Applied Spectroscopy

A coaxial arrangement of a ring-shaped ceramic magnet and a disk-shaped rare-earth magnet is used to trap plasma electrons in circular orbits near the surface of the planar cathode. This configuration results in the formation of a ring-shaped plasma which is stable at all pressures in the range 0.0007 Torr to over 10 Torr. A model is presented which describes the sensitivities of plasma emission intensities to the discharge current for atoms and ions of both the plasma gas and the sputtered flux of cathodic material. The model is based on the number of electron collisions needed to generate the radiating species. Experimental data for the species Ar, Ar⁺, Cu, and Cu⁺ are compared to the model.

We present a noncontact temperature-sensing method for microdroplets of water, which relies on temperature dependence of the OH stretching band of the spontaneous Raman spectrum. Temperatures of freely moving evaporating droplets with mean particle diameters of approximately 35 µm have been measured with an accuracy of ±1°C in the range from 10 to 65°C. The method allows the observation of droplet cooling during the initial unsteady phase of evaporation.

Evolving factor analysis (EFA) is a new approach to the resolution of individual component spectra on the basis of fluorescence lifetime differences. Phase-resolved fluorescence spectra collected at several excitation modulation frequencies are well suited to EFA resolution because the frequency-response curve of phase-resolved intensity for each lifetime component has a single, unique maximum. Unlike other self-modeling spectral matrix resolution methods, EFA has no fundamental limitation upon the number of components that can be resolved. In this work, the resolution of two-component and three-component mixtures of polycyclic aromatic hydrocarbons is demonstrated.

This paper presents an application of locally weighted regression (LWR) in diffuse near-infrared transmittance spectroscopy. The data are from beef and pork samples. The LWR method is based on the idea that a nonlinearity can be approximated by local linear equations. Different weight functions (for the samples) as well as different distance measures for “closeness” are tested. The LWR is compared to principal component regression and partial least-squares regression. The LWR with weighted principal components is shown to give the best results. The improvements with respect to linear regression are up to 15% of the prediction errors.

We studied the absorption spectrum of 7-aminocephalosporanic acid between 312 and 238 nm (32,000–42,000 cm⁻¹) at different pH values. The spectrum was deconvoluted into log-normal curves that showed three different transitions; the first two bands are due to π-π* transitions between the HOMO and LUMO, and from the HOMO to the LUMO + 1, respectively. We also recorded the spectrum of the acid in dioxane/water mixtures and found band I to shift to shorter wavelengths with an increase in the medium polarity. Theoretical calculations allowed us to ascribe this behavior to the fact that the β-lactam nitrogen atom possesses a high electron density in the ground state. As the medium polarity increases, so does the stability of this state, which results in the band being shifted to higher energy values. The theoretical calculations (MNDO) performed yielded 36.8 and 39.8 kK (1 kK = 10³ cm⁻¹) for the first two electron transitions, i.e., very close to the experimental values (∼36 and 39 kK).

Fourier transform infrared (FT-IR) data of pure and mixture samples of benzene and nitrobenzene are used to investigate and improve methods for interferogram-based qualitative analyses. For use in dedicated monitoring applications, the methodology employed is based on the application of pattern recognition analysis to short, digitally filtered interferogram segments. In the work described here, the impact of the interferogram data sampling rate on the analysis is studied. The results of this study indicate that optimal pattern recognition prediction performance is achieved by use of linear discriminants developed from faster sampled interferogram data. These findings suggest that improved performance can be obtained in FT-IR monitoring applications through the use of spectrometer designs based on a decreased interferogram scan length, coupled with faster sampling electronics.

Raman microspectroscopy can be used effectively to study very small samples or to study small areas within a transparent sample. With the application of the technique of confocal microscopy to a Raman microscope, the depth resolution of the instrument can be enhanced considerably. Confocal microscopy uses a pinhole, placed in the back image plane of the microscope objective, to block light from outside the focal plane. In this way the signal from the small volume element one wants to study can be better separated from the signals arising from the surrounding material. In this paper we show that the performance of the confocal Raman microscope can be described satisfactorily by geometrical optics. Furthermore, we have performed measurements to determine the depth resolution of our system for different combinations of objectives and pinholes. Finally, we report on the applications of this technique to different polymer systems, such as multilayer foils, fibers, and fiber composites.

