Applied Spectroscopy

An acousto-optic diffraction device is mounted within a polychromator to perform spectral shifting for the purpose of background correction for atomic spectrometry. Controlling the applied frequency to the acousto-optic allows “on-line” and “off-line” spectral observations. Preliminary investigations included characterization of the behavior of the photomultiplier tube (PMT) signal output at various acoustic frequencies and applied powers to verify the applicability of the system. Low-rate (0.1 Hz) frequency modulation was performed to successfully test the applicability of the background-correction system.

The concepts of a double polarization modulation interferometer and the experimental measurements verifying these concepts are presented. This interferometer serves as an analytical tool for the characterization of polarization modulation devices. In addition, for polarization difference measurements this interferometer offers four times better efficiency over widely used amplitude-division interferometers.

This paper analyzes the influence of the departure from thermodynamic equilibrium on three spectroscopic methods for determining the excitation temperature: the classical Boltzmann plot method, the method of absolute intensity measurements, and the method of the ratio of atomic to ionic lines for one element. Use of these methods is illustrated with experiments conducted in a helium microwave plasma in nonthermodynamic equilibrium produced by an axial injection torch at atmospheric pressure. The excitation temperature was determined at a variable microwave power and different positions along the flame. The results are discussed in terms of the atomic-state distribution function (ASDF) calculated for the plasma.

Domoic acid, an amino acid neurotoxin, produces a single intense resonance Raman peak observed at 1652 cm⁻¹ from aqueous solution when excited at either 242 or 257 nm. The detection limits for domoic acid in water are found to be well within those concentrations determined to be representative of values in toxic phytoplankton cells. Examination of cells known to contain identified large amounts of domoic acid shows that domoic acid spectra are sufficiently intensely excited to allow detection in the presence of normal phytoplankton cell constituents within the cell. Single cells from species established as producers of domoic acid, cultured under controlled conditions favorable to the production of domoic acid, produce spectra consistent with the presence of domoic acid in low concentrations.

Previous Fourier transform infrared (FT-IR) spectroscopic studies on neoplastic and normal cells have shown different band profiles and intensity associated with absorptions of proteins and nucleic acids. In the present study, an interpretation of such differences has been attempted by comparing the spectra of DNA/RNA/protein mixtures with the spectra, particularly, obtained for lymphocytes from B-chronic lymphatic leukemia (B-CLL) patients and normal donors. FT-IR microspectroscopy analysis showed a good agreement between the intensity and the band profile of the spectra of leukemic lymphocytes and those of the binary mixture made up of 75% human serum albumin and 25% DNA. The addition of small amounts of RNA (1–5%) modified the band shape, making it more similar to the spectrum of normal lymphocytes. An attempt was also made to estimate the relative amounts of DNA and RNA. The results demonstrated an increase in the DNA/RNA ratio value in neoplastic lymphocytes with respect to that reported in literature for normal ones.

A method for the optical detection of the resonance ionization signal of mercury atoms in a buffer gas is described that is based on the emission from buffer gas atoms that are collisionally excited by interactions with electrons in a strong electric field. The first observations of this phenomenon are reported here, along with comparisons between optical and electrical detection. Advantages of a pulsed electric field over a continuous field are described. A wide range of possible applications for this type of gas phase detector are suggested.

Benzyl disulfide (BDS) can be adsorbed on HCl- or H₂SO₄-etched iron plate to form self-assembled film. The films were characterized by Fourier transform surface-enhanced Raman scattering spectroscopy. It is proposed that the bands in the range of 290 to 296 cm⁻¹in the spectra were due to the S–Fe bond. The disappearance of the S–S band, the large shifts of the C–S band, and the appearance of the Fe–S band from free BDS to the adsorbed molecule on the iron surface suggested that the BDS molecule was chemisorbed perpendicularly on the iron surface in the form of thiolate. The thiolates formed a film dotted with ferric hydroxide and silver cluster. The SERS effect of iron covered with BDS can be enhanced by the chemically deposited silver film, and the enhancing effect was related to the deposition time of silver.

Ion mobility spectrometry (IMS) has been successfully developed to yield an advanced portable instrument. However, the formation of pure or heterogeneous cluster ions introduces nonlinear variances into the data. Cluster ions may arise from the sample in addition, and competition to the standard anticipated product ions and may deleteriously affect quantitative determinations. The SIMPLISMA (simple-to-use interactive self-modeling mixture analysis) method is demonstrated for detecting and modeling these nonlinear variances in IMS data, which is especially useful when vapor mixtures are encountered. Furthermore, SIMPLISMA may assist in the resolution of overlapping peaks that are characteristic of low-resolution IMS drift tubes. The synergistic combination of IMS and SIMPLISMA is shown for the detection of heterogeneous cluster ions produced from vapor mixtures of 1-pentanol and 1-octanol.

