Showing papers in "Applied Spectroscopy in 2003"

Automated Method for Subtraction of Fluorescence from Biological Raman Spectra

Abstract: One of the challenges of using Raman spectroscopy for biological applications is the inherent fluorescence generated by many biological molecules that underlies the measured spectra. This fluorescence can sometimes be several orders of magnitude more intense than the weak Raman scatter, and its presence must be minimized in order to resolve and analyze the Raman spectrum. Several techniques involving hardware and software have been devised for this purpose; these include the use of wavelength shifting, time gating, frequency-domain filtering, first- and second-order derivatives, and simple curve fitting of the broadband variation with a high-order polynomial. Of these, polynomial fitting has been found to be a simple but effective method. However, this technique typically requires user intervention and thus is time consuming and prone to variability. An automated method for fluorescence subtraction, based on a modification to least-squares polynomial curve fitting, is described. Results indicate that the presented automated method is proficient in fluorescence subtraction, repeatability, and in retention of Raman spectral lineshapes.

Absorption of water by room-temperature ionic liquids: effect of anions on concentration and state of water.

Abstract: Near-infrared (NIR) spectrometry was successfully used for the non-invasive and in situ determination of concentrations and structure of water absorbed by room-temperature ionic liquids (RTILs). It was found that RTILs based on 1-butyl-3-methylimidazolium, namely, [BuMIm]+ [BF4]-, [BuMIm]+ [bis((trifluoromethyl)sulfonyl)amide, or Tf2N]- and [BuMIm]+ [PF6]-, are hydroscopic and can quickly absorb water when they are exposed to air. Absorbed water interacts with the anions of the RTILs, and these interactions lead to changes in the structure of water. Among the RTILs studied, [BF4]- provides the strongest interactions and [PF6]- the weakest. In 24 hours, [Bu-MIm]+ [BF4]- can absorb up to 0.320 M of water, whereas [Bu-MIm]+ [PF6]- can only absorb 8.3 x 10(-2) M of water. It seems that higher amounts of water can be absorbed when the anion of the RTIL can strongly interact and hence stabilize absorbed water molecules by forming hydrogen bonds with them or inducing hydrogen bonds among water molecules. More importantly, the NIR technique can be sensitively used for the noninvasive, in situ determination of absorbed water in RTILs, without any pretreatment, and at limits of detection as low as 3.20 x 10(-3) M.

Short-Wavelength Near-Infrared Spectra of Sucrose, Glucose, and Fructose with Respect to Sugar Concentration and Temperature

Abstract: Short-wavelength near-infrared (SW-NIR) (700-1100 nm) spectra of aqueous solutions of sucrose, D-glucose, and D-fructose were monitored with respect to change in temperature and sugar concentration. Sugar OH and CH related vibrations were identified by analysis of the spectra of sugar solutions in deuterium oxide (D2O), and of sucrose-d8 solutions in D2O. Absorption spectra were explained in terms of the second and third overtones of OH stretching vibrations and the third overtone of CH2 and CH stretchings. In deuterated solutions, CH and CH2 higher overtone vibration bands became apparent. The major spectral effect of decreased temperature or increased sugar concentration was a decrease in absorbance at 960 nm and an increase in absorbance at 984 nm, interpreted as an increase in the degree of H bonding. Partial least-squares (PLS) calibrations on sugar concentrations (with spectra collected at several sample temperatures) relied strongly on the 910 nm sugar CH related bands, whereas calibrations on temperature depended equally on all OH associated vibrations (750, 840, 960, and 985 nm).

New opportunities in micro- and macro-attenuated total reflection infrared spectroscopic imaging: Spatial resolution and sampling versatility

Abstract: New opportunities exist to obtain chemical images using attenuated total reflection infrared (ATR-IR) spectroscopy. This paper shows the feasibility of obtaining FT-IR images with a spatial resolution of at least 3-4 μm using a Ge ATR objective coupled with an infrared microscope. The improved spatial resolution compared to FT-IR images obtained by the transmission method is due to the high refractive index of the ATR crystal, which gives a high numerical aperture and hence, a higher spatial resolution. FT-IR imaging with a conventional diamond ATR accessory has been investigated. This is the first time that FT-IR imaging is reported using such a versatile accessory based on a diamond ATR crystal. These results showed that a spatial resolution up to 13 μm can be achieved without the use of infrared microscope objectives. One advantage of the diamond element is that it allows pressure to be applied and hence, good contact to be obtained over the whole field of view.