The polybutylene terephthalate (PBT) concentration and intrinsic viscosity of polycarbonate (PC) polymer blends were determined by means of near-infrared (NIR) reflectance and absorption spectrometry coupled with multiple regression analysis. The cross-search method was used in the selection of analytical wavelengths, whereas the correctness of the determinations was confirmed by cross validation. The correlation coefficients between the reference and NIR-determined values for the reflectance and absorption techniques were 0.979 and 0.955, respectively. The apparent number of orthogonal degrees of freedom in the polymer standards was affected by the number of samples employed in the NIR regression analysis.

An infrared (IR) emission cell which is capable of operation up to 1500°C is described. The cell is based on an atomic absorption graphite furnace and is coupled to a Fourier transform infrared spectrometer. The spectrometer has been used to measure the emission spectrum of quartz from 200 to 1400°C, and the changes in the spectrum occurring with temperature can be related to the formation of cristobalite; transitions between low and high forms (alpha and beta forms) can also be monitored. Aragonite has also been analyzed through the temperature range 100 to 600°C, and the aragonite/calcite transition is clearly evident. The transformation of kaolinite to metakaolinite and through to mullite and cristobalite has also been studied with this in situ technique. The formation of mullite is evident in the spectrum at temperatures as low as 900°C, and the formation of cristobalite is clearly seen at 1200°C.

This paper describes the optical analysis, construction, testing, and application of a cell for measuring in situ IR reflection spectra in the C-H stretching region of monolayer films of long-chain alkanethiolates on evaporated Au films beneath a thin overlayer (∼1 µm) of aqueous solution. The cell facilitates the interchange of sample (monolayer-coated) and reference substrates—a major improvement over previous designs. This allows the acquisition of an IR reflection spectrum of a monolayer without the application of electrochemical or polarization-modulation methods. The optical properties of the cell are analyzed within the context of classical electromagnetic theory; the effects of angle of incidence, solution layer thickness, and solution composition (H₂O and D₂O) and the refractive indices of the IR-transparent window material on the observed spectrum are specifically examined. Calculations of both the mean-square electric fields and the reflection spectra for a monolayer of octadecanethiolate on Au are presented. From these results, conditions to achieve high surface detectability are identified, and optically induced alterations in the intensities and shapes of the monolayer spectrum are delineated. Spectra for the same monolayer are measured to test these predictions as well as to assess the practical limitations of the experimental technique.

By the coupling of thermogravimetry (TG) and Fourier transform infrared spectroscopy (FT-IR), the thermal behavior of polymeric materials can be elucidated from their mass loss combined with characterization of the evolved components. A hyphenated TG/FT-IR technique is presented, which enables the direct detection and characterization of evolving components in TG experiments, i.e., without transferring these components from the TG to the FT-IR equipment. In this on-the-spot TG/FT-IR technique, the IR beam of the FT-IR spectrometer is led directly into the TG equipment. The IR beam is reflected by a mirror mounted inside the TG equipment and subsequently detected by a standard FT-IR detector. In this way, the evolved components are detected both in gas and condensed phase directly in the TG equipment. The detection limits are in the sub µg/s range. The experimental setup and conditions are presented together with advantages and applications in the field of polymeric research.

The thermal degradation of calcium oxalate monohydrate is studied by using diffuse reflectance Fourier transform infrared spectroscopy combined with quadrupole mass spectrometry (DRIFTS/MS). DRIFTS/MS results are correlated with thermogravimetric analyses to verify the thermal properties of samples studied. Infrared spectral evidence for CaC₂O₄H₂O dehydration and infrared spectra for three different CaC₂O₄ phases are given. In addition, the significance of a band at 1085 cm⁻¹ assigned to CO₂²⁻ which appears during the decomposition of CaC₂O₄ to CaCO₃ is discussed.