Raman spectra of aluminum oxide, Al₂O₃, have been a subject of interest over the last two decades. The origin of the fluorescence background in the Raman spectra of Al₂O₃ is still a subject of discussion. It has been believed to be due to hydrocarbon, iron ion impurities, or hydroxy groups on the surface of Al₂O₃. In this paper, Raman spectra of Al₂O₃ are reported that were acquired with different excitation frequencies as well as different spectrometers and different temperatures. It is shown that, whereas at an excitation frequency of 1064 nm a strong fluorescence background is observed, a sample of Al₂O₃ heated at 1250 °C when excited with a 632.8-nm frequency exhibits strong and remarkable fluorescence ne structure. It is concluded that the origin of the fluorescence background in the Raman spectra of Al₂O₃ is iron, and Fe impurity, and the intensity of this fluorescence depends upon the structure of Al₂O₃ and the excitation frequency used.

Raman shifting to the near-infrared, when possible, provides a simple and economical alternative to the optical parametric oscillator (OPO) or difference frequency mixing approach. We report the production of 1.30- to 1.55-μm radiation from first Stokes Raman shifting in a single-pass, open (no capillary waveguide), hydrogen-filled Raman cell (constructed in-house) pumped with a Nd:YAG/dye laser combination operating near 900 nm. A maximum of 10 mJ (19% efficiency) of first Stokes energy was measured for the highest cell pressure (490 psia) and input pulse energy (53 mJ). The quality of the first Stokes output is similar to the dye laser output as indicated by polarization, shot-to-shot energy fluctuation, beam diameter, and linewidth. A characterization of the Stokes and anti-Stokes output was also conducted including one-dimensional spatial intensity profiles and output line dependence on input pulse energy and cell pressure. A large first Stokes conversion efficiency has been attributed to little production of higher-order Stokes and anti-Stokes lines.

A chemical method for separating diesel engine piston deposits into various organic and inorganic components is described. The components are subsequently analyzed by infrared spectroscopy. The method is illustrated with examples of deposits removed from the upper lands and upper ring grooves of pistons taken from heavy-duty diesel engine tests. Subtle chemical differences were observed between two deposits removed from upper lands of pistons taken from diesel engine tests which had been carried out with the use of lubricating oils with different formulations. More obvious differences were found between the deposits taken from the upper land areas and the upper ring grooves. These results indicate that a combined chemical and spectroscopic approach has the potential to provide detailed chemical information about engine deposits.

Near-infrared (near-IR) fluorescence has been used to develop a solid-phase immunoassay that detects trace amounts of human immunoglobulin (HuIgG). Various concentrations of HuIgG bound to a nitrocellulose surface were determined from the fluorescence generated by near-IR labeled goat anti-human antibody (GAHG) bound to the HuIgG. The GAHG was labeled with a heptamethine cyanine fluorophore that has spectral properties in the near-IR region (above 780 nm). These fluorophores are versatile because they can be modified for several bioanalytical applications. Fluorescence was detected with a near-IR fluorescence instrument previously developed in the laboratory. Two cyanine fluorophore labels were evaluated for the ability to selectively bind to GAHG on a nitrocellulose matrix with a minimal amount of background interference. After the most appropriate near-IR fluorophore was selected, the labeling of GAHG was optimized under aqueous conditions. The most effective GAHG–dye conjugates were used to develop an immunoassay to detect various concentrations of HuIgG. The results are presented, here. Solutions of HuIgG with concentrations as low as 10⁻¹⁰ molar have been detected with a minimum of interference.

The thickness dependence of the Berreman effect for naturally grown oxide films on chrome is analyzed theoretically and experimentally. The shift of the spectral position of the Berreman minimum can be described by the Fuchs–Kliewer theory of virtual modes. Both the absorption and the shift of the position can be used for thickness determination. The experimental results compared with calculated values based on different optical constants for Cr₂O₃ indicate their influence on the position and the absorption of the Berreeman minimum.

An experimental system to investigate the feasibility of using laser Raman spectroscopy to detect stack gas pollutants is described. With spontaneous Raman spectroscopy, one unit is capable of measuring all gas species simultaneously and, when applied to a continuous emissions monitoring (CEM) system, could both simplify it and improve reliability. Minimum detectabilities with the use of this weak light scattering process were determined for the three primary pollutants from coal-based power plants: carbon monoxide (CO), sulfur dioxide (SO₂), and nitric oxide (NO). A frequency-doubled, pulsed Nd:YAG laser was used to excite the test gas held in a sample cell. A charge-coupled device connected to a spectrometer was used as the detection system, providing complete spectral information. A set of experiments was carried out for different concentrations of the three test gas species. With a 200-s integration time, the following minimum detectability limits were obtained: 100 ppm for SO₂, 250 ppm for CO, and 250 ppm for NO. A mixture of all three pollutants in a base of N₂ was made to simulate typical stack gas mixtures. Baseline strength and associated shot noise are more severe when gas mixtures are being analyzed. One possible explanation is the broad-band fluorescence of NO₂, perhaps produced photolytically. System modifications to improve minimum detectabilities are discussed.

