Applied Spectroscopy

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The biomedical uses for the spectroscopy of scattered light by micro and nanoscale objects can broadly be classified into two areas. The first, often called light scattering spectroscopy (LSS), deals with light scattered by dielectric particles, such as cellular and sub-cellular organelles, and is employed to measure their size or other physical characteristics. Examples include the use of LSS to measure the size distributions of nuclei or mitochondria. The native contrast that is achieved with LSS can serve as a non-invasive diagnostic and scientific tool. The other area for the use of the spectroscopy of scattered light in biology and medicine involves using conducting metal nanoparticles to obtain either contrast or electric field enhancement through the effect of the surface plasmon resonance (SPR). Gold and silver metal nanoparticles are non-toxic, they do not photobleach, are relatively inexpensive, are wavelength-tunable, and can be labeled with antibodies. This makes them very promising candidates for spectrally encoded molecular imaging. Metal nanoparticles can also serve as electric field enhancers of Raman signals. Surface enhanced Raman spectroscopy (SERS) is a powerful method for detecting and identifying molecules down to single molecule concentrations. In this review, we will concentrate on the common physical principles, which allow one to understand these apparently different areas using similar physical and mathematical approaches. We will also describe the major advancements in each of these areas, as well as some of the exciting recent developments.

A new algorithm for the automatic recognition of peak and baseline regions in spectra is presented. It is part of a study to devise a baseline correction method that is particularly suitable for the simple and fast treatment of large amounts of data of the same type, such as those coming from high-throughput instruments, images, process monitoring, etc. This algorithm is based on the continuous wavelet transform, and its parameters are automatically determined using the criteria of Shannon entropy and the statistical distribution of noise, requiring virtually no user intervention. It was assessed on simulated spectra with different noise levels and baseline amplitudes, successfully recognizing the baseline points in all cases but for a few extremely weak and noisy signals. It can be combined with various fitting methods for baseline estimation and correction. In this work, it was used together with an iterative polynomial fitting to successfully process a real Raman image of 40 000 pixels in about 2.5 h.

Changes in the infrared spectra of bronchial epithelia in victims of fatal burns were investigated. The mechanism of spectral changes on the basis of cellular morphological changes was considered. The ability of spectral parameters to diagnose fatal burns was assessed. Ten cases of fatal burns and 20 control cases were selected. Their lung tissues were removed, and sections were cut and mounted on glass and barium fluoride slides. Spectra of polarized bronchial epithelia were obtained by microscopy based on their morphological changes. In the spectra, 16 major absorbance bands were evaluated to determine their ability to act as positive markers for exposure to fire. Compared with the control group, the bronchial epithelia of the fatal burn victims showed three spectral results. (1) The absorbance of 16 major bands from the spectra of polarized bronchial epithelia in fatal burn victims significantly increased. (2) For the same cell number, the absorbance at 2850, 2920, 2959, and 3084 cm⁻¹ decreased. (3) The degree of increased or decreased absorbance of bands is related to the degree of polarization. These spectral results suggest that there is a vital reaction induced by the inhalation of hot fumes that includes an increase in the number of bronchial epithelia and a polarization effect. Overall, Fourier transform infrared (FT-IR) microspectroscopy was shown to be a convenient and reliable method to provide objective spectral markers to assist the diagnosis of fatal burns by simultaneously monitoring several specific parameters, although these observations have yet to be applied at forensic scenes.

