Variable temperature operation

Permanent magnets are used for some of the less expensive spectrometers, but suffer from the disadvantage that the stability of the field depends on extremely accurate control of the temperature of the magnet enclosure this temperature may be affected by variable-temperature operations, unless the probe is thermally well-insulated.37... 

Is the spectrometer/probe capable of variable temperature operation Dynamic samples need to be measured at low temperatures to freeze out the dynamic processes. Qnadrupolar nnclei are often best observed at high temperatures the increased molecnlar tumbling rates help to average ont the qnadrupolar line-broadening interaction. 

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 second genera] protocol uses variable-temperature MAS probes to observe the reaction while in progress. These experiments became feasible owing to the continued evolution of probes and experimental techniques for variable temperature operation [29-32]. The reliability of variable-temperature MAS probes has improved considerably in recent years. Commercial probes are available from several vendors with demonstrated temperature ranges of ca. 77 K to 523 K or higher. Using specially designed probes, Yannoni has performed MAS studies at temperatures as low at 5 K [331, and Stebbins has reported MAS spectra of a mineral sample at 773 K [34]. 

The present design of our MAS probe is clearly amenable to variable temperature operation. Hence, one can study site exchange processes as a function of temperature and possibly separate rotational motion from translational motion of the molecule on the surface. Anisotropic rotational motion can be studied by nmr, e.g. deuterated pyridine and n-butyl amine. The ease of the experiment suggested that nmr with enriched samples would be equally as straightforward. In the near future, the Al, Ag, Rh nmr spectroscopy of the surface acceptor can also be studied. 

In order for the reader to gain some insight into the instrumentation required for DD/CP/MAS experiments, a brief description of the spectrometer we have employed to obtain HR-NMR spectra in solids is given in this Section. Particular emphasis is placed on the MAS apparatus which is designed for variable temperature operation. 

Temperature and pressure are not considered as primary operating variables temperature is set sufficiendy high to achieve rapid mass-transfer rates, and pressure is sufficiendy high to avoid vaporization. In Hquid-phase operation, as contrasted to vapor-phase operation, the required bed temperature bears no relation to the boiling range of the feed, an advantage when heat-sensitive stocks are being treated. 

Volumetric heat generation increases with temperature as a single or multiple S-shaped curves, whereas surface heat removal increases linearly. The shapes of these heat-generation curves and the slopes of the heat-removal lines depend on reaction kinetics, activation energies, reactant concentrations, flow rates, and the initial temperatures of reactants and coolants (70). The intersections of the heat-generation curves and heat-removal lines represent possible steady-state operations called stationary states (Fig. 15). Multiple stationary states are possible. Control is introduced to estabHsh the desired steady-state operation, produce products at targeted rates, and provide safe start-up and shutdown. Control methods can affect overall performance by their way of adjusting temperature and concentration variations and upsets, and by the closeness to which critical variables are operated near their limits. 

Once identified, the costs and/or savings are placed into their appropriate categories and quantified for subsequent analysis. Equipment cost is a function of many variables, one of the most significant of which is capacity. Other important variables include operating temperature and/or pressure conditions, and degree of equipment sophistication. Preliminary estimates are often made using simple cost-capacity relationships that are vahd when the other variables are confined to a narrow range of values. 

The present description is based on previous publications from this laboratory56-59 and the interested reader will find additional details and references in that work. Two different ion-source reaction chambers are used. One of these sources which operates at room temperature is shown in Figure 4. The second source, a variable temperature source will also be described. The electrospray generator and the ion-source reaction chamber are shown in Figure 4, while the mounting of the ion source and the front end of the mass spectrometer are shown in Figure 5. 

In general, then, an examination of the effects of the operational variables temperature and frequency and of changes in the nature of the polymer is closely tied ty) TK and which set the location of the transition zone in plots such as Figure 5B and A, respectively. 

The procedure developed by Joris and Kalitventzeff (1987) aims to classify the variables and measurements involved in any type of plant model. The system of equations that represents plant operation involves state variables (temperature, pressure, partial molar flowrates of components, extents of reactions), measurements, and link variables (those that relate certain measurements to state variables). This system is made up of material and energy balances, liquid-vapor equilibrium relationships, pressure equality equations, link equations, etc. 

Such chelates have been detected also by variable temperature multinu-clear NMR spectroscopy, which showed that basically the above mechanism is also operating in solution [47,55,58]. 

The third block in Fig. 2.1 shows the various possible sensing modes. The basic operation mode of a micromachined metal-oxide sensor is the measurement of the resistance or impedance [69] of the sensitive layer at constant temperature. A well-known problem of metal-oxide-based sensors is their lack of selectivity. Additional information on the interaction of analyte and sensitive layer may lead to better gas discrimination. Micromachined sensors exhibit a low thermal time constant, which can be used to advantage by applying temperature-modulation techniques. The gas/oxide interaction characteristics and dynamics are observable in the measured sensor resistance. Various temperature modulation methods have been explored. The first method relies on a train of rectangular temperature pulses at variable temperature step heights [70-72]. This method was further developed to find optimized modulation curves [73]. Sinusoidal temperature modulation also has been applied, and the data were evaluated by Fourier transformation [75]. Another idea included the simultaneous measurement of the resistive and calorimetric microhotplate response by additionally monitoring the change in the heater resistance upon gas exposure [74-76]. 

This chapter will only deal with the possible gas transport mechanisms and their relevance for separation of gas mixtures. Beside the transport mechanisms, process parameters also have a marked influence on the separation efficiency. Effects like backdiffusion and concentration polarization are determined by the operating downstream and upstream pressure, the flow regime, etc. This can decrease the separation efficiency considerably. Since these effects are to some extent treated in literature (Hsieh, Bhave and Fleming 1988, Keizer et al. 1988), they will not be considered here, save for one example at the end of Section 6.2.1. It seemed more important to describe the possibilities of inorganic membranes for gas separation than to deal with optimization of the process. Therefore, this chapter will only describe the possibilities of the several transport mechanisms in inorganic membranes for selective gas separation with high permeability at variable temperature and pressure. 

Normal safety precautions for laboratory work and for the use of electrical equipment, especially variable temperature accessories, must be observed. The thermal analysis experiment involves high temperatures and there is a danger of being burned. Consult instrument operating manual for specific cautions regarding operation. 

Variable temperature measurements of the enriched samples show that exchange processes are operating and different states of aggregation are observed. It is not clear at this point if inter- or intra-aggregate exchange is dominating. 

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