Desorption reproducibility

Figure 10.26. Adsorption isotherm of CH on MgO (10 0) at 87.4 K. Open symbols, adsorption solid symbols, desorption (reproduced courtesy of Gay et al., 1990).

Figure 1. Adsorption isotherms for a pure iron synthetic ammonia catalyst for various gases near their boiling points. Curve lA is for physical plus chemical adsorption of CO. Curve IB is for physical adsorption occurring at -1830C after the evacuation of the samples at -78 C for an hour. The solid symbols are for desorption. (Reproduced from Ref. 22. Copyright 1937, American Chemical Society.)...

Figure 4 TPD spectra on H-mordenite (SiOa/AlaOa 15.0) and H-ZSM-5 (Si02/Al203 = 23.8), I- and h-peaks due to low and high temperatures of desorption. (Reproduced with permission from Niwa M and Katada N (1997) Measurement of acidic property of zeolites by temperature programmed desorption of ammonia. Catalysis Surveys from Japan 1 215-226.)...

Fig. 2.47. (a) Conventional thermal desorber (b) direct thermal desorption. Reproduced by permission of ATAS GL International, B.V., Veldhoven. The Netherlands. 

A characteristic feature of a Type IV isotherm is its hysteresis loop. The exact shape of the loop varies from one adsorption system to another, but, as indicated in Fig. 3.1, the amount adsorbed is always greater at any given relative pressure along the desorption branch FJD than along the adsorption branch DEF. The loop is reproducible provided that the desorption run is started from a point beyond F which marks the upper limit of the loop. 

As an example of a multilayer system we reproduce, in Fig. 3, experimental TPD spectra of Cs/Ru(0001) [34,35] and theoretical spectra [36] calculated from Eq. (4) with 6, T) calculated by the transfer matrix method with M = 6 on a hexagonal lattice. In the lattice gas Hamiltonian we have short-ranged repulsions in the first layer to reproduce the (V X a/3) and p 2 x 2) structures in addition to a long-ranged mean field repulsion. Second and third layers have attractive interactions to account for condensation in layer-by-layer growth. The calculations not only successfully account for the gross features of the TPD spectra but also explain a subtle feature of delayed desorption between third and second layers. As well, the lattice gas parameters obtained by this fit reproduce the bulk sublimation energy of cesium in the third layer. 

When the temperature of the analyzed sample is increased continuously and in a known way, the experimental data on desorption can serve to estimate the apparent values of parameters characteristic for the desorption process. To this end, the most simple Arrhenius model for activated processes is usually used, with obvious modifications due to the planar nature of the desorption process. Sometimes, more refined models accounting for the surface mobility of adsorbed species or other specific points are applied. The Arrhenius model is to a large extent merely formal and involves three effective (apparent) parameters the activation energy of desorption, the preexponential factor, and the order of the rate-determining step in desorption. As will be dealt with in Section II. B, the experimental arrangement is usually such that the primary records reproduce essentially either the desorbed amount or the actual rate of desorption. After due correction, the output readings are converted into a desorption curve which may represent either the dependence of the desorbed amount on the temperature or, preferably, the dependence of the desorption rate on the temperature. In principle, there are two approaches to the treatment of the desorption curves. 

Fig. 1. Effect of pumping speed on a desorption peak at a fixed heating rate. Experimental parameters given in the text. Reproduced from Ehrlich (27), with permission.

Fig. 2. Effect of heating rate on a desorption peak at a fixed pumping speed. Hyperbolic heating schedule, l/To = 9.95 X 10-1 K-1 S/V = 4.8 sec-1 E — 80 kcal mole-1 x = 1. Reproduced from Hansen and Mimeault (30), with permission.

Its main features are given by the use of a stream of inert carrier gas which percolates through a bed of an adsorbent covered with adsorbate and heated in a defined way. The desorbed gas is carried off to a detector under conditions of no appreciable back-diffusion. This means that the actual concentration of the desorbed species in the bed is reproduced in the detector after a time lag which depends on the flow velocity and the distance. The theory of this method has been developed for a linear heating schedule, first-order desorption kinetics, no adsorbable component in the entering carrier gas (Pa = 0), and the Langmuir concept, and has already been reviewed (48, 49) so that it will not be dealt with here. An analysis of how closely the actual experimental conditions meet the idealized model is not available. 

