Applied Spectroscopy

This paper develops expressions for the Raman power emitted by liquid core optical fiber (LCOF) sample cells in six simple excitation/collection geometries and the fraction of that power that can be utilized in a conventional Raman spectrometer. From these expressions a “figure of merit” is developed that can be used to predict the relative intensity enhancement provided by LCOFs having different inside diameters and loss characteristics. For the apparatus used here, we show theoretically and experimentally that the figure of merit takes the simple form (αd)⁻¹ where α and d are, respectively, the loss coefficient and inside diameter of the LCOF. Given the varying optical quality of Teflon®-AF LCOFs currently in use, the analysis presented here should be useful in optimizing the performance of LCOF accessories in Raman and related spectroscopic applications.

Diamond is evaluated as an internal reference for remote Raman spectroscopy using three different fiber-optic probe designs. The three probe designs include (1) a six-around-one fiber-optic probe with a flat diamond window; (2) a six-around-one fiber-optic probe with a quartz lens window with an embedded diamond particle, and (3) a filtered probe design with a flat diamond window. It is found that the second probe design provides a compact and inexpensive probe head which allows quantitative measurements of Raman-active analytes independent of changes in the incident laser power and independent of changes in the refractive index of the solution. In addition, the second probe design minimizes the glass Raman background from the fiber optics without the use of filters.

A study has been made of the use of polynomial curve fitting for removal of nonlinear background and high-spatial-frequency noise components from Raman spectra. Two variations on polynomial curve fitting through a least-squares calculation are used. One, involving fitting data x values to corresponding y values, was used to approximate background functions, which are subtracted from the original data. For smoothing, a reference matrix of six vectors that contains a unity d.c. level, a ramp made up of x values, a quadratic made up of x² values, etc., is fitted to a section of data. The reference vectors are scaled by the fit values and added to give the smoothed estimate of a spectral peak. It is demonstrated, with factor analysis as a test procedure, that the background removal procedure does remove nonlinearities that were present in the original data. The smoothing procedure rejects high-spatial-frequency noise without introducing detectable nonlinearities.

We have investigated the surface geometry of azobenzene-containing long-chain fatty acids (nAmH; n = 8, 12, m = 3, 5) adsorbed on silver and gold colloids as well as the interaction between nAmH and the two kinds of metal colloidal surface by means of near-infrared and visible surface-enhanced Raman scattering (SERS). The 488.0 and 514.5 nm excited Raman spectra of nAmH adsorbed on silver colloid and the 1064 nm excited Fourier transform (FT)-Raman spectrum adsorbed on gold colloid show a marked SERS effect. The lowest concentration at which the SERS signals were observed was 10⁻¹⁰ and 10⁻⁸ M for the silver and gold colloids, respectively, in the present study. The 488.0 nm excited SERS spectrum of 12A3H adsorbed on silver colloid is very similar to a Raman spectrum of 12A3H in chloroform, while the 1064 nm excited SERS spectrum of 12A3H adsorbed on gold colloid is quite different; a number of new bands appear, and the relative intensity of bands changes markedly in the 1064 nm excited SERS spectrum. Probably, a chemical mechanism contributes significantly to the emergence of the SERS effect for 12A3H on gold colloid. Particularly striking in the SERS spectrum of 12A3H adsorbed on gold colloid is the observation that bands at 1586, 1166, and 834 cm⁻¹ due to the phenyl ring modes show remarkable enhancement. It seems likely that the COO⁻ group is adsorbed directly on the silver colloid surface, while one of the phenyl groups is adsorbed directly on the gold colloid surface. The effect of the particle size on the SERS intensity has been explored for 12A3H on gold colloid. It was found from the frequency of the -N=N- stretching band that 12A3H assumes a cis conformation on the gold colloid with the smaller particle size.

A method based on the electrode-less deposition of silver islands on an infrared substrate is proposed and examined for surface-enhanced infrared (SEIR) measurements. The simplicity of this metal island-forming method can largely reduce the cost of surface preparation. Meanwhile, this preparation method also provides the advantage of being applicable to substrates with nonplanar surfaces. The influence of formula and reaction time on silver ion reduction was studied to obtain optimum conditions for island formation suitable for SEIR measurements. The morphologies of the forming silver islands or clusters were examined by a scanning electron microscope and correlated with the level of the enhancement effect. Small silver clusters (∼ 40 nm) with a round to rod shape produced the greatest surface enhancements. Large silver crystals (or clusters) of around a few micrometers were formed during long reaction times. These crystals showed no effect on surface enhancement and only served to block IR energy. Precisely controlled reaction times were critical to prevent the stacking of unwanted silver clusters. Both formula and concentration of reactants influenced the reduction rates of silver ions; hence the IR enhancement. With an increase of the concentration of reduction agents, the formed silver surface provided higher enhancement effect. With the use of the optimized reaction conditions, the silver films deposited by electrode-less solution provided levels of enhancement similar to those for to metallic surfaces prepared by conventional physical vapor deposition.

