Sample holder maintaining

The method used to observe the radical ions is very simple, only a sample holder maintained at liquid nitrogen temperature being required. 

The following experimental details apply both to the determination of tungsten with bromine as internal standard, and to the experiments of Table 7-2. The solutions filled a 3-ml container made by sectioning a 10-ml beaker. To prevent" evaporation and to maintain a fixed distance between x-ray tube window and sample surface, the beaker-section was covered with Mylar film, 0.0025 cm thick, placed in a plastic sample holder and pressed firmly against the sample drawer. The Mylar film attenuated the x-rays uniformly enough so as not to affect the precision of the results. 

More modern systems (diffractometers) follow the same principles but the diffracted X-rays are detected with a solid state detector, as described earlier. Typically, the X-ray source is static and the sample and detector are rotated, with the detector moving at twice the angular velocity of the sample to maintain the equivalent angle. Such instruments typically make use of relatively large samples compressed into the window of a 35 mm sample holder. However, where the sample size is restricted, as is common with archaeological applications, a smaller sample (a few mg) can be attached to a silica wafer. In all cases the sample needs to be hnely ground to ensure a uniform diffracted beam. 

Infrared Procedures. Acidity Measurements. Zeolites treated as for catalytic experiments (heating in a stream of dry air and equilibration with H20 vapor at room temperature) were compressed at 1000 kg/cm2. The resulting disks (5 mg/cm2) were mounted in a quartz sample holder which was introduced into an IR cell as previously described (18). They were heated slowly under vacuum (the temperature was raised stepwise up to 450° in 5 hours). At 450°C, 02 was admitted, and the cell, connected to a liquid nitrogen trap, was maintained at this temperature for 4-5 hours. Finally the wafers were evacuated overnight at 450° C. The vapor of thoroughly dried pyridine was allowed to equilibrate with the wafer at room temperature. Afterwards, the pyridine was desorbed at a series of increasing temperatures for 15 hours each time. 

The whole system should be maintained at a constant temperature. This can be achieved by immersing the sample holder in a constant-temperature water bath or by placing the whole system in a thermostatted cabinet. A correction factor needs to be used for the first case. 

Addition of a thin-film heater to the back of a metal sample holder allowed studies of thin polymer films by GRAS at up to 200 °C (121). In a study of an ultrathin film of polymethylmethacrylate, the two doublets near 1240/1270 and 1150/1190 cm"1 exhibited changes in relative intensity above the glass transition temperature of 100 °C, indicating that the polymer glass structure was maintained even in such thin films. 

Light Exposures. Silk fabric samples, 0.25 m x 0.17 m, were mounted in Atlas Electric Devices aluminum sample holders according to AATCC Test Method 16E-1982 (7). An Atlas Ci-35 Weather-Ometer xenon-arc was used on continuous light cycle. Exposures were conducted at an irradiance of 0.35 W/m2 measured at 340 nm and the irradiance was monitored and controlled automatically. Borosilicate inner and outer filters were used to simulate the solar spectrum. The relative humidity was maintained at 65% and the black panel temperature was 50°C. The actual fabric temperature during the irradiation was measured, using small thermocouples threaded into the fabric, and was found to be 35 C. Control samples for these tests were kept in the dark at 35°C and 65% RH for the same time period as the illuminated samples. 

Three kinds of sample holders are available for DTA and DSC (Fig. 4.8.6). Type 1 holders are commonly used for a DTA apparatus. In this type, the sample and reference holders are placed on the same metal block and heated by the same heater the temperature difference between the two holders is indicated by a thermocouple. Type 2 holders are generally used in a quantitative DTA (heat-flux DSC) apparatus. Both sample and reference holders are maintained at the same temperature by two individual internal heaters, which, in turn, are heated by the same main heater. The temperature difference between the sample and the reference material is measured by a thermocouple placed outside of the holders. Type 3 holders are customarily used for a power-compensated DSC apparatus. This apparatus has separate heaters for heating the sample and reference holders thus maintaining the sample and the reference... 

In order to reach a low temperature, the sample may be placed inside a metal block which is flushed with a cooling medium. Such devices reach approximately -78°C. Even lower temperatures are achieved if the sample holder is in thermal contact with a heat exchanger. Liquid nitrogen is usually vaporized in the heat exchanger to cool the sample. To maintain the desired temperature, most sample holders can be heated electrically. Vaporization of liquid helium, typically in a closed cycle, also has a cooling effect. To avoid heat loss and to protect the cell windows from condensed water, the cells are surrounded by a vacuum chamber or a chamber containing a dry gas at atmospheric pressure. 

