Fiber optic fluorescence spectrometers

The intrinsic sensors are based on the direct recognition of the chemicals by its intrinsic optical activity, such as absorption or fluorescence in the UV/Vis/IR region. In these cases, no extra chemical is needed to generate the analytical signal. The detection can be a traditional spectrometer or coupled with fiber optics in those regions. Sensors have been developed for the detection of CO, C02 NOx, S02, H2S, NH3, non-saturated hydrocarbons, as well as solvent vapors in air using IR or NIR absorptions, or for the detection of indicator concentrations in the UV/ Vis region and fluorophores such as quinine, fluorescein, etc. 

Fiber-optic fluorometry. Fiber-optic waveguides provide excellent means for delivering excitation energy to fluorescing media in remote, hostile, or inaccessible environments (8) such as reactors and plant streams, and for guiding the emitted fluorescence to a detector or a spectrometer, thus facilitating in-situ monitoring of the resin fluorescence (5,6). In our case, the autoclave represents a hostile environment (350°F and 100 psl) which is inaccessible for optical measurements by conventional methods. 

Fluorescence sensors for saccharides are of particular interest in a practical sense. This is in part due to the inherent sensitivity of the fluorescence technique. Only small amounts of a sensor are required (typically 10-6 M), offsetting the synthetic costs of such sensors. Also, fluorescence spectrometers are widely available and inexpensive. Fluorescence sensors have also found applications in continuous monitoring using an optical fiber and intracellular mapping using confocal microscopy. 

Figure 3.40 shows the layout of a typical Raman analyzer that uses fiber optics for process application. In a Raman process system, light is filtered and delivered to the sample via excitation fiber. Raman-scattered light is collected by collection fibers in the fiber-optic probe, filtered, and sent to the spectrometer via return fiber-optical cables. A charge-coupled device (CCD) camera detects the signal and provides the Raman spectrum. To take advantage of low-noise CCD cameras and to minimize fluorescence interference, NIR diode lasers are used in process instruments. 

Fiber optics may be used as probes for conventional spectrophotometric and fluorescence measurements. Light must be transmitted from a radiation source to the sample and back to the spectrometer. While there are couplers and designs that allow light to be both transmitted and received by a single fiber, usually a bifurcated fiber cable is used. This consists of two fibers in one casing, split at the end that goes to the radiation source and the spectrometer. Often, the cables consist of a bundle of several dozen small fibers, and half are randomly separated from the other at one end. For absorbance measurements, a small mirror is mounted (attached to the cable) a few millimeters from the end of the fiber. The source radiation penetrates the sample solution and is reflected back to the fiber for collection and transmission to the spectrometer. The radiation path length is twice the distance between the fiber and the mirror. 

The range of applications of Raman spectroscopy has also been extended by several important recent developments, such as Raman microscopy, which makes it possible to study extremely small samples. One can also analyze the surface of an extended inhomogeneous sample to obtain very high spatial resolution, or scan across a surface using fiber optics. It is also possible to use specially developed interference filters or holographic notch filters in certain applications as an alternative to a dispersing spectrometer, provided that one suppresses the fluorescence that would otherwise interfere with the measurements. 

Initially, experiments by Freeman et al. [19] in the laboratory using reagent-grade samples and both 532-nm and 632.8-nm visible excitation demonstrated that Raman analysis of the phosphorus trichloride system was feasible. When typical process materials were used, sample fluorescence prevented the analysis. Freeman et al. subsequently demonstrated that using 1064-nm excitaion and an FT-Raman spectrometer, it was possible to monitor on-line heel chlorination. The initial setup used an unfiltered windowed probe coupled to the analyzer (a FT-Raman spectrometer, excitation wavelength = 1064 nm) via a fiber-optic cable. 

The 78S-nm line of the diode laser used in these experiments is transmitted voy efficiently by optical fibers. Moreover, this wavelength is ideal for measuring SER spectra of highly luminescent con unds that have visible absmption bands. This wavelength is far removed from most electronic absorption bands and thus produces little or no fluorescence. Furthermore, the spectral range of a typical Raman spectrum using 78S-nm excitation is within the range of photomultiplier tubes thus, diode lasers can be used with conventional spectrometers. 

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