Desorption parameters

With the aid of (B1.25.4), it is possible to detennine the activation energy of desorption (usually equal to the adsorption energy) and the preexponential factor of desorption [21, 24]. Attractive or repulsive interactions between the adsorbate molecules make the desorption parameters and v dependent on coverage [22]- hr the case of TPRS one obtains infonnation on surface reactions if the latter is rate detennming for the desorption. 

Attractive or repulsive interactions between the adsorbate molecules make the desorption parameters Edes and V dependent on coverage [17],... 

Figure 2.13 Activation energies and prefactors for the desorption of Ag from Ru(001) as determined with the complete analysis. The desorption parameters become essentially constant for coverages above 0.15 ML, indicative of zero-order kinetics. This suggests that Ag atoms desorb from the edges of relatively large two-dimensional islands (data from Niemantsverdriet et al. [18]). 

Release methodology may lead to incorrect desorption parameters, which in turn may be (erroneously) interpreted as hysteresis. For example, Hodges and Johnson (1987) used two experimental techniques (rapidly stirred batch and miscible... 

In conclusion, TIRF promises to be exceedingly useful in the study of protein-substrate interactions. It gives in situ, possibly remote, real-time information about protein adsorption-desorption parameters, conformational changes upon adsorption and hopefully, nanosecond time-resolved fluorescence lifetime information about adsorbed proteins 156). 

J. L. Brand and S. M. George, Effects of laser-pulse characteristics and thermal desorption parameters on laser-induced thermal desorption, Surf. Sci. 167, 341-362 (1986). 

Trefz, P., Kischkel, S., Hein, D., James, E.S., Schubert, J.K., Miekisch, W. Needle trap micro-extraction for VOC analysis effects of packing materials and desorption parameters. J. Chromatogr. A 1219, 29-38 (2012)... 

P is the pressure increase, V is the volume of the chamber, S is Che pumping speed of the pumping system, and C is a constant depending on sample area, gas constant and gas phase temperature. If the desorption rate is small compared with the pumping speed, it will be proportional to the pressure P. The desorption rate is related Co desorption parameters through the following Polanyi-Wigner equation ... 

The shortcoming of Redhead s method is chat it is basd on che assumption that all the desorption parameters are coverage-independent. This is often not true in practice. 

According to equation 2.4, a plot of ln(r/tf ) verses 1/T will yield a straight line if desorption parameters vary moderately with coverage. With an assumed value of n, a set of data points of ln(r/9") and 1/T can be calculated from a single TPD experimental curve. If Che plot of the points is a straight line, then Che assumed desorption order parameter is correct. 

The TPD data analytical methods Introduced In Section 2.2.3 were used to calculate the desorption parameters of CO desorption for Che COj/K/AgClll). The calculation of parameters was limited Co peak a of COj K(fl—l/3)/Ag(lll) dosed with saturation coverage of COj. 

It is still unknown how che desoirption parameters depend on COj and K coverages. More TPD data analysis will certainly provide useful information on this aspect. A computer program with che ability of varying che desorption parameters has Co be developed for curve simulations. 

Several papers investigated the use of SPME for VFA analysis in wastewater and in air. Briefly, a fiber is exposed to the sample headspace or inserted directly into the sample. Analytes adsorb onto the fiber and are subsequently desorbed at high temperatures in the GC injection port. SPME is a solvent-free technique which introduces less potential contaminants into the GC compared to direct injections. SPME is also rapid since no further sample preparation steps are required. It may be used for routine analysis provided that the specific autosampler required for this method is available and that the optimized method conditions are suitable for autosampler application. Further information on principles and other applications of this technique can be found elsewhere. " " Parameters which have been optimized for VFA analysis are fiber coating, fiber exposure time, sample temperature, sample pH, sample agitation, potential salt addition, and desorption parameters. Surrogate standards employed for VFA analysis were 2-ethylbutyric acids for GC/FID or GC/MS and C-labeled organic acids for GC/MS. The method was optimized using standards in deionized water and only a few wastewater samples were analyzed as examples. 

The desorption step takes place by a rapid increase in temperature and by a convenient volume of inert gas in order to transport the analytes to the chromatographic system in a short time and through a narrow band. The faster the heating is, the quicker the analytes are desorbed, making possible adsorption in a short time with a small volume of inert gas, which improves the chromatographic separation and analytical signal. Table 14.2 also shows the desorption parameters for the analysis of BTEX in water samples. 

An elegant method of obtaining desorption parameters involves the use of molecular beams. Here, the crystal is maintained at the desorption temperature and the requirement for rapid heating is avoided. Two methods are used. If the beam is chopped, the decay rate of the desorbed particles can be directly monitored [217, 218]. Alternatively, the beam may be modulated and the phase shift between the input and output measured [219], These methods were originally used to determine lifetimes for species desorbing as ions, such as Ba+ and Cs+ from tungsten [217, 219, 220] and for metal atoms [221], but CO, 02 and H2 desorption have also been examined in this way [223—225]. 

Desorption parameters desorption order, pre-exponential constant, desorption activation energy, Ed, saturation coverage, 7Vmax, and adsorption state (a, j3 etc)... 

Further, combining equations (7) and (9) yields an equation to compute gas generated from the source rock in terms of the gas desorption parameters and expulsion efficiency as... 

Figure 1. Uniform surface model prediction of temperature programmed desorption data from a nonuniform surface for reactant A of Scheme I requires low values of desorption parameters (Aj = 3.22 X 10 s , Ej = 9.01 kcal mol") to capture peak breadth.

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