Laser-induced breakdown spectroscopy detection and classification of biological aerosols.

Abstract: Laser-induced breakdown spectroscopy (LIBS) is examined as a potential method for detecting airborne biological agents. A spectrally broadband LIBS system was used for laboratory measurements on some common biological agent simulants. These measurements were compared to those of common, naturally occurring biological aerosol components (pollen and fungal spores) to determine the potential of LIBS for discriminating biological agents from natural background aerosols. A principal components analysis illustrates that linear combinations of the detected atomic lines, which are present in different ratios in each of the samples tested, can be used to discriminate biological agent simulants from other biological matter. A more sensitive, narrowband LIBS instrument was used to demonstrate the detection of single simulant (Bg) particles in the size range 1-5 μm. Ca, Mg, and Na, which are present in varying concentrations between 0.3 and 11% (by mass) in the Bg particles, were observed in single particles using LIBS.

In vivo noninvasive measurement of blood glucose by near-infrared diffuse-reflectance spectroscopy.

Abstract: This paper reports in situ noninvasive blood glucose monitoring by use of near-infrared (NIR) diffuse-reflectance spectroscopy. The NIR spectra of the human forearm were measured in vivo by using a pair of source and detector optical fibers separated by a distance of 0.65 mm on the skin surface. This optical geometry enables the selective measurement of dermis tissue spectra due to the skin's optical properties and reduces the interference noise arising from the stratum corneum. Oral glucose intake experiments were performed with six subjects (including a single subject with type I diabetes) whose NIR skin spectra were measured at the forearm. Partial least-squares regression (PLSR) analysis was carried out and calibration equations were obtained with each subject individually. Without exception among the six subjects, the regression coefficient vectors of their calibration models were similar to each other and had a positive peak at around 1600 nm, corresponding to the characteristic absorption peak of glucose. This result indicates that there is every possibility of glucose detection in skin tissue using our measurement system. We also found that there was a good correlation between the optically predicted values and the directly measured values of blood samples with individual subjects. The potential of noninvasive blood glucose monitoring using our methodology was demonstrated by the present study.

Surface plasmon resonance imaging as a tool to monitor biomolecular interactions in an array based format.

Abstract: N early all processes within a living organism are driven by biomolecular interactions. In order to fully understand the role of a newly discovered gene or protein within a biological system it is necessary to know what molecules it interacts with and what the possible outcomes of these interactions are. One consequence of biological diversity is that there are many potential interacting partners in living systems (e.g., DNA, RNA peptides, proteins, lipids, and carbohydrates)

Development of a broadband picosecond infrared spectrometer and its incorporation into an existing ultrafast time-resolved resonance raman, UV/visible, and fluorescence spectroscopic apparatus

Abstract: We have constructed a broadband ultrafast time-resolved infrared (TRIR) spectrometer and incorporated it into our existing time-resolved spectroscopy apparatus, thus creating a single instrument capable of performing the complementary techniques of femto-/picosecond time-resolved resonance Raman (TR3), fluorescence, and UV/visible/infrared transient absorption spectroscopy. The TRIR spectrometer employs broadband (150 fs, approximately 150 cm(-1) FWHM) mid-infrared probe and reference pulses (generated by difference frequency mixing of near-infrared pulses in type I AgGaS2), which are dispersed over two 64-element linear infrared array detectors (HgCdTe). These are coupled via custom-built data acquisition electronics to a personal computer for data processing. This data acquisition system performs signal handling on a shot-by-shot basis at the 1 kHz repetition rate of the pulsed laser system. The combination of real-time signal processing and the ability to normalize each probe and reference pulse has enabled us to achieve a high sensitivity on the order of deltaOD approximately 10(-4) - 10(-5) with 1 min of acquisition time. We present preliminary picosecond TRIR studies using this spectrometer and also demonstrate how a combination of TRIR and TR3 spectroscopy can provide key information for the full elucidation of a photochemical process.

Hyphenated techniques for elemental speciation in biological systems.

Abstract: he recognition of the fact thatin environmental chemistry,occupational health, nutrition,and medicine the chemical, biologi-cal, and toxicological properties ofan element are critically dependenton the form in which the element oc-curs in the sample has spurred rapiddevelopment of an area of analyticalchemistry referred to as speciationanalysis.

