Thermal desorption technique

With porous materials, a slow diffusion in the pores can sometimes control the rate of desorption. This may give rise to complications because diffusion in the pores may be complex and difficult to treat mathematically. Cvetanovi6 and Amenomiya (48) gave a model treatment for their modification of the thermal desorption technique. 

In the thermal desorption technique excavated soil is heated to around 200 to 1000°F (93 to 538°C). Volatile and some semivolatile contaminants are vaporized and carried off by air, combustion gas, or inert gas. Off-gas is typically processed to remove particulates. Volatiles in the off-gas may be burned in an afterburner, collected on activated carbon, or recovered in condensation equipment. Thermal desorption systems are physical separation processes that are not designed to provide high levels of organic destruction, although some systems will result in localized oxidation or pyrolysis. 

Because of its low affinity for water we have used TENAX TA, poly-(2,6-diphenyl-p-phenylene oxide). This very porous polymer has a high thermostability and may be used with a thermal desorption technique. It is also possible to dry it to avoid plugging the GC column, with only minimum losses of other collected components. 

Elemental composition C 7.81%, Cl 92.19%. Carhon tetrachloride may be analyzed by GC or GC/MS. For GC determination, an FID or a halogen-specific detector such as ECD or HECD may he used. Trace concentrations in aqueous matrix or soil, sediments or sohd wastes may he determined by purge and trap or thermal desorption techniques followed hy GC or GC/MS measurements. The characteristic masses for identification of CCI4 by GC/MS are 117, 119 and 121. 

Similarly, cost estimates should include such items as preparation of work plans, permitting, excavation, processing, quality assurance/quality control verification of treatment performance, and reporting of data (D15673U, p. 7). For more specific cost estimates for ex situ thermal desorption techniques, refer to the individual technologies in the RIMS 2000 library/database. 

Thermal desorption techniques have been used to determine plant volatiles [51]. 

Fig. 15. Linear ramp thermal desorption of oxidized stainless steel samples implanted with 300 eV hydrogen at room temperature (Clausing, R. E., et. al. in Ref.39), p. 573). The thermal desorption technique shows that (1) a large amount of hydrogen is adsorbed in or near the sample surface, (2) the hydrogen is easily desorbed...

Adedeji et al. (1993) used a direct thermal desorption technique (220°C) to analyse the volatiles from beans that might cause the thermal degradation and transformation of sugar into common volatile compounds such as 3,5-dimethyl-2,4(3H,5H)-furandione and 3,5-dihydroxy-6-methyl-2,3-dihydro-4H-pyran-4-one. This last compound was detected at a high concentration (3880 ppm) in Mexican vanilla, being the third most abundant compound after vanillin and 2-furfural, and far more abundant than vanillic acid, p-hydroxy-benzaldehyde or p-hydroxybenzoic acid. 

Thermal desorption techniques have gained great popularity owing to their comparatively easy operation. Various different mathematical analyses of the thermodesorption profiles have been proposed by different authors to provide kinetic information about the distribution of the surface acid strength [22]. A simple approach is based on the analysis of thermodesorption curves collected at different heating rates ( 3). The shift of the temperature of the maximum rate of desorption (Tmax) as a function of P can be exploited to derive activation parameters (i.e. TJ... 

Lower (i.e.,better) detection limits are achieved with the thermal desorption method owing to the higher aliquot transferred to the analytical device. The shortcoming of the charcoal method is the injection of only a very small part (aliquot) of a sample, which can only be partly compensated by sampling a larger air volume. The disadvantage of the thermal desorption technique is that quality assurance requires the collection of at least duplicate samples. Another disadvantage of this method is the need for special equipment and specially trained personnel. 

Han, J.I. and Lee, J.Y., An investigation of the intrinsic degradation mechanism of LaNi5 by thermal-desorption technique, International Journal of Hydrogen Energy, 1988, 13(9) p. 577-581. 

The requirement that E is continuous function of q means that, in the limit, point heterogeneity (or one site per element ds) is assumed. Hence, at any time, there is a Maxwellian distribution of probabilities for adsorption on all sites with the maximum probability centered at Et. We now examine the effect of such a distribution on rates of desorption in view of the fact that thermal desorption techniques are frequently being employed to obtain information about the distribution of site energies and about the different adsorbed states on an adsorbent surface. 

Isobutanol may be analyzed by GC-FID and GC/MS techniques. Air analysis may be performed by NIOSH Method 1401. GC/MS volatile organic analysis using a purge and trap or thermal desorption technique may be suitable for analyzing isobutanol in water, soil, and solid wastes (U.S. EPA 1986, Method 8240) (see Section 4.6). Characteristic mass ions from electron-impact ionization are 41, 42, 43, and 47. 

Chloroethyl vinyl ether in potable and nonpotable waters and solid and hazardous wastes may be analyzed by EPA Methods based on GC and GC/MS instrumentation (U.S. EPA 1992 1997 Methods 611, 625, 8010, 8270) using a purge and trap or thermal desorption technique. Characteristic masses to identify this compound by GC/MS using electron-impact ionization are 106, 63, and 65. 

Benzene may be analyzed by several tech-niqnes, which include GC, MS, IR spectroscopy, and hydrocarbon analyzer. Its analysis at the ppb level in wastewater, groundwater, and potable water may be performed by a GC equipped with a photoionization detector and using the purge and trap or thermal desorption techniques. GC-FID may be employed to analyze benzene at ppm range. GC/MS is an excellent confirmatory test. Benzene may be identified by its primary characteristic ion with an m/z ratio of... 

In this article, the commonly used thermal desorption technique is first discussed in detail. Then other related methods are introduced briefly. Surface analyses based on electronic transition-induced desorption that provide unique information on the surface structure are presented. 

By nature, thermal desorption techniques are limited to lower mass species amenable to vaporization and thermally stable compounds. Several approaches are not commercially available so far. New ion sources working under ambient conditions are not yet status quo in the most MS laboratories and must be bought specifically for TLC/HPTLC-MS, which might be hindered by an expensive price. These circumstances lessen the impact of the new development. On the other hand, having a forced-flow technique in the laboratory or the TLC-MS interface, coupling is readily feasible with any HPLC-MS system. 

Deuteron diffusion in acceptor-doped ceramics was studied as a function of the Ni content by using a thermal desorption technique. The results, for 850 to 1150K, could be described by ... 

The states of similarly adsorbed nitrogen were studied by thermal desorption technique on the surface of Mo-W and Re-W. The desorption heat of adsorbed nitrogen measured were 41kJ mol for 7 states on Mo (100), 364kJ-mol (Ref. 82) for / states on Mo (100), and 364 kJ mol for / state on Mo (110). The / state on Re single crystal was considered to be an intermediate for ammonia synthesis. 

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