Applied Spectroscopy

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Over the past five years, new developments in the field of plasmonics have emerged with the goal of finely tuning a variety of metallic nanostructures to enable a desired function. The use of plasmonics in spectroscopy is of course of great interest, due to large local enhancements in the optical near field confined in the vicinity of a metal nanostructure. For a given metal, such enhancements are dependent on the shape of the structure as well as the optical properties (wavelength, phase, polarization) of the impinging light, offering a large degree of control over the optical and spatial localization of the plasmon resonance. In this focal point, we highlight recent work that aims at revealing the spatial position of the localized plasmon resonances using a variety of optical and non-optical methods.

The use of formamidine sulfinic acid in the textile industry goes back many years, particularly as an agent for the reduction or “vatting” of vat dyes, to form the water-soluble leuco species, when dyeing or printing cellulosic fibers. Many workers have labeled this agent as thiourea dioxide and theorized that its reducing power developed through the existence of two isomeric forms: thiourea dioxide (the sulfone), and formamidine sulfinate. This article has used solid and solution Fourier transform infrared (FT-IR) spectroscopy to show that the name thiourea dioxide is a misnomer since the compound does not exist in the six-valent state, but is stabilized in the four-valent state by formation of a strong internal zwitterion as shown in the article.

A concept based on a combination of photofragmentation laser-induced fluorescence (PF-LIF) and two-photon laser-induced fluorescence (LIF) is for the first time demonstrated for simultaneous detection of hydrogen peroxide (H₂O₂) and water (H₂O) vapor. Water detection is based on two-photon excitation by an injection-locked krypton fluoride (KrF) excimer laser (248.28 nm), which induces broadband fluorescence (400-500 nm) from water. The same laser simultaneously photodissociates H₂O₂, whereupon the generated OH fragments are probed by LIF after a time delay of typically 50 ns, by a frequency-doubled dye laser (281.91 nm). Experiments in six different H₂O₂/H₂O mixtures of known compositions show that both signals are linearly dependent on respective species concentration. For the H₂O₂ detection there is a minor interfering signal contribution from OH fragments created by two-photon photodissociation of H₂O. Since the PF-LIF signal yield from H₂O₂ is found to be at least ∼24 000 times higher than the PF-LIF signal yield from H₂O at room temperature, this interference is negligible for most H₂O/H₂O₂ mixtures of practical interest. Simultaneous single-shot imaging of both species was demonstrated in a slightly turbulent flow. For single-shot imaging the minimum detectable H₂O₂ and H₂O concentration is 10 ppm and 0.5%, respectively. The proposed measurement concept could be a valuable asset in several areas, for example, in atmospheric and combustion science and research on vapor-phase H₂O₂ sterilization in the pharmaceutical and aseptic food-packaging industries.

Lignins and their cross-linking to hemicelluloses detrimentally affect the cellulose-to-ethanol conversion of grass lignocelluloses. Screening appropriate grass cell walls and their compositional changes during the various steps of the process calls for a high-throughput analytical technique. Such a performance can be fulfilled by Fourier transform mid-infrared (FT-MIR) spectroscopy. In the present paper, a set of maize cell walls from mature stems were selected, including brown midrib samples. Lignin fractions were isolated by mild acidolysis to obtain a set of purified maize lignin standards. The lignin content and the percentage of lignin-derived p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) thioacidolysis monomers were determined. In addition, the composition of cell wall polysaccharides, as well as the amount of ester-linked p-coumaric (CA) and ferulic (FA) acids, was measured by wet chemistry. Partial least square (PLS) analyses were applied to infrared and chemical data of cell walls. The resulting models showed a good predictive ability with regard to the lignin content, to the frequency of S (or G) thioacidolysis monomers, and to the level of ester-linked CA of maize cell walls. The loading plots and regression coefficients revealed relevant infrared absorption bands.

Changes in raw materials and process wear and tear can have significant effects on the prediction error of near-infrared calibration models. When the variability that is present during routine manufacturing is not included in the calibration, test, and validation sets, the long-term performance and robustness of the model will be limited. Nonlinearity is a major source of interference. In near-infrared spectroscopy, nonlinearity can arise from light path-length differences that can come from differences in particle size or density. The usefulness of support vector machine (SVM) regression to handle nonlinearity and improve the robustness of calibration models in scenarios where the calibration set did not include all the variability present in test was evaluated. Compared to partial least squares (PLS) regression, SVM regression was less affected by physical (particle size) and chemical (moisture) differences. The linearity of the SVM predicted values was also improved. Nevertheless, although visualization and interpretation tools have been developed to enhance the usability of SVM-based methods, work is yet to be done to provide chemometricians in the pharmaceutical industry with a regression method that can supplement PLS-based methods.

