Fiber-optic Raman Probe Research Articles

Multi-wavelength raman imaging using a small-diameter image guide with a dimension-reduction imaging array.

A commercially available fiber-optic Raman probe was modified for high-resolution spectral Raman imaging using a 350 microm diameter optical fiber image guide coupled to a dimension-reduction imaging array (DRIA). The DRIA comprised 672 optical fibers, arranged as a square array (21 x 32 fibers) on one end and a linear array (672 x 1 fibers) on the other. An imaging spectrograph was used with the DRIA to acquire multi-wavelength Raman images from -250 to 1800 cm(-1) at a spectral resolution of approximately 5 cm(-1). The utility of this technique for in situ and remote Raman imaging is demonstrated by monitoring the polymerization of a model polymer, dibromostyrene (DBS), while simultaneously measuring the Raman Stokes/ anti-Stokes ratio as a function of sample heating time, over a sample area of approximately 4 x 1.6 mm.

In situ raman spectroscopy studies of metal ion complexation by 8-hydroxyquinoline covalently bound to silica surfaces.

Raman spectroscopy is applied to an investigation of the interfacial chemistry of silica-immobilized 8-hydroxyquinoline (8HQ) for binding of metal ions over a wide range of solution conditions. Since the derivatized silica has a high specific binding capacity, the mass of silica equilibrated with solution needs to be small for studies of reactions with trace-level (microM) metal ions; otherwise, the solution volume required to reach equilibrium becomes excessive. To address this problem, a small-volume flow cell is designed for this work using a fiber-optic Raman probe inserted directly into the packed end of a microcolumn, allowing excitation and collection of Raman scattering from less than 10 mg of derivatized silica. This cell is attached to a flow system that allows control of solution conditions while the response of the 8HQ-silica material is acquired by continuous monitoring of Raman scattering from the sample. Raman spectra of the deprotonated, neutral, protonated, and copper-complexed forms of the ligand can be distinguished, allowing proton-transfer and metal ion binding reactions of the ligand to be investigated. To account for the effects of changing surface potential on these reactions, zeta-potential measurements are made on the 8HQ-silica particles under the same solution conditions that are employed in the Raman scattering measurements. The observed pH dependence of metal ion binding was corrected for the effect of surface potential using the Boltzmann equation, and the resulting equilibrium constant for binding of Cu2+ was independent of metal ion concentration over a 100-fold range from 30 microM to 5 mM.

Performance of Fiber-Optic Raman Probes for Analysis of Gas Mixtures in Enclosures

The feasibility of using fiber-optic Raman probes to identify and quantify gases in enclosures is investigated by measuring and comparing detection thresholds using several probe and enclosure designs. Unfiltered, non-imaging, fiber-optic probes are shown to achieve lower detection thresholds than a filtered, imaging, fiberoptic probe, provided that light scattering within the sample enclosure is minimized and provided that a window is not used between the probe and the analyte gas. Achievable thresholds for hydrogen, oxygen, nitrogen, carbon monoxide, and methane in gas mixtures are demonstrated to be below 1 kPa with ten seconds signal acquisition and 0.1 kPa with twenty minutes signal acquisition with the use of 0.4 W of 532-nm excitation. Ambient carbon dioxide in air (.03 kPa) is shown to be detectable in a twenty minute acquisition, and ambient water vapor is well above the detection threshold. Background signals generated within the optical fibers remain the principal factors limiting detection thresholds. Factors affecting the magnitudes of these signals reaching the detector are investigated and discussed. A flat piece of light-absorbing colored glass tilted to direct reflected light away from the fiber-optic probe performs well as a beam stop to reduce background signal in a simple, cylindrical sample enclosure.