Figure 19. Thermal desorption spectra of water adsorbed on (I) Ag(l 10), (2) PK111), (3) Ru(001), and (4) Ni(UO). (Reproduced from P.A. Thiel and T.E Madey, Surf. Sci. Reports 7 258, Fig. 29,1987, Ci 1987 with permission of Elsevier Science.)...

Figure 5. Perspective drawing of the laser desorption FTMS instrument showing the relative positions of the crystal, the laser beam, and the FTMS analyzer cell. Reproduced with permission from Ref. 18. Copyright 1985, North-Holi and Physics Publishing.

Figure 6. The sequence of events in a laser desorption FTMS experiment, (a) The laser beam enters the cell and strikes the crystal, (b) Some of the desorbed molecules are ionized by an electron beam, (c) Ions are trapped in the analyzer cell by the magnetic and electric fields, (d) Ions are accelerated by an RF pulse and the resulting coherent image current signal is detected. Reproduced with permission from Ref. 18. Copyright 1935, North-Holland Physics Publishing.

Figure 2.2 Reactivity of oxygen states chemisorbed at Ni(210) (a) at 295 K and (b) at 77 K to water adsorbed at 77 K. The oxygen concentration ct is calculated from the 0(1 s) spectra. The oxygen state preadsorbed at 295 K is unreactive with water desorption complete at 160K whereas that at 77 K is reactive, resulting in surface hydroxylation.37 (Reproduced from Refs. 37, 42).

Figure 8.9 Dynamics of STM-driven desorption and dissociation of chlorobenzene at Si(lll) (7 x 7) (a) before and (b) after a desorption scan the circles indicate the positions of chlorobenzene molecules before and after desorption (c) appearance of a chlorine adatom formed by dissociation of chlorobenzene with corresponding 3D image (d) measured rates of desorption and dissociation as a function of tunnelling current for a sample bias of + 3 V. (Reproduced from Ref. 26).

To use the DCI probe, 1-2 xL of the sample (in solution) are applied to the probe tip, composed of a small platinum coil, and after the solvent has been allowed to evaporate at room temperature, the probe is inserted into the source. DCI probes have the capability of very fast temperature ramping from 20 to 700 °C over several seconds, in order to volatilise the sample before it thermally decomposes. With slower temperature gradients, samples containing a mixture of components can be fractionally desorbed. The temperature ramp can be reproduced accurately. It is important to use as volatile a solvent as possible, so as to minimise the time required to wait for solvent evaporation, which leaves a thin layer of sample covering the coil. The observed spectrum is likely to be the superposition of various phenomena evaporation of the sample with rapid ionisation direct ionisation on the filament surface direct desorption of ions and, at higher temperature, pyrolysis followed by ionisation. 

Table 6.16 summarises the main characteristics of FI-MS. FT uses high voltages and was once restricted to sensitive double-focusing magnetic sector instruments of relatively high cost. Field ionisation is considered to be the softest ionisation mode. The reproducibility of the non-standard techniques, such as FI-MS and FD-MS, is less well assessed than that of EI-MS. A noticeable drop in FI use occurred after the mid-1980s because of the advent of FAB and other desorption/ionisation methods. FI-MS is only used in a few laboratories worldwide. 

In direct insertion techniques, reproducibility is the main obstacle in developing a reliable analytical technique. One of the many variables to take into account is sample shape. A compact sample with minimal surface area is ideal [64]. Direct mass-spectrometric characterisation in the direct insertion probe is not very quantitative, and, even under optimised conditions, mass discrimination in the analysis of polydisperse polymers and specific oligomer discrimination may occur. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible (e.g. Hostanox 03, MW 794). Good quantitation is also prevented by contamination of the ion source by pyrolysis products of the polymeric matrix. For polymer-based calibration standards, the homogeneity of the samples is of great importance. Hyphenated techniques such as LC-ESI-ToFMS and LC-MALDI-ToFMS have been developed for polymer analyses in which the reliable quantitative features of LC are combined with the identification power and structure analysis of MS. 

