Sample heating effects

West, Y.D. 1996. Study of sample heating effects arising during laser Raman spectroscopy. Int. J. Vibr. Spectrosc. 1, 5. 

These same activation energies describe our conductivity data for these samples. Indeed, we can go farther. We have also measured the current-voltage characteristics of semiconducting ITF-TCNQ at higher fields, using a pulsed technique designed to eliminate sample heating effects. The results do... 

Thus, efficient decoupler operation is crucial especially at the higher frequencies where certain samples may absorb more power than at the lower frequencies. Sample heating effects for ionic solutions have been discussed by Led and Petersen (1978) and by Bock, et al. (1980) and a more efficient coil design by Alderman and Grant (1979). A key fact to remember is that ionic heating is an electric field effect like the piezoelectric resonance discussed in VI.B.5. so that some sort of an electrostatic shielding as described there should work here, too. 

Currently the experiment is limited by sample heating effects on the hyperfine relaxation time of the spins, as evidenced by an apparent T] time which varies by up to 20% on timescales of order 5 min. This seems to indicate that too much excess heat is being generated inside the sample volume by the pump pulse, causing temperature fluctuations. In addition, there are various mechanical and electronic problems that limit our ability to take reproducible data over the timescales necessary to do the measurement. 

Capillary Electrophoresis. Capillaries were first appHed as a support medium for electrophoresis in the early 1980s (44,45). The glass capillaries used are typically 20 to 200 p.m in diameter (46), may be filled with buffer or gel, and are frequendy coated on the inside. Capillaries are used because of the high surface-to-volume ratio which allows high voltages without heating effects. The only limitations associated with capillaries are limits of detection and clearance of sample components. 

To protect the sample from stray electrons from the anode, from heating effects, and from possible contamination by the source enclosure, a thin (-2 pm) window of aluminum foil is interposed between the anode and the sample. For optimum X-ray photon flux on the surface (i. e. optimum sensitivity), the anode must be brought as close to the sample as possible, which means in practice a distance of -2 cm. The entire X-ray source is therefore retractable via a bellows and a screw mechanism. 

Each of the two beamlines are focussed by a single electromagnetic quadrupole doublet, and a beam size of about 1 pm2 is achieved with a beam current of approximately 50 pA. Electrostatic deflectors are mounted to permit beam sweeping on the sample both to produce images and to limit heating effects. 

The measurement of an enthalpy change is based either on the law of conservation of energy or on the Newton and Stefan-Boltzmann laws for the rate of heat transfer. In the latter case, the heat flow between a sample and a heat sink maintained at isothermal conditions is measured. Most of these isoperibol heat flux calorimeters are of the twin type with two sample chambers, each surrounded by a thermopile linking it to a constant temperature metal block or another type of heat reservoir. A reaction is initiated in one sample chamber after obtaining a stable stationary state defining the baseline from the thermopiles. The other sample chamber acts as a reference. As the reaction proceeds, the thermopile measures the temperature difference between the sample chamber and the reference cell. The rate of heat flow between the calorimeter and its surroundings is proportional to the temperature difference between the sample and the heat sink and the total heat effect is proportional to the integrated area under the calorimetric peak. A calibration is thus... 

A disadvantage of 1ST measurements is that the experiments take time (days to weeks). Also, several experiments at different temperatures are necessary to get information with respect to the kinetics of the exothermic decomposition. Finally, it may take several hours to reach equilibrium after inserting a sample due to the time-lag of the system. Thus the recorded heat effect may be inaccurate. This is a particular disadvantage in the case of rapid reactions. 

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