Raman Spectroscopy for the in Situ Identification of Cocaine and Selected Adulterants

We demonstrate the in situ identification of crack cocaine and cocaine·HCl by using a fiber-optic Raman probe and a portable Raman spectrograph. The Raman spectrum of freebase cocaine (crack) is obtained in just seconds without any sample preparation, and differs significantly from that of cocaine·HCl. We also show that the Raman spectra of these drugs are easily distinguishable from common cutting agents and impurities such as benzocaine and lidocaine. Another advantage of using Raman spectroscopy is that the drugs can be identified while contained in transparent containers, such as clear plastic evidence containers that are used to store drug evidence and to maintain chain of custody. We also demonstrate the in situ Raman identification of drugs separated by thin-layer chromatography. We discuss the utility of surface-enhanced Raman spectroscopy (SERS) in toxicological drug screening and present preliminary SERS data for cocaine in solution using colloidal silver. We believe this to be the first published SERS spectrum of freebase cocaine.

Caries Detection by Diode Laser Raman Spectroscopy

Carious lesions can be distinguished from sound hard tooth tissues because their near-infrared Raman spectra exhibit an increased luminescence. A comparison of the ratio between luminescence and Raman band intensities for diseased and healthy areas of the tooth gives a measure for the stage of the disease. Intraoral analyses can be made in a reasonable time with the use of a diode laser, a fiber-optic Raman probe, and a compact spectrograph without moving optics. An excitation wavelength of 785 nm gives sufficient contrast between sound and carious tissue and allows a sensitive detection of the weak Raman scattering by silicon detectors.

Some new uses for filtered fiber-optic Raman probes:in situ drug identification andin situ and remote Raman imaging

The use of a commercially available filtered fiber-optic Raman probe is discussed for the in situ identification of illicit drugs. This probe is also shown to be useful for remote Raman measurements in a collimated beam geometry. Fiber-optic Raman imaging is discussed using single small-diameter image guides and also image guides with attached imaging GRIN lenses. Finally, a demonstration of remote Raman imaging is presented using a simple modification of the same commercially available Raman probe that was used for illicit drug measurements. Copyright © 1999 John Wiley & Sons, Ltd.

Surface and bulk stress/strain measurements in composite laminates with a fibre-optic Raman probe

Fibre stress/strain measurements in unidirectional, as well as, multidirectional aramid/epoxy composites have been conducted with the use of a laser Raman microprobe. The composite was incrementally loaded in tension while Raman measurements were taken. Fibre-optic probes sandwiched between adjacent laminae were employed for channelling the laser excitation light to a specified location within the bulk of the composite. The direction of the fibre-optic was either perpendicular or parallel to the reinforcing fibres. For comparison purposes, the same fibre-optic probe was used to scan the surface of the laminates. The perpendicular configuration was found to reduce the tensile strength of the as-received composite coupon by 10% whereas the parallel second configuration had no effect. In the unidirectional coupons the stress or strain in the principal fibre direction could be measured prior to loading and at every increment of applied tensile load up to fracture. The take-up of fibre strain for both bulk and surface set of measurements was identical with that obtained from the attached electrical resistance strain gauges. In the case of multidirectional coupons the stress or strain in the principal direction could be measured within successive plies situated at angles θ to the loading direction. The results for the 0° plies were in good agreement with those obtained by conventional laminate analysis whereas small deviations from linearity were observed in the angle plies. The proposed methodology paves the way for simultaneous in-service stress/strain measurements on the reinforcing fibres situated on the surface or within the bulk of a composite laminate.

Capillary isotachophoresis with fiber-optic Raman spectroscopic detection: Performance and application to ribonucleotides

A fiber-optic Raman probe fitted with a microscope objective was used to obtain on-line normal Raman spectra of adenosine 5′-monophosphate, cytidine 5′-monophosphate, guanosine 5′-monophosphate and uridine 5′-monophosphate separated by capillary isotachophoresis. With multimode optical fiber, the system interrograted a 40-μm length of capillary. Fiber-optic coupling facilitated use of an unmodified spectrograph and conventional capillary mounting systems. Raman spectra were excited with a 2W 532 nm NdYVO 4, laser as the excitation source, with collection of 1 spectrum per second. Even at 2·10 − 5 M initial concentration, Raman spectra were obtained at a good signal-to-noise ratio.

