Temperature control, sample

This is based on a sample fast loop with a sample take-off probe in the main process line, and a retnm to either a pnmp-snction or ambient pressnre sample recovery system (Fignre 5.22). The fast loop provides a filtered slip-stream to the analyzer, along with flow control and monitoring (low-flow alarm), and freqnently some form of sample antograb facility for sample capture. The slipstream flow to the internal analyzer sample flow cell is relatively low volnme and allows for a very high degree of sample temperature control (typically... 

In the worse case, where either sample temperature, pressure or reactor integrity issues make it impossible to do otherwise, it may be necessary to consider a direct in situ fiber-optic transmission or diffuse reflectance probe. However, this should be considered the position of last resort. Probe retraction devices are expensive, and an in situ probe is both vulnerable to fouling and allows for no effective sample temperature control. Having said that, the process chemical applications that normally require this configuration often have rather simple chemometric modeling development requirements, and the configuration has been used with success. 

One of the disadvantages concerns the fact that in axial magnets it is rather difficult to use probes with solenoid RF coils. The difficulties are related to sample insertion/removal complications and to numerous spatial constraints, exacerbated by the presence of a glass dewar for sample-temperature control (see Section VI). This is unfortunate because the alternative saddle coils are substantially less efficient, especially at the relatively low excitation/detection frequencies used in FFC NMR. 

In our final realization (Fig. 18), the probes use the Helmholtz coil geometry, favoring ease of use and efficient sample temperature control over a wide range of temperature values. The tunable, broad-band probe is inserted into the magnet from below and fixed to the bottom part of the magnet assembly in a simple way reminiscent of most high-resolution NMR systems. Thanks to this design, it is possible to use standard 10 mm NMR sample tubes which are inserted comfortably from above without any need to manipulate the probe. 

Finally, we should mention the sample temperature control. It is a direct consequence of all relaxation theories that, in any FFC NMRD application. 

The importance of temperature control is significant for laboratory comparisons, accelerated shelf-life studies, and packaging requirements. Also, temperature may be essential when measuring aw levels near critical values, especially for government regulations or critical control points. The precision required in water activity applications will determine the need for temperature control. Several reasons for sample temperature control are ... 

Even when DSC and XRD are both used, quite separate instruments are involved. This leads to difficulties in reconciling results owing to differences in sample thermal history/conditioning, sample dimensions and sample temperature control and uniformity. These difficulties can be entirely overcome by coupling XRD and DSC together in the same instrument and making both types of measurement simultaneously on the same sample. 

Pyda, M. Kwon, Y.K. Wunderlich, B. Heat capacity measurements by saw-tooth modulated standard heat-flux differential scanning calorimetry with sample temperature control. Thermochim. Acta 2001, 367 (8), 217-227. 

Pyda M, Kwon YK, Wunderlich B (2001) Heat Capacity Measurement by Sawtooth Modulated Standard Heat-flux Differential Scanning Calorimeter with Sample-temperature Control. Thermochim Acta 367/368 217-227. 

The frequency range of devices of this sort is of the order of 10 to 5000 Hz. Since neither the frequency response curve nor the logarithmic decrement requires an absolute measurement of amplitude, the relative amplitudes are rather easily measured by optical or electrical elements or even by observation with a microscope. The limited choice of frequencies is a serious drawback, however. Usually only a few harmonics can be applied, and, in contrast to the flexibility of the compound resonance devices in Chapter 6, whose frequencies can be adjusted by changing the mass or moment of inertia of the apparatus, a new set of values can be obtained only by shaping a new sample. Temperature control can be satisfactorily arranged, since the mechanical system can easily be isolated in a thermostat, and in fact measurements have been made at temperatures down to 4.2 K. 

Dependence on a large reference set Influence of sample morphology Slow and costly method development Need for quantitative calibration model Troublesome calibration transfer Strict sample temperature control required Spectroscopic complexity (lack of specificity no characteristic absorption bands)... 

The thermal desorption methods discussed in this Chapter are based on heating the material below the decomposition temperature of the polymer, as illustrated by Wampler [1004] in TD-GC of 1 mg PVC/DEHP heated at 300° C. At higher temperatures, less volatile organic additives will be desorbed more readily, but polymer decomposition products (and perhaps additive pyrolysates) will add complexity to the chromatogram, as in case of the analysis of 2,6-di-r-butyl-/ -cresol (BHT) in SBR [784], The obvious lesson here is to heat the polymer only to the temperature necessary to vaporise the materials of interest. The method is also less useful for compounds which are too unstable for GC analysis. Wampler [1010a] has illustrated direct multi-step polymer and additive analysis by sample temperature control on-line with the GC injector for removal of semivolatiles followed by polymer identification by PyGC-MS. 

The instrument is normally calibrated against distilled water and air. Once calibrated, the instrument should not need recalibration unless the sample tube is replaced. Oscillation is continuous and the period of oscillation is updated every two seconds, making the instrument essentially continuous. The sample cell is thermostatted for accurate sample temperature control. Models are available with four, five, or six place precision. 

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