Applied Spectroscopy

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We describe a new compact acousto-optically Q-switched diode-pumped solid-state (DPSS) intracavity frequency-tripled neodymium-doped yttrium vanadate laser capable of producing ~100 mW of 213 nm power quasi-continuous wave as 15 ns pulses at a 30 kHz repetition rate. We use this new laser in a prototype of a deep ultraviolet (UV) Raman standoff spectrometer. We use a novel high-throughput, high-resolution Echelle Raman spectrograph. We measure the deep UV resonance Raman (UVRR) spectra of solid and solution sodium nitrate (NaNO₃) and ammonium nitrate (NH₄NO₃) at a standoff distance of ~2.2 m. For this 2.2 m standoff distance and a 1 min spectral accumulation time, where we only monitor the symmetric stretching band, we find a solid state NaNO₃ detection limit of ~100 μg/cm². We easily detect ~20 μM nitrate water solutions in 1 cm path length cells. As expected, the aqueous solutions UVRR spectra of NaNO₃ and NH₄NO₃ are similar, showing selective resonance enhancement of the nitrate (NO₃⁻) vibrations. The aqueous solution photochemistry is also similar, showing facile conversion of NO₃⁻ to nitrite (NO₂⁻). In contrast, the observed UVRR spectra of NaNO₃ and NH₄NO₃ powders significantly differ, because their solid-state photochemistries differ. Whereas solid NaNO₃ photoconverts with a very low quantum yield to NaNO₂, the NH₄NO₃ degrades with an apparent quantum yield of ~0.2 to gaseous species.

Near-infrared spectroscopy (NIR) was used to analyze synthetic hydroxyapatite calcined at various temperatures, synthetic carbonated hydroxyapatite, and human hard dental tissues (enamel and dentin). The NIR bands of those materials in the combination, first-overtone, and second-overtone spectral regions were assigned and evaluated for structural characterization. They were attributed to adsorbed and structural water, structural hydroxyl (OH) groups and surface P–OH groups. The NIR spectral features were quantitatively discussed in view of proton solid-state magic-angle spinning nuclear magnetic resonance (¹H MAS NMR) results. We conclude that the NIR spectra of apatites are useful in the structural characterization of synthetic and biogenic apatites.

Nutrient assessment and management are important to maintain productivity in citrus orchards. In this study, laser-induced breakdown spectroscopy (LIBS) was applied for rapid and real-time detection of citrus anomalies. Laser-induced breakdown spectroscopy spectra were collected from citrus leaves with anomalies such as diseases (Huanglongbing, citrus canker) and nutrient deficiencies (iron, manganese, magnesium, zinc), and compared with those of healthy leaves. Baseline correction, wavelet multivariate denoising, and normalization techniques were applied to the LIBS spectra before analysis. After spectral preprocessing, features were extracted using principal component analysis and classified using two models, quadratic discriminant analysis and support vector machine (SVM). The SVM resulted in a high average classification accuracy of 97.5%, with high average canker classification accuracy (96.5%). LIBS peak analysis indicated that high intensities at 229.7, 247.9, 280.3, 393.5, 397.0, and 769.8 nm were observed of 11 peaks found in all the samples. Future studies using controlled experiments with variable nutrient applications are required for quantification of foliar nutrients by using LIBS-based sensing.

The most critical steps during the conservation-restoration treatment applied in Moroccan libraries are the deacidification using immersion in a saturated aqueous calcium hydroxide (Ca(OH)₂) solution and the consolidation of degraded manuscripts using Japanese paper. The present study aims to assess the effciency of this restoration method using a multi-analytical approach. For this purpose, three ancient Arabic Moroccan manuscript papers dating back to the 16th, 17th, and 18th centuries were investigated to characterize the paper support and make a comparative study between pre-restoration and post-restoration states. Three structural and molecular characterization techniques including solid-state nuclear magnetic resonance spectroscopy on ¹³C with cross-polarization and magic-angle spinning nuclear magnetic resonance (¹³C CP-MAS NMR), attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR), and X-ray diffraction (XRD) were used to elucidate the cellulose main features, to identify the inorganic composition of the papers, and to study the crystallinity of the samples. Inductively coupled plasma atomic emission spectrometry (ICP-AES) allowed us to obtain a qualitative and quantitative characterization of the mineral fillers used in the manufacturing of the papers. Scanning electron microscopy coupled to energy dispersive spectrometry (SEM-EDS) ascertained the state of conservation of the different papers and helped us to study the elemental composition of the samples. After restoration, it was shown that the deacidification improved the stability of papers by providing an important alkaline buffer, as demonstrated using FT-IR and energy dispersive spectrometry (EDS) results. However, XRD and ICP-AES did not confirm the pertinence of the treatment for all samples because of the unequal distribution of Ca on the paper surface during the restoration. The consolidation process was studied using SEM analysis; its effectiveness in restoring torn areas was found to be significant.

