Applied Spectroscopy

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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 of interest to readers of Applied Spectroscopy, contact David J. Butcher, Applied Spectroscopy News Editor, 340C Stillwell Science Building, 246 Central Drive, Western Carolina University, Cullowhee, NC 28723, Ph: (828) 227–7646, email: david.butcher@analytchem.org.

Cadmium selenide/zinc sulfide (CdSe/ZnS) core-shell quantum dots (QDs) embedded in biocompatible materials were thermally and optically characterized with a thermal lens (TL) technique. Transient TL measurements were performed with a mode-mismatched, dual-beam (excitation and probe) configuration. A thermo-optical study of the CdSe/ZnS QDs was performed for different core diameters (3.5, 4.0, 5.2, and 6.6 nm) in aqueous solution and synthetic saliva, and three different core diameters (2.4, 2.9, and 4.1 nm) embedded in restorative dental resin (0.025% by mass). The thermal diffusivity results are characteristic of the biocompatible matrices. The radiative quantum efficiencies for aqueous solution and biofluid materials are dependent on the core size of the CdSe/ZnS core-shell QDs. The results obtained from the fluorescence spectral measurements for the biocompatible materials support the TL results.

This work presents a novel method for obtaining surface infrared spectra with sub-second time resolution during atomic layer deposition (ALD). Using a rapid-scan Fourier transform infrared (FT-IR) spectrometer, we obtain a series of synchronized interferograms (120 ms) during multiple ALD cycles to observe the dynamics of an average ALD cycle. We use a buried metal layer (BML) substrate to enhance absorption by the surface species. The surface selection rules of the BML allow us to determine the contribution from the substrate surface as opposed to that from gas-phase molecules and species adsorbed at the windows. In addition, we use simulation to examine the origins of increased reflectivity associated with phonon absorption by the oxide layers. The simulations are also used to determine the decay in enhancement by the buried metal layer substrate as the oxide layer grows during the experiment. These calculations are used to estimate the optimal number of ALD cycles for our experimental method.

We measured the photothermal lens signal in samples exhibiting high turbidity using a pump-probe scheme. We show that the photothermal lens signal properties remain nearly unchanged up to values of turbidity of 6 cm⁻¹ despite the signal reduction due to the decrease of excitation power associated to turbidity losses. The signal starts decreasing abruptly for values of turbidity larger than 6 cm⁻¹. Multiple light scattering yields a reduction of the temperature gradients' which results in a decrease of the effective signal. However, the signal-to-noise ratio remains above 50 for turbidity values of 9 cm⁻¹, which corresponds to a reduction of light transmission by more than four orders of magnitude. We report on the detection of the photothermal lens signal through a 2 mm layer of organic tissue with a signal-to-noise ratio of about 500. This technique appears promising for imaging applications in organic samples, which usually exhibit high turbidity for visible and near-infrared light.

The prevalence of optical spectroscopy techniques being applied to the online analysis of continuous processes has increased in the past couple of decades. The ability to continuously “watch” changing stream compositions as operating conditions change has proven invaluable to pilot and world-scale manufacturing in the chemical and petrochemical industries. Presented here is an application requiring continuous monitoring of parts per million (ppm) by weight levels of hydrogen chloride (HCl), water (H₂O), and carbon dioxide (CO₂) in two gas-phase streams, one nitrogen-rich and one ethylene-rich. Because ethylene has strong mid-infrared (IR) absorption, building an IR method capable of quantifying HCl, H₂O, and CO₂ posed some challenges. A long-path (5.11m) Fourier transform infrared (FT-IR) spectrometer was used in the mid-infrared region between 1800 and 5000 cm⁻¹, with a 1 cm⁻¹ resolution and a 10 s spectral update time. Sample cell temperature and pressure were controlled and measured to minimize measurement variability. Models using a modified classical least squares method were developed and validated first in the laboratory and then using the process stream. Analytical models and process sampling conditions were adjusted to minimize interference of ethylene in the ethylene-rich stream. The predictive capabilities of the measurements were ±0.5 ppm for CO₂ in either stream; ±1.1 and ±1.3 ppm for H₂O in the nitrogen-rich and ethylene-rich streams, respectively; and ±1.0 and ±2.4 ppm for HCl in the nitrogen-rich and ethylene-rich streams, respectively. Continuous operation of the instrument in the process stream was demonstrated using an automated stream switching sample system set to 10 min intervals. Response time for all components of interest was sufficient to acquire representative stream composition data. This setup provides useful insight into the process for troubleshooting and optimizing plant operating conditions.

