Fiber-Optic Sampling Probes

It is clear from previous discussions that fiber-optic probe designs differ substantially in sensitivity, background interference, acceptable fiber length, size, and fragility, not to mention cost. As always, the choice of probe depends on the requirements of the application, so there is no best design. Nevertheless, it is useful to address some general comparisons of probe performance. 

In principle, the signal and signal/noise ratio (SNR) figures of merit discussed in Sections 3.4 and 4.4 can be applied to fiber-optic sampling. Equation (3.11) 

The observed F depends on several variables, particularly the fiber numerical aperture, sample depth, interfiber distances, filter losses, and so fourth Similarly, an SNR figure of merit may be determined empirically, which would Incorporate SNR degradation due to fiber background. Equation (12.7) for is identical to Eq. (4.25), and both are based on experimetnally determined variables  

It is theoretically possible to predict F and for different probes, but it is difficult to achieve generality because the results depend on sample properties such as transparency and inelastic scattering efficiency. Nevertheless, for a given type of sample, such as a clear, deep liquid, Fs or F can provide a direct prediction of relative signal strength. 

Type Design Excitation Radius fim Collection radius fim Interfiber spacing (center to center) fim F s (relative to type 1) 

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COMPARISONS OF FIBER-OPTIC SAMPLING PROBES Table 12.2. Calculated Fs > for Several Probe Designs ... 

COMPARISONS OF FIBER-OPTIC SAMPLING PROBES acetaminophen tablet... 

It has already been noted that a Raman analyzer can be useful for studying liquid distillation products [30,32,83], The flexibility afforded by the use of fiber-optic sampling probes allows for composition measurement of either vapor-phase materials directly on the distillation tower or the distillation liquid as it is being removed from the tower. A proof of concept example of this type has been shown by Pelletier [107] for a test mixture of water and acetone. 

Design and selection of the sample interface is vital to provide the best-quahty data for an analysis. The sample interface may be located in the sample cavity of a spectrophotometer, as in the cases of laboratory cuvettes, vials, and flow cells. The sample interface may also be fiber-coupled and located closer to the process. Fiber-optic sample interfaces include flow cells, insertion probes, and reflectance probes. 

J.H. Cho, P.J. Gemperline, P.K. Aldridge, S.S. Sekulic, Effective mass sampled by NIR fiber-optic reflectance probes in blending processes. Anal Chim. Acta, 348, 303-310 (1997). 

Some of the industrial process analyzers are of the probe type (capacitance, fiber-optic IR probes), others can look through the process stream in the pipe (microwave), and the majority require some form of a sampling system. Table 3.27 provides a summary of the ranges and other features of many of the process moisture analyzers. 

In some cases, these environmental requirements are met by locating the spectrometer remotely from the sampling point, in a cleaner, more controlled environment. Fiber-optic probes, described in Chapter 12, have been used successfully when the sampling point is located 100 m or more from the spectrometer. Depending on the application, fiber-optic sampling may be a critical performance requirement. 

Figure 6.2. Schematic of fiber-optic sampling configuration. Fiber lengths between probe and laser/spectrometer can be quite long, up to several hundred meters.

With a fiber optic dip probe, many liquids and solutions can be analyzed with no sample preparation. The use of a dip probe for transmission measurements requires that the liquid or solution be free from small particles or turbidity. Suspended particles scatter light and reduce the sensitivity of the measurement. The already low absorptivity of NIR bands makes transmission measurements of limited use for liquid samples that are not clear. 

The use of coupled onhne spectroscopies (ATR-FTIR/Raman) for the development and the monitoring of industrial chemical processes was demonstrated by Wiss and ZiUan [14], For the investigation of two types of reaction, respective fiber optical spectroscopic probes were installed in a commercial reaction calorimeter. The authors pointed out that such a setup is more appropriate for industrial applications than bypass systems for sampling, and can be used in a pilot plant or production plant without major modification of the available equipment. 

Figure 7 Nonfocusing fiber optic Raman probe. The sample is placed in the region where the excitation and collection paths overlap (A) illustrates the overlap for a nonfocusing probe having a single excitation fiber and a single collection fiber (B) illustrates a comrmn configuration using a single excitation fiber and multiple collection fibers.

Figure 7 illustrates a nonfocusing fiber optic Raman probe. Diverging laser light from the delivery fiber illuminates the sample. Adjacent optical fibers collect light scattered back from the sample and deliver the light to the spectrometer. The collection... 

Figure 8 illustrates a focusing fiber optic Raman probe. Light from the laser delivery fiber is sent through a miniature monochromator inside the probehead before being focused onto the sample. Light from the sample is filtered to remove laser light and then injected into the collection fiber for return... 

Fiber optic spectrometers with reflectance probes build an additional degree of flexibility into reflectance spectroelectrochemical measurements. Fiber optic reflectance probes are typically made of multiple fiber optics which direct the light from the source to the sample surrounding one or more fiber optics which collect the reflected light and direct it to the detector. These fibers are usually combined within a single cable which is bifurcated... 

Advances have been made in fiber-optic sampling techniques that overcome problems with sampling high temperature and high viscosity materials. As a result, in situ sampling with fiber-optic probe systems is being implemented for these types of applications. 

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