Applied Spectroscopy

Applied Spectroscopy News is a monthly feature in the journal. It includes information from the Society for Applied Spectroscopy, news from other societies and institutions, announcements of meetings, schools, or other activities, and reports of symposia from recent conferences. If you have news items, a meeting announcement, or a report from a symposium that would be in interest to readers of Applied Spectroscopy, contact Deborah K. Bradshaw, Applied Spectroscopy News Editor, P.O. Box 536307, Orlando, Florida 32853-6307, email: d.k.bradshaw@att.net, Ph: (407) 898-3823.

The capability to make simultaneous neutron and Raman scattering measurements at temperatures between 1.5 and 450 K has been developed. The samples to be investigated are attached to one end of a custom-made center-stick suitable for insertion into a 100 mm-bore cryostat. The other end of the center-stick is fiber-optically coupled to a Renishawt® inVia Raman spectrometer incorporating a 300 mW Topticat® 785 nm wavelength stabilized diode laser. The final path for the laser beam is ∼1.3 m in vacuo within the center-stick followed by a focusing lens close to the sample. Raman scattering measurements with a resolution of 1 to 4 cm⁻¹ can be made over a wide range (100–3200 cm⁻¹) at the same time as a variety of different types of neutron scattering measurements. In this work we highlight the use of inelastic neutron scattering and neutron diffraction in conjunction with the Raman for studies of the globular protein lysozyme.

Accurately computing molecular Raman spectra would enable rapid development of inexpensive and extensive Raman libraries. This is especially beneficial for chemicals that are regulated, toxic, or otherwise difficult to handle. Numerous quantum mechanical methods have been developed that enable computation of Raman spectra. Here, we study the B3LYP exchange correlation functional with various combinations of basis sets, polarization functions, and diffuse functions to determine which combination best computes the Raman spectra for explosive and nonexplosive molecules. In comparing spectra, three metrics were utilized: the root mean square error, the earth mover's distance, and the weighted cross-correlation average. The earth mover's distance and weighted cross-correlation metrics are shown to have significantly greater power at detecting spectral similarities and differences than the root mean square error. Across all methods and molecules examined, B3LYP/6-311++G(d,p) was found to provide the best match between measured and computed Raman spectra. Spectra generated at the B3LYP/6-311++G(d,p) level were found to be accurate enough to correctly identify each molecule out of a set of measured molecular spectra.

The ability to accurately and noninvasively analyze illicit drugs is important for criminal investigations and prosecution. Current methods involve significant sample pretreatment and most are destructive. The goal of this work is to develop a method based on Raman spectroscopy to classify simulated street drug mixtures composed of one drug component and up to three cutting agents including those routinely found in confiscated illicit street drug mixtures. Spectra were collected on both a homebuilt instrument using a HeNe laser and on a handheld commercial instrument with a 785 nm light source. Mixtures were prepared with drug concentrations ranging from 10 to 100 percent. Optimal preprocessing for the data set included truncating, Savitzky–Golay smoothing, normalization, differentiating, and mean centering. Using principal component analysis (PCA), it was possible to resolve the spectral differences between benzocaine, lidocaine, isoxsuprine, and norephedrine and correctly classify them 100 percent of the time.

The seawater neutralization process is currently used in the alumina industry to reduce the pH and dissolved metal concentrations in bauxite refinery residues through the precipitation of Mg, Al, and Ca hydroxide and carbonate minerals. This neutralization method is very similar to the co-precipitation method used to synthesize hydrotalcite (Mg₆Al₂(OH)₁₆CO₃-4H₂O). This study looks at the effect of temperature on the type of precipitates that form from the seawater neutralization process of Bayer liquor. The Bayer precipitates have been characterized by a variety of techniques, including X-ray diffraction (XRD), Raman spectroscopy, and infrared spectroscopy. The mineralogical composition of Bayer precipitates largely includes hydrotalcite, hydromagnesite, and calcium carbonate species. Analysis with XRD determined that Bayer hydrotalcites that are synthesized at 55 °C have a larger interlayer distance, indicating that more anions are removed from Bayer liquor. Vibrational spectroscopic techniques have identified an increase in hydrogen bond strength for precipitates formed at 55 °C, suggesting the formation of a more stable Bayer hydrotalcite. Raman spectroscopy identified the intercalation of sulfate and carbonate anions into Bayer hydrotalcites using these synthesis conditions.

Near-infrared (NIR) spectroscopy and chemometrics were applied to analyze the degradation mechanism of hardwood following hydrothermal treatment. NIR spectra, chemical composition, oven-dried density, equilibrium moisture content, compressive Young's modulus parallel to grain, and cellulose crystallinity of artificially degraded beech as an analogue of archaeological wood were systematically measured. Partial least squares (PLS) regression analysis was employed to predict compressive Young's modulus using NIR spectra and various properties as independent variables. Results are also compared with previous data obtained from similar treatment of softwood (Hinoki cypress). The increase in cellulose crystallinity of hardwood during the initial stage of hydrothermal treatment (up to 5 hours) was correlated with an improvement in the mechanical properties of wood. Young's modulus for both hardwood and softwood showed a gradual decrease over five hours of hydrothermal treatment, which is proposed to be due to the degradation of polysaccharide.