Near-infrared spectroscopic monitoring of the diffusion process of deuterium-labeled molecules in wood. Part I: softwood.

Abstract: Fourier transform near-infrared (FT-NIR) transmission spectroscopy was applied to monitor the diffusion process of deuterium-labeled molecules in hardwood (Beech). The results are compared with previous data obtained on softwood (Sitka spruce) in order to consistently understand the state of order in cellulose of wood. The saturation accessibility and diffusion rate varied characteristically with the OH groups in different states of order in the wood substance, the diffusants, and the wood species, respectively. The variation of saturation accessibility should be associated with the fundamental difference of the fine structure such as the microfibrils in the wood substance. The effect of the anatomical cellular structure on the accessibility was reflected in the variation of the diffusion rate with the wood species. The size effect of the diffusants also played an important role for the diffusion process in wood. Since the volumetric percentage of wood fibers and wood rays is relatively similar, the dichroic effects due to the anisotropy of the cellulose chains were apparently diminished. Finally, we proposed a new interpretation of the fine structure of the microfibrils in the cell wall by comparing a series of results from hardwood and softwood. Each elementary fibril in the hardwood has a more homogeneous arrangement in the microfibrils compared to that in the softwood.

Femtosecond broadband stimulated Raman: a new approach for high-performance vibrational spectroscopy.

Abstract: Femtosecond stimulated Raman spectroscopy (FSRS) is a new technique that produces high-quality vibrational spectra free from background fluorescence. FSRS combines a narrow-bandwidth picosecond Raman pump pulse with an ~80 fs continuum probe pulse to produce stimulated Raman spectra from the pump-induced gain in the probe spectrum. The high intensity of the Raman pump combined with the broad bandwidth of the probe produces high signal-to-noise vibrational spectra with very short data acquisition times. FSRS spectra of standard solutions and solvents such as aqueous Na2SO4, aqueous KNO3, methanol, isopropanol, and cyclohexane are collected in seconds. Furthermore, stimulated Raman spectra can be obtained using just a single pump-probe pulse pair that illuminates the sample for only ~ 1 ps. Fluorescence rejection is demonstrated by collecting FSRS spectra of dyes (rhodamine 6G, chlorophyll a, and DTTCI) with varying degrees of fluorescence background and resonance enhancement. The high signal-to-noise, short data acquisition time, fluorescence rejection, and high spectral and temporal resolution of femtosecond stimulated Raman spectroscopy make it a valuable new vibrational spectroscopic technique.

Laser-induced breakdown spectroscopy for on-line engine equivalence ratio measurements.

Abstract: Laser-induced breakdown spectroscopy (LIBS) has been used to measure the equivalence ratio of a spark-ignited engine in a laboratory setting. Spectral features of C (711.3 nm), O (776.6 nm), N (746.3 and 743.8 nm), and CN (broad emission 707–734 nm) were used to quantify the equivalence ratio over a range from P = 0.8 to P = 1.2. The C/N and C/O peak ratios were found to be successful measurement metrics, compared with a standard exhaust gas oxygen analyzer, for averaged measurements. Some variation in the measurements was observed as a function of engine load. Single-shot data based on a CN/air peak ratio were evaluated using a separate calibration from averaged measurements, and the average of the single-shot data was found to agree well with the exhaust gas oxygen analyzer. The scatter in the single-shot data was substantially higher at lower equivalence ratios. The measurements including the CN peak were slightly sensitive to load, possibly due to pressure changes in the sample as the load increases, or possibly due to changes in the particle size distribution in the gas stream.

Micro-Raman Densimeter for CO2 Inclusions in Mantle-Derived Minerals

Abstract: We investigated the applicability of Raman microprobe spectroscopy for determining the density of CO2 in fluid inclusions in minerals of mantle-derived xenolith samples. A separation (delta) between two Raman bands of CO2 due to Fermi resonance can be a reliable densimeter for CO2 fluid. The relationship between the density of CO2 (g/cm3) and delta (cm-1) can be expressed as: d = -0.03238697 delta 3 + 10.08428 delta 2 - 1046.189 delta + 36163.67. This equation was obtained from the Raman data on CO2 fluid with densities from 0.1 to 1.21 g/cm3, including super critical fluids at 58-59 degrees C. The delta value was constant with increasing temperature from room temperature to 200 degrees C. This indicates that the Raman densimeter is not affected by a possible rise in temperature, an artifact induced by the high flux of the incident laser. The minimum size of measurable inclusions is 1 micron, and the precision in the determination of delta is 0.1 cm-1, corresponding to 0.02 g/cm3 for inclusions of 1 micron in size. The precision can be better for larger inclusions. The micro-Raman densimeter can determine the density of CO2 fluid inclusions over a wide range. In particular, densities of gas and mixtures of gas and liquid phases, which cannot be measured by microthermometry, can be determined.

