Probe, sampling high temperature

To avoid the surface catalytic effects, a water-cooled probe must be used when sampling high temperature combustion products. This is of particular importance to oxyfuel combustion where measured species concentrations are much higher due to the elimination of nitrogen. Recommended probe materials and cooling requirements to avoid reaction of the various gases inside the probe are given elsewhere [48]. 

Advances have been made in fiber-optic sampling techniques that overcome problems with sampling high temperature and high viscosity materials. As a result, in situ sampling with fiber-optic probe systems is being implemented for these types of applications. 

In practice, direct insertion of samples requires a somewhat more elaborate arrangement than might be supposed. The sample must be placed on an electrode before insertion into the plasma flame. However, this sample support material is not an electrode in the usual meaning of the term since no electrical current flows through it. Heating of the electrode is done by the plasma flame. The electrode or probe should have small thermal mass so it heats rapidly, and it must be stable at the high temperatures reached in the plasma flame. For these reasons, the sort of materials used... 

Diffuse reflectance FTIR (DRIFT) spectra were recorded on a Bio-Rad FTIR spectrometer (EXCALIBUR FTS3000). A high-temperature cell was attached to a flow system that allows in-situ sample treatment, adsorption and desorption of probe molecules at different temperatures. 

Problems associated with restricted rotation (discussed later) also seem to be worse in D6-DMSO, and being relatively nonvolatile (it boils at 189 °C, though some chemical decomposition occurs approaching this temperature so it is always distilled at reduced pressure), it is difficult to remove from samples, should recovery be required. This nonvolatility however, makes it the first choice for high temperature work - it could be taken up to above 140 °C in theory, though few NMR probes are capable of operating... 

High-temperature flow-reactor studies [60,61] on benzene oxidation revealed a sequence of intermediates that followed the order phenol, cyclopentadiene, vinyl acetylene, butadiene, ethene, and acetylene. Since the sampling techniques used in these experiments could not distinguish unstable species, the intermediates could have been radicals that reacted to form a stable compound, most likely by hydrogen addition in the sampling probe. The relative time order of the maximum concentrations, while not the only criterion for establishing a mechanism, has been helpful in the modeling of many oxidation systems [4,13]. 

To measure the effect of pressure on structures and dynamics, a good stability and homogeneity of the sample temperature has to be guaranteed. Therefore, variable pressure probes for high resolution NMR are in general thermostatted by... 

Figure 2.9 Left Overall view of a high pressure, high temperature probe for NMR studies of supercritical water. Right detail of the ceramic sample tube and free piston pressure balancing arrangement. The stainless-steel cylinder and piston are far from the heated region and remain cool.

We will not discuss microscopy and structure determinations since special monographs are available. Let us mention, however, that hot stages are available these days which allow imaging and diffraction work to be done at high temperatures. The limits for high spatial resolution are often not set by the temperature but rather by the ambient atmospheres. For example, the electron probe beam requires vacuum, whereas the component chemical potentials of a sample are undefined in a vacuum. 

If, however, solid electrolytes remain stable when in direct contact with the reacting solid to be probed, direct in-situ determinations of /r,( ,0 are possible by spatially resolved emf measurements with miniaturized galvanic cells. Obviously, the response time of the sensor must be shorter than the characteristic time of the process to be investigated. Since the probing is confined to the contact area between sensor and sample surface, we cannot determine the component activities in the interior of a sample. This is in contrast to liquid systems where capillaries filled with a liquid electrolyte can be inserted. In order to equilibrate, the contacting sensor always perturbs the system to be measured. The perturbation capacity of a sensor and its individual response time are related to each other. However, the main limitation for the application of high-temperature solid emf sensors is their lack of chemical stability. 

Chemiluminescent probes enable highly sensitive quantitative analysis of proteins blotted from electrophoretic gels onto a supporting matrix. For a quantitative comparison, it is important to be able to correct for introduced variables such as antibody titre, temperature, substrate etc. Comparison of blots completed on different days requires a chemiluminescent standard. The situation is more complex with 2D gels, where only one sample per gel/blot is used. A method has been published for preparing a chemiluminescent standard for quantitative comparison of 2D Western blot (89). 

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