A diffuse reflectance spectroscopy-based method to score fibrosis in paraffin-preserved human liver specimens has been developed and is reported here. Paraffin blocks containing human liver tissue were collected from the General Hospital of Mexico and included in the study with the patients' written consent. The score of liver fibrosis was determined in each sample by two experienced pathologists in a single-blind fashion. Spectral measurements were acquired at 450–750 nm by establishing surface contact between the optical probe and the preserved tissue. According to the histological evaluation, four liver samples showed no evidence of fibrosis and were categorized as F0, four hepatic specimens exhibited an initial degree of fibrosis (F1—F2), five liver specimens showed a severe degree of fibrosis (F3), and six samples exhibited cirrhosis (F4). The human liver tissue showed a characteristic diffuse reflectance spectrum associated with the progressive stages of fibrosis. In the F0 liver samples, the diffuse reflection intensity gradually increased in the wavelength range of 450–750 nm. In contrast, the F1–F2, F3, and F4 specimens showed corresponding 1.5-, 2-, and 5.5-fold decreases in the intensity of diffuse reflectance compared to the F0 liver specimens. At 650 nm, all the stages of liver fibrosis were clearly distinguished from each other with high sensitivity and specificity (92–100%). To our knowledge, this is the first study reporting a distinctive diffuse reflectance spectrum for each stage of fibrosis in paraffin-preserved human liver specimens. These results suggest that diffuse reflectance spectroscopy may represent a complementary tool to liver biopsy for grading fibrosis.

Several families of catfish species are extensively aquacultured around the world; however, only those from the family Ictaluridae can be labeled as catfish in the United States. Non-Ictalurid catfish species that are marketed as “catfish” in the USA are considered misbranded. Misbranding in general has led to an increased interest in developing deoxyribonucleic acid (DNA)-based methods such as DNA barcoding, polymerase chain reaction restriction fragment length polymorphism, and DNA microarrays with fluorescence detection for the identification of fish species. In this proof-of-concept study, DNA microarrays coupled with a newly developed mid-infrared imaging detection method were applied to the identification of seven species of catfish for the first time. Species-specific DNA probes targeting three regions per species of the cytochrome c oxidase 1 (barcoding) gene were developed and printed as microarrays on glass slides. Deoxyribonucleic acid targets labeled with biotin were hybridized to their complementary probes using a strategy that allowed the selective formation of a silver layer on hybridized spots needed for detection. Using this three-probe format, the seven species were all identified correctly, even when a limited number of false positive spots were observed. Raman spectroscopy was employed to further characterize the arrays.

The dynamic properties of photothermal processes occurring after the initial photogenerated carrier recombination in the flash photolysis of a semiconductor plate sample probed with a reflected infrared beam are reported in this work. Transient kinetics pertaining to the photothermal processes always appear as interfering signals in that of the photogenerated carrier recombination and should be distinguished and excluded. We observed that the photothermal-induced Rayleigh wave occurs immediately after the photogenerated carrier recombination that is then followed by the photothermal-induced flexural vibration of the sample substrate with a set of intrinsic frequencies as reported in our previous work (Appl. Spectrosc. 2013. 67(5): 506–512). When these two faster types of waves decay, the transient decaying signal from the plate deformation due to the inhomogeneous temperature field remains for much longer than 22 ms. Thus, all three types of the photothermal dynamic processes of different temporal properties induced by the pulsed laser on an absorbing semiconductor thin plate are clearly identified.

Improvements in diode laser, fiber optic, and data acquisition technologies are enabling increased use of Raman spectroscopic techniques for both in lab and in situ water analysis. Aqueous media encountered in the natural environment often contain suspended solids that can interfere with spectroscopic measurements, yet removal of these solids, for example, via filtration, can have even greater adverse effects on the extent to which subsequent measurements are representative of actual field conditions. In this context, this study focuses on evaluation of turbidity effects on Raman spectroscopic measurements of two common environmental pollutants in aqueous solution: ammonium nitrate and trichloroethylene. The former is typically encountered in the runoff from agricultural operations and is a strong scatterer that has no significant influence on the Raman spectrum of water. The latter is a commonly encountered pollutant at contaminated sites associated with degreasing and cleaning operations and is a weak scatterer that has a significant influence on the Raman spectrum of water. Raman observations of each compound in aqueous solutions of varying turbidity created by doping samples with silica flour with grain sizes ranging from 1.6 to 5.0 μm were employed to develop relationships between observed Raman signal strength and turbidity level. Shared characteristics of these relationships were then employed to define generalized correction methods for the effect of turbidity on Raman observations of compounds in aqueous solution.