Virtual enantiomers as the solution of optical activity's deterministic offset problem.

Abstract: Deterministic offsets have remained one of optical activity's most intractable problems. To the extent that the mechanisms by which they are produced do not depend on the chiroptical properties of the sample, they can be eliminated by the subtraction of measurements done on both enantiomers. We show that it is possible to create, by purely optical means, by the sole use of half-wave retarders, the optical antipode of a chiral molecule, to measure the chiroptical properties of the molecule and of its optically generated antipode, and to recover, by subtracting the measurements, the offset-free data of the enantiomer which is physically present. We moreover show that it is possible to do the measurements in a way that eliminates offsets that might occur through the influence of the differing chiroptical properties of the two antipodes. The procedure can be repeated, and by doing so, an almost arbitrarily high precision can be reached. The method is demonstrated by offset-free Raman optical activity back-scattering spectra measured in the so-called scattered circular polarization mode, one of optical activity's so far largely unsolved measurement problems. Such measurements can now be done with 2 mg of substance, in throw-away capillary cells, and on compounds sealed in cylindrical vials.

FT-IR spectroscopic analysis of rainbow trout liver exposed to nonylphenol.

Abstract: Nonylphenol (NP) is a biodegradation product of nonylphenol ethoxylates (NPEs) belonging to the alkylphenol ethoxylates (APEs) group. APEs are widely used nonionic surfactants in detergents, herbicides, pesticides, paints, and cosmetics. The present work investigates the effects of NP on rainbow trout (Oncorhynchus mykiss) livers at the molecular level using Fourier transform infrared (FT-IR) spectroscopy. The FT-IR spectra revealed dramatic differences between the NP-treated and control tissues, which mainly indicated that the level of triglycerides increased, the lipid order increased, and the protein concentration decreased in the treated samples. Moreover, it was also found that glycogen levels significantly decreased and the relative content of nucleic acids increased in NP-treated fish. The 17β-Estradiol-treated fish liver spectra were found to be quite similar to those of NP-treated fish. All these results implied that rainbow trout may offer considerable promise to be used as a bioindicator for NP in the future.

Ultra Violet Resonance Raman Spectroscopy in Lignin Analysis: Determination of Characteristic Vibrations of p-Hydroxyphenyl, Guaiacyl, and Syringyl Lignin Structures

Abstract: Raman spectroscopy of wood and lignin samples is preferably carried out in the near-infrared region because lignin produces an intense laser-induced fluorescence background at visible excitation wavelengths. However, excitation of aromatic and conjugated lignin structures with deep ultra violet (UV) light gives resonance-enhanced Raman signals while the overlapping fluorescence is eliminated. In this study, ultra violet resonance Raman (UVRR) spectroscopy was used to define characteristic vibration bands of model compounds of p-hydroxyphenyl, guaiacyl, and syringyl lignin structures at three excitation wavelengths (229, 244, and 257 nm). The intensities of each band, relative to the intensity of the aromatic vibration band at 1600 cm-1, were defined and the most suitable excitation wavelength was suggested for each structure. p-Hydroxyphenyl structures showed intensive characteristic bands at 1217-1214 and 1179-1167 cm-1 with excitation at 244 nm, whereas the bands of guaiacyl structures were more intensive with 257 nm excitation. Most intensive characteristic bands of guaiacyl structures were found at 1289-1279, 1187-1185, 1158-1155, and 791-704 cm-1. Syringyl structures had almost identical spectra with 244 and 257 nm excitations with characteristic bands at 1514-1506, 1333-1330, and 981-962 cm-1. The characteristic bands of the three structural units were also found from the compression wood, softwood, and hardwood samples, indicating that UVRR spectroscopy can be applied for the determination of chemical structures of lignin.