To study the effect of roughness of a supporting substrate to Raman enhancement, silver nanoparticles (AgNPs) were prepared on Si with different degrees of roughness. To roughen the surface of silicon, electroless displacement was used first to grow AgNPs on smooth Si. By chemically removing the resulting AgNPs, an electrolessly roughened Si surface can be exposed. A second electroless displacement then was performed to grow new AgNPs on the roughened Si crystal to form surface-enhanced Raman scattering substrates. Another approach, called the protecting method, also was proposed and demonstrated to structure AgNPs on surface-roughened Si. In this second method, electroless displacement also was used to grow AgNPs on the Si crystal. The resulting AgNPs then were protected by thio compounds to control removal of the outer layer of AgNPs, thereby exposing the underlying AgNPs located directly on the electroless roughened Si surface. Results indicate that the structure of AgNPs on roughened Si surfaces provides approximately two orders of magnitude higher enhancement than AgNPs on non-roughened Si, and the substrates prepared in this work are highly sensitive, with enhancement factors reaching 10⁸.

A quantitative and simultaneous measurement of K, KCl, and KOH vapors from a burning fuel sample combusted in a single particle reactor was performed using collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) with a time resolution of 0.2 s. The previously presented CPFAAS technique was extended in this work to cover two consecutive fragmentation pulses for the photofragmentation of KCl and KOH. The spectral overlapping of the fragmentation spectra of KCl and KOH is discussed, and a linear equation system for the correction of the spectral interference is introduced. The detection limits for KCl, KOH, and K with the presented measurement arrangement and with 1 cm sample length were 0.5, 0.1, and 0.001 parts per million, respectively. The experimental setup was applied to analyze K, KCl, and KOH release from 10 mg spruce bark samples combusted at the temperatures of 850, 950, and 1050 °C with 10% of O₂. The combustion experiments provided data on the form of K vapors and their release during different combustion phases and at different temperatures. The measured release histories agreed with earlier studies of K release. The simultaneous direct measurement of atomic K, KCl, and KOH will help in the impact of both the form of K in the biomass and fuel variables, such as particle size, on the release of K from biomass fuels.

Charge-coupled device detectors are vulnerable to cosmic rays that can contaminate Raman spectra with positive going spikes. Because spikes can adversely affect spectral processing and data analyses, they must be removed. Although both hardware-based and software-based spike removal methods exist, they typically require parameter and threshold specification dependent on well-considered user input. Here, we present a fully automated spike removal algorithm that proceeds without requiring user input. It is minimally dependent on sample attributes, and those that are required (e.g., standard deviation of spectral noise) can be determined with other fully automated procedures. At the core of the method is the identification and location of spikes with coincident second derivatives along both the spectral and spatiotemporal dimensions of two-dimensional datasets. The method can be applied to spectra that are relatively inhomogeneous because it provides fairly effective and selective targeting of spikes resulting in minimal distortion of spectra. Relatively effective spike removal obtained with full automation could provide substantial benefits to users where large numbers of spectra must be processed.

An accurate technique has been developed to calculate the equivalent width of absorption lines. The calculations have been carried out for the pure Doppler and pure Lorentz limiting forms of the equivalent width. A novel expression for the equivalent width for Lorentz profile is given from direct integration of the line profile. The more general case of a Voigt profile leads to an analytical formula that permits a rapid estimate of the equivalent width for a wide range of maximum optical depths. The reliability of the approach is verified using a numerical application calculating the equivalent width for nickel resonance lines at 232.0 and 352.3 nm from atomic absorption (AA) measurements. The dependence of equivalent width on the number density of absorbing atoms is also provided. The results obtained for the equivalent width for the Voigt profile were compared with the data in the available literature obtained by different approaches.

We report an approach for selecting an internal standard to improve the precision of laser-induced breakdown spectroscopy (LIBS) analysis for determining calcium (Ca) concentration in water. The dissolved Ca²⁺ ions were pre-concentrated on filter paper by evaporating water. The filter paper was dried and analyzed using LIBS. By adding strontium chloride to sample solutions and using a Sr II line at 407.771 nm for the intensity normalization of Ca II lines at 393.366 or 396.847 nm, the analysis precision could be significantly improved. The Ca II and Sr II line intensities were mapped across the filter paper, and they showed a strong positive shot-to-shot correlation with the same spatial distribution on the filter paper surface. We applied this analysis approach for the measurement of Ca²⁺ in tap, bottled, and ground water samples. The Ca²⁺ concentrations determined using LIBS are in good agreement with those obtained from flame atomic absorption spectrometry. Finally, we suggest a homologous relation of the strongest emission lines of period 4 and 5 elements in groups IA and IIA based on their similar electronic structures. Our results indicate that the LIBS can be effectively applied for liquid analysis at the sub-parts per million level with high precision using a simple drying of liquid solutions on filter paper and the use of the correct internal standard elements with the similar valence electronic structure with respect to the analytes of interest.