A dual-beam FT-IR instrument with optical null has been developed for the study of polymers adsorbed on low-surface-area solids. By the use of narrow-bandpass filters coupled with the dual-beam FT-IR, a sensitivity of 2 × 10⁻⁵ absorption units per scan has been achieved. Transmission infrared spectra are shown for poly(ethylene oxide-b-styrene) and didodecyl dimethyl ammonium bromide (DODAB) adsorbed on silicon.

Fluorometric detection of analytes using batch injection analysis (BIA) has been investigated. BIA involves the injection of microliter samples toward a nearby detector which is immersed in a large-volume, nonflowing, blank solution. The characteristics and advantages of employing fiber-optic fluorometric detection for BIA are described. Similar to analogous flow injection measurements, batch injection fluorometric analysis offers high speed, reproducibility and simplicity, while eliminating the need for pumps, valves, and associated tubings. With injection rates at 120–500 samples/h, there is no observable carryover, and the precision is typically 2–3% (RSD).

The Spectropus™ system,‡ a versatile optical sampling system for performing infrared spectroscopic analyses on remotely located samples with little regard to their size and with spectral quality and sampling rates comparable to, or better than, those achievable with spectrometer sample compartment accessories, is described. The Spectropus™ uses a “glass-tube beam-line” optical transfer system for delivering the collimated beam of an FT-IR spectrometer to one of several dedicated accessories, complete with its own detector, that may be located several meters distance from the spectrometer. The accessories include the barrel ellipsoid diffuse reflectance and emission accessory, the grazing-incidence external reflection accessory, and the gas analysis accessory. One system described here supports up to four accessories in laboratory air and another supports two accessories in two different glove boxes. Application examples include the monitoring of the formation of LiOH and LiOD on LiH and LiD in glove boxes as a result of moisture corrosion; the analysis of ink stains on paper; the analysis of inks, parylene films, and oil stains on metals; the analysis, with monolayer resolution, of oxides on aluminum foil and on uranium being oxidized in air; the analysis of wood and of graphite-epoxy composites, including the rapid sequential determination of diffuse reflectance and emission spectra from one point on one sample; and sub-part-per-million gas analyses using an evacuated cell reference spectrum that is free of pressurization stress artifacts.

New fiber-optic instrumentation for phosphorescence-lifetime determinations and for steady-state phosphorescence measurements is shown to perform comparably to conventional instrumentation. The importance of optimal glass formation in the fiber-optic sample cell is considered in some detail. Detection limits for acetophenone, benzophenone, and p-nitrophenol are reported, and phosphorescence lifetimes are measured for several compounds, with values ranging from 2.5 ms to 4.7 s.

The variable-exit-angle ultrasoft x-ray fluorescence technique has been evaluated for Ni/Fe thin-film multilayers. The theoretical and experimental agreement is reasonable. The experimental results demonstrate that on these Ni/Fe samples quantitative information can be obtained in the presence of surface contamination.

A computerized method is described for correcting systematic distortion of images from slab gel electrophoresis. Such distortions can lead to misinterpretation of the information contained in the gel. The method is useful for data analysis in one-dimension slab gel electrophoresis where the information is manifested in rectangular shaped bands, such as conventional restriction digest or sequencing gels, and the distortions can be adequately described by continuous low-order polynomial functions. The purpose is to eliminate human skill and judgement from the process and to minimize human interaction, which would be useful in any future attempts to automate the analysis of electrophoretic gels.