We propose a robust technique called Savitzky–Golay second-derivative (SGSD) fitting for modeling the in situ Raman spectrum of graphitic materials in rock samples such as carbonaceous chondrite meteorites. In contrast to non-derivative techniques, with assumed locally linear or nth-order polynomial fluorescence backgrounds, SGSD produces consistently good fits of spectra with variable background fluorescence of any slowly varying form, without fitting or subtracting the background. In combination with a Monte Carlo technique, SGSD calculates Raman parameters (such as peak width and intensity) with robust uncertainties. To explain why SGSD fitting is more accurate, we compare how different background subtraction techniques model the background fluorescence with the wide and overlapping peaks present in a real Raman spectrum of carbonaceous material. Then, the utility of SGSD is demonstrated with a set of real and simulated data compared to commonly used linear background techniques. Researchers may find the SGSD technique useful if their spectra contain intense background interference with unknown functional form or wide overlapping peaks, and when the uncertainty of the spectral data is not well understood.

Previous research has produced conclusive evidence to show that soil hydrophobicity is affected by soil organic matter (SOM) and soil water content (WC). Soil hydrophobicity that responds to WC is a unique form of hydrophobicity called reversible hydrophobicity. The mechanistic processes by which water and SOM interact continue to be a subject of investigation. This article presents a novel application of photoacoustic Fourier transform infrared spectroscopy (FT-IR) for the investigation of reversibly hydrophobic soils. Photoacoustic FT-IR data show that, in response to hydration, surface molecules on wetted soil particles interacted differently with mid-infrared radiation than surface molecules present on air-dried soil. This can be interpreted as an effect of the reorienting of amphiphilic molecules in response to hydrationdriven entropic processes. These results suggest that the photoacoustic FT-IR method can be used to elucidate how SOM and water interact at the molecular scale to drive soil hydrophobicity.

The aim of this study was to evaluate in-line Raman spectroscopy for monitoring the progress of particle size reduction in real time during wet-stirred media milling of two Biopharmaceutics Classification System (BCS) Class II drugs, griseofulvin and naproxen. To develop a validated online Raman method, Raman analyses were carried out offline by taking samples from the mill at various milling times. A multivariate linear model (partial least squares, PLS) was fitted to the raw data obtained from the Raman measurements and good linearity between online and offline Raman spectra was found. Line intensities (I) of the in-line spectra obtained during the wet media milling were processed by multivariate analyses and correlated with both the median size (d₅₀) and the 90% passing particle size (d₉₀), which were measured offline by laser diffraction. A two-parameter exponential growth model of the form d = exp[A(I – I₀)] was found to establish a good correlation (R ² > 0.90) as a statistically significant model with statistically significant parameters (P < 10⁻⁴). The correlations were applicable to milled suspensions with particles in the approximate size range of 0.1–6 μm for griseofulvin and 0.1–8 μm for naproxen. These results suggest that in-line Raman spectroscopy can be used to successfully monitor the progress of particle size reduction during wet media milling.

An omission regretfully occurred in the caption for the first figure of the paper entitled, “Laser-Induced Plasma Spectroscopy of Hydrogen Balmer Series in Laboratory Air”, by Lauren D. Swafford and Christian G. Parigger [Applied Spectroscopy, vol. 68(9): 1016-1020. doi: 10.1366/13-07388]. It should instead read as the following:

Fig. 1. Schematic representation of the apparatus used during this experiment, which is a typical LIBS arrangement: (1) beam splitter, (2) mirrors, (3) focusing lens, (4) plasma, (5) focusing lenses. [Reprinted with permission from C.G. Parigger, A.C Woods, M.J. Witte, L.D. Swafford, D.M. Surmick. “Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C₂, CN, and TiO Spectra Following Laser-induced Optical Breakdown”. Journal of Visualized Experiments. 2014. 84: e51250. doi:10.3791/51250.]