Trapping and desorption

The trap serves not only as pre-concentration device, but also for band sharpening which is a prerequisite for optimal separation in the chromatographic column. When the gas from the purge chamber passes through the trap, the halocarbons are either absorbed by a resin or adsorbed on to the walls of a tube. The halocarbons are trapped at low temperatures and desorbed by heating the trap. 

Recent experiments on the coverage dependence of the sticking of 02 to Ag(l 1 0) revealed a remarkable drop shown in Fig. 20 [172-179]. This drop has been explained by electrostatic effects [179]. Butler et al. have attributed the drop to the build-up of added rows at the surface [180], If the picture of adsorption into a metastable intermediate is valid, it implies that the intermediate should be stabilized at a step edge. Subsequently the molecule can dissociate at the site and form an Ag-O pair, that is inserted into an added row. This involves mass transport at the surface, which has been observed in 

STM-experiments [181, 182]. Butler et al. have modeled the coverage dependence of the sticking coefficient on Ag(l 1 0) in a simple model, which is summarized in Fig. 21. An analytical representation for the dependence of sticking on coverage was derived, which matches the data remarkably well. This analysis shows, that it is not enough to include only energetic barriers in the modeling of dissociative 

A variety of processes can occur in the interaction of 02 molecules and Ag(l 11). At first scattering from and trapping in the physisorption potential can occur. Secondly, scattering from the chemisorption (02 ) potential occurs, together with transient trapping-desorption. The chemisorption potential well is very shallow. From being transiently trapped the molecule can be captured in the molecular chemisorption well presumably surface imperfections are necessary to stabilize the molecular adsorbate in this case. From the molecular chemisorption well the molecule can proceed to dissociation. In this step ad atoms may be involved on Ag(l 11). Finally, there is a small probability for direct dissociative chemisorption of 02 at Ag(l 11). The formation of added Ag-O rows (fences) at the surface inhibit further sticking at the surface. 

A lot of the work described in this article was carried out in the research group of the author at the FOM Institute of Atomic and Molecular Physics in Amsterdam for the period of 1980-2000. The author gratefully acknowledges all his co-workers, who contributed to this work. Michael Gleeson is thanked for his careful reading of the manuscript. 

Dynamics of precursors in activated dissociative chemisorption systems 

---

Head-Gordon M, Tuiiy J C, Rettner C T, Muiiins C B and Auerbach D J 1991 On the nature of trapping and desorption at high surface temperatures theory and experiments for the Ar-Pt(lll) system J. Chem. Phys. 94 1516... 

As with the volatility range, recovery generally is most effectively increased by raising the sample temperature. Additional factors affecting sensitivity include trapping and desorption efficiencies, column resolution, interferences, and detector sensitivity. For oils the lower limit of detection for the majority of the compounds listed in Tables I and II is on the order of 1 to 100 ppb. For oil samples, nonane, which is often added as an internal standard, is detectable to less than 5ppb. 

In dynamic headspace extraction (purge and trap extraction mode), the constant passage of carrier gas through a warmed sample (purge), followed by trapping of the purged volatiles on a sorbent (trap) and desorption into a gas chromatograph take place. 

Purge, Trap, and Desorption Processes Factors Affecting the Technique... 

Oysters and clams Homogenization purged with nitrogen for 2 hours at 25 °C, then 2 hours at 70 °C onto Tenax/silica gel trap thermal desorption Capillary GC/MS Not specified Not specified Ferrario et al. 1985a... 

Particles may be trapped on the biofilm surface or in voids of the biofilm where any organics may be hydrolyzed and further take part in the transformation processes. A number of factors influence adsorption and desorption of particles, such as particle size, surface charge, pH, etc., as well as biofilm surface properties and bulk water flow pattern. Studies of model biofilms have shown that water flows into the biofilm in small channels, making the prediction of transport of particles as well as soluble compounds complex (Norsker et al., 1995). 

Kester [5] has discussed the application of the purge and trap gas chromatographic method to the determination of aliphatic chloro-compounds in soil. Following methanol extraction of the soil the extract is gas purged and the purge gases trapped on a Tenax silica gel/ charcoal trap followed by thermal desorption from the trap and examination by gas chromatography and mass spectrometry. Compounds that have been determined by this method are listed in Table 5.1. 

Studies involving mass spectral identifications were carried out using a VG 7035 coupled to a Dani gas chromatograph (VG Analytical, Manchester). Modifications were made to the analytical system as the high vacuum of the mass spectrometer was incompatible with the air vent system normally employed between the cold trap and the analytical column. Consequently, the flow of gas through the Tenax during thermal desorption was reduced... 

Blood, urine purge-and-trap, thermal desorption cap GC/MS No data No data Barkley et al. 1980... 

In contrast, on the surface of the amino-containing polymeric materials, protonated amino groups introduced in a small proportion under physiological conditions, destroy their surrounding hydrogen bonds to produce, here and there, gaps in the network [127, 128]. Thus, the network structures are considered to become more or less unstable. As a consequence, the residence time of protein molecules trapped by these defective networks will be shorter than in the case of polyHEMA or cellulose. On the surface of these amino-containing materials, reversible protein adsorption and desorption, and also replacement (Vroman effect) - or even protein rejection - will become possible. 

Our previous study [130] on the adsorption and desorption behavior of bovine IgG has shown that the protein adsorbed to HA surface would be eluted quantitatively by 0.1 M PBS (phosphoric buffer solution) as shown in Table 11. Presumably, IgG molecules had been trapped in the destabilized network of water molecules on the surface of the HA copolymer. 

Translation to lattice energy transfer is the dominant aspect of atomic and molecular adsorption, scattering and desorption from surfaces. Dissipation of incident translational energy (principally into the lattice) allows adsorption, i.e., bond formation with the surface, and thermal excitation from the lattice to the translational coordiantes causes desorption and diffusion i.e., bond breaking with the surface. This is also the key ingredient in trapping, the first step in precursor-mediated dissociation of molecules at surfaces. For direct molecular dissociation processes, the implications of Z,X,Y [Pg.158]

---