Thomas M. Vess

Remote-Raman Spectroscopy at Intermediate Ranges Using Low-Power cw Lasers

A portable Raman system is described that has been developed for line-of-site spectral measurements of remotely located samples at intermediate ranges. Raman spectra were measured at distances up to 20 m with the use of a 40-mm-diameter collection optic (f/500) and at 16.7 m with a 22-mm-diameter collection optic (f/750). In all cases, low-power cw lasers were used with powers ranging from 23 to 100 mW. The system consists of a small f/4 image-corrected spectrograph with a liquid-nitrogen-cooled CCD detector and has been demonstrated with both an argon-ion laser, emitting at 488 nm, and an 809-nm diode laser. Applications of the system include monitoring of organic and inorganic compounds at toxic waste sites during remediation, process monitoring, and remote detection of highly toxic materials.

Remote Fiber-Optic Raman Analysis of Xylene Isomers in Mock Petroleum Fuels Using a Low-Cost Dispersive Instrument and Partial Least-Squares Regression Analysis

We report the use of a low-cost dispersive Raman instrument with charge-coupled-device (CCD) detection, near-infrared (near-IR) diode laser excitation, and remote fiber-optic sampling to analyze mock petroleum samples which contain high benzene, toluene, ethylbenzene, and xylene (BTEX) concentrations. Partial least-squares regression (PLSR) analysis is used to correlate the individual xylene isomer concentrations to the Raman spectral signal without the use of an internal standard. The resulting PLSR model is used to predict the concentration of individual xylene isomers, and it is found that, at a 95% confidence level, samples containing between ∼1.5 and 15% xylene isomer can be predicted to better than ±0.1% for meta- and para-xylene, and to ±0.15% for ortho-xylene. The use of PLS model leverage plots provides a facile statistical method by which to identify Raman spectra which involve diode laser mode hops or significant fiber backscatter.

Simultaneous multipoint fiber optic Raman sampling for chemical process control using diode lasers and a CCD detector

A diode-laser-based portable Raman spectrometer is described that uses a 2-dimensional-array detector (charge-coupled device--CCD) to simultaneously measure chemical processes at several remote points using fiber optics. Optical multiplexing with the CCD allows simultaneous measurements with little loss of sensitivity and the low-energy near-visible (NVIS) excitation (e.g., 786 to 820 nm) prevents most sample luminescence. Several fiber- optic sensor configurations optimized for particular applications, including surface-enhanced Raman (SER), are presented. Applications are described including in-situ epoxy cure chemistry monitoring in a curing oven, monitoring a distillation apparatus, and mixed waste monitoring. Some of the limitations of the system are discussed.

Elimination of Mode Hopping and Frequency Hysteresis in Diode Laser Raman Spectroscopy: The Advantages of a Distributed Bragg Reflector Diode Laser for Raman Excitation

A comparison is made between an index-guided (Fabry-Perot type) diode laser and a distributed Bragg reflector (DBR) diode laser as excitation sources for fiber-optic Raman spectroscopy utilizing charge-coupled device (CCD) detection and an image-corrected spectrograph. The DBR diode laser was superior to the index-guided diode laser for elimination of mode hopping, elimination of frequency hysteresis as a function of both temperature and current changes, and reduction in laser broadband emission. These advantages may allow the DBR laser to be used in industrial process control applications which are too demanding for index-guided diode lasers.

Remote fiber optic Raman analysis of benzene, toulene, and ethylbenzene in mock petroleum fuels using partial least squares regression analysis

Abstract Remote fiber optic Raman spectroscopy utilizing CCD detection and near IR diode laser excitation is used to analyze mock petroleum fuel samples with individual benzene, toulene, and ethylbenzene (BTE) concentrations ranging from 0 to 16%. Specific Raman spectral regions which contain BTE vibrational modes, as well as the entire spectrum, are correlated to the known concentrations of BTE in the samples by Partial Least Squares (PLS) regression analysis. The error associated with the models using only specific spectral regions is less than those using the entire spectral regions in the regression analysis. The use of specific spectral regions minimizes both the time of the regression analysis and the amount of noise associated within the most highly correlated regression model factors; thus over fitting of the data is avoided and the reliability of subsequent predictions is improved.

Real-Time In Situ Monitoring of the Thermal Cure of a Bisphenol Cyanate: A View Toward Intelligent Processing

SYNOPSIS A dispersive fiber-optic Raman spectrometer was used to remotely monitor, in real-time, the local temperature and the extent of reaction of a commercial cyanate ester polymer (AroCy L-10). The local temperature was determined by solving the Boltzmann relation governing the intensity ratio of the Raman Stokes and anti-Stokes scattering of a reference mode which does not vary with the reaction chemistry. The extent of the reaction can be monitored using either individual peaks associated with the reactant or product or by using the entire spectrum and principal component multivariate calibration. The use of principal component analysis has distinct advantages over the single-peak method. cdr 1996 John Wiley & Sons. Inc

Near-visible Raman instrumentation for remote multipoint process monitoring using optical fibers and optical multiplexing

A portable Raman instrument is presented which measures spectra simultaneously from 10 separate fiber-optic probes. The instrument is being developed for a number of applications including multipoint process monitoring and characterizing mixed waste tanks.

Remote Raman spectroscopy using diode lasers and fiber-optic probes

The authors explore analytical applications of remote Raman spectroscopy using fiber optics. In these applications, emphasis is placed on the use of portable near-infrared excitation sources, including diode and diode-pumped lasers, for the purpose of developing a truly portable instrument. This paper addresses the instrumentation that is being developed for this purpose, including the portable Raman spectrometer and some fiber-optic probe designs.

Total light loss fiber optic spectroscopy: Progress towards a fiber optic raman organic sensor

A Raman probe has been developed utilizing a single optical fiber as both a light pipe and an active sensing element. By coating a small segment of the surface of an exposed glass fiber core with a thin polymer film, an inverted waveguide is formed where light transmitted down the fiber is stripped out of the core and into the polymer film. The polymer coating is used both as a waveguide and as a medium for concentrating small organic molecules to be interrogated by Raman spectroscopy. The ability of the fiber optic thin-film waveguide probe to detect organic vapors is demonstrated. The utility of the probe in the detection of nonaqueous phase liquids (NAPLs) is also described.