Fiber-Optic Probe Technique

Fig. 13.13 The Fiber-Optic Probe Technique [73], a A typical optical probe system, b A U-shaped fiber-optical sensor, c A typical optical-probe signal and discrimination method. These figures are drawn based on smUar figures in Goldstein [73] (pp. 518 and 520)...

The movement of a fiber-optic probe and/or the hbers leading to/from a probe or cell can also induce baseline shifts in the recorded spectra. These shifts are usually minor and can be eliminated with proper preprocessing of the data (e.g. derivatives). (A detailed description of preprocessing techniques can be found in Chapter 12)... 

With the appropriate fiber-optic probe and data processing techniques, UV-vis spectroscopy may be used to determine the optical thickness of a transparent thin film. It is possible to simultaneously measure thickness of different layers in a multilayer structure as long as each layer falls within the analysis range of the instrument. Typically, this means layers in the 0.5-150/rm range. A further constraint on this technique is that the layer structure of the film must be smooth on the scale of the spot size of the fiber-optic probe. Neighboring layers must have different indices of refraction in order for them to appear as distinct layers to the analyzer. 

S.2.2.2 ICLS Example 2 This example discusses the determination of sodium hydroxide (caustic) concentration in an aqueous sample containing sodium hydroxide and a salt using NIR spearoscopy. An example of this problem in a chemical process occurs in process scrubbers where CO, is converted to Na,CO and H,S is converted to Na,S in the presence of caustic. Although caustic and salts have no distinct bands in the NIR, it has been demonstrated that they perturb the shape of the water bands (Watson and Baughman, 1984 Phelan et al., 1989)-Near-infrared spectroscopy is therefore a viable measurement technique. This method also has ad tages as an analytical technique for process analysis because of the stability of the instrumentation and the ability to use fiber-optic probes to multiplex tlie interferometers and Icx ate them rcm< >tely from the processes. 

There are many examples of second-order analyzers that are used in analytical chemistry including many hyphenated spectroscopic tools such as FTIR-TGA, IR-microscopy, as well as GC-MS, or even two-dimensional spectroscopic techniques. Another hyphenated technique that is being developed for the study of solid-state transitions in crystalline materials is dynamic vapor sorption coupled with NIR spectroscopy (DVS-NIR).26 DVS is a water sorption balance by which the weight of a sample is carefully monitored during exposure to defined temperature and humidity. It can be used to study the stability of materials, and in this case has been used to induce solid-state transitions in anhydrous theophylline. By interfacing an NIR spectrometer with a fiber-optic probe to the DVS, the transitions of the theophylline can be monitored spectroscopically. The DVS-NIR has proven to be a useful tool in the study of the solid-state transitions of theophylline. It has been used to identify a transition that exists in the conversion of the anhydrous form to the hydrate during the course of water sorption. 

Research performed by Charles DiMarzio s students [29] at Northeastern University is on the study of blood oxygenation. For the first application, a fiber optic device has been built to measure blood oxygen in the brain. A NIR beam is shone on the patient s head at one point via a fiber optic probe, and a second probe is used to collect the energy from a second point. This technique is expected to be useful in brain surgery in newborns. The other application for a noninvasive NIR probe is for use in measuring oxygen levels in the skin. This second device would be useful for patients with bums, skin ulcers, and other skin problems. 

A second area of on-line spectroscopy that has experienced breakthrough technology improvements is near-infrared. Three areas of instrumentation have seen this technology breakthrough (a) computer controlled scanning and detection capabilities, (b) development of fiber optic probes, and (c) implementation of chemometric techniques. As in the FT-IR instrumentation, near-IR has benefited immensely from computer control to either scan the entire spectrum or to monitor specific areas. Equally important from a process perspective is the ability to use optical fibers to transmit the near-IR light between the process and the instrument. This allows the instrument to be placed in an area that does not... 

The Raman spectrum shown in Figure 18-10 illustrates how a fiber-optic probe can be used to monitor chemical processes. In this ca.se a fiber-optic probe w-as used to monitor the hanging drop crystalli/ation of aprotinin (a serine protease inhibitor) and (Nll4)-S04 in aqueous solution. Kaman bands were attributed lo both the protein and the salt. By using chemomet-ric techniques, changes in the spectrum during crystallization were correlated with depletion of both the protein and the salt. The authors were able to determine accurately supersaturation of aprotinin using this technique. 

FT-NIR spectroscopy in combination with a fiber-optic probe was successfully used to monitor living isobutylene, ethylene oxide and butadiene polymerizations using specific monomer absorptions. In the case of EO a temperature dependent induction period was detected when 5ec-BuLi/ BuP4 were used as an initiating system. This demonstrates the usefulness of this technique because this phenomenon had not been observed so far by other methods. We have also successfully conducted experiments in controlled radical polymerization. Then we were able to monitor the RAFT polymerization of A -isopropylacrylamide (NIPAAm). Thus it can be expected that with the help of online NIR measurements detailed kinetic data of many polymerization systems will become available which will shed more light onto the reaction mechanisms. Consequently, FT-NIR appears to be a method, which can be applied universally to the kinetics of polymerization processes. 

Although a lot of information can be obtained with the analysis of diluted polymer lattices samples, as shown previously, the advantages that in situ analysis of the emulsion polymerization processes represent over systems that require dilution are well documented. A dilution step implies sample manipulation and increased measurement dead times. Therefore, regardless the effectiveness of the dilution strategy, there is incentive in continuing the development of in-line and in-situ measurement probes. Of the light based techniques reported only Raman and Near Infrared probes have been used for in situ to monitor conversion." The use of light and specifically fiber optic probes as a particle concentration and particle size detectors is a concept that has become widely accepted. ... 

---