A new silver-coated alumina/glass substrate was prepared by a chemical reduction method at room temperature. The substrate was found to exhibit strong surface-enhanced scatterings for crystal violet (CV), p-nitrophenol (PNP), p-nitrobenzoic acid (PNBA), and pyrene. Optimization of silver deposition time was achieved by using CV as an analyte. Lower limits of detection were determined for these compounds to demonstrate the analytical potential of the new substrate. Enhancement factors of ∼10⁶ and ∼10⁷ were determined from comparisons of the surface-enhanced Raman scattering (SERS) intensities of mono-molecular layers with the normal Raman intensities for PNP and PNBS, respectively. Three different methods of sample applications were adapted and tested. The reusability of the substrates was tested by recording the surface-enhanced resonance Raman scattering (SERRS) spectra of CV at different conditions.

An analytical application for Surface-Enhanced Raman Spectroscopy at a silver electrode is described. Real-time SER spectra of adenine and cytosine have been recorded in a 10-µL spectroelectrochemical flow cell under flowing conditions. Charge-coupled-device detection allowed high-quality spectra spanning a ∼1200 cm⁻¹ region to be recorded with integration times of 4 seconds. A low-power He Ne laser was used as a source. SERS at the silver electrode offers rapid time response to adsorption/desorption by appropriate potential modulation. The technique is extremely reproducible and insensitive to temperature and flow rate. The effects of incident photon energy and applied potential on the intensity of the Raman signal are discussed.

The infrared spectra of blends of various ortho-phthalate esters with polyvinyl chloride) and vinylidene chloride/(20%)acrylonitrile copolymer were recorded. The data indicate that the lowering of the ester carbonyl absorption frequency is due to a dipole-dipole/complex type interaction between the carbonyl of the ester and the chlorine of the polymer in addition to hydrogen bonding with acidic hydrogens. The lowering of the absorption frequence increases with an increase in the amount of chlorine in the polymer chain.

A polarization-modulation technique has been combined with Fourier transform infrared (FT-IR) spectrometry to eliminate the large background radiation which interferes with the infrared emission from thin organic films on metal surfaces. A step scan and a continuous scan interferometer which were combined with a photoelastic modulator were also studied, and the step scan system was found to be stabler and have a better S/N. The spectrum of a 12-nm perfluoropolyether film was obtained at 120°C by this system.

An analytical technique for aerosol samples which utilizes the resonance Raman effect is described. The aerosol particles were generated at 50 kHz by a vibrating orifice. The nominal particle size was 45 µm in diameter. The visible lines (4579, 4765, 4880, and 5145 Å) of a continuous argon-ion laser were used as the excitation source. Within the coverage of the laser wavelengths, effects of pre-resonance, resonance, and post-resonance Raman scattering were studied. Under the resonance condition, the enhancement of p-nitrosodimethylaniline is 5.5 × 10⁴ when compared to that of nitrate ions. The corresponding concentration of detection limit is 10⁻⁶ molar.

A kilowatt-plus microwave-induced plasma (KiP-MIP) system was modified for better analytical performance. Modifications included the use of a redesigned plasma torch and a new plasma resonator cavity. The use of the redesigned plasma torch improved the detection of aqueous chloride by 2 orders of magnitude. With the redesigned plasma torch, the performance of the KiP-MIP utilizing the original 3-cm-depth and 2-cm-depth resonator cavities were studied. Results indicated that the KiP-MIP with the 2-cm-depth resonator cavity exhibited better characteristics in terms of plasma energy coupling, excitation characteristics, and determination of aqueous chloride.

Steady-state and frequency-domain fluorescence spectroscopy are used to study the inclusion complexation of six anilinonaphthalene sulfonates (ANSs) with silica-immobilized beta-cyclodextrin (βCD). To fit the experimental data, we compared several modified Lorentzian lifetime distribution models. However, in all cases studied, a simple unimodal Lorentzian distribution best fit the fluorescence decay kinetics. The recovered distribution shapes did not change substantially as the surface was titrated with ANS; however, subtle trends suggest that silica-immobilized βCDs in more hydrophobic environments complex first. Furthermore, the recovered distribution widths (a measure of the environmental heterogeneity) for the silica-immobilized βCD-ANS complexes are always broader than those for ANS-βCD complexes in bulk solution.