Drop-coating deposition Raman (DCDR) spectroscopy is based on the measurement of a sample that has been preconcentrated by being dried on a special hydrophobic plate. In addition to its higher sensitivity, the advantage of DCDR over the conventional Raman spectroscopy is the small sample volume needed, the lack of interference from solvents, and the capability of segregating any impurities present and separating components in more complex samples. In this study, DCDR spectroscopy was employed to investigate the complex of the cationic copper(II) 5,10,15,20-tetrakis(1-methyl-4-pyridyl) porphyrin (CuTMPyP) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes. Drop-coating deposition Raman spectra were treated using factor analysis (FA), which led to the following conclusions: (i) the distribution of CuTMPyP in the complex is not homogenous, (ii) the DCDR technique segregates complexed and noncomplexed parts of the sample, (iii) the spectral changes caused by the drying process and by the interaction of CuTMPyP with the DPPC liposomes can be distinguished, and (iv) the porphyrin molecules interacting with DPPC affect both the order–disorder properties of the lipid chains and the lipid head.

A modified design of a chromatically resolved optical microscope (CROMoscope), a grating-based spectral imaging microscope, is described. By altering the geometry and adding a beam splitter, a twisting aberration that was present in the first version of the CROMoscope has been removed. Wavelength adjustment has been automated to decrease analysis time. Performance of the new design in transmission–absorption spectroscopy has been evaluated and found to be generally similar to the performance of the previous design. Spectral bandpass was found to be dependent on the sizes of apertures, and the smallest measured spectral bandpass was 1.8 nm with 1.0 mm diameter apertures. Wavelength was found to be very linear with the sine of the grating angle (R ² = 0.9999995), and wavelength repeatability was found to be much better than the spectral bandpass. Reflectance spectral imaging with a CROMoscope is reported for the first time, and this reflectance spectral imaging was applied to blue ink samples on white paper. As a proof of concept, linear discriminant analysis was used to classify the inks by brand. In a leave-one-out cross-validation, 97.6% of samples were correctly classified.

Hyperspectral chemical imaging technologies are frequently applied in the pharmaceutical industry to assess the distribution of ingredients in product intermediates and finished products. This article discusses two recognized measures of spatial uniformity in the context of pharmaceuticals. Synthetic images are used to introduce the utility of Ripley's K-function and the Herfindahl-Hirschman index (HHI) for describing image content. These metrics were applied to a commercial-like product to demonstrate the practical interpretation for product development. The simple approaches presented here offer the possibility of reporting objective measures of intra-tablet compositional uniformity with minimal supervision.

We examine the use of multiwavelength ultraviolet (UV) resonance–Raman signatures to identify the effects of growth phase and growth medium on gram-positive and gram-negative bacteria. Escherichia coli(E. coli), Citrobacter koseri(C. koseri), Citrobacter braakii (C. braakii), and Bacillus cereus (B. cereus)were grown to logarithmic and stationary phases in nutrient broth and brain heart infusion broth. Resonance Raman spectra of bacteria were obtained at multiple wavelengths between 220 and 260 nm; a range that encompasses the resonance frequencies of cellular constituents. We find that spectra of the same bacterial species exhibit differences due to both growth condition and growth phase, but the larger differences reflect changes due to growth phase. The differences in the Raman spectra correlate with genetic differences among the species. Using a Pearson correlation based algorithm, we achieve successful identification of these bacteria in 83% of the cases.

This paper reports methods for obtaining time-dependent reduced isotropic Raman spectra of aqueous species in quartz capillary high-pressure optical cells under hydrothermal conditions, as a means of determining quantitative speciation in hydrothermal fluids. The methods have been used to determine relative Raman scattering coefficients and to examine the thermal decomposition kinetics of the non-complexing anions bisulfate (HSO₄⁻), perchlorate (CIO₄⁻), perrhenate (ReO₄⁻), and trifluoromethanesulfonate, or “triflate” (CF₃SO₃⁻) in acidic and neutral solutions at temperatures up to 400 °C and 30 MPa. Arrhenius expressions for calculating the thermal decomposition rate constants are also reported. Thermal stabilities in the acidic solutions followed the order HSO₄⁻ (stable) > ReO₄⁻ > CIO₄⁻ > CF₃SO₃⁻ with half-lives (t_1/2) > 7 h at 300 °C. In neutral solutions, the order was HSO₄⁻ (stable) > CF₃SO₃⁻ > ReO₄⁻ > CIO₄⁻, with t_1/2 > 8 h at 350 °C. CF₃SO₃⁻ was extremely stable in neutral solutions, with t_1/2 > 11 h at 400 °C.

