Quartz Dewar

Fig. 7. Diagram of spectrophosphorimeter.28 L, light source Mi, Ms, Hilger D 247 quartz prism monochromators Di, D2, chopper discs driven by synchronous motors B, silica plat beam splitter F, 0.5-mm. silica optical cell containing fluorescent screen solution Pi, monitoring photomultiplier P2, fluorescence-phosphorescence photomultiplier Q, fused quartz dewar containing sample cell.

Visible Spectra. These were taken on frozen isopentane solutions in a rectangular quartz cell cooled in liquid nitrogen in a quartz Dewar flask, using a Cary ultraviolet spectrophotometer. 

The support of this work by the Robert A. Welch Foundation (Grant F-042) and the Petroleum Research Fund of the American Chemical Society is gratefully acknowledged. The authors also thank Ben A. Shoulders for his patient help with the low temperature NMR determinations and Clifford Becker for the use of his low temperature quartz Dewar flask for visible and ultraviolet spectra. 

As the e.s.r. spectra of the deposits cannot be observed in situ a sample has to be removed from the drum and transferred to a glass tube which can be inserted into the sample cavity of the spectrometer. In order to prevent the trapped radicals from diffusing and reacting the deposit must be kept close to 77°K and under high vacuum (ca. torr) during the transfer and while the spectrum is being recorded. The transfer system involves the removal of the deposit from the smrface of the drum by a mechanical scraper (Bennett and Thomas, 1964). The sample tubes are placed in a quartz Dewar that contains liquid nitrogen and can be inserted into the sample cavity. The e.s.r. spectra are recorded on a standard Varian EPR spectrometer. 

A solution of 20 mg (0.07 mmol) of 1.1.2a in 1 mL of 2,2-dimethylbutane/n-pentane (8 3, Rigisolve, Merck) was irradiated at -196 °C in a quartz tube with a low-pressure mercury lamp (Vycor-reactor, Grantzel quartz dewar flask, charged with liquid nitrogen). After evaporation of the solvent 12 mg (66%) of 1.1.2b could be obtained. 1.1.2b could be purified by sublimation at 0.01 Torr and 35 °C. 

Variable temperature operation was achieved by placing the quartz sample tube within a standard quartz dewar insert through which nitrogen flowed at a controlled temperature in the range -130 to 30°C. Compared with the majority of previous cell designs, the Bond cell offers several advantages in that variable temperature experiments are possible, only small volumes of solution are used (helpful where materials are precious),... 

The optical spectra were measured with the aid of a special quartz Dewar flask. Its construction practically precluded boiling of the liquid nitrogen in the cell in which the sample was located (48). This prevented accumulation of frozen water and carbon dioxide on the samples. The frozen samples were irradiated in quartz cells of 2-4 mm. in thickness. 

Pope Scientific, Inc., N90 W14337 Commerce Dr., Menomonee Falls, WI 53051. (glass and quartz dewars)... 

Fig. 2. Low-temperature photolysis apparatus. A, 2S4-nm lamp B, quartz photolysis tube (see Fig. 1) C, Rayonet photoreactor D, double-walled quartz dewar E, magnetic stirrer F, Platinum RTD (available from Omega Engineering, Inc., No. KGC-0107) G, resistance heater (available from Watlow Electric Mfg. Co., St. Louis, Missouri 63146 No. G2J46, Firerod Cartridge Heater, 120 V, 250 W) H, double-walled reservoir Dewar for liquid nitrogen I, insulated stainless steel tubing J, variable transformer K, proportional controller, L, glass wool and cheese cloth.

A schematic drawing of the typical X-band ( 9.35 GHz) ODMR spectrometer used to obtain the results reviewed in this chapter is shown in Figure 7.4. The sample is inserted in the quartz dewar of an Oxford Instruments He gas flow cryostat, at the centre of the optically-accessed microwave cavity, which is positioned between the poles of a conventional electromagnet. 

For EL-. (7-, and C7ph-detected resonances (see Section 1.4.3), thin copper wire leads were inserted into the quartz dewar of the lie gas flow cryostat to provide bias. The cr- and cTpu-detected resonances were recorded by measuring the microwave-induced changes in the current resulting in similar changes in the voltage across a standard resistor connected in series with the LED. 

Photoluminescence spectra were obtained with a SPEX Fluorolog 2 instrument (JobinA von) in which the monochromators were equipped with single gratings of 1200 lines/mm, blazed at wavelengths of A= 330 nm and A= 500 nm for excitation and emission, respectively. The instrument s focal length was / = 0.22 m for the commonly selected slit widths of 0.25 mm, the spectral bandwidth of the instrument was typically 1 nm. The powdered solid samples were inserted into the center tube (ca. 3 mm inner diameter) of a quartz dewar with which measurements could be performed at room temperature and at the temperature of liquid nitrogen (77 K). 

The room temperature fluorescence excitation and emission spectra were recorded for the 1000 ppm solutions of the compounds. The same solutions were then transferred to the thick-walled sample tubes aligned in the quartz Dewar flask containing liquid nitrogen. The low temperature luminescence excitation and emission spectra were then recorded. The phosphorescence spectra at -196 °C were then obtained after the phosphoroscope rotating can had been fitted. The phosphorescence life-time was determined by observation of the decay curve of the phosphorescence on the X-Y recorder or with the oscilloscope. 

Fig. 14.2 Silvered quartz Dewar and phosphorescence tube for low temperature work. The sample is placed in the narrow tube, which is then inserted into the vertically held liquid nitrogen filled Dewar. Excitation and emission is carried out through the unsilvered nipple on the Dewar. The 5p UK coin, included to give some idea of scale, has a diameter of 18 mm...

A quartz Dewar (Fig. 14.2) as described in Sect. 14.3.3 is a relatively cheap alternative for low temperatme work. 77 K is the most convenient temperature to work at, but thermostatting at various other temperatures down to 113 K is possible using solid CO2 or liquid nitrogen slush baths [1, 38]. If a thermocouple or thermistor can be placed in the Dewar the temperature can be monitored, and measurements made, as the whole assembly warms to ambient temperature. 

Low temperature SERRS spectra were obtained by adsorbing the purified flavodoxin onto the electrochemically roughened Ag electrode and flash freezing the electrode in liquid N2. The electrode was then transferred to a quartz Dewar flask containing liquid N2. The Dewar flask was constructed with a transparent body to allow direct acquisition of the SERRS spectrum of the electrode in liquid N2. 

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