Optical interfaces

Partial reflections at the iimer optical interfaces of the interferometer lead to so-called secondary and tertiary fringe patterns as can be seen from figure B 1.20.4. These additional FECO patterns become clearly visible if the reflectivity of the silver mirrors is reduced. Methods for analysis of such secondary and tertiary FECO patterns were developed to extract infonnation about the topography of non-unifonn substrates [54]. 

A grism consists of a prism with a grating optically interfaced to one side. The usual configuration is shown in Fig. 5. The prism deflects the zero-order... 

Tena et al. [52] carried out a screening of polyaromatic hydrocarbon types in soil by on-line fibre optic interfaced supercritical fluid extraction spectrofluorimetry. 

The apparatus incorporates a fibre optic interface for the spectrofluorimetric measurement on the supercritical carbon dioxide emerging from the extraction cell of a supercritical fluid extractor, prior to depressurization from up to 350 bar. Recoveries of polyaromatic hydrocarbons are between 89 and 107%, and measurements can be carried on with a relative standard deviation of less than 5%. 

Young and Weber [397] presented an equilibrium and rate study of analyte-matrix interactions in SFE in aqueous matrices, while correlation of SFE with supercritical fluid chromatography (SFC) in aqueous media has been reported by Yu et al. [398]. Tena et al. [399] screened PAHs in soil by on-line fiber-optic-interfaced SFE spectrometry. 

Section 5.3 described a number of alternative design and implementation strategies for near-infrared analyzers, suitable for operation in a process analytical environment. However, none of these analyzers can operate without a robust, maintainable and repeatable sampling interface with the process sample under consideration. In addition to this question of the optical interface to the sample, there is a whole wider area of concern, which is how far the particular sample interface is representative of the sample in the process as a whole. This complex issue is not addressed here, and is dealt with separately in Chapter 3. 

In terms of practicality the optical interface to the sample is certainly the most demanding aspect of all. It requires a number of issues to be addressed and clearly understood for each specific application to be attempted. These include ... 

The latter is particularly the case for laser instruments where attention to optical interfacing is essential, and often features specialized optical hber based optical coupling. 

Figure 3.24 Absorption, scattering and reflection of light as it passes through matter. Iq is the incident beam intensity, It the transmitted beam intensity, Ir and If are the intensities of light reflected at the optical interfaces and Is is the total intensity of scattered light...

Note that the examples above are over-simplifications to some extent, and other critical elements, such as optics, the associated electronics and the sampling interfaces are also important for a final implementation. The latter is particularly the case for laser instruments, where attention to optical interfacing is essential and often features specialized optical fiber-based optical coupling. 

Figure 13.10 Photograph showing fiber-optic interface for collecting noninvasive near-infrared spectra from a rat skin model.

To obtain vibrational spectra tuned by external pressure one needs - in addition to the infrared or Raman spectrometer - a special high-pressure optical sample cell, an optical interface between the sample cell and the spectrometer, and a device to measure the pressure on the sample [i]. 

In view of the small sample size, the optical aperture used for pressure tuning vibrational spectroscopic measurements is also very small. This does not create serious problems for high pressure Raman spectroscopic measurements since the laser beam can be focused to 30-40 pm. Thus, the optical system employed in a standard Raman scattering experiment can be used and no special optical interface is required for the pressure tuning Raman spectroscopic measurements. 

The FAMOS initiative, introduced above, presented a flow cell (Figure 4.75) for optical inspections connected to one of the micro structured reactors in their new micro reaction system [124], The flow cell measures only a few cubic centimeters and the actual optical interface to a spectrometer consists of a macro-scale table-top device with a clamping mechanism which positions the flow cell in the optical pathway. 

Fig. 2.20. High-sensitivity cell incorporated in the optical interface.

Initial tests, as shown in Figures 14-19, demonstrate clearly that the performance goals have been achieved. These goals are shown in Table 2. While Fig. 19, a composite of photographic recording and a photometric scan of the line spectrum at the fiber optic interface, demonstrates the result of these tests. 

Figure 19. Composite of photographic and scanning photometric microscope recording at the fiber optic interface of a partial spectrum of PFK from approximately mass 525 to mass 786...

Figure 2-15 Schematic of FT -Raman instrumentation fitted with a bifurcated fiber optic interface. (Reproduced with permission from Ref. 26.)...

Fiber optic interfaces are ideal for monitoring chemical processes. The fiberoptic FT-Raman system shown in Fig. 2-15 was used to demonstrate the... 

Leverette, C.L. and Dluhy, R.A. (2000) A novel fiber-optic interface for unenhanced external reflection Raman spectroscopy of supported monolayers. Langmuir, 16, 3977-3983. 

The spatial resolution of FT-IR microspectroscopy, without sacrificing spectral quality and resolution, makes imaging possible. Shortly after the introduction of the first research-quality IR microscope by Messerschmidt and Sting in 1986, Wetzel, Messerschmidt and Fulcher reported spectra obtained from wheat kernel transverse sections in situ, and compared them with flour milling fractions [7]. This was achieved with an accessory IR-PLAN microscope optically interfaced to a Nicolet interferometer bench. Subsequently, at the Agriculture Canada laboratory the same model IR-PLAN was interfaced to a Bomen Michelson IR 100 spectrometer such that, over the period of a year, transverse sections of wheat kernels, vanilla beans, peppercorns and soybeans were manually line-mapped to reveal any differences in microchemical structural characteristics between their different botanical parts [8]. 

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