Dynamic thermal desorption

Dynamic thermal desorption (DTD), which closely resembles the DHS technique, is also commonly used in combination with GC in the analysis of (semi)volatile compounds in both solid and liquid matrices (85-88). In DTD,... 

Applications The potential of a variety of direct solid sampling methods for in-polymer additive analysis by GC has been reviewed and critically evaluated, in particular, static and dynamic headspace, solid-phase microextraction and thermal desorption [33]. It has been reported that many more products were identified after SPME-GC-MS than after DHS-GC-MS [35], Off-line use of an amino SPE cartridge for sample cleanup and enrichment, followed by TLC, has allowed detection of 11 synthetic colours in beverage products at sub-ppm level [36], SFE-TLC was also used for the analysis of a vitamin oil mixture [16]. 

Various sample enrichment techniques are used to isolate volatile organic compounds from mammalian secretions and excretions. The dynamic headspace stripping of volatiles from collected material with purified inert gas and trapping of the volatile compounds on a porous polymer as described by Novotny [3], have been adapted by other workers to concentrate volatiles from various mammalian secretions [4-6]. It is risky to use activated charcoal as an adsorbent in the traps that are used in these methods because of the selective adsorption of compounds with different polarities and molecular sizes on different types of activated charcoal. Due to the high catalytic activity of activated charcoal, thermal conversion can occur if thermal desorption is used to recover the trapped material from such a trap. 

Valuable information can be obtained from thermal desorption spectra (TDS) spectra, despite the fact that electrochemists are somewhat cautious about the relevance of ultrahigh vacuum data to the solution situation, and the solid/liquid interface in particular. Their objections arise from the fact that properties of the double layer depend on the interaction of the electrode with ions in the solution. Experiments in which the electrode, after having been in contact with the solution, is evacuated and further investigated under high vacuum conditions, can hardly reflect the real situation at the metal/solution interface. However, the TDS spectra can provide valuable information about the energy of water adsorption on metals and its dependence on the surface structure. At low temperatures of 100 to 200 K, frozen molecules of water are fixed at the metal. This case is quite different from the adsorption at the electrode/solution interface, which usually involves a dynamic equilibrium with molecules in the bulk. 

The foregoing experiments using visible or near-infrared lasers hoped to explore desorption dynamics within a framework characterized by a thermal desorption picture. Hiere are two complementary studies that undertook to explore nonthermal desorptions. 

For molecular desorption, laser spectroscopic studies of the desorbing molecule can give full internal state distributions, Df Ef, 6f, v, J, f M ), Ts), where f M ) is some distribution function describing the rotational orientation/alignment relative to the surface normal. For thermal desorption in non-activated systems, most atoms/molecules have only modest (but important) deviations from a thermal distribution at Ts. However, in associative desorption of systems with a barrier, the internal state distributions reveal intimate details of the dynamics. Associative desorption results from the slow thermal creation of a transition state, with a final thermal fluctuation causing desorption. Partitioning of the energy stored in V into... 

Dynamic mode instead of working in a closed environment, a carrier gas such as helium is either passed over the surface of the sample or bubbled through it in order to carry the volatile components into a trap where they are adsorbed and concentrated (Fig. 20.5). The sample is then introduced into the chromatograph by thermal desorption. This purge-and-trap technique is semi-quantitative and delivers a sample without residue. 

Figure 20.5—Dynamic mode of headspace sample analysis. The sample is recovered by thermal desorption ( stripping ) from a cartridge appropriate for the compound being measured.

The TPD experimental technique is alternatively, but less suitably, termed thermal desorption spectroscopy (TDS). It is a very useful complement to vibrational spectroscopy and can be applied to adsorption on single-crystal or finely divided metal surfaces. TPD involves the dynamic analysis, usually by mass spectrometry, of the gases desorbed from the surface as the temperature is raised at a uniform rate, starting from a known state of adsorption. In addition to... 

Zaika Yu.V., Chernov I.A. (2003) Nonlinear dynamical boundary-value problem of hydrogen thermal desorption, Int. J. of Mathematics and Math. Sciences 23, 1447-1463. 

Villasenor, M. J., Valero, E., Sanz, J., Martinez Castro, I. 2000. Analysis of volatile components of Manchego cheese by dynamic headspace followed by automatic thermal desorption-GC-... 

Thermal desorption is a dynamic (non-equilibrium) technique in which a sample of hydrated corneum is heated at a constant rate in a dry atmosphere. The water desorption rate is plotted as a function of temperature. The general shape and temperature maxima of the desorption rate vs. temperature curves (Figure 12) are characteristic of the material s diffusion and equilibrium sorption behavior as well as experimental conditions such as heating rate. In a simple desorption process where... 

E. K. Grimmelmann, J. C. Tully, and E. Helfand, Molecular Dynamics of Infrequent Events Thermal Desorption of Xenon from a Platinum Surface, J. Chem. Phys., 74 (1981) 5300. 

J. C. Tully, Dynamics of gas-surface interactions Thermal desorption... 

Determination of volatiles at the trace level is also possible by pre-concentrating the headspace volatiles on a suitable adsorbent. The trapped compounds are subsequently recovered by thermal desorption in front of a cooled trap connected to the capillary column or by solvent elution followed by splitless or on-column injection. These methods, called dynamic headspace enrichment or purge-and-trap , have been applied to trace level analysis of volatiles, using conventional electrically heated systems [ 31, 32 ], a Curie-point Pyrolyser... 

Insertion/introduction of the needle into the GC port, depression of the plunger, and thermal desorption of the analytes. Alternatively, the analytes are washed out of the fiber by the HPLC mobile phase via a modified HPLC six-port injection valve and a desorption chamber that replaces the injection loop in the HPLC system. The SPME fiber is introduced into the desorption chamber, under ambient pressure, when the injection valve is in the load position. The SPME-HPLC interface enables mobile phase to contact the SPME fiber, remove the adsorbed analytes, and deliver them to the separation column. Analytes can be removed via a stream of mobile phase (dynamic desorption) or, when the analytes are more strongly adsorbed to the fiber, the fiber can be soaked in mobile phase or another stronger solvent for a specific period of time (e.g., 1 min) before the material is injected onto the column (static desorption) (Fig. 6). 

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