The impact of kidney stone disease is significant worldwide, yet methods for quantifying stone components remain limited. A new approach requiring minimal sample preparation for the quantitative analysis of kidney stone components has been investigated utilizing attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FT-IR). Calcium oxalate monohydrate (COM) and hydroxylapatite (HAP), two of the most common constituents of urinary stones, were used for quantitative analysis. Calibration curves were constructed using integrated band intensities of four infrared absorptions versus concentration (weight %). The correlation coefficients of the calibration curves range from 0.997 to 0.93. The limits of detection range from 0.07 ± 0.02% COM/HAP where COM is the analyte and HAP is the matrix, to 0.26 ± 0.07% HAP/COM where HAP is the analyte and COM is the matrix. This study shows that linear calibration curves can be generated for the quantitative analysis of stone mixtures provided the system is well understood especially with respect to particle size.

The spectral properties of the SH2 and active site-directed sequences of the bivalent Src kinase inhibitor Ac-EELL(F₅)Phe-(GABA)₃-pYEEIE-amide (1) have been determined. Ac-pYEEIE-amide (2) and Ac-EELL(F₅)Phe-amide (3), as well as the amino acids phosphotyrosine (pTyr) and pentafluorophenylalanine (F₅)Phe, have been characterized by electronic absorption, fluorescence, and vibrational spectroscopy. Specific and unique marker bands that originate from the phosphate group of pTyr and the fluorinated aromatic ring of (F₅)Phe have been identified, with the latter showing some solvent dependence. Peptide 2 was found to have excitation and emission wavelengths emanating from pTyr at 268 and 295 nm, respectively, whereas peptide 3 displayed excitation and emission peaks attributable to (F₅)Phe at 274 and 315 nm, respectively. Fourier transform infrared (FT-IR) analysis of the amino acid pTyr identified distinct marker bands at approximately 930, 1090, and 1330 cm⁻¹ that could be attributed to the phosphate group. These markers were also observed in the IR spectrum of peptide 2. Likewise, peptide 3 displayed a characteristic C–F stretching mode at 961 cm⁻¹ due to the presence of (F₅)Phe, including two C–F reporting ring modes at 1509 and 1527 cm⁻¹. Identifying and monitoring spectroscopic changes in these marker bands may afford a means to observe the molecular interactions that occur when peptides 1–3 bind to the Src kinase.

A dispersive vibrational circular dichroism (VCD) instrument has been designed and optimized for the measurement of mid-infrared (MIR) bands such as the amide I and amide II vibrational modes of peptides and proteins. The major design considerations were to construct a compact VCD instrument for biological molecules, to increase signal-to-noise (S/N) ratio, to simultaneously collect and digitize the sample transmission and polarization modulation signals, and to digitally ratio them to yield a VCD spectrum. These were realized by assembling new components using design factors adapted from previous VCD instruments. A collection of spectra for peptides and proteins having different dominant secondary structures (alpha-helix, beta-sheet, and random coil) measured for identical samples under the same conditions showed that the new instrument had substantially improved S/N as compared with our previous dispersive VCD instrument These instruments both provide protein VCD for the amide I that are comparable to or somewhat better than those measurable with commercial Fourier transform (FT) VCD instruments if just the amide I band in the spectra is obtained at modest resolution (8 cm⁻¹) with the same total data collection time on each type of instrument.

In this paper, we present a method to estimate the power spectral distribution of a source from input data acquired by an interferometric-based spectrometer. Our spectrometer shows distortions in the fringe pattern and a lack of data, making it impossible to apply the Fourier transform approach, which is the gold standard as a spectral recovery method for interferometric spectrometers. We combined linear inverse problem solving and iterative methods instead, considering that each detector of the spectrometer has a specific and known spectral response. Iterative methods are used to overcome problems caused by lack of input data. We show that a good spectral estimation of relatively simple spectra having a resolution of 400 points is typically achieved using fewer than 10 detectors with such a method. Since the quality of spectral restitution with such an approach relies both on signal processing and an optimal selection of the detector's spectral responses, the paper also shows that some sets of spectral responses selected for the detection and consequently a spectral repartition of the detectors are more successful than others in the spectral recovery process. We chose the condition number of the inversion matrix as an optimization criterion and evaluated how this criterion can be used within this framework. We found that maximizing it achieves better spectral restitution, within a range where noise remains low.