A simple, accurate, rapid, and sensitive spectrofluorimetric method for the determination of ofloxacin in urine was developed by means of first derivative matrix isopotential synchronous fluorescence spectrometry (MISF). The calibration curve was found to be linear in the concentration range 40–320 ng/mL. The method allows the determination of compound in samples with unknown background fluorescence without the need for tedious pre-separation. Synchronous scans are performed along a trajectory that connects points of identical intensity in a three-dimensional fluorescence spectrum. The unknown analytical signal of the urine is suppressed from the MISF spectrum by calculating its first derivative at λ_ex = 319.2 and λ_em = 465 nm. In order to ensure maximum sensitivity and adequate selectivity, the experimental variables affecting fluorescence intensity were studied in the ofloxacin band centered at λ_ex = 333 and λ_em = 460 nm. As result, the determination was performed in a water medium at pH 7.2, adjusted by using sodium dihydrogen phosphate as a buffer solution. Calibration graphs were subjected to a comprehensive statistical analysis. The detection limit according to Long and Winefordner was 8.4 ng/mL, and the detection limit proposed by Clayton was 13.9 ng/mL.

This paper presents studies on the pulsed optogalvanic effect and isotope-selective excitation of Yb 555.648 nm (0 cm⁻¹ → 17 992.007 cm⁻¹) and 581.067 nm (17 992.007 cm⁻¹ → 35 196.98 cm⁻¹) transitions, in a Yb/Ne hollow cathode lamp. The Yb atoms were excited by narrow linewidth (500–1000 MHz) Rh110 and Rh6G dye based pulsed lasers. Optogalvanic signal inversion for ground state transition at 555.648 nm was observed beyond a hollow cathode discharge current of 8.5 mA, in contrast to normal optogalvanic signal at 581.067 nm up to maximum current of 14 mA. The isotope-selective excitation studies of Yb were carried out by recording Doppler limited optogalvanic signals as a function of dye laser wavelength. For the 581.067 nm transition, three even isotopes, ¹⁷²Yb, ¹⁷⁴Yb, and ¹⁷⁶Yb, and one odd isotope, ¹⁷¹Yb, were clearly resolved. These data were compared with selective isotope excitation by 10 MHz linewidth continuous-wave dye laser. For 555.648 nm transition, isotopes were not clearly resolved, although isotope peaks of low modulation were observed.

Phenylhydrazone derivatives of 2,6-diarylpiperidin-4-ones viz., 3-methyl-2,6-diphenylpiperidin-4-one (PMP-PH) and 3-methyl-2,6-di-(9H-fluorenyl)piperidin-4-one (FMP-PH) when illuminated and protonated perform the functions of AND, OR, NOR, NAND Boolean logic gates with all optical outputs. The functions of the AND and the NAND gates rely on changes in absorption and emission of the phenylhydrazone upon isomerization of the photochromic unit. We report novel heterocyclic compounds where setting up of threshold absorption or emission can be made as a logic device.

Uranium ore concentrates (UOCs) are produced at mining facilities from the various types of uranium-bearing ores using several processes that can include different reagents, separation procedures, and drying conditions. The final UOC products can consist of different uranium species, which are important to identify to trace interdicted samples back to their origins. Color has been used as a simple indicator; however, visual determination is subjective and no chemical information is provided. In this work, we report the application of near-infrared (NIR) spectroscopy as a non-contact, non-destructive method to rapidly analyze UOC materials for species and/or process information. Diffuse reflectance spectra from 350 to 2500 nm were measured from a number UOC samples that were also characterized by X-ray diffraction. Combination and overtone bands were used to identify the amine and hydroxyl-containing species, such as ammonium uranates or ammonium uranyl carbonate, while other uranium oxide species (e.g., uranium trioxide [UO₃] and triuranium octoxide [U₃O₈]) exhibit absorption bands arising from crystal field effects and electronic transitions. Principal component analysis was used to classify the different UOC materials.

Grating couplers for planar silver halide waveguides were designed and fabricated by using focused ion beam (FIB) milling technology, facilitating coupling of mid-infrared radiation from quantum cascade lasers into thin-film waveguide structures. An optimized rectangular grating structure for an emitted wavelength of 10.4 μm, with a grating constant of 16.4 μm was integrated into a silver halide waveguide substrate via an optimized FIB fabrication procedure. Efficient uncoupling and radiation propagation through the waveguide was confirmed by analyzing droplets of acetic acid at different concentrations, deposited at the waveguide surface via evanescent field absorption spectroscopy.

The adulteration and traceability of olive oils are serious problems in the olive oil industry. In this work, a method based on laser-induced breakdown spectroscopy (LIBS) and neural networks (NNs) has been developed and applied to the identification, quality control, traceability, and adulteration detection of extra virgin olive oils. Instant identification of the samples is achieved using a spectral library, which was obtained by analysis of representative samples using a single laser pulse and treatment by NNs. The samples used in this study belong to four countries. The study also included different regions of each country. The results obtained allow the identification of the oils tested with a certainty of more than 95%. Single-shot measurements were enough for clear identification of the samples. The method can be developed for automatic real-time, fast, reliable, and robust measurements, and the system can be packed into portable form for non-specialist users.

