Temperature Control of the Sample

An alternative to a saddle coil would be a solenoid coil which, however, would have to be oriented perpendicularly to the magnet bore and thus to the physical axis of the probe assembly. Due to spatial constraints, such an arrangement complicates considerably sample insertion, especially when the sample temperature has to be controlled and the assembly has to include an enveloping dewar for temperature control of the sample. ... 

The injection temperature can be a signiflcant issne for thermally unstable samples or where samples are stored for hours in an antosampler prior to injection. For this reason, most manufacturers sell autosamplers with optional thermostated sample compartments. This can be done either by placing the sample tray in an air bath oven or by a condnctive temperature control of the sample rack. The need to keep samples cool prior to injection when conpled with elevated temperature separation increases the complexity of the flow system reqnired. For such application, a separate mobile phase pre-heater with a low volnme placed between the injector and the column is a good choice. Alternatively, the injector valve wonld need to be monnted ontside the antosampler or in the column oven to insure preheating of the mobile phase before the colnmn. 

Most laboratories have a standard low-speed centrifuge used for routine sedimentation of relatively heavy particles. The common centrifuge has a maximum speed in the range of 4000 to 5000 rpm, with RCF values up to 3000 X g. These instruments usually operate at room temperature with no means of temperature control of the samples. Two types of rotors, fixed angle and swinging bucket, may be used in the instrument. Centrifuge tubes or bottles that contain 12 or 50 mL of sample are commonly used. Low-speed... 

For the purposes of conversion monitoring of emulsion polymerization, we have found the DMA40D with a precision of +1 x 10 g/cm capable of resolving monomer conversion to +0.2% in the absence of thermostating and sampling errors. On-line, in the presence of such possible errors, a resolution of at least +0.5% can be expected. Care must be taken to ensure a representative sample and good temperature control of the sample stream before introducing it into the instrument. Some example results with this instrument are presented below. 

Temperature control of the sample must be precise the experiment is sensitive to thermal convection currents within the sample. 

The scattering chamber consists of the sample holder and the associated optical components. For most work, temperature control of the sample must be included (at least to 0.1°). If only a small amount of the sample is available necessitating use of a small volume scattering cell, the cell should be surrounded with a fluid such as deca-hydronapthalene which matches the index of refraction to that of the scattering cell. In this way the scatter caused from the change in refractive index at the air/glass interface can be substantially removed from the vicinity of the scattering volume. 

Fred and Scribner describe a variation of the fractional distillation technique. The analysis of uranium oxides or uranium ores is complicated by the complex spectrum of uranium. To obtain good temperature control of the sample during arcing, an electrode assembly as shown in Figure 8-13 was devised and constructed of graphite. An anode cup was prepared with fairly thick walls and mounted on a tapered pedestal. The sample was placed in the lower one-fourth of the electrode cavity. The arrangement of Figure... 

Figures 4.179 and 4.180 illustrate two furnaces for TGA for different temperature ranges. The figures are self-explanatory. Several different sample holders are shown in the bottom portions of the figures. The multiple holders can be used for simultaneous thermogravimetry and DTA, the single cmcibles are used for simple thermogravimetry. The major problem for the combined thermogravimetry and DTA technique is to bring the thermocouple wires out of the balance without interference with the weighing process. Even the temperature control of the sample holder may be a major problem in vacuum experiments since the thermocouple does not touch the sample. The cmcibles are made of platinum or sintered aluminum oxide. Typical sample masses may vary from a few to several hundred milUgrams.

Temperature control of the sample is one of the most basic reqnirements for an experiment. This is especially the case when one performs time-resolved experiments in which one wants to determine the stmctnral phases in a temperature-composition diagram (64,108). The required accin-acy is dependent... 

Another technique in an early stage of development - spin polarised electron energy loss spectroscopy (SPEELS) - is expected to have particularly strong impact on rare earth surface science. In electron spectrometers with good temperature control of the sample the technique should unfold a wealth of interesting magnetic phases at surfaces and in ultra thin films on substrates. 

This method has been used by Gallagher (1978) and by Price ef flZ. (1980). In these units the temperature control of the sample and temperature programming is provided. The method decomposes the sample under a very high vacuum. The product gases have little time to decompose, so side reactions are eliminated. It should be noted that because of these unconventional features the results should not necessarily coincide with conventional data. The product gases are analyzed by mass number, but the solid residue cannot be determined in situ. 

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