Liquid cells variable-thickness cell

Three types of sample cells are used in liquid analysis sealed, demountable, and variable thickness cells (fixedand rotating windows). Fig. 7 shows a circular... 

The sample to be analyzed, say C60 fullerene, is mixed with an appropriate amount of KBr in an agate mortar and then transferred into a press and compressed at 4,000 Kg into a pellet with a diameter of 1.2 cm and a thickness of 0.2 cm. The pellet was mounted into the sample holder of the Specac variable temperature cell and inserted into the cell. The cell was then evacuated with the aid of a pump to a vacuum of 0.1 torr and then heated gradually at 120°C in order to permit the humidity absorbed on the internal surfaces of the cell and in the KBr pellet to evaporate. The sample was then cooled to the desired temperature to record the infrared spectrum. In order to go below room temperature, use was made of liquid nitrogen, added cautiously and in small amount in the cavity present inside the cell. Such cavity is connected with the sample holder and permits to cool the sample to the desired temperature. The temperature of the sample was monitored with adequate thermocouples. The lowest temperature reached with this apparatus was -180°C (93K) while the highest temperature was +250°C. Heating is provided by the Joule effect and an external thermal control unit. 

A variety of cells is available for the liquid samples. A liquid cell may be as simple as a drop of liquid sandwiched between two windows to form a capillary film, or as refined as a variable-path cell costing several hundred dollars (see Fig. 8.5). The optical path length required for a liquid is usually less than 1 mm, and spacers of different thicknesses are used to establish a suitable path, often by trial and error. Liquid cells whose path length may be altered are called demountable or sandwich, cells (Fig. 8.6) while others with one fixed path length are termed fixed-path cells (Fig. 8.7). If a liquid has to be distilled directly into a cell, then it must be connected to the transfer system by a pipe connection. 

Many types of cells are available from manufacturers for use with liquids. Various window materials are available (NaCl, KBr, CsBr, AgCl, CaFz, BaFz, IRTRAN-2, thallium bromoiodide, etc.) m both the demountable and the sealed cells. For these cells there are available spacers of various thicknesses of lead. Teflon, polyethylene, etc. Since the fixed-thickness sealed cells often contain lead spacers cemented to the windows by a mercury amalgam (and these metals are biological poisons) it is wise to consider whether the use of such cells is advisable in a particular instance (e.g., an aqueous enzymic system in one such cell). Cells are available for semimicro, micro, and ultramicro work. Variable path cells of the micrometer or wedge type are available, and these are particularly useful for solvent compensation. 

Xiong et al. proposed a microlens possessing both tunable focal length and variable light transmission [11]. The structure of the device is depicted in Figure 6.12. It contains a liquid cell sealed with a 100 pm thick membrane layer of elastic PDMS on its upper surface. The cell is connected to a syringe via a tube for tuning. The liquid filled into the liquid cell in this work was an aqueous solution of 2.5% poly-N-isopropylacrylamide (PNlPAAm). 

Hirt and King have described the use of variable thickness (micrometer Baly cells for short-path measurements in the ultraviolet. Mitzner used thin cells to get the ultraviolet spectra of pure liquids in order to avoid solvent effects. Jones and Keir described a low-temperature cell for the Cary. Hamner et aL described a 50-cm heated gas cell for the ultraviolet. Cylindrical silica gas cells of 100 mm pathlength are commercially available. So are continuous-flow cells, short-path cells (to 0.3 mm), 50-cm... 

In addition to the above two types of liquid cells, there is another type, in which the path-length is continuously variable from close to 0 to 10 mm. This variable-thickness cell is also commercially available. It is particularly useful for measuring the concentration dependence of the absorption intensity of a solute in a solution over a wide concentration range. Such measurements can be carried out only in a wavenumber region where the solvent does not have a strong absorption band. Because of this limitation, the variable-thickness cell is more useful in the near-infrared than in the mid-infrared region. 

Figure 7 Diagram of mucous blanket of the conducting airways Although the exact structure of the mucous layer of the airways is unknown, the diagram shows some of the features that have been demonstrated in recent light and electron microscopic studies. An osmiophilic film is seen at the air-liquid interface, which has a multilaminated appearance and exists in various degrees of thickness (see inset). Beneath this surfactant film lies an aqueous hypophase, again of variable thickness, in which are found macrophages, mucus, and osmiophilic lamellar structures. CC, Clara cell CEP, ciliated epithelial cell GC, goblet cell.

Refractive index data are very useful for the quantitation of isotropic (liquid and cubic liquid crystal) phases, and for the calibration of cell thickness and nonflatness. Hovever, the analysis of birefringent phases using refractive index data has been found to be unreliable (9). A problem arises from the fact that the orientation of such phases relative to the direction of the light path, as veil as the system variables, influence refractive indices. In order to use refractive index data for quantitation, a phase must spontaneously orient in a reproducible fashion. Such orientation does occur in the case of fluid lamellar phases (as in short chain polyoxyethylene nonionic systems (7)), but viscous lamellar phases, hexagonal phases, and crystal phases do not orient to a sufficient degree. 

Liquid or solution samples are the most straightforward to analyze, for they can be deposited directly as thin films between the two windows, in a cell with fixed or variable sample thickness. The effects of solvent absorption in a solution can be compensated for by comparing the spectra of the solution and the pure solvent. However, it might be necessary to use several solvents so that regions obscured by bands of one are not obscured in the spectrum of another. 

As the rate of twist in a liquid crystal cell increases, the ability of the layer to rotate the plane of polarization of light is diminished. The light becomes elliptically rather than linearly polarized, and the major axis of the ellipse is rotated. This is the situation in the liquid crystal cells used for the super-twisted nematic display. Such displays are therefore normally operated in a variable birefringence mode [25]. In order to allow for the optical rotatory power of the liquid crystal layer, special choices of the optical thickness of the layer and the angles at which the polarizing filters are set allow efficient... 

Yang and coworkers [35, 897-899] performed detailed studies of the electro-chromism of P(ANi) in liquid and solid electrolytes, with polymeric and other dopants, and in multilayer systems. They monitored effects of variables such as pH, film thickness, multilayer configuration and combination dopants. Again, however, few of these studies, at least in the public domain, described actual, working devices, as opposed to laboratory studies in electrochemical cells. 

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