Desorption direction

Hysteresis may be caused by experimental artifacts or degradation. Also, to assess hysteresis fairly from the desorptive direction requires that samples be at true equilibrium. 

As a partial compromise between the use of on-site instrumental analysis and laboratory analysis, a passive sampler can be immersed into the soil (at a specified depth or at several depths) to collect the evolved gases that are adsorbed onto a solid-phase support. The sampler is then removed to the laboratory, where the gases are transferred by Curie point desorption, directly into the ion source of an interfaced quadrupole mass spectrometer. This procedure has its origin in the petroleum exploration industry, and the samplers can be used at a considerable range of depths (Einhom et al., 1992). 

The theoretical description of photochemistry is historically based on the diabatic representation, where the diabatic models have been given the generic label desorption induced by electronic transitions (DIET) [91]. Such theories were originally developed by Menzel, Gomer and Redhead (MGR) [92,93] for repulsive excited states and later generalized to attractive excited states by Antoniewicz [94]. There are many mechanisms by which photons can induce photochemistry/desorption direct optical excitation of the adsorbate, direct optical excitation of the metal-adsorbate complex (i.e., via a charge-transfer band) or indirectly via substrate mediated excitation (e-h pairs). The differences in these mechanisms lie principally in how localized the relevant electron and hole created by the light are on the adsorbate. 

The reaction direction, taken as forward, in particular cases may correspond to either the forward or reverse direction of stage 1 of scheme (188). Therefore, we distinguish the directions of stage 1 as follows. The direction that results in the occupation of a free site on the surface is called the adsorption direction, and the direction that results in a site becoming unoccupied is called the desorption direction. The rate of stage 1 in adsorption direction [i.e., in the forward direction in our record (188)] is denoted as rA and the rate in desorption direction [i.e., in the reverse direction in scheme (188)] is denoted as rB. When applied to concrete reactions one of these values will stand for the forward reaction rate, r+, and the other will stand for the reverse reaction rate, r. Transfer coefficients for adsorption and desorption directions will be denoted as a and / , respectively so a + / = 1. 

We accept the simplifying Assumption 4 of Section IX (i.e., assume that the transfer coefficient a is identical for both stages). Then, from the linear relationship between AG+ and AGa° for each stage and bearing in mind that rate constants and k 2 refer to adsorption directions, and jc j and k2 refer to desorption directions, we obtain, in a manner similar to... 

This information was obtained from samples of less than 5 pg by electron impact/desorption direct probe mass spectrometry. On the basis of complete structural analyles, to be presented elsewhere, we have been able to determine that the four fractions isolated thus far actually contain six different glycosphingolipids with the following structures ... 

The transport of species along the chromatography column is assumed to be dependent only on the solid angle of the desorption direction and on the dimensions of the column. The solid angles of desorption from a plane surface into the vacuum are calculated according to Knudsen [19] assuming a cosine law ... 

The stability of these compounds in the desorbed state has to be in the right range to allow reversibility under moderate conditions. Furthermore, the stability of these compounds formed upon desorption directly determines the change in reaction enthalpy of the multicomponent hydride system in comparison to the singlecomponent hydride. 

Six types of Eu(dik)3 complexes were accurately analyzed in the absence and presence of phen, using both infusion ESI-MS and ELD. The LOD by FLD for compounds extracted with acetonitrile were 1-100 ppb. Using ESI-MS, high sensitive and prompt discrimination of luminescent europium S-diketonates can be achieved. The combination of laser samphng and ion trap MS is a powerful analytical technique for the direct analysis of complex samples. Thus, the generation of both negative and positive ions by laser desorption directly within a quadrupole ion trap was apphed to analyze complex samples such as Cr(acac)3. ... 

Thermal Desorption (direct) Vacuum/thermal desorption/extraction via direct MS direct insertion probe - no separation. (5)... 

Thermal Desorption (direct) Direct thermal desorption from sample located within the injection port finer (see discussion below). ... 

The area (integral) under each curve in Figure 5.9 represents the total amount of adsorbate released, regardless of the presence of readsorption. The differential of the time function of this amount is the rate of desorption. The upshot is that, once the raw data curves on Figure 5.9 are normalized and scaled, they report the rate of desorption directly. 

By establishing the scaling factors relating Ca to the fraction of the surface covered, one can read the kinetics of adsorption/desorption directly from Figure 5.9. Rates from the full experiment can then be sieved for isothermal, isokinetic, or other sets, just as they were in the case of the TS-PFR. 

If one wishes to study the pure rate of desorption directly one needs to minimize the rate of re-adsorption. This is done by minimizing the concentration CA in the output stream by increasing the rate of flow of the sweeping stream. This in turn creates analytical difficulties in detection of A in the output stream. It will also necessitate an additional control function to implement changes in flow rate during the experiment in order to keep the output concentration below a selected value in the face of a rising rate of desorption as temperature increases. 

Desorption directly after branching can be assumed strongly constrained because of a much more spatially demanding transition state as desorption after linear growth. The assumption of slow desorption after branching has been introduced by Wojciechowsky et al [23], Of course, this argument holds for both, iron- and cobalt catalysts, and the phenomenon is persistent with both sorts of catalysts. 

For solvent desorption direct steaming of the activated carbon bed is the most widely used regeneration technique because it is cheap and simple. Steam (110-130°C) is very effective in raising the bed temperature quickly and is easily condensed to recover the solvent as a liquid. A certain flow is also required to reduce the partial pressure of the adsorbate and carry the solvent out of the activated carbon bed. A flow diagram for steam desorption for toluene recovery is given in Figure 22.1.5. 

The condensed-phase ionization of polyether ionophore coccidiostats and antibiotics mostly results in [M+Na] -ions. This has been reported for FAB-MS of lasalocid, septamycin and monensin with desorption directly from TLC plates [124] and for maduramicins and other polyether antibiotics [125], In ESI-MS, they are also mostly analyzed as [M+Na]. The presence of multiple cychc ether units, a free caiboxyhc acid group at one end, and a terminal alcohol group at the other enable them to form stable complexes with various metal cations, e.g., alkah cations, Ca, Mg, and Cu, via intramolecular head-to-tail hydrogen bonding [126], In MALDI-MS using DHB as matrix, such coccidiostats show [M+Na] as well as some [M+K]+and [(M-H+Na)+Na]+ [127],... 

The transport of species along the chromatography column is assumed to be dependent only on the solid angle of the desorption direction and on the dimensions of the column. 

DI, desorption Direct generation of ions from a condensed phase sample. 

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