Variable path length cells

Fig. 3.5(c) Carbon disulphide (recorded in a variable path length cell, set at 0.015 mm path length). 

The most accurate way of compensating for solvent absorption is to use the more expensive variable path length cells (Fig. 3.7) in which one of the plates which constitute the liquid cell can be moved with the aid of a micrometer device to allow adjustment to any required path length. This allows the accurate matching in the spectrophotometer of two such cells filled with the appropriate solvent. One cell may then be emptied, cleaned and refilled with solution so that the spectrum of the solute may be recorded. 

Fig. 7. A detailed drawing of the W-cell (Vacuum Variable path length cell). The cell mount may be attached to adaptors for the Cary model 17 or an IR spectrophotometer. The optical path goes through the center of the cell, passing through two sapphire windows and an annular opening in the calibrated handle used to turn the micrometer in order to vary the path length.

Silvera, I. F., Variable-temperature, variable path-length cell for infra-red studies of liquefied and solidified gases. Rev. Sci. Instrum. 41, 1592-1594 (1970). 

A very short path length cell can be installed, or a variable path length cell can be used. Another option is to set the wavelength at that which the solutes have very small extinction coefficients. If possible, a detector specifically designed for preparative work should be used, but there are a limited number of these available. 

The demountable is by far the easiest to maintain as it can be readily dismantled and cleaned. The windows can be repolished, a new spacer supplied and the cell reassembled. The permanent cells are difficult to clean and can become damaged by water. The pathlengths need to calibrated regularly if quantitative work is to be undertaken. Variable path-length cells suffer from similar disadvantages and they are difficult to take apart. The calibration therefore suffers and the cells have to be calibrated regularly. 

Liquid samples are easily studied with the aid of a wide variety of liquid cells, including heatable. flow through, and variable path length cells. These cells are constructed of two infrared transparent windows with a spacer between them, thereby forming a cavity for the sample. As the samples are often strong infrared absorbers, for this spectral range the liquid transmission cell must be constructed with short optical path lengths (0.025 -1 mm). 

Although this spectrum does not correspond to any particular ruthenium carbonyl complex, it is consistent with the presence of one or more anionic ruthenium carbonyl complexes, perhaps along with neutral species. Work is in progress with a variable path-length, high pressure infrared cell designed by Prof. A. King, to provide better characterization of species actually present under reaction conditions. 

A high pressure cell with a variable path length for quantitative absorption measurements up to 2,5 kilobar and 250 °C has been described by Buback (1977). This cell makes it possible to determine absorption band intensities with an error of less than 2%. A high pressure cell for absorption studies in the far-infrared region up to 10 kilobar at temperatures between 300 K and 10 K has been described by Medina (1980). The advantages and disadvantages of different window materials are discussed in detail (see Sec. 6.7). 

Romanach and de Haseth [3] have used, in CCC, a how cell for LC-FTIR (liquid chromatography-Fourier transform infrared) spectrometry. The main difflculty is the absorbance of the liquid mobile phase. This problem is exacerbated in EC by low solute-to-solvent ratios in the eluates. On the contrary, CCC leads to a high solute-to-solvent ratio so that it can be used with a very simple interface with a CCC column, without any complex solvent removal procedures. High sample loadings are possible by using the variable path length of the IR detector (from 0.025 to 1.0 mm). 

Spectroscopic and phase behavior studies were conducted using a fixed path length cell. Pressure was increased by adding pure solvent to the cell, so molar concentrations were constant while mole fractions varied. Additional phase behavior studies and solubility studies were conducted in a variable volume view cell, based on an existing design(24), in which mole fractions were held constant but molarities varied. 

Gas analyzer consisting of a variable path length gas cell and infrared spectrometer can be used for the analysis of any gas or vapor having infrared absorption in concentrations ranging from a few parts per million to several percent. 

Figure 8.5. Variable path-length infrared cell. Courtesy of Beckman Instruments, Inc.

Fig. 14.1 Various absorption and emission cells, a Thin pathlength demountable flow cell b 0 cm pathlength cylindrical absorption cell c triangular emission cell for use with highly absorbing solutions d variable path length absorption flow cell with stainless steel sleeve and screw e standard 1 and 5 mm path length absorption cells, and micro cell f 0.1 mm path length cylindrical cell (the 0.1 mm path length is behind the cell face placed down on the table) g 5 cm cylindrical absorption cell h 4 cm absorption cell i standard 1 cm emission cell. The 5p UK coin, included to give some idea of scale, has a diameter of 18 mm...

Often the seal height or gap is variable and unknown. In this case, kh, the product of permeability, k, and seal height, //, can be replaced by A , a permeability per unit of seal perimeter length. For a given stack geometry, the seal volume basis can also be defined to include the seal path length, z, and perimeter, w, to define a permeability per cell or stack as is convenient. 

Liquids are usually analysed with cells which have dismountable IR windows. For qualitative analysis, a droplet of the sample is compressed between two NaCl or KBr disks without a divider. However, for quantitative analysis, either Infrasil quartz cells (with an optical path from 1 to 5 cm) or cells that have a variable or fixed width, generally smaller than 1 mm (see Fig. 10.17), can be used. In the mid-infrared, the latter consist of two KBr or NaCl windows with a spacer. The optical path length must be calibrated and periodically controlled. 

Experimental. All photodimerizations were carried out in a stainless steel fixed volume cell (1.75 cm ID with a 1.0 cm path length) with sapphire windows under the irradiation of a Hanovia medium pressure mercury lamp filtered through water and Pyrex for a 13.5 hour exposure. The cell and lamp assembly have been described previously (31). For selected runs a custom built 0.9 mL variable-volume pump was connected to the cell and the pressure was varied to determine the exact location of the phase boundary, based on light scattering measured in a Cary 2290 UV-Vis spectrophotometer (Varian Inst.). The spectrophotometer was also used to measure the concentrations of the monomeric cyclohexenone before and after reaction. 

The optical high pressure cell shown in figure 2 was developed for spectroscopic investigations at pressures ranging from 1 to 400 bar and temperatures from 20 to 150 °C. The optical cell has a low-cost construction and shows versatile application possibilities (e g. variable optical path length, fluorescence measurements). The temperature is measured in direct contact with the fluid. 

Figure 6.7-6 High-pressure cell with variable optical path length 1 and 2 window, 3 screw, 4 stainless steel ring, 5 stainless steel plug. 6 sheathed thermocouple, 7 packing, 8 stainless steel ring, 9 heating, 10 and 11 screw, 12 movable pi.ston.

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