Long optical pathlength

Spectroelectrochemical experiments can be used to probe various adsorption and desorption processes. In particular, changes in the absorbance accrue from such processes can be probed utilizing the large ratio of surface area to solution volume of OTEs with long optical pathlength (30). Additional information on such processes can be attained from the Raman spectroelectrochemical experiments described below. 

With only weakly absorbing solvents, low concentrations of the species to be detected or weak effects (i.e. small extinction coefficient), a long optical pathlength might be desirable. In a design described by Zak et al. [43], a cell body holding both the counter and the reference electrode is manufactured from PTFE. The working electrode is attached to the cell body with a spacer of about 0.1-mm thickness. The... 

Near-Parallel Configuration Long Optical Pathlength Thin-Layer Cell and Spatially Resolved Spectroelectrochem istry... 

The long effective pathlength and high surface area afforded by these colloidal semiconductor materials allow spectroscopic characterization of interfacial electron transfer in molecular detail that was not previously possible. It is likely that within the next decade photoinduced interfacial electron transfer will be understood in the same detail now found only in homogeneous fluid solution. In many cases the sensitization mechanisms and theory developed for planar electrodes" are not applicable to the sensitized nanocrystalline films. Therefore, new models are necessary to describe the fascinating optical and electronic behavior of these materials. One such behavior is the recent identification of ultra-fast hot injection from molecular excited states. Furthermore, with these sensitized electrodes it is possible to probe ultra-fast processes using simple steady-state photocurrent action spectrum. 

Winer, A. M and H. W. Biermann, Long Pathlength Differential Optical Absorption Spectroscopy (DOAS) Measurements of Gaseous HONO, N02, and HCHO in the California South Coast Air Basin, Res. Chem. Intermed., 20, 423-445 (1994). 

Transfer optics The interface of the IR beam with the sample, especially in a classified hazardous environment, can be a major challenge. Gas samples are not too difficult, although it is important to pay attention to the corrosivity of the gases, relative to the windows and any internal optics, as with a folded pathlength cell. Liquids offer different challenges. For on-line applications, users prefer to minimize the use of valves, bypass streams and auxiliary pumps, especially over long distances between the stream and the analyzer. At times there is a benefit to sample the stream either directly or as close as possible to the stream, and in such cases there is the need for transfer optics between the sample cell and the spectrometer/analyzer. 

Optically smooth samples with 0.3-10 xm thicknesses are placed on a gold covered surface or other high reflecting substrate. Similar to the transmission method except that the pathlength is twice as long. The physical properties are uncompromised. In some cases, optically smooth protein and starch films can be prepared (without cryogenic slicing). 

The last subject in the discussion of inherently chiral compounds deals with the analysis of the aminoacids, peptides, and proteins. Most all of the remarks that were made about the steroids and carbohydrates regarding CD detection apply equally well to these. The enantiomeric purity of aminoacids is usually determined by their optical rotations at the sodium-D line. Rotations are normally so small that concentrated solutions and long pathlengths are needed. The detection is enhanced a little if laser illumination is used [66] or if ORD detection is done around 230 nm [71]. Without derivatization, only aminoacids with aromatic substituents are CD active in the near UV. Signals are generally weak and enantiomeric purity measurements are not quantitative. 

Operation in the near-UV and visible regions is inexpensive since quartz or glass optics may be used. One major concern is the intrinsic fluorescence properties of the fibre core material which, because of the long pathlength of fibre, may interfere with the analytical signal. The same fibre may be used for excitation and emission with an external beam splitter to enable the separation of the two beams. This is illustrated in Figure 3.41, which shows a system... 

No racemization was observed when the electrode potential was scanned only to a value where the dianion is formed. Upon formation of the tetraanion, subsequent chemical reactions were found. With a slightly different electrolyte salt (Mc4NBF4 instead of BU4NF6), reversibility without racemization was found even up to the tetraanion formation. Further examples include the spectroelectrochemistry of vitamin D2 [139], which has been studied with a long pathlength cell similar to the design described by Zak et al. [44]. Optical rotary dispersion and CD of optically active polybithiophene that has been electropolymerized in a cholesteric electrolyte have been studied [140]. The optical rotation of this chiral polymer could be controlled via the electrode potential. 

Figure 1 Examples of gas cells for mid-infrared transmission measurements (A) photograph of a multiple-pass gas cell ( Long Path Miniceir), with a high path-to-volume (530 ml) ratio. Allows paths from 1.2 m (eight passes) to 7.2 m (48 passes) (B) schematic of a multipass cell with transfer optics for use in a center-focus sample compartment (C) and (D) photographs of Pyrex and stainless steel bodied 10-cm pathlength cells, respectively. ((A and B) Reproduced by kind permission of Infrared Analysis, Inc., Anaheim CA, USA. (C and D) Reproduced by kind permission of Specac Ltd., Orpington, Kent, UK.)...

Geometry of optical pathway as parallel as possible within the cell, otherwise problems with long pathlength close to bottom (however, above a necessary stirrer in the cell). 

If the gain medium is an optical fiber a long pathlength can be realized and the threshold is therefore low, which means that a low-power pump laser can be used. Since the most of the pump power is confined inside the core of the fiber by total reflection at the boundary between cladding and core (Fig. 6.32), the pump intensity inside the core is high. Even cw operation of Raman lasers has been demonstrated with silicon as the gain medium [621]. 

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