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.
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.
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.
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