The powder obtained with the MHQ technique, which has to be maintained below 140 K to prevent further reaction, is more difficult to handle than that produced by the RFQ technique because of its finer structure. To apply the spectrocopies listed above to such a powder, one often has to construct specific adaptations to sample holders and/or sample compartments, in particular, to get the powder where required, without warming during handling and operation. 

The sample holder was then put into a joint PEVD/sensor testing facility, as shown schematically in Eigure 20b. The temperature during both PEVD processing and sensor testing was set at 500°C. Atmospheric pressure was maintained throughout the experiment in both chambers. In order to check the consistency of the results, the study was repeated on two sample systems. 

In thermogravimetry (TG or TGA) the change in sample mass is determined as a function of temperature and/or time. The instrument is a thermobalance that permits the continuous weighing of a sample as a function of time. The sample holder and a reference holder are bounded to each side of a microbalance. The sample holder is in a furnace, without direct contact with the sample, the temperature of which is controlled by a temperature programer. The balance part is maintained at a constant temperature. The instrument is able to record the mass loss or gain of the sample as a function of temperature and time [m = /( )]. Most instruments also record the DTG curve, which is the rate of the mass change dm/dt = f(T). 

Different practical constructions of a Curie point pyrolyzer are commercially available. In these systems, the sample is put in direct contact with the ferromagnetic alloy, which is usually in the shape of a ribbon that can be folded over the sample forming a sample holder. The sample and its holder are maintained in a stream of inert gas in a similar way as for resistively heated filaments. The housing where the sample and its ferromagnetic holder are introduced is also heated to avoid the condensation of the pyrolysate but without decomposing the sample before pyrolysis. Autosample capabilities for Curie point pyrolyzers are also commercially available (e.g. DyChrom modelJPS-330) [11, 12]. 

The magnetic field is maintained for 2 h, and the sample is cooled rapidly on a cold metal surface or with liquid nitrogen as soon as the power supply of the magnet is turned off. The film is then placed in the sample holder of a suitable X-ray diffractometer (with axis vertically aligned) and the diffraction pattern is recorded with a flat-plate camera. 

Cryostat. A cryostat designed for MBssbauer measurements of species isolated in a low temperature matrix was cooled down by a Cryomini D closed-cycle helium refrigerator (Osaka Sanso Kogyo Co. Ltd.) (Fig. 5). The sample holder in the cryostat was maintained at a desired temperature between 15 and 100 K by means of a 40-flanganin heater and a DTC-2 digital temperature controller... 

The behavior of silver ion represents a different type of ashing loss. As opposed to the other metals in this study, only 75 to 90% of the 110mAg could be recovered from the borosilicate sample holder by rinsing with dilute nitric acid. To recover the remainder of the 110mAg, the glassware had to be repeatedly rinsed with a warm mixture of nitric and hydrofluoric acids. By maintaining the system at low temperature this difficulty was ameliorated, but not eliminated. 

In so-called power-compensation DSC, sample and reference are completely isolated from each other (Fig. 3.2). Both the sample and reference crucible have their own heating element and temperature sensing element. With the aid of a temperature programmer, both sample and reference are heated and always have the same temperature. As soon as changes in the sample occur, extra (or less) heat will be needed to maintain the set heating rate. With the aid of specialized electronic circuitry, extra (or less) power is now sent to the sample holder in order to keep the temperature difference zero. In this way, power and consequently heat flow and enthalpy changes are measured. 

Thermal Treatment. A sample-holder of quartz containing 30 grams of Type 4A zeolite is introduced into a stove in which the temperature is regulated at 1°C. The temperature of the zeolite is checked by a thermocouple placed inside the sample and protected by a quartz sheath. The heating and cooling rates are 300 °C per hour. The sample is maintained at the chosen temperature for 2 hours in air at atmospheric pressure. 

A typical DTA apparatus is illustrated schematically in Figure 6.1. The apparatus generally consists of (1) a furnace or heating device, (2) a sample holder, (3) a low-level dc amplifier, (4) a differential temperature detector, (5) a furnace temperature programmer, (6) a recorder, and (7) control equipment for maintaining a suitable atmosphere in the furnace and sample holder. Many modifications have been made of this basic design, but ail instruments measure the differential temperature of the sample as a function of temperature or time (assuming that the temperature rise is linear with respect to time). 

Barrall and Rogers (70) described a constant-pressure device mol a sealed-tube type sample holder) which is illustrated in Figure 6.20. Total cell volume was about 0.8 ml a glass capillary tube provides a reservoir for the gases released during a reaction and prevents significant dilution of the sample atmosphere with air or the loss of gas. The small mercury seal provides for sample atmosphere expansion and maintains an essentially constant pressure in the cell. 

---