Desorption, thermal

Thermal desorption studies have the attraction of comparatively simple experimentation, but face severe problems in the evaluation of unambiguous, unique rate parameters from the measurements. The subject has been reviewed several times recently (see, for example, refs. 57—61), particularly in relation to gas—metal systems, so here we will concentrate on its specific applications to semiconductors, where it has been used almost exclusively to study metal absorbate-isemiconductor surface interactions. Since this topic provides the subject matter for Sect. 5, we will limit the discussion in this section to the basic experimental approach and available methods of data analysis. We will leave to Sect. 5 the critical appraisal of the validity of these methods as applied to solid adsorbates, and the interaction models which have been postulated. 

The basic problem, by whatever means the experiment is performed, is to establish the correct form of the rate equation which describes the desorption. The usual procedure is to postulate the simple, empiric form described as the Polanyi—Wigner equation 

Given such postulated, but not general, rate equations, a variety of methods is available for evaluating the rate parameters. We will consider the uniqueness or otherwise of the rate parameters subsequently, but first we will describe briefly the analytical procedures with specific reference to their application to metal—semiconductor systems. The simplest, and in many cases most accurate, is the peak temperature method first described by Redhead [63]. AE can be directly obtained by measuring the temperature, Tp, at which the desorption rate is a maximum, then by setting d2n/dt2 = 0, an expression for AE can be derived in terms of v, Tp and the known heating rate 3 v is usually assumed to be 1013 s-1. 

If we return now to the question of the uniqueness of the rate parameters determined from thermal desorption measurements, we see that all of the analytical methods depend on the assumption of a rate equation whose validity, in general, is not tested. In particular, when there are adsorbate—adsorbate (lateral) interactions, or where desorption occurs via a precursor state, the coverage dependence in the pre-exponential term is not a simple function and the concept of reaction order is not meaningful. 

Other effects which can influence the form of the rate equation include a coverage dependence of the activation energy and an apparent order which changes with coverage as a function of initial coverage. It is clear that great caution must be exercised in the interpretation of desorption spectra if a unique, unambiguous solution is to be obtained. Unfortunately, this is all too rare an occurrence, but the reader is referred to the article by Petermann [57] for an excellent appraisal of the overall situation. 

Depending on the sorbents used, solvent desorption or thermal desorption may be applied to the sampled analytes. For silica gel and carbon-based sorbents, solvent desorption [40,50,51,56,57] and microwave desorption [5,58] are the preconcentration methods of choice. 

Acetonitrile is frequently used for the desorption of 2,4-dinitrophenylhydra-zones of carbonyl compounds collected on silica gel [39,40,59], while CS2 is used for samples collected onto charcoal and dichloromethane for samples collected onto Anasorb 747 [59]. Carbon disulphide is particularly suitable for the desorption of nonpolar compounds but gives less satisfactory outcomes for the polar compounds. To overcome this shortcoming, polar cosolvents such as dimethylformamide, dimethylsulfoxide and ethanol are added to CS, to increase the recovery of polar analytes [36]. In addition, the use of CS2 suffers from a number of other drawbacks, including the facts that (1) it reacts with amines and volatile chlorocarbons (2) it is unsuitable when electron detectors (e.g. electron capture detectors, ECDs) are used, (3) it is toxic and (4) has an unpleasant odour [36]. 

Compared with thermal desorption, solvent desorption is plagued by a number of shortcomings. For example, VVOCs are lost when the liquid sample is reconcentrated prior to its analysis. Moreover, solvent peaks may overlap with the peaks of VVOCs. A recent comparison of thermal and solvent desorption efficiencies also showed that with few exceptions solvent desorption consistently underestimates various classes of VOCs found in typical indoor air [59]. According to Wolkoff [5], solvent desorption leads to considerable loss in analytical sensitivity. 

This is a very popular method of transferring indoor VOC samples trapped by 

The most common approach for semivolatile detection is through the thermal desorption of samples from the solid state to produce neutral vapors, which are then introduced into the IMS as discussed. The first thermal desorption method was simply a platinum wire onto which 1 or 2 pL of the liquid sample, or dissolved sample solution, was placed and the solvent evaporated, leaving the semivolatile components adsorbed on the wire s surface. This wire was then inserted into the heated injection port of an IMS, where the sample was thermally desorbed into the carrier gas and the vapors transferred to the ionization region of the spectrometer. 