As indicated in Section 6.2.2, DI-CIMS suffers from poor reproducibility. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible. The method depends on polymer formulation standards that are reliably mixed. Wilcken and Geissler [264] described rapid quality control of l- xg paint samples by means of temperature-programmable DI-EIMS with PCA evaluation. 

For PMMA/additive dissolutions, it was not possible to identify any additive characteristic mass peaks, either by direct laser desorption or with matrix-assistance (dithranol, DHBA or sinapinic acid, 4-hydroxy-3,5-dimethoxy-cinnamic acid). This has again been ascribed to very strong interaction between PMMA and additives, which suppresses desorption of additive molecules. Also, partial depolymerisation of pho-tolytically labile PMMA by laser irradiation may play a role, which leads to saturation of the detector by PMMA fragment-ions and disappearance of additive mass peaks below noise level. Meyer-Dulheuer [55] has also reported MALDI-TOFMS analysis of a coating/2-ethylhexyldiphenylphosphate sample. Quantitative determination of the additives by means of MALDI-ToFMS proved impossible. Possibly the development of reproducible (automated) sample handling procedures or thin films might overcome this problem. 

Monte Carlo simulations have been also used to reproduce the dynamics of adsorbates associated with NO reduction reactions. As mentioned above, complex desorption dynamics have been observed experimentally in some instances. For example, the N2 produced from decomposition of N20 on Rh(110) leaves the surface in five peaks associated with both the N20 dissociation events and the desorption of the adsorbed products. Monte Carlo simulations of those spectra was possible by using a model that takes into account both channels of N2 desorption and also N20 O lateral interactions to stabilize N20 adsorption [18],... 

Figure 3.10. Schematic representation of the elementary steps used in microkinetic simulations of the reduction of NO on supported metal particles [23]. The mechanism represented here incorporates adsorption and desorption steps, surface reactions such as NO dimerization and dissociation and N2, N20 and C02 formation, surface oxidation, and mobility of adsorbates. (Figure provided by Professor Libuda and reproduced with permission from Elsevier, Copyright 2005).

The same group has looked into the conversion of NO on palladium particles. The authors in that case started with a simple model involving only one type of reactive site, and used as many experimental parameters as possible [86], That proved sufficient to obtain qualitative agreement with the set of experiments on Pd/MgO discussed above [72], and with the conclusion that the rate-limiting step is NO decomposition at low temperatures and CO adsorption at high temperatures. Both the temperature and pressure dependences of the C02 production rate and the major features of the transient signals were correctly reproduced. In a more detailed simulation that included the contribution of different facets to the kinetics on Pd particles of different sizes, it was shown that the effects of CO and NO desorption are fundamental to the overall behavior... 

Experimental probes of Born-Oppenheimer breakdown under conditions where large amplitude vibrational motion can occur are now becoming available. One approach to this problem is to compare theoretical predictions and experimental observations for reactive properties that are sensitive to the Born-Oppenheimer potential energy surface. Particularly useful for this endeavor are recombinative desorption and Eley-Rideal reactions. In both cases, gas-phase reaction products may be probed by modern state-specific detection methods, providing detailed characterization of the product reaction dynamics. Theoretical predictions based on Born-Oppenheimer potential energy surfaces should be capable of reproducing experiment. Observed deviations between experiment and theory may be attributed to Born-Oppenheimer breakdown. 

Saenz, A. J. Petersen, C. E. Valentine, N. Gantt, S. L. Karman, K. H. Kingsley, M. T. Wahl, K. L. Reproducibility of matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry for replicate bacterial culture analysis. Rapid Comm. Mass Spectrom. 1999,13,1585-1585. 

Wang, Z. Russon, L. Li, L. Roser, D. C. Long, S. R. Investigation of spectral reproducibility in direct analysis of bacteria proteins by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Rapid Comm. Mass Spectrom. 1998,12,456-464. 

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