A spectroscopic near-infrared imaging system, using a focal plane array (FPA) detector, is presented for remote and on-line measurements on a macroscopic scale. On-line spectroscopic imaging requires high-speed sensors and short image processing steps. Therefore, the use of a focal plane array detector in combination with fast chemometric software is investigated. As these new spectroscopic imaging systems generate so much data, multivariate statistical techniques are needed to extract the important information from the multidimensional spectroscopic images. These techniques include principal component analysis (PCA) and linear discriminant analysis (LDA) for supervised classification of spectroscopic image data. Supervised classification is a tedious task in spectroscopic imaging, but a procedure is presented to facilitate this task and to provide more insight into and control over the composition of the datasets. The identification system is constructed, implemented, and tested for a real-world application of plastic identification in municipal solid waste.

A strategy for the infrared spectroscopic analysis of drug metabolites in high-pressure liquid chromatography (HPLC) effluent is presented with a primary focus on delivery of metabolite structure as opposed to development of dedicated or automated instrumentation. This paper stresses the complementary nature of on-line (solution phase) and off-line (condensed phase) HPLC Fourier transform infrared (FT-IR) implementations for structural elucidation of drug metabolites.

The free radical intermediates from the photolysis of eleven substituted styrenes (1–11) with both tertiary and secondary amines were studied by spin trapping and the high-pressure liquid chromatography electron paramagnetic resonance (HPLC-EPR) technique. Eleven α-methylbenzyl radicals were trapped by 2-methyl-2-nitro-sopropane (MNP) and separated by HPLC. Resolution enhancement was applied to obtain long-range hyperfine splitting constants (hfsc). The nitrogen hfsc are linearly correlated with the Hammett substituent constants. The major mass spectral fragments for the trapped aminoxyl radicals (12–22) include [M+1]⁺, [M–1]⁺, [M– ^t Bu]⁺, [M– ^t BuOH]⁺, [M– ^t BuNO]⁺, and [(CH₃)₃C]⁺. The structures of 12–22 were also confirmed by chemical synthesis by adding Grignard reagent to α-phenyl-N-tert-butylnitrone (PBN) and its derivatives.

Miniature, low-cost sensors are in demand for a variety of applications in industry, medicine, and environmental sciences. As a first step in developing such a sensor, we have etched a grating into a GaAs rib waveguide to serve as a wavelength-dispersive element. The device was fabricated with the techniques of metal-organic chemical vapor deposition, electron-beam lithography, optical lithography, and reactive ion-beam etching. While full integration is the eventual goal of this work, for the present, a functional spectrometer was constructed with the addition of a discrete source, sample cell, lenses, and detector. The waveguide spectrometer has a spectral resolution of 7.5 nm and a spectral dispersion of 0.11°/ nm. As presently configured, it functions in the spectral range of 1500 to 1600 nm. A demonstration of the analytical capability of the waveguide spectrometer is presented. The problem posed is the determination of diethanol amine in an ethanol solution (about 10 to 100 g/L). This procedure involves the detection of the first overtone of the NH stretch at 1545 nm in a moderately absorbing solvent background. The standard error of prediction for the determination was 5.4 g/L.

Spatially resolved and integrated spectroscopies, especially temporal emission profile measurements, were used to examine the plume emissions associated with copper plasma generated by 308-nm excimer laser ablation at reduced pressures (< 1.0 × 10⁻¹ Pa), by which the composition and dynamics of the plasma were investigated. It is found that, even for the same emitting species, the temporal and spatial behaviors of the emissions originating from different transitions can be very different. Three types of emission lines are clearly evident, which reveal the characteristics of the plasma. The plasma forms as a result of laser ablation and evolves from breakdown of the ablation-created copper vapor to electron-collisional expansion dynamics. At the initial stage, the plasma involves a large amount of Cu(II) and high energy-excited Cu(I) species, and then evolves to consist mainly of low energy-excited Cu(I) species. The results also show that the plasma maintains higher temperature for a quite a long time, and cool electron-impacting excitations determine the plasma behavior while it expands far away from the target.

Principal component analysis (PCA) is used here to determine percent composition of the four monomer units styrene, 1,2-butadiene (vinyl), trans-1,4-butadiene (trans), and cis-1,4-butadiene (cis) in styrene/butadiene rubber (SBR) copolymers and the three monomer units (vinyl, trans, and cis) in butadiene rubber (BR) polymers. Comparison of PCA with current methods is described. Cluster analysis is used to classify the SBR and BR into groups.

In a previously published work, a procedure based on both line correlation and normalized line intensity ratios was described to perform qualitative analysis in inductively coupled plasma atomic emission spectrometry based on the use of solutions. In this work, this procedure was applied to dry aerosols obtained by UV laser ablation. Since the procedure relies on the comparison of experimental normalized intensity ratios with stored normalized intensity ratios usually obtained by using synthetic solutions, a limitation was observed because it was found that wet and dry aerosols resulted in different plasma conditions and, therefore, different normalized intensity ratios. This limitation was overcome in the present work by adding a small amount of hydrogen (40 mL min⁻¹) to the gas which carried the ablated material. Plasma conditions were then similar to those observed with aqueous solutions. Good agreement in terms of the normalized intensity ratios was obtained for different types of ablated materials such as steel, glass, and polymers.

The ellipsometric infrared spectra of a metallic island film indicate its potential for surface-enhanced infrared absorption (SEIRA). Such a film is characterized by unique optical constants, and these can be simulated by using an effective-medium approach.