Detection of the nerve agent Sarin is investigated by using diffuse reflectance infrared spectroscopy with magnesium oxide (MgO) as a preconcentrating medium. Magnesium hydroxide, produced from magnesium oxide and water, initially hydrolyzes the Sarin to isopropyl methylphosphonic acid (IMPA). The IMPA then reacts with magnesium hydroxide (a slightly soluble product of the addition of MgO to water) to form an insoluble salt. Gas chromatography mass spectrometry (GC-MS) analysis of the Sarin/water solution indicates hydrolysis of the Sarin after 5 min of addition of the Nantek MgO and within 15 min after addition of the Aldrich MgO. Capillary electrophoresis analysis of the same Sarin/water solution shows that approximately 30% of the IMPA produced by the Nantek MgO hydrolysis of Sarin is not detected in solution, while most of the IMPA produced by the Aldrich MgO is detected. The diffuse reflectance infrared (IR) spectrum of the IMPA/MgO sample clearly indicates the presence of IMPA for both the Nantek MgO and Aldrich MgO samples. Well-resolved bands for methylphosphonic acid (MPA) are also evident for the Nantek MgO/Sarin sample, while less resolved, weaker bands due to MPA are seen for the Aldrich MgO/Sarin sample. These data suggest that a significant percentage of the Sarin is hydrolyzed to MPA by using the Nantek MgO. A detection limit of 98 ppb (Aldrich MgO) and 170 ppb (Nantek MgO) Sarin in water is calculated on the basis of the band heights and signal-to-noise ratio.

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used to study the pyrolysis of polyimide (Kapton®). The samples were prepared in a KBr matrix, which did not show pronounced interferences, such as increased emissivity, during the measurements. The pyrolysis of Kapton® reveals pronounced differences from laser-induced (UV) decomposition. The polyimide system decomposes thermally in distinct steps, i.e., first the imide ring, without elimination of the carbonyl groups, followed by the aromatic system and then the carbonyl groups. Several intermediates, such as nitriles and alkynes, are identified. The quantitative analysis of the spectra suggests that Kapton® decomposes in two steps, i.e., as a growing particle with shrinking core followed by a shrinking particle. The growing particle with shrinking core is the nonreacted polymer as core and a polyamic structure as the growing part, while the shrinking particle is the complete pyrolysis of the polymer. The activation energies for these two steps were determined. The reaction rate appears to be diffusion controlled at low temperatures and surface reaction controlled at higher temperatures.

Mid-infrared chalcogenide fiber optics coupled with Fourier transform infrared (FT-IR) benches has made it possible to perform noninvasive reflectance measurements of layers of paint. The technique has potential applications for noninvasive analysis of works of art. These measurements can present large distortions in the spectrum, both in band shape and absorption frequency, which may depend on the band strength, on the concentration of the sample, or on the optical layout of the measuring system. Therefore, it is difficult to compare reflectance spectra with those collected in the transmission mode and, consequently, with the available databases. The work deals with an overall survey of the limits and problems involved in the utilization of this analytical technique, an estimate of the reproducibility of the measurements, and the development of a correct measurement procedure.

Mid-infrared fiber-optics reflectance spectroscopy supported by classification procedures based on the Mahalanobis distance in the principal component space was applied to investigate laboratory samples simulating actual paintings. The spectral data obtained were analyzed by means of principal component analysis (PCA). The application of PCA to first-derivative spectra resulted as a robust method of processing spectral data and made it possible to distinguish the binding medium and/or the pigment/dye, and to classify test samples by means of the Mahalanobis distance discrimination method.

The reagent-free multicomponent analysis of components in urine using mid-infrared spectroscopy possesses many attractions. A population of 67 individual urine samples from children and adults, collected over 24 h, was analyzed for urea, creatinine, uric acid, glucose, total protein, phosphate, and sulfate by using clinical reference methodology. The urine pH value was potentiometrically measured by a glass electrode. Partial least-squares (PLS) calibration models were calculated over optimized, component-specific ranges from attenuated total reflection spectra of the urine samples measured by a micro-Circle cell. Apart from glucose and total protein, for which the spread in urine sample concentrations was too small, calibrations were successful for metabolites such as urea, creatinine, and uric acid. Additionally, concentrations of sulfate and phosphate anions, which show significant mid-infrared absorption bands, could also be quantified. The acid secreted with the urine influences the equilibrium between di- and monobasic phosphate in this biofluid, which is used as the spectroscopic basis for the pH assay presented here. The analytical performance of the reference methods is discussed with regard to evaluating the limitations of the spectroscopic assay. Additionally, aqueous solutions of individual urine components with a spread of concentrations similar to those found in native urine samples were analyzed by using PLS calibrations.