Comparative Study of Some Fiber-Optic Remote Raman Probe Designs. Part II: Tests of Single-Fiber, Lensed, and Flat- and Bevel-Tip Multi-Fiber Probes

We compare relative performances of flat-tipped, beveled (two-fiber and six-around-one), and single-lensed focused fiber-optic Raman probes and, where feasible, evaluate the utility of optical filters for reducing fiber background. The sensitivity profile of each probe is determined by measuring the relative intensity of light backscattered off a flat surface as a function of distance from the probe tip. The experimental results are compared with a simple light-cone-overlap model incorporating fiber numerical aperture, fiber and immersion medium refractive indices, separation between excitation and collection fibers, number of fibers, and fiber bevel angle and/or lens focal length. The model and sensitivity profiles are used to interpret the sampling regions for Raman spectra obtained by using each of the probes with a clear, transparent sample (single-crystal sparry calcite), a white, partially transparent sample (acetaminophen tablet), and a set of organic liquids of varying refractive index. The sensitivity of the tested commercial lensed probe drops off symmetrically about the focal point. For both solid samples, the intensity of fiber background follows a profile determined primarily by laser backscattering off the surface, whereas the sample Raman signal follows a profile dependent upon sampling depth.

Comparative Study of Some Fiber-Optic Remote Raman Probe Designs. Part I: Model for Liquids and Transparent Solids

We have developed models describing the sensitivity and sampling volume of various remote fiber-optic Raman probes—single-fiber, lensed, dual-fiber beveled-tip, dual-fiber flat-tipped, and multi-fiber flat-tipped. The models assume clear samples and incorporate radii, separation, bevel angle, and numerical aperture of the fibers; overlap geometry of illumination and excitation light cones; and refractive index of immersion medium. For the Raman spectra of solid samples in air, single-fiber and lensed probes are predicted to yield the highest Raman signal. Beveled probes should provide greater Raman signal strength than do flat-tipped probes because beveled probes can collect light from a restricted volume closer to the probe end. Although multiple collection fibers improve Raman signal strength, progressively distant concentric fiber rings contribute less and sample material further from the probe.

Optical-Fiber Raman Probe with Low Background Interference by Spatial Optimization

A Raman probe was set up with optical fibers and a graded refractive index (GRIN) lens. It was found that the Raman background arising from optical fiber was spatially dependent, while normal Raman (NR) scattering, surface-enhanced Raman scattering (SERS), and surface-enhanced resonance Raman scattering (SERRS) were spatially independent. Spatial optimization was carried out to minimize the background interference of the optical fiber Raman probe with the use of benzoic acid as a test sample. The best configuration of the probe could also be applied to both SERS and SERRS. SER spectra of p-nitrophenol (1.0 × 10−3 M) and SERR spectra of methyl red (1.0 × 10−6 M) were obtained with the use of this probe to check its performance.

Detection of low levels of trichloroethylene vapor with Raman spectrometry

The response of a novel fiber-optic Raman probe to low levels of trichloroethylene (TCE) vapors is characterized. The detection limit of the current probe for TCE vapor is 34 mg/L, and the probe exhibits a fully reversible response. The probe uses an organic-polymer, low-density polyethylene to concentrate TCE vapors in the optical path of the fiber-optic Raman spectrometer. The relative standard deviation for measurement of 677 mg/L of TCE in the vapor is 0.3%.

Rare-Earth-Doped Glass Fiber for Background Rejection in Remote Fiber-Optic Raman Probes: Theory and Analysis of Holmium-Bearing Glass

We have investigated the feasibility of using rare-earth-doped glasses in “self-filtering” optical fibers used for remote Raman spectral collection. We have derived a theoretical treatment and have used the measured relevant sample and glass parameters (e.g., optical absorption of the doped glass and ratio of the intensities of inelastic scattering to elastic scattering plus reflection) to evaluate the usefulness of such fibers. With the use of these parameters, the optical absorption bands of Ho-doped glass, in particular, are found to be sufficiently intense and sharp to enable this glass to be used in the collection fiber of a remote Raman probe. Ho-doped glass fiber of as little as 2 cm in length is sufficient to filter undesirable laser radiation while permitting a high proportion of the sample Raman signal to pass. Use of the 488.0-nm Ar+ laser line or green or red laser wavelengths from a “tunable” laser can ensure that the excitation is within an absorption band and close to the long-wavelength transmission cut-on for the doped glass.