Raman chemical mapping was used to determine the distribution of magnesium stearate, a lubricant, on the surface of tablets. The lubrication was carried out via a punch-face lubrication system with different spraying rates applied on placebo and active-containing tablets. Principal component analysis was used for decomposing the matrix of Raman mapping spectra. Some of the loadings associated with minuscule variation in the data significantly overlap with the Raman spectrum of magnesium stearate in placebo tablets and allow for imaging the domains of magnesium stearate via corresponding scores. Despite the negligible variation accounted for by respective principal components, the score images seem reliable as demonstrated through thresholding the one-dimensional representation and the spectra of the hot pixels that show a weak but perceivable magnesium stearate band at 1295 cm⁻¹. The same approach was applied on the active formulation, but no magnesium stearate was identified, presumably due to overwhelming concentration and spectral contribution of the active pharmaceutical ingredient.

Simple molecules such as HCOOH, or formic acid, are suggested to have played important roles in planetary physics due to their possibility for high pressure and temperature chemistry under impact conditions. In this study, we have investigated the effect of pressure (up to 50 GPa) on H-bonding and reactivity of formic acid using synchrotron far infrared spectroscopy. Based on the pressure-induced changes to H-bond ν(O–H···O) stretching and γ(O–H···O) deformations, we observe significant reorganization of H-bonding network beginning at ∼20 GPa. This is in good agreement with reports of symmetrization of H-bonds reported at 16–21 GPa from X-ray diffraction and Raman spectroscopy studies as well as molecular dynamics simulations. With further increase in pressure, beyond 35 GPa, formic acid undergoes a polymerization process that is complete beyond 45 GPa. Remarkably, upon decompression, the polymeric phase reverts to the crystalline high-pressure phase at 8 GPa.

Spectral peak overlapping is a basic problem in analytical data processing of laser-induced breakdown spectroscopy (LIBS). Curve fitting is the typical method of resolving overlapped peaks. For preventing ambiguous fitting, appropriate initial values must be known. The aim of this work was to present a method that could be used to determine appropriate initial values of the curve-fitting method by using fractional differential theory. According to the variation of characteristic points of Lorentzian peaks at different fractional differential orders, parameter estimators were obtained that were used to calculate the initial values of the curve-fitting method. As it is a widely used optimization method, the Levenberg–Marquardt method was used in curve fitting. Simulation and LIBS experimental results proved that the proposed method of the initial value estimation can effectively resolve the overlapped peaks in LIBS data processing.

A spectral filtering approach for improving the sensitivity of two-color laser-induced incandescence measurements is proposed. The commonly used bandpass filters providing wavelength selection, and hence temperature sensitivity, are replaced by shortpass and longpass filters, respectively, allowing significantly higher signal intensities to be detected. This modification is of particular interest when nanoparticles with low emissivity, for instance, metal and metal oxide particles, are investigated. An example case in which the conventional optical components are compared with the new approach reveals an improvement by more than one order of magnitude.

The chemical form of phosphate phases in sewage sludge ash (SSA)-based fertilizers was determined by Raman microspectroscopy. Raman mapping with a lateral resolution of 5 × 5 μm² easily detected different compounds present in the fertilizers with the help of recorded reference spectra of pure substances. Quartz and aluminosilicates showed Raman bands in the range of 450–520 cm⁻¹. Phosphates with apatite structure and magnesium triphosphate were determined at around 960 and 980 cm⁻¹, respectively. Furthermore, calcium/magnesium pyrophosphates were detected in some samples.

Reactive nitrogen (Nr) is a term used to describe non-nitrogen gas (non-N₂) forms of nitrogen (N) in the biosphere. It causes major pollution problems when it occurs in excess, and it has many sources, including fertilizers used in production agriculture. Currently there is no on-the-go soil nitrate sensor that could guide the application of the optimal amount of fertilizer, which often varies significantly within a field. We report for the first time nitrate-in-soil measurements performed on moving soil samples at concentration levels relevant for fertilizer application. An infrared emission technique called transient infrared spectroscopy (TIRS) was tested on soil samples spiked with different nitrate concentrations in the parts-per-million range and moving at a velocity of 2.6 m/s (5.8 miles per hour) in the laboratory. The TIRS Fourier transform infrared (FT-IR) spectra were modeled by partial least squares and produced a standard error of cross-validation (SECV) of 6.3 parts per million (ppm) N and an R ² of 0.938 for 512-scan spectra. These results are compared to those using fewer TIRS scans and to those from photoacoustic spectroscopy (PAS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements on stationary samples. TIRS 128-, 32-, and 8-scan spectra yielded SECVs of 11.2, 11.4, and 18.4 ppm N and R ² values of 0.800, 0.831, and 0.583, respectively. The PAS and DRIFTS measurements produced SECVs of 12.4 and 9.0 ppm N and R ² values of 0.766 and 0.876, respectively.