Overlapped bands often appear in applications of infrared spectroscopy, for instance in the analysis of the amide I band of proteins. Fourier self-deconvolution (FSD) is a popular band-narrowing mathematical method, allowing for the resolution of overlapped bands. The filter function used in FSD plays a significant role in the factor by which the deconvolved bands are actually narrowed (the effective narrowing), as well as in the final signal-to-noise degradation induced by FSD. Moreover, the filter function determines, to a good extent, the band-shape of the deconvolved bands. For instance, the intensity of the harmful side-lobule oscillations that appear in over-deconvolution depends importantly on the filter function used. In the present paper we characterized the resulting band shape, effective narrowing, and signal-to-noise degradation in infra-, self-, and over-deconvolution conditions for several filter functions: Triangle, Bessel, Hanning, Gaussian, Sinc², and Triangle². We also introduced and characterized new filters based on the modification of the Blackmann filter. Our conclusion is that the Bessel filter (in infra-, self-, and mild over-deconvolution), the newly introduced BL3 filter (in self- and mild/moderate over-deconvolution), and the Gaussian filter (in moderate/strong over-deconvolution) are the most suitable filter functions to be used in FSD.

Maintaining multivariate calibrations is essential and involves keeping models developed on an instrument applicable to predicting new samples over time. Sometimes a primary instrument model is needed to predict samples measured on secondary instruments. This situation is referred to as calibration transfer. This paper reports on using a Tikhonov regularization (TR) based method in both cases. A distinction of the TR design for calibration maintenance and transfer is a defined weighting scheme for a small set of new (transfer or standardization) samples augmented to the full set of calibration samples. Because straight application of basic TR theory is not always possible with calibration maintenance and transfer, this paper develops a generic solution to always enable application of TR. Harmonious (bias/variance tradeoff) and parsimonious (effective rank) considerations for TR are compared with the same TR format applied to partial least squares (PLS), showing that both approaches are viable solutions to the calibration maintenance and transfer problems.

A similarity measure was developed that can differentiate between two-dimensional fluorescent spectra based on their similarities and differences. The two-dimensional fluorescent spectra are digitalized into matrices. The difference between the two spectra is defined by a difference matrix, whose elements contain the difference of one two-dimensional fluorescent spectrum minus the other. The similarity measure is transformed into hypothesis tests of the similarity and difference between the two spectra. The scalar mean of the difference matrix is used as the statistical variable for the hypothesis test. The Bayesian prior odds ratio was estimated from multiple spectra of the same reference sample. A threshold for the hypothesis test that the spectra are different is proposed. The posterior odds ratio was used to quantify the similarity measure of the two spectra. Two-dimensional fluorescent spectra of Changyu red wine samples were used to demonstrate this method. The results show that this new method can detect differences between the spectra.

Experimental results for photothermal lens measurements are compared to finite elemental analysis models for commercial colored glass filters. Finite elemental analysis software is used to model the photothermal effect by simulating the coupling of heat both within the sample and out to the surroundings. Modeling shows that heat transfer between the glass surface and the air coupling fluid has a significant effect on the predicted time-dependent photothermal lens signals. For comparison with experimental signals, a simple equation based on the finite element analysis result is proposed for accounting for the variance of experimental data where this type of heat coupling situation occurs. The colored glass filters are found to have positive thermo-optical coefficients. The net positive dn/dT of CdS_xSe_1–x doped glass filters is considered to be the consequence of counteracting factors: optical nonlinearity, stress-induced birefringence, and the structural network of glass. Finite element analysis modeling results are also used to correlate experimental measurements of different sample geometries. In particular, the glass samples are compared to ethanol solutions of iron (II) dicylopentadiene in a sample cuvette even though heat transfer is different for these two samples.

In the present work, an argon microwave (2.45 GHz) plasma flame created at the end of a surface-wave-sustained discharge column in a helium environment has been experimentally studied. This is a plasma with new possibilities because under some experimental conditions it expands, being less contracted than the plasma flame created in open air. The new expanded discharge could offer additional advantages for applications in which larger extensions of plasma were required. The expansion phenomenon of this plasma flame was studied under different experimental conditions. In every case, the characteristic parameters of this expanded plasma such as electron density, electron and gas temperatures, or density population of excited atomic levels were measured by using optical emission spectroscopic techniques. From these results, the main advantages of this plasma source were pointed out.

Particle identification is an important analytical procedure for quality control and assurance in the biopharmaceutical industry. Rapid and reliable identification of micro-particles helps in evaluating the nature of particle contamination and its consequences on the product quality regulated by internal and external standards. Raman microscopy is one of the microspectroscopic techniques that can be used to identify micro-particles with the advantage of in situ detection. In this paper we demonstrate that a visible laser Raman microscope was particularly useful to identify micro-particles that were inside glass containers such as glass syringes, vials, and test tubes, which are commonly used as containers for aqueous formulated drugs. The examples include the identifications of a droplet-like particle inside a pre-filled glass syringe, a fibrous particle inside a glass test tube, and a white particle inside a glass vial; all of these examples usually demand challenging or time-consuming sample manipulation for other techniques. The Raman microscopic technique was shown to be able to solve these challenging micro-particle identifications due to its ability to carry out detection in situ. Particularly in the example of micro-droplet identification, the Raman microscopic technique was the only choice for a fast and successful particle detection. For all three identifications, Raman in situ detection has significantly accelerated particle analysis and avoided potential sample secondary contamination or losses owing to none or minimal sample manipulation.