Cosmic rays (CRs) occasionally affect charge-coupled device (CCD) detectors, introducing large spikes with very narrow bandwidth in the spectrum. These CR features can distort the chemical information expressed by the spectra. Consequently, we propose here an algorithm to identify and remove significant spikes in a single Raman spectrum. An autocorrelation analysis is first carried out to accentuate the CRs feature as outliers. Subsequently, with an adequate selection of the threshold, a discrete wavelet transform filter is used to identify CR spikes. Identified data points are then replaced by interpolated values using the weighted-average interpolation technique. This approach only modifies the data in a close vicinity of the CRs. Additionally, robust wavelet transform parameters are proposed (a desirable property for automation) after optimizing them with the application of the method in a great number of spectra. However, this algorithm, as well as all the single-spectrum analysis procedures, is limited to the cases in which CRs have much narrower bandwidth than the Raman bands. This might not be the case when low-resolution Raman instruments are used.

This paper shows the application of Raman and infrared hyperspectral imaging combined with multivariate curve resolution (MCR) to the analysis of the constituents of commercial chocolate samples. The combination of different spectral data pretreatment methods allowed decreasing the high fluorescent Raman signal contribution of whey in the investigated chocolate samples. Using equality constraints during MCR analysis, estimations of the pure spectra of the chocolate sample constituents were improved, as well as their relative contributions and their spatial distribution on the analyzed samples. In addition, unknown constituents could be also resolved. White chocolate constituents resolved from Raman hyperspectral image indicate that, at macro scale, sucrose, lactose, fat, and whey constituents were intermixed in particles. Infrared hyperspectral imaging did not suffer from fluorescence and could be applied for white and milk chocolate. As a conclusion of this study, micro-hyperspectral imaging coupled to the MCR method is confirmed to be an appropriate tool for the direct analysis of the constituents of chocolate samples, and by extension, it is proposed for the analysis of other mixture constituents in commercial food samples.

Cotton fibers are routinely harvested from cotton plants (in planta), and their end-use qualities depend on their development stages. Cotton fibers are also cultured in controlled laboratory environments, so that cotton researchers can investigate many aspects of experimental protocols in cotton breeding programs at reduced expenses. In this work, attenuated total refection Fourier transform infrared (ATR FT-IR) spectra of cotton fibers grown in planta and in culture were collected to explore the potential of FT-IR technique as a simple, rapid, and direct method for characterizing the fiber development. Complementary to visual inspection of spectral variations, principal component analysis (PCA) of ATR FT-IR spectra revealed the occurrence of phase transition from primary to secondary cell wall synthesis and also the difference of starting the phase transition between two types of fibers. Like PCA observation, three simple algorithms were capable of monitoring the secondary cell wall formation effectively. Interestingly and uniquely, simple algorithms were able to detect the subtle discrepancies in fibers older than 25 days post-anthesis, which was not apparent from PCA results. The observation indicated the feasibility of FT-IR technique in rapid, routine, nondestructive, and direct assessment of fiber development for cotton physiology and breeding applications.

The authors of the work entitled, “Determination of Photoluminescence Quantum Yields of Scattering Media with an Integrating Sphere: Direct and Indirect Illumination”, Christian Würth and Ute Resch-Genger [Applied Spectroscopy, vol. 69(06): 749-759. doi: 10. 1366/14-07679] have provided corrections to the discussion pertaining to Eq. 9 and to the order of Eq. 11.

An integral sign is missing in the published explanation for Eq. 9 on page 752. It should instead read:

Here, Φ_f is the photoluminescence quantum yield and S_λ(λ_em) the area-normalized (∫S_λ(λ_em)dλ = 1) emission profile.

Equation 11 on page 752 should instead be (11) l S d e m l B d e x - l S d e x λ e m λ e x s ( λ e x ) s ( λ e x ) = Φ f M e m ( λ e m ) M 0 ( λ e x ) 1 t S λ ( λ e m )