Geographic Classification of Extra Virgin Olive Oils from the Eastern Mediterranean by Chemometric Analysis of Visible and Near-Infrared Spectroscopic Data

Abstract: Visible and near-infrared reflectance spectra have been examined for their ability to classify extra virgin olive oils from the eastern Mediterranean on the basis of their geographic origin. Classification strategies investigated were partial least-squares regression, factorial discriminant analysis, and k-nearest neighbors analysis. Discriminant models were developed and evaluated using spectral data in the visible (400-750 nm), near-infrared (1100-2498 nm), and combined (400-2498 nm) wavelength ranges. A variety of data pretreatments was applied. Best results were obtained using factorial discriminant analysis on raw spectral data over the combined wavelength range; a correct classification rate of 93.9% was obtained on a prediction sample set. Though the overall sample set was limited in numbers, these results demonstrate the potential of near-infrared spectroscopy to classify extra virgin olive oils on the basis of their geographic origin.

Use of fluorescently labeled phage in the detection and identification of bacterial species.

Abstract: Phages are viruses whose hosts are bacterial cells. They identify their hosts by specific receptor molecules on the outside of the host cell. Once the phages find their specific receptors, they bind to the bacterial cell and inject their nucleic acid inside the cell. The binding between phage and host can be so specific that only certain strains of a single species can be infected. In this communication, the specificity of phage P22 for Salmonella typhimurium LT2 is exploited to allow the detection of Salmonella in the presence of other bacterial species. In particular, the dsDNA of P22 is bound to SYBR gold, a highly sensitive, fluorescent nucleic acid stain. When multiple phages infect the same cell, the fluorescence emissions of the phage DNA inside the cell allow it to be imaged using an epifluorescence microscope. The advantages of using phages as the bacterial recognition element in a sensor over antibodies are discussed.

Analysis of single bacterial spores by micro-Raman spectroscopy

Abstract: The spectroscopic analysis of individual living cells in their native state provides a powerful tool for the investigation of complex biological systems. Micro-Raman spectroscopy, in which confocal microscopy is combined with Raman spectroscopy, offers a promising route to achieving this, because it provides a means to study individual cells and cellular components.1–9 Here we describe the analysis of individual bacterial endospores from four species in the genus Bacillus by micro-Raman spectroscopy. Previous Raman studies on Bacillus spores resulted in spectra with strong scattering from calcium dipicolinate (CaDPA), which is the calcium chelate of dipicolinic acid (2,6-pyridinedicarboxylic acid, DPA); however, these earlier studies were conducted on concentrated samples of spores.10–14 By using micro-Raman spectroscopy, we demonstrate the ability to obtain similar information from individual spores. The Raman spectra for most spores studied were dominated by scattering from CaDPA, although Raman scattering assignable to protein bands and to phenylalanine was also observed. Approximately 4% of the spores analyzed did not exhibit Raman intensity from CaDPA, possibly due to incomplete sporulation. The results presented indicate that micro-Raman spectroscopy is a promising technique for in-

New method for pollen identification by FT-IR spectroscopy.

Abstract: A new methodology for identification of pollen was developed based on FT-IR spectroscopy. Pollen samples of twenty different plant species were collected and the diffuse reflectance infrared Fourier transform (DRIFTS) and KBr pellet spectra were recorded. Libraries of spectra were created. Spectra of unknown plant origin pollen were recorded and compared with those of the corresponding pollen library and the match value was measured automatically using the appropriate software (OMINC ver. 3.1). From the same pollen samples, microscopic slides were prepared and the photographs of the pollen grains were used as a second comparison method. Using light microscopy, the pollen identification is usually limited to the family or generic name, while FT-IR spectroscopy can distinguish species belonging to the same genus. This method is simple and fast, and when the DRIFTS technique is used the sample is not destroyed.

Band-target entropy minimization (BTEM) applied to hyperspectral Raman image data.

Abstract: Band-target entropy minimization (BTEM) has been applied to extraction of component spectra from hyperspectral Raman images. In this method singular value decomposition is used to calculate the eigenvectors of the spectroscopic image data set. Bands in non-noise eigenvectors that would normally be used for recovery of spectra are examined for localized spectral features. For a targeted (identified) band, information entropy minimization or a closely related algorithm is used to recover the spectrum containing this feature from the non-noise eigenvectors, plus the next 5-30 eigenvectors, in which noise predominates. Tests for which eigenvectors to include are described. The method is demonstrated on one synthesized Raman image data set and two bone tissue specimens. By inclusion of small amounts of signal that would be unused in other methods, BTEM enables the extraction of a larger number of component spectra than are otherwise obtainable. An improvement in signal/noise ratio of the recovered spectra is also obtained.

