Molecular species, desorption

Temperature-programmed desorption (TPD) is amenable to simple kinetic analysis. The rate of desorption of a molecular species from a uniform surface is given by Eq. XVII-4, which may be put in the form... 

By bombarding a surface consisting of species A with primary ions, the surface coverage of A is reduced. Particles of A can he removed hy desorption, hy driving them into a deeper layer or, for molecular species, hy fragmentation. The ratio of the number of sputtered particles to the number of primary ions is given by the disappearance yield Y (A) ... 

To derive an explicit expression of the rate of desorption we restrict ourselves to nondissociative adsorption, listing references to other systems— such as multicomponent and multilayer adsorbates with and without precursors—for which such a treatment has been given, later. We look at a situation where the gas phase pressure of a molecular species, P, is different from its value, P, which maintains an adsorbate at coverage 6. There is then an excess flux to re-establish equilibrium between gas phase and adsorbate so that we can write [7-10]... 

The concept of H2 chemisorbing on Rh as a molecular species may appear at first to be contradictory to other studies e.g.. Isotope exchange and TPD but It Is not, given the existence of klnetlcally distinct sites. An adsorption/desorption process can occur on a specific site via ... 

Thermal desorption spectroscopy and temperature programmed reaction experiments have provided significant insight into the chemistry of a wide variety of reactions on well characterized surfaces. In such experiments, characterized, adsorbate covered, surfaces are heated at rates of 10-100 K/sec and molecular species which desorb are monitored by mass spectrometry. Typically, several masses are monitored in each experiment by computer multiplexing techniques. Often, in such experiments, the species desorbed are the result of a surface reaction during the temperature ramp. 

In a separate study, a protocol for Matrix-assisted laser desorption-ionization (MALDI) imaging mass spectrometry (IMS) has been proposed.18 This IMS technique provides a new approach to visualize spatial distribution of thousands of molecular species, including peptides, proteins, and their metabolites in two- or three-dimensional levels. This approach may also provide a straightforward method of determining the tissue distribution of multiple peptides or proteins in a quantitative manner.18 Chu et al.19 reported a nondestructive molecular extraction method to obtain proteins from a single FFPE or frozen tissue section, without destroying the tissue morphology, such... 

One of the standard surface science methods for assessing the concentration and stability of a chemisorbed species is thermal desorption spectroscopy (TDS). An early paper by Redhead ( 7) developed the conceptual framework for certain cases. Many papers since then have expanded the applicability of this method. Recent work of Madix Q8) > Weinberg (9) and Schmidt CIO) is particularly noteworthy. Most of this work focuses on the desorption of a single molecular species and not on reactions in desorbing systems. However, qualitative features of the temperature dependence of reactions can be assessed using this method. Figures 1 and 2 taken from the... 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

Fig. 11.6. Diagram depicting desorption ionization (MALDI, FAB or SIMS). The operating principles of the three techniques are similar. The initiating event is exposure of the analyte to a beam of photons, atoms or ions. In order to prevent damage to the fragile analyte molecules and enhance the conversion of the involatile molecules into gas-phase ions, a matrix is employed. For MALDI, the matrix compounds are UV absorbing compounds such as hydroxycinnamic acid. The most commonly used FAB matrix was glycerol and ammonium chloride was employed by some investigators in SIMS experiments (although at low ion beam fluxes molecular species could be effectively ionized for many analytes with minimal evidence of damage by the primary ion beam).

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

Figure 11. Field desorption mass spectrum of sphingomyelin obtained at high emitter current (28 ma) and therefore dominated by peaks that correspond to transfer of choline (mass 104) to the three major molecular species present n = 16, MW 730 n = 22.1, MW 812 and n = 22, MW 814. The (M + choline) adducts are observed at m/e 834, 916, and 918, respectively. For the higher MW compounds, the fragment at m/e 548 when n = 16 occurs at m/e 630 and 632.

Studies of corrosion processes, detailed in Section 4.4.3, have demonstrated the capability of SAW devices to monitor relatively low rates of chemisorption, including the conversion of a thin copper film to CU2S at an initial rate of 4% of one molecular monolayer/day. The use of SAW devices to monitor the real-time desorption of species from a metal film in response to a temperature ramp has been shown to yield information about both the energy and extent of chemisorption [114]. TSM studies of chemisorption of O2 and CO on very thin Ti films were used to determine that the oxide being formed is Ti203 and that the oxidation depth is approximately one nm [137]. For further discussion and additional examples of chemisorption, the reader is referred to Section 5.4.4.3, where these... 

Alternatively, the technique of laser-induced desorption takes advantage of the adsorption properties of surfaces. As previously discussed, the optical skin depth of most metals is on the order of 50 A for IR frequencies. As a result, local heating can occur (up to 10 ° K s ) when incident radiation is focused to a small spot on a surface. By analogy to temperature programmed desorption (TPD), molecular species present in this region can be thermally desorbed and detected with a mass spectrometer. The rapid local heating of the surface induced by... 

To date the surface science approach and techniques such as those described above have been used to study structure of ceria surfaces, the adsorption and desorption of several molecular species on ceria and model ceria supported catalysts, and the co-adsorption and reaction of certain of these molecular species. The results provide a basis for clarifying the elementary reaction steps underlying catalytic processes occurring on ceria based catalysts. In this Chapter it is attempted to review and summarize this research. 

Polycrystalline Platinum.—Three desorption peaks are observed with Pt powders (170, 250, and 360 K) and films (120, 200, 330 K) [compare Pt(lll)]. Calculated heats of desorption for Pt films are 34, 50, and 88 kJ moP Similar values are found with Pt wire (71 kJ moP at 0 = 0.37 decreasing to 46 kJ moP at 0 = 0.46). Surface potential measurements on Pt films are consistent with work function data for single crystals. In both cases the most strongly bound H, located at surface imperfections, has a partial negative charge, while H on terrace sites has a partial positive charge. Dus and Tompkins also report a weakly bound (5 kJ moP ) molecular species, H2, which only adsorbs at high pressures. 

As a soft ionization technique thermospray mass spectrometry often provides little fragmentation of molecular species. More importantly from the viewpoint of a metabolism chemist, thermospray accomplishes desorption ionization of extremely low vapor pressure analytes including intact glucuronide and sulfate conjugates. As demonstrated in the chapter by Brown and Draper in this proceedings, particle beam mass spectrometry does not have this capability. 

When studying mixtures, it is necessary to increase the emitter current slowly, in order to induce the sequential desorption of the components of the mixture. Thus, in addition to these molecular species, there also appear decomposition ions (probably by pyrolysis) of the molecular species (protonated, cationized or not) which were emitted initially. 

Figure 25.2 Thermal desorption spectrum of hydrogen deu-teride (HD, mass 3) from a Pd(210) surface that had received a simultaneous exposure of hydrogen, Hj, and deuterium, Dj, at 40 K. The ft states represent atomically adsorbed hydrogen (deuterium), while the y states are due to the molecular species. Apparently, practically no isotopic scrambling occurs in the y states (absence of HD), while the exchange is complete in the atomic f states. After Schmidt et al. [9,10].

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