Collector Electrode Focusing Rings Drift Region Gating Grid 

Reaction Region Ionizing Source Repelling Ring 

FIGURE 3.4 Block diagram of an ion mobility spectrometer used for thermal desorption introduction of samples. (From Fetterolf and Clark, Detection of trace explosive evidence by ion mobility spectrometry, J. Forensic Sci. 1993, 38(1), 28-39. With permission.) 

The desorber heater and the drift tube with the drift gas can be operated at a variety of temperatures. In general, the desorber temperature is 210°C, but some explosives will decompose at these temperatures and thus must be desorbed at lower temperatures. Programmed temperature desorption appears to be a promising approach for obtaining IMS spectra of a broad range of explosives as well as for simple and quick graduated thermal desorption to differentiate between compounds of different volatility. 

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TDS, FDS Thermal desorption spectroscopy. Flash desorption spectroscopy [173] Similar to TPD Similar to TPD... 

Thermal desorption. The vaporization of ionic or neutral species from the condensed state by the input of thermal energy. The energy input mechanism must be specified. 

Desorption ionization (DI). General term to encompass the various procedures (e.g., secondary ion mass spectrometry, fast-atom bombardment, californium fission fragment desorption, thermal desorption) in which ions are generated directly from a solid or liquid sample by energy input. Experimental conditions must be clearly stated. 

SPME has been utilized for deterrnination of pollutants in aqueous solution by the adsorption of analyte onto stationary-phase coated fused-siUca fibers, followed by thermal desorption in the injection system of a capillary gas chromatograph (34). EuU automation can be achieved using an autosampler. Eiber coated with 7- and 100-p.m film thickness and a nitrogen—phosphoms flame thermionic detector were used to evaluate the adsorption and desorption of four j -triazines. The gc peaks resulting from desorption of fibers were shown to be comparable to those obtained using manual injection. 

A more simplified description is a unit that combusts materials in the presence of oxygen at temperatures normally ranging from 800 to 1650°C. A typical configuration of an incinerator is shown in Figure 9. Typical types of incineration units that are discussed herein are catalytic oxidation, fluidized beds, hquid injection, multiple hearth furnaces, and rotary kiln. Thermal desorption is also discussed. However, an overview of the main factors affecting incinerator performance is presented first, below. 

Thermal Desorption. Thermal desorption is an innovative treatment that has been appHed primarily to soils. Wastes are heated to temperatures of 200 to 600°C to increase the volatilization of organic contaminants. Volatilized organics in the gas stream are removed by a variety of methods including incineration, carbon adsorption, and chemical reduction. 

Other Techniques. Other methods, more conventional in type, are employed for ex-situ treatment. These include solvent extraction and thermal desorption, which are detailed under the "Physical/Chemical Treatment" and "Thermal Treatment" sections, respectively. 

U.S. EPA, Eco Eogic International Gas-Phase Chemical Reduction Process, The Thermal Desorption Enit Applications Analysis Report, EPA/540/AR-94/504, Washington, D.C., 1994. 

U.S. EPA, SITE Demonstration Bulletin, Thermal Desorption System Clean Berkshires, Inc., EPA/540/MR-94/507, Cincinnati, Ohio, 1994. 

This article discusses why one would choose nonresonant multiphoton ionization for mass spectrometry of solid surfaces. Examples are given for depth profiling by this method along with thermal desorption studies. 

Table 10.14 Thermal desorption of sorbed gas from sample tubes...

Lab method using porous polymer adsorption tube and thermal desorption with gas chromatography Lab method using porous polymer diffusive samplers with thermal desorption and gas chromatography Lab method using pumped acid-coated filters, desorption and liquid chromatography... 

Diethyl sulphate and dimethyl sulphate Lab method using Tenax sorbent tube, thermal desorption and gas chromatography with mass spectrometry 89... 

Hydrocarbons (mixed C3-C,q) Lab method using pumped porous polymer and carbon sorbent tubes, thermal desorption and gas chromatography 60... 

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