Desorption continued thermal

Heating of the adsorbent in the thermal desorption method can be performed either in a continuous or in a stepwise manner. Most usually a continuous heating is applied. 

Alternatively, peak asymmetry could arise from thermal effects. During the passage of a solute along the column the heats of adsorption and desorption that are evolved and adsorbed as the solute distributes itself between the phases. At the front of the peak, where the solute is being continually adsorbed, the heat of adsorption will be evolved and thus the front of the peak will be at a temperature above its surroundings. Conversely, at the rear of the peak, where there will be a net desorption of solute, heat will be adsorbed and the temperature or the rear of the peak will fall below its surroundings. 

Solid-phase microextraction (SPME) consists of dipping a fiber into an aqueous sample to adsorb the analytes followed by thermal desorption into the carrier stream for GC, or, if the analytes are thermally labile, they can be desorbed into the mobile phase for LC. Examples of commercially available fibers include 100-qm PDMS, 65-qm Carbowax-divinylbenzene (CW-DVB), 75-qm Carboxen-polydimethylsiloxane (CX-PDMS), and 85-qm polyacrylate, the last being more suitable for the determination of triazines. The LCDs can be as low as 0.1 qgL Since the quantity of analyte adsorbed on the fiber is based on equilibrium rather than extraction, procedural recovery cannot be assessed on the basis of percentage extraction. The robustness and sensitivity of the technique were demonstrated in an inter-laboratory validation study for several parent triazines and DEA and DIA. A 65-qm CW-DVB fiber was employed for analyte adsorption followed by desorption into the injection port (split/splitless) of a gas chromatograph. The sample was adjusted to neutral pH, and sodium chloride was added to obtain a concentration of 0.3 g During continuous... 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

Nelson and Eggertsen/in 1958, extended the Loebenstein and Dietz technique by continuously flowing a mixture of helium and nitrogen through the powder bed. They used a hot-wire thermal conductivity detector to sense the change in effluent gas composition during adsorption and desorption, when the sample cell was immersed into and removed from the bath, respectively. 

Lowell and Karp measured the effect of thermal diffusion on surface areas using the continuous flow method. Figure 15.14 illustrates a fully developed anomalous desorption signal caused by thermal diffusion. 

Semonian and Manes have devised an approach which provides continuous data from which the desorption isotherm can be constructed. Their method utilizes a calibrated thermal conductivity detector for sensing the effluent concentration from a cell filled with adsorbate and slowly purged with a carrier gas. The amount desorbed at any relative pressure is calculated by integrating the effluent flow rate and thermal conductivity signal. 

Decontamination of soils using supercritical fluids is an attractive process compared to extraction with liquid solvents because no toxic residue is left in the remediated soil and, in contrast to thermal desorption, the soils are not burned. In particular, typical industrial wastes such as PAHs, PCBs, and fuels can be removed easily [7 to 21]. The main applications are in preparation for analytical purposes, where supercritical fluid extraction acts as a concentration step which is much faster and cheaper than solvent-extraction. The main parameters for successful extraction are the water content of the soil, the type of soil, and the contaminating substances, the available particle-size distribution, and the content of plant material, which can act as adsorbent material and therefore prolong the extraction time. For industrial regeneration, further the amount of soil to be treated has to taken into account, because there exists, so far, no possibility of continuous input and output of solid material for high pressure extraction plants, so that the process has to be run discontinuously. 

I. Chromathermography. The term coined by Zhukhovitskii et al(Ref 41) for a chromatographic analysis in which a stream of air is applied while the firnace(which heats consecutive sections of the adsorbing column, and causes desorption) is moved down the column. The air stream thus distributes the components at different spots of the temperature field, and keeps them separated. The method, first proposed in 1951, was discontinuous(Ref 41 45), but later(Ref 46), the continuous modification, called "thermal dynamic method was devised (Compare with "programmed temperature gas chromatography listed unde r item D)... 

Thermal desorption, on the other hand, makes use of the fact that the ability of a sorbent to retain compounds dramatically decreases at elevated temperatures. Therefore, heating of the sorbent under a continuous stream of an inert carrier gas can be used to transfer the adsorbed compounds into the GC system. In general the desorption temperature should be at least 20 °C above the boiling point of the adsorbed compounds, so the range of compounds which can be analyzed with thermal desorption is limited by the thermal stability of the sorbent. In order to avoid a loss of analyte due to decomposition on the sorbent s surface at elevated temperatures the surface activity needs to be carefully chosen. 

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