The robustness of models developed for the near-infrared spectroscopic prediction of mycelial biomass, total sugars, and ammonium in a submerged Penicillium chrysogenum bioprocess was assessed by rigorously challenging them with artificially introduced analyte and background matrix variations, so that analyte concentrations were varied in an invariant matrix and vice versa. The models were also challenged by using a data set from a process operated at a different scale from that used in the original model formulation. Simple univariate and bivariate linear regression models, and partial least-squares (PLS) models with as few factors as three and four, performed sufficiently well for predicting analyte concentrations and were robust with respect to the matrix variations tested. However, models based on relatively weaker absorptions, or those that were likely to be influenced by stronger absorbers present in the same matrix, were vulnerable to changes in the matrix. A change in the scale of operation affected models that would be influenced by biomass, possibly due to an influence of the morphology of the mycelial biomass. An analysis of the loading vectors of some PLS models revealed details that were useful in understanding the type of information modeled and the behavior of these models to the variations tested.

A dual-beam near-infrared (NIR) spectrometer was constructed and evaluated. The instrument was designed to work with the two monochromatic beams produced by each one of the two acousto-optic tunable filters (AOTFs) evaluated, capable of scanning the wavelength range 800–1600 or 1500–2400 nm. Two temperature-controlled, cooled InAs detectors were employed for comparison of the two monochromatic beams produced by the AOTF. Data were obtained in a controlled ambient temperature, (25 ± 1) °C, and were compared for their intensity and wavelength coincidence for each of the 300 values of radio frequency (85–160 and 56–92 MHz) applied to the AOTF. The results show that the two beams present good concordance as a function of the wavelength (mean Δλ <0.30 nm) for both AOTFs investigated. The normalized intensities of the two beams were quite different, and their ratio values (0.9 to 1.2) depend on the wavelength. Inversion of intensities was observed in the present case and can be attributed to the unmatched position of the two detectors and to the difference in responsivity. On the other hand, the use of the two beams revealed that it can provide a robust way to correct for light-source intensity fluctuation as high as 10%. The mean standard deviation for long-term operation and for a zero absorbance baseline was reduced three times when the dual-beam instrument was employed. The instrument has also been evaluated for determination of water in fuel ethanol. The results for water content demonstrate the good analytical performance of the spectrometer with a relative error of 0.07 and 0.03% (w/w) for water determination in the ranges 0.20–10.0 and 0.20–3.00%, respectively.

When in-line or on-line spectroscopy is performed on a process, different types of variations are present in the measured spectrum. The variation due to the chemistry of the process is the wanted variation, which should be used for monitoring the process. Also unwanted variation is present in the measured spectrum. One example is variation due to changes in the temperature of the sample. This unwanted variation will cause deterioration of the performance of the spectroscopic analyzer. In this paper the properties of continuous piecewise direct standardization (CPDS) for temperature corrections in near-infrared (NIR) spectra are discussed. A procedure to obtain sensible settings of the meta-parameters in the CPDS algorithm has been developed. Initial values of the meta-parameters can be found for each data set by looking at spectra as a function of temperature. Particularly, difference spectra are shown to be useful in order to determine the degree of linearity of the spectral disturbances caused by the temperature changes. A cross-validation procedure around these initial settings is proposed to establish the optimum meta-parameters. This procedure was successfully applied to two NIR data sets. For each data set, it was found that CPDS corrected for temperature disturbances in an independent test data set and maintained the prediction errors on the same low level as for a calibration model built at the calibration temperature.

A new method involving the use of chemometric techniques was developed on a process nuclear magnetic resonance (NMR) spectrometer for measurement of total styrene in styrene/butadiene (Sty/BD) block copolymers. The method uses partial least-squares (PLS) regression to correlate the total styrene data produced by solutions H-1 NMR to the data produced by process NMR. The new method has comparable accuracy and precision to the solution NMR method, but it is much faster and easier to perform. It also has the additional capability of being selective for a specific process and/or polymer. The NMR method and chemometrics models are discussed, and results for validation of the models and prediction of the total styrene in unknown samples are presented.

