Variable temperature cells

The low temperature apparatus consisted of a variable temperature cell from Specac model P/N 21525 equipped with KBr windows and sample holder which is able to work in the range comprised between +250°C and -196°C. 

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. 

The Figs. 10.1 and 10.2 show the variable temperature cell from Specac outside and mounted inside the FT-IR spectrometer. 

Fig. 10.2 The variable temperature cell installed inside the FT-IR spectrometer. The temperature control reports -176°C (97 K)...

The variable-temperature cell in which reactive intermediates can be studied at temperatures down to — 130°C (Figure 6.18) is designed with opposed, coaxial working, and Luggin-probe electrodes surrounded by a platinum-coil counter electrode. The cylindrical symmetry provides uniform current and po-... 

Specialist suppliers offer variable temperature cells which operate in the range —185 to 250°C. Micro cells with volume of 6/d are also available. The use of microscope cells of low aperture size (3.5 x 0.5 mm) and a volume of less than 0.4//1 requires a beam condenser. 

The solvent used by Marmur and Doty (1962) was 0.15 M NaCl + 0.015 M sodium citrate adjusted to pH 7.0+0.3 containing about 20 pg/ml DNA. The optical density with a 1 cm light path at 260 mp of the solution was measured as a function of temperature using a spectrophotometer equipped with a variable temperature cell. Evaporation must be prevented and a correction for thermal expansion is required. is the temperature at which the absorbance increase is 50% of the final, maximum, absorbance increase. The use of other sodium ion concentrations drastically affects the results. The necessary corrections are described by Owen et al. (1969). 

Ultrasonic sound velocity measurements provide an easy method of determination of the elastic properties as a function of temperature by using a suitable variable temperature cell. 

Variable temperature cells are available which can be controlled to within 0.1°C over the range from -180 to 250°C. An electrical heating system is used for temperatures above ambient, with liquid nitrogen and a heater for low temperatures. These cells can be used to study phase transitions and the kinetics of reactions. As well as transmission temperature cells, variable temperature ATR cells and cells for microsampling are also available. 

Variable temperature spectroscopy, both IR and Raman, can be achieved either by using a variable temperature cell or chamber in a standard spectrometer or by including a variable temperature stage when using an IR or Raman microscope. Variable temperature cells for FT-IR use DRIFTS... 

The principal method under this subheading is the photochemical initiation of reaction. The purpose of photoinitiation could be either to cause further photochemical reactions or initiate a process that is followed by a thermal reaction. Common methods of monitoring the subsequent processes are UV/visible spectrophotometry or time-resolved IR (TRIR), whether the reaction is fast or slow, or at ambient or elevated pressures. In some highly specialized fields, reactions are not initiated by a flash of the flash photolysis method, but by a laser, and subsequent processes can be in the micro- or nano- or picosecond range. The detailed technical aspects of such apparatus are beyond the scope of this article, but references are cited for reader s examination. Sophisticated apparatus such as the high-pressure/variable temperature cell and flow system enable fine details of mechanisms to be delineated in suitable reactions. Examples of reactions in this genre will be described below. 

The kinetics of the reaction of this product with a mixture of CO and -PrBr (halocarbons arc used as promoters in the molybdenum-catalyzed hydroxycarbonylation mechanisms) have been monitored. Analysis of the results of TRO spectroscopy showed that the transient decay could be fitted differently depending on the CO concentration, and two reactions could be distinguished. One was the reaction of Mo(CO)s(S) with CO to reform Mo(CO)6 and the other was the reaction of the hexacarbonyl to yield Mo2(CO)n. These findings were consistent with earlier results. In the presence of -PrBr, the value of obs increased linearly with increasing [ -PrBr], yielding iRBr = (5.3 0.7) x 10 dm moF s and the non-zero intercept of (5.2 0.6) x 10 s equal to the value of /fobs in the absence of -PrBr for Pco = 1 aim. Higher pressures of CO could be used in parallel TRIR experiments with the special high-pressure/ variable temperature cell. The activation parameters derived from /fco measurements were AHco = 33 3 kj moF and ASco = -7 11 J moF K extracted for the reaction... 

Variable temperature cells generally have plenty of free volume to allow for expansion of the solvent and to prevent high pressures building up, leading to leaks. 

Figure 2-8. Variable-temperature cell utilized for the spectroscopic characterization of the electric field-induced alignment of liquid crystals, a) Complete assembly of the accessory, (b) Expanded view of the sample cell.

The variable-temperature cell was the same as described in Section 2.5.2, the only difference being that a spacer with a uniform thickness of 5 pm was used between the electrodes. The empty cell and the FLCP were heated to 90 °C to introduce the... 

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