Leakage of injected carbon dioxide (CO₂) or resident fluids, such as brine, is a major concern associated with the injection of large volumes of CO₂ into deep saline formations. Migration of brine could contaminate drinking water resources by increasing their salinity or endanger vegetation and animal life as well as human health. The main objective of this study was to investigate the effect of sodium chloride (NaCl) concentration on the detection of calcium and potassium in brine samples using laser-induced breakdown spectroscopy (LIBS). The ultimate goals were to determine the suitability of the LIBS technique for in situ measurements of metal ion concentrations in NaCl-rich solution and to develop a chemical sensor that can provide the early detection of brine intrusion into formations used for domestic or agricultural water production. Several brine samples of NaCl–CaCl₂ and NaCl–KCl were prepared at NaCl concentrations between 0.0 and 3.0 M. The effect of NaCl concentration on the signal-to-background ratio (SBR) and signal-to-noise ratio (SNR) for calcium (422.67 nm) and potassium (769.49 nm) emission lines was evaluated. Results show that, for a delay time of 300 ns and a gate width of 3 μs, the presence of and changes in NaCl concentration significantly affect the SBR and SNR for both emission lines. An increase in NaCl concentration from 0.0 to 3.0 M produced an increase in the SNR, whereas the SBR dropped continuously. The detection limits obtained for both elements were in the milligrams per liter range, suggesting that a NaCl-rich solution does not severely limit the ability of LIBS to detect trace amount of metal ions.

A method employing an integrated femtosecond (fs) and nanosecond (ns) dual-laser system was developed to generate plasma with desired radical species from gas mixtures via a fs laser pulse and then to excite selected radical species to higher electronic states using a wavelength-tunable ns laser pulse. An optical spectrometer was used to measure the emission spectra and identify the transition from the excited electronic state to the ground state. The proposed technique has been demonstrated for an N₂–CO₂ mixture with various time delays between the two fs and ns pulses. The results have indicated that the population of selected radical species at the excited electronic state can be increased using the subsequent ns laser pulse, which also enhances the intensity of emission spectra allowing better identifications of the radical species. This technique holds a promise of detection and identification of signature plasma species, particularly for trace elements and long-distance standoff detections.

In an effort to augment the atomic emission spectra of conventional laser-induced breakdown spectroscopy (LIBS) and to provide an increase in selectivity, mid-wave to long-wave infrared (IR), LIBS studies were performed on several organic pharmaceuticals. Laser-induced breakdown spectroscopy signature molecular emissions of target organic compounds are observed for the first time in the IR fingerprint spectral region between 4–12 μm. The IR emission spectra of select organic pharmaceuticals closely correlate with their respective standard Fourier transform infrared spectra. Intact and/or fragment sample molecular species evidently survive the LIBS event. The combination of atomic emission signatures derived from conventional ultraviolet–visible-near-infrared LIBS with fingerprints of intact molecular entities determined from IR LIBS promises to be a powerful tool for chemical detection.

Despite the fact that non-covalent interactions between various aromatic compounds and carbon nanotubes are being extensively investigated now, there is still a lack of understanding about the nature of such interactions. The present paper sheds light on one of the possible mechanisms of interaction between the typical aromatic dye proflavine and the carbon nanotube surface, namely, π-stacking between aromatic rings of these compounds. To investigate such a complexation, a qualitative analysis was performed by means of ultraviolet visible, infrared, and nuclear magnetic resonance spectroscopy. The data obtained suggest that π-stacking brings the major contribution to the stabilization of the complex between proflavine and the carbon nanotube.