n-Octadecyltriethoxysilane (C₁₈-TEOS) and tetraethoxysilane (TEOS) were used as the precursors to fabricate an organic-inorganic hybrid composite by sol-gel processing. The hydrolysis and initial condensation reactions of TEOS and C₁₈-TEOS/TEOS mixtures were studied by ²⁹Si-NMR spectroscopy. The rate of disappearance of the silane precursors and the evolution of the various silicon species during the reaction indicate that TEOS hydrolyzes and condenses faster than C₁₈-TEOS. The rate of hydrolysis for C₁₈-TEOS, however, increases when reacting in the C₁₈-TEOS/TEOS hybrid mixture, indicating that the condensation reactions involving TEOS species drive the reaction of C₁₈-TEOS. The formation of cross-condensed products leads to the composite with octadecyl groups incorporated into the final glass material. The maximum degree of condensation in the C₁₈-TEOS/TEOS mixture was observed after 2 h of reaction. Fused-silica capillary columns were coated with the C₁₈-TEOS/TEOS mixture while in its liquid-like stage. After drying, the columns were tested for open tubular capillary electrochromatography (OT-CEC). Coating of the columns was performed after the maximum degree of condensation was achieved, which has been shown to correlate with maximum chromatographic retention.

The use of laser scanning confocal microscopy (LSCM) to view textile substrates is a relatively new field. It has been used to determine fiber shapes and concentrations of a fluorescent dye in a fiber. However, this technique is limited to very low concentrations of strongly absorbing species. A new technique based on LSCM is developed for obtaining concentration profiles at much higher concentrations where the effects of absorption of the incident light cannot be ignored. Sample preparation is particularly simple. Although this new approach is specific to planar films, it permits the study of dye diffusion in polymers at normal dye concentrations and with dyes that are not traditionally considered fluorescent dyes. The concentration profiles are used to obtain the diffusion coefficient of C.I. Disperse Blue 3 in nylon 6,6 film.

The effects of oxygen over a wide concentration range on the solid-matrix phosphorescence (SMP) of perdeuterated phenanthrene (D₁₀) adsorbed on partially hydrophobic paper (1PS) were investigated. The SMP lifetime results of D₁₀ adsorbed on Whatman 1PS paper and Whatman No. 1 paper were considered, and the SMP intensities and SMP lifetimes of D₁₀ on 1PS paper were compared. The SMP quenching model employed showed that diffusional quenching was not very extensive for D₁₀ adsorbed on both types of paper. In addition, a large fraction of the D₁₀ phosphor molecules did not undergo diffusional quenching in either paper. The changes in the SMP intensities for D₁₀ adsorbed on 1PS paper were much greater than the corresponding SMP lifetime changes, which indicated that static quenching by oxygen was very effective for D₁₀ on 1PS paper. Also, the SMP intensity data indicated that a relatively large fraction of the D₁₀ molecules were not quenched statically. The quenching data were interpreted and compared with equations that permitted both linear and nonlinear models to be employed.

Room-temperature phosphorimetry of ternary β-cyclodextrin complexes of the model compound 1-bromonaphthalene has been examined. The selected third partners were alcohols (cyclohexanol, cyclopentanol, and 1-pentanol) and surfactants (Triton X-100 and sodium dodecyl sulfate). A thorough analysis of all equilibria involved in the interaction between the components is performed and discussed. A computational procedure, not previously employed to study this type of complex systems, is used for the determination of the equilibrium constants and stoichiometries of the formed species. It is shown that a rigorous analysis of the equilibria is necessary for a proper optimization of analytical methods based on roomtemperature phosphorescence of ternary complexes. The best working conditions are inferred from the examination of three-dimensional and contour plots of the ternary complex concentration as a function of those of the reagents. The latter plots are constructed once the relative stabilities of all intervening species are known. On the basis of the calibration results, the relative advantages of employing the examined systems for determining the studied compound are presented.

A robust method of analysis that uses multichannel array detector technology is presented for high-precision, accurate measurement of Pt, Pd and Rh in catalytic converters. Catalytic converter samples, National Institute of Standards and Technology (NIST) reference standards, and a fresh converter sample are dissolved and analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Complete dissolution of sample sizes up to 2 g of NIST reference standards is achieved with the use of high-pressure, high-temperature carius tubes. Carius tube dissolution of a modern catalytic converter sample is found to be incomplete because of the presence of higher concentrations of Ce, Ni, and Zr. For this sample, the addition of FeCl₃ increases the dissolving power of the carius tube for base metals and noble metals, particularly Rh. Although interference problems are encountered in samples bearing high concentrations of Fe, use of a high-resolution ICP-AES system alleviated these problems. Precise quantitation of Pt, Pd, and Rh is accomplished by using the method of standard additions, with high accuracy and a precision of 1–2% RSD (relative standard deviation). This analysis also confirmed the values determined by NIST by using isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) for standard reference materials (SRMs) 2556 and 2557.