Mars Analysis by Laser-Induced Breakdown Spectroscopy (MALIS): Influence of Mars Atmosphere on Plasma Emission and Study of Factors Influencing Plasma Emission with the use of Doehlert Designs

Abstract: A project called MALIS (Mars Analysis by Laser-Induced breakdown Spectroscopy) is under progress to perform in situ analysis of Mars soils and rocks. This paper reports on the behavior of plasma in Martian conditions, i.e., in a CO2 atmosphere at pressures between 5 and 12 mbar. Plasma expansion and lifetime have been studied in order to compare plasma evolution under standard conditions (air at atmospheric pressure) and in a Mars atmosphere. We have shown that the Mars environment favors plasma expansion and lifetime. The second part of the study concerns optimization of the emission signal from the plasma. An original approach has been chosen, as we used a Doehlert design for the first time in laser-induced breakdown spectroscopy (LIBS). The best conditions obtained are for a laser wavelength of 1064 nm with the maximum energy available due to space limitations, which is 40 mJ at 15 Hz. The other factors studied are delay, angle of incidence, and CO2 pressure. We have shown that these factors do not have the same influence depending on which spectroscopic line is used, i.e., the atomic line or the ionic line.

Near-infrared surface-enhanced-Raman-scattering-mediated detection of single optically trapped bacterial spores.

Abstract: A novel methodology has been developed for the investigation of bacterial spores. Specifically, this method has been used to probe the spore coat composition of two different Bacillus stearothermophilus variants. This technique may be useful in many applications; most notably, development of novel detection schemes toward potentially harmful bacteria. This method would also be useful as an ancillary environmental monitoring system where sterility is of importance (i.e., food preparation areas as well as invasive and minimally invasive medical applications). This unique detection scheme is based on the near-infrared (NIR) surface-enhanced Raman scattering (SERS) from single, optically trapped, bacterial spores. The SERS spectra of bacterial spores in aqueous media have been measured using SERS substrates based on ~60-nm-diameter gold colloids bound to 3-aminopropyltriethoxysilane derivatized glass. The light from a 787-nm laser diode was used to trap and manipulate as well as simultaneously excite the SERS of an individual bacterial spore. The collected SERS spectra were examined for uniqueness and the applicability of this technique for the strain discrimination of Bacillus stearothermophilus spores. Comparison of normal Raman and SERS spectra reveals not only an enhancement of the normal Raman spectral features but also the appearance of spectral features absent in the normal Raman spectrum.

Radiation dose distribution in polymer gels by Raman spectroscopy.

Abstract: The Raman spectroscopy of polymer gel dosimeters has been investigated with a view to developing a novel dosimetry technique that is capable of determining radiation dose within a micrometer of spatial resolution. The polymer gel dosimeter, known as the PAG dosimeter, is typically made up of acrylamide, N,N'-methylene-bis--acrylamide, gelatin, and water. A polyacrylamide network within the gelatin matrix forms in response to an absorbed dose. The loss of monomers may be monitored by corresponding changes to the Raman spectrum. Principal component analysis offers a simple method of quantifying the absorbed radiation dose from the Raman spectrum of the polymer gel. The background luminescence in the spectrum increased significantly with dose and is shown to originate in the glass of the sample vial. The competing effects of elastic scatter, which increases with dose due to the formation of polymer, and sample absorption were quantified and found to introduce errors of up to 5% under certain conditions. Raman spectra as a function of distance from the air-surface interface have been measured for samples that were subjected to doses delivered by a clinical linear accelerator. The depth dose profile thus obtained compared favorably with "gold standard" ion-chamber measurements.

Detection of anions by normal Raman spectroscopy and surface-enhanced Raman spectroscopy of cationic-coated substrates.