The water-gas shift (WGS) reaction has been studied by pulsing carbon monoxide (CO) into a steady-state water (H₂O)-Ar flow over nickel(II) oxide–zinc oxide (NiO–ZnO) catalysts using in situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) coupled with a mass spectrometer method using the pulse technique (in situ pulse DRIFTS-MS) for different flow rates (gas hourly space velocity [GHSV] of 24 000–72 000 h⁻¹) and reaction temperatures (250–350 °C). The results obtained from the in situ pulse DRIFTS-MS revealed that there are two types of water adsorption bands on the surface of the catalyst: (i) molecular adsorption (infrared [IR] bands in the 2500–3600 cm⁻¹ range and at 1640 cm⁻¹), and (ii) dissociative adsorption at 3700 cm⁻¹, where carboxyl bands are formed at 1461 and 1368 cm⁻¹ and the gas-phase CO is adsorbed at 2187 and 2111 cm⁻¹ on the surface of the catalyst. After using a GHSV = 24 000 h⁻¹ H₂O/Ar flow, we probed the existence of two active intermediates via the formation of two hydrogen production peaks. The products of hydrogen gas (H₂) and carbon dioxide (CO₂) had two pathways: the redox process and the associative process via the intermediate of the carboxyl group. In situ pulse DRIFTS-MS proves to be an effective approach for studying the nature of adsorbed species on the catalyst surface and the nature of the reaction product.

A novel near-infrared spectroscopy (NIRS) method has been researched and developed for the simultaneous analyses of the chemical components and associated properties of mint (Mentha haplocalyx Briq.) tea samples. The common analytes were: total polysaccharide content, total flavonoid content, total phenolic content, and total antioxidant activity. To resolve the NIRS data matrix for such analyses, least squares support vector machines was found to be the best chemometrics method for prediction, although it was closely followed by the radial basis function/partial least squares model. Interestingly, the commonly used partial least squares was unsatisfactory in this case. Additionally, principal component analysis and hierarchical cluster analysis were able to distinguish the mint samples according to their four geographical provinces of origin, and this was further facilitated with the use of the chemometrics classification methods—K-nearest neighbors, linear discriminant analysis, and partial least squares discriminant analysis. In general, given the potential savings with sampling and analysis time as well as with the costs of special analytical reagents required for the standard individual methods, NIRS offered a very attractive alternative for the simultaneous analysis of mint samples.

In the present study, the possibility of employing spatially offset Raman spectroscopy (SORS) in the qualitative and quantitative characterization of quality parameters of salmon through the skin has been explored. A laboratory-based SORS setup comprising an 830 nm laser was employed, and intact samples and model samples made of salmon tissue constituents were used to investigate the penetration of Raman signals through the dark and light part of salmon skin. Intact salmon samples with both dark and light skin were measured at different spatial offsets. When using spatial offsets in the range of 5–6 mm, the results clearly show that information regarding fatty acid composition and carotenoid content could be obtained from both dark and light parts of the skin. Similar information could not be obtained using conventional backscattering Raman spectroscopy. Model samples of ground salmon spiked with either solutions of carotenoids or a range of vegetable oils were also measured, and at a spatial offset of 5 mm, a clear relationship between Raman carotenoid band intensities and carotenoid concentrations in the model samples was revealed. In addition, high correlations for the estimation of iodine values (i.e., fatty acid unsaturation) could be obtained for SORS measurements through light and dark parts of the salmon skin. A crude estimate suggested that information from around 5 mm beneath the surface area of the salmon skin could be obtained. The choice of a laser line in the near-infrared region is a major prerequisite for successful through-skin analysis of salmon. This feasibility study could pave the way for future Raman analysis of intact salmon.