The results are compared with those of an ionic liquid-encapsulated RM system. The formation, bonding characteristics, and thermal stability of the RM droplets were studied by solubilization, dynamic light scattering, rheology, and Raman spectroscopy experiments. Furthermore, it is shown that hydrogen bonding between the DES and the surfactant leads to a stable RM system. Interestingly, the viscosity of the RM system is significantly lower than that of the neat DES suggesting that DESs have a much wider practical applicability in the form of RMs.

Cluster Formation through Hydrogen Bond Bridges across Chloride Anions in a Hydroxyl-Functionalized Ionic Liquid.

Several recent studies of hydroxyl-functionalized ionic liquids (ILs) have shown that cation-cation interactions can be dominating these materials at the molecular level when the anion involved is weakly interacting. The hydrogen bonds between the like ions led to the formation of interesting chain-like, ring-like, or distinct dimeric (i. e. two ion pairs) supermolecular clusters. In the present work, vibrational spectroscopy (ATR-IR and Raman) and density functional theory (DFT) calculations of the hydroxyl-functionalized imidazolium ionic liquid C2 OHmimCl indicate that anion-cation hydrogen bonding interactions are dominating, leading to the formation of distinct dimeric ion pair clusters. In this arrangement, the Cl- anions function as a bridge between the cations by establishing bifurcated hydrogen bonds with the OH group of one cation and the C(2)-H of another cation. Cation-cation interactions, on the other hand, do not play a significant role in the observed clusters.

Principal component analysis to enhance enantioselective Raman spectroscopy.

Enantioselective Raman (esR) spectroscopy is an innovative technique with a high potential for online process monitoring in chiral media, e.g. in the pharmaceutical industry. A prerequisite for an effective application is to combine the experimental approach with suitable concepts for data analysis. In this work, we present a chemometric approach to analyze the esR spectra recorded in an automatized polarization-resolved Raman set-up. It is demonstrated that the proposed method is capable of distinguishing between the enantiomers of the chiral alcohol 4-methylpentan-2-ol in a fully unsupervised fashion. Furthermore, it is shown that the difficulty of facing only small intensity differences between the esR spectra of the enantiomers can be overcome by feeding difference spectra between the pure enantiomers and the racemate into the principal component analysis (PCA) algorithm. The enantiomers are clearly discriminable along the first principal component.

Dual-Wavelength Raman Fusion Spectroscopy.

Spatially compressed dual-wavelength Raman spectroscopy allows recording the full Raman spectrum using a detection system with limited spectral range. The common approach is to record the spectra with the two excitation lasers consecutively and then concatenate the full spectrum. However, with this approach, quantitative analysis for process monitoring is not possible as the investigated object may change between the two acquisitions. In this Note, spectral fusion is proposed as a concept to overcome this problem. The sample is illuminated by the two lasers simultaneously, hence leading to an on-chip fusion of the different parts of the Raman spectrum. It is shown that the resulting data are suitable for quantitative evaluation using univariate and multivariate methods. Dual-wavelength Raman fusion spectroscopy offers new opportunities for building highly compact devices for analytical chemistry.

Clusters of the Ionic Liquid 1-Hydroxyethyl-3-methylimidazolium Picrate: From Theoretical Prediction in the Gas Phase to Experimental Evidence in the Solid State.

Interonic interactions determine the macroscopic properties of ionic liquids (ILs). Hence, unravelling the relationships between the microscopic and macroscopic scales is key for rational design. Combining density functional theory (DFT) calculations of isolated ion pairs and vibrational spectroscopy of the condensed phase (fluid or solid) has become a very common approach. In the present work, we make a step towards understanding how the physicochemical effects in small gas phase clusters of a hydroxyl functionalized imidazolium-picrate IL relate with the molecular structure and interactions of the corresponding solid material taking 1-hydroxyethyl-3-methylimidazolium picrate, C2 OHmimPic, as an example. In the isolated ion pair, strong alkyl-OH⋅⋅⋅Pic hydrogen bonding interactions are found rather than the commonly observed hydrogen bonding interactions at the slightly acidic C(2)-H site of the imidazolium ring. However, this part of the cation plays an important role when clusters of ion pairs in the gas phase and inside a crystal lattice are considered. For example, in the dimeric ion-pair cluster, one centre (O*) with two interaction sites (C(2)-H-O* and alkyl OH-Pic) is observed. This configuration is suggested by single crystal X-ray diffraction (XRD), vibrational spectroscopy, and the dispersion-corrected DFT calculations. Hence, the study provides evidence for the appearance of theoretical gas phase clusters in an actual solidified ionic liquid. This ion pair dimer formation may be a general behavior of hydroxyl functionalized imidazolium ILs, but further research is needed to draw a final conclusion. Moreover, the Raman spectra confirm the exclusive gauche conformation of the hyroxyl functionalized alkyl chain.

Optical Spectroscopy for Analysis and Monitoring of Metalworking Fluids.

For various industrial manufacturing processes, water-based metalworking fluids (MWFs) are of high relevance due to their cooling and lubricating ability. They commonly form oil-in-water emulsions or solutions and hence their composition and stability is crucial for their performance in the metalworking process. To ensure a long service life of the MWF, intense monitoring is obligatory. However, examination techniques which display comprehensive and precise information about the actual state of the cooling lubricant in use are currently not available. The present study aims at testing the suitability of spectroscopic methods in terms of Fourier transform infrared, Raman, and laser-induced fluorescence spectroscopy for analyzing and monitoring MWFs. It is shown that all three techniques are capable of determining the initial composition, i.e., the ratio of water and concentrate. Fourier transform infrared provides the best performance regarding monitoring the state of the fluid over an extended period of time. The spectral signatures show distinct changes during a five-month service life in a technical environment.