Abstract: The use of normal Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS) of cationic-coated silver and gold substrates to detect polyatomic anions in aqueous environments is examined. For normal Raman spectroscopy, using near-infrared excitation, linear concentration responses were observed. Detection limits varied from 84 ppm for perchlorate to 2600 ppm for phosphate. In general, detection limits in the ppb to ppm concentration range for the polyatomic anions were achieved using cationic-coated SERS substrates. Adsorption of the polyatomic anions on the cationic-coated SERS substrates was described by a Frumkin isotherm. The SERS technique could not be used to detect dichromate, as this anion reacted with the coatings to form thiol esters. A competitive complexation method was used to evaluate the interaction of chloride ion with the cationic coatings. Hydrogen bonding and π-π interactions play significant roles in the selectivity of the cationic coatings.

Multivariate Curve Resolution in the Analysis of Vibrational Spectroscopy Data Files

Abstract: T he ability of chemometric techniques to reduce and resolve large data matrices into responses and corresponding weights of these responses has been shown with a variety of data sets, and there exist a variety of approaches capable of assessing such bilinear data sets measured with vibrational spectroscopy.1–4 We have focused on using a form of principal factor analysis, which uses matrix decomposition as the initial step to generate abstract factors and scores. This initial estimate is then reŽ ned in a second step to produce spectra and intensity matrices. The second step uses an iterative constrained least-squares method referred to as alternating least squares. The technique, which is particularly useful in extracting estimates of pure analyte spectra and concentration proŽ les from spectral data matrices, is generally referred to

Optimization of a Microphone for Photoacoustic Spectroscopy

Abstract: The principles of optimizing a microphone for a photoacoustic detector in the nonresonant mode of operation are presented. Performance of the constructed optical cantilever microphone is compared to the commercial electret microphone by measuring the photoacoustic spectrum of methane with both systems. The sensitivity of the presented photoacoustic detector in the nonresonant mode of operation is 100 times higher with the optical cantilever microphone than with the electret microphone.

Influence of Temperature on Water and Aqueous Glucose Absorption Spectra in the Near- and Mid-Infrared Regions at Physiologically Relevant Temperatures

Abstract: Near- and mid-infrared absorption spectra of pure water and aqueous 1.0 g/dL glucose solutions in the wavenumber range 8000-950 cm-1 were measured in the temperature range 30-42 degrees C in steps of 2 degrees C. Measurements were carried out with an FT-IR spectrometer and a variable pathlength transmission cell controlled within 0.02 degree C. Pathlengths of 50 microns and 0.4 mm were used in the mid- and near-infrared spectral region, respectively. Difference spectra were used to determine the effect of temperature on the water spectra quantitatively. These spectra were obtained by subtracting the 37 degrees C water spectrum from the spectra measured at other temperatures. The difference spectra reveal that the effect of temperature is highest in the vicinity of the strong absorption bands, with a number of isosbestic points with no temperature dependence and relatively flat plateaus in between. On the basis of these spectra, prospects for and limitations on data analysis for infrared diagnostic methods are discussed. As an example, the absorptive properties of glucose were studied in the same temperature range in order to determine the effect of temperature on the spectral shape of glucose. The change in water absorption associated with the addition of glucose has also been studied. An estimate of these effects is given and is related to the expected level of infrared signals from glucose in humans.

Attenuated total reflection surface-enhanced infrared absorption spectroscopy of carboxyl terminated self-assembled monolayers on gold.

Abstract: A new recipe for surface-enhanced infrared absorption (SEIRA) active island Au films with improved adhesion in aqueous solution, low resistivity, and enhancement of the infrared (IR) absorption of about 300 was developed. The Au films prepared were utilized in studies of the ionization of self-assembled monolayers of 11-mercaptoundecanoic acid in Na2 SO4 aqueous solutions by attenuated total reflection surface-enhanced infrared absorption (ATR-SEIRA) spectroscopy. It was found that the carboxyl end groups of the self-assembled monolayer turn into carboxylate anions on going from anodic to cathodic potentials or from acidic to alkaline pH. The water molecules close to the self-assembled monolayer in acidic solutions or at anodic potentials are preferentially aligned with their dipole moments parallel to the interface. This type of alignment can be ascribed to the dipole-dipole interaction between the carboxyl groups and the water molecules. On the other hand, in alkaline solutions or at cathodic potentials the structure of water close to the self-assembled monolayer is essentially bulk-like, with randomly oriented water molecules. This observation suggests that in alkaline solutions or at cathodic potentials the charge of the carboxylate anions is almost completely compensated for by strongly adsorbed counter cations. As a result, the electric field close to the surface of the ionized self-assembled monolayer is weak and has little influence on the orientation and hydrogen bonding of the water molecules.