Probe, sampling high pressure

Since the data-acquisition time for a given S/N scales inversely with the square of the number of molecules sampled, high pressures and large tube volumes are desirable. NMR tubes are produced in a range of diameters, and the largest of these that is compatible with the instrument probe design should be chosen. Tubes of 5-, 10-, and 15-mm diameter with an in-line Teflon valve, which permits easy attachment to a vacuum line via i-in. or -in. Ultra-Torr fittings and which can be spun in the normal way, are available from various suppliers. 

Various types of probes are employed to monitor compositional changes. Among these are pH probes, dissolved oxygen probes, and high-pressure infrared probes. Such probes can be off-line, on-line, or in-line. Off-line probes require that samples be withdrawn from the reactor for analysis. In-line probes are typically installed on a slipstream that can be diverted from the reactor. In-line probes are installed directly into the reactor for real-time monitoring. Signals from the probes are interpreted by probe-specific electronics that, in turn, send a signal to the process controller for possible action. 

The take-home lesson is that the vast majority of high-pressure studies are on solids or other rigid media and are not done under hydrostatic conditions. The stresses and stress-related properties may vary throughout the sample. Unless the probes are very local and focus on a small region of the sample, measurements are averages over a range of, often uncharacterized, conditions. 

For the elucidation of chemical reaction mechanisms, in-situ NMR spectroscopy is an established technique. For investigations at high pressure either sample tubes from sapphire [3] or metallic reactors [4] permitting high pressures and elevated temperatures are used. The latter represent autoclaves, typically machined from copper-beryllium or titanium-aluminum alloys. An earlier version thereof employs separate torus-shaped coils that are imbedded into these reactors permitting in-situ probing of the reactions within their interior. However, in this case certain drawbacks of this concept limit the filling factor of such NMR probes consequently, their sensitivity is relatively low, and so is their resolution. As a superior alternative, the metallic reactor itself may function as the resonator of the NMR probe, in which case no additional coils are required. In this way gas/liquid reactions or reactions within supercritical fluids can be studied... 

Although most early analytical and experimental studies focused on NO formation, more information now exists on N02 and the conditions under which it is likely to form in combustion systems. Some measurements in practical combustion systems have shown large amounts of N02, which would be expected under the operating conditions. Controversy has surrounded the question of the extent of N02 formation in that the N02 measured in some experiments may actually have formed in the probes used to capture the gas sample. Indeed, some recent high-pressure experiments have revealed the presence of N20. 

In chemistry and biochemistry NMR is, in general, applied to liquid samples. The quality of the experimental data is mainly influenced by the spectral resolution and by the signal-to-noise ratio obtained. Building high pressure, high resolution NMR probes for liquid samples has to take into account these constraints. Special probes for variable pressure experiments differ from commercial probes by the presence... 

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 2.1 Schematic drawing of a high pressure NMR probe designed for wide-bore cryo-magnets and a sample cell [10].

Figure 2.5 Sample container for normal bore high pressure NMR probe.

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.

High-pressure NMR studies for catalysis and with supercritical fluids will lead to a much broader application of sapphire NMR cells and to special applications of toroidal probes. The sapphire tube technique can today be considered as a standard, cheap and easily applicable technique to study samples under medium gas pressures, up to 100 MPa. 

Internal reflection spectroscopy is widely applied for on-line process control. In this type of application, the chemical reactor is equipped with an internal reflection probe or an IRE. The goal of this type of application is the quantification of reactant and/or product concentrations to provide real-time information about the conversion within the reactor. In comparison with other analytical methods such as gas chromatography, high-pressure liquid chromatography, mass spectrometry, and NMR spectroscopy, ATR spectroscopy is considerably faster and does not require withdrawal of sample, which can be detrimental for monitoring of labile compounds and for some other applications. 

If the fluid to be sampled is liquid which is to be vaporised, or is a gas at high pressure, then a narrow bore pipe (typically 2 mm ID) is inserted through the original low pressure probe (Fig. 6.59b). 

Phase equilibria and pressure-temperature coordinates of critical points in ternary systems were taken with a high-pressure apparatus based on a thermostated view cell equipped with two liquid flow loops which has been described in detail elsewhere [3]. The loops feed a sample valve which takes small amounts of probes for gas-chromatographic analysis. In addition to temperature, pressure and composition data, the densities of the coexisting liquid phases are measured with a vibrating tube densimeter. Critical points were determined by visual oberservation of the critical opalescence. 

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