Interface analysis surface species

Surface Characterization. Most modem techniques for the characterization of surfaces have been developed since 1970 (74,75). Surface techniques allow for both qualitative and quantitative characterization of trace levels of molecular species (see Surface AND INTERFACE ANALYSIS). Most recently an extension of surface analysis utilizing laser ionization has been introduced (76). In surface analysis by laser ionization (sah), a probe beam, composed of ions, electrons, or laser light, is directed to the surface under examination to remove a sample of material. An untuned, high intensity laser passes dose to, but paralld and above the surface. The laser has sufficient intensity to induce a high degree of nonresonant, and hence nonselective, photoionization of the vaporized sample of material within the laser beam. The nonselectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species. A highlight of this technique is the use of efficient, nonresonant, and therefore nonselective photoionization by pulsed imtuned laser radiation. The commercial availabiUty of intense laser radiation makes this technique viable. The mass spectrometer, not the laser, performs the chemical differentiation. 

It is also common for pol3rmeric compoimds to form surface regions with compositions different from the bulk material, by selective diffusion of components. This process is termed blooming when the surface component is solid, and bleeding if it is liquid. Sulfur and fatty acid blooms can inhibit adhesion in rubber laminates (3). Laser desorption mass spectroscopy has been employed to identify surface species on vulcanized rubber (4). X-ray scattering methods for the study of polymer surfaces and interfaces have been reviewed (5). Other surface analysis techniques commonly used with polymers include attenuated total reflectance (6-8), electron microprobe (9), Auger electron spectroscopy (10), x-ray photoelectron spectroscopy (11), and scanning probe microscopic methods (12). Overviews on polymer surface analysis have been published (13,14). 

Relative photoionization cross sections for molecules do not vary gready between each other in this wavelength region, and therefore the peak intensities in the raw data approximately correspond to the relative abundances of the molecular species. Improvement in quantification for both photoionizadon methods is straightforward with calibration. Sampling the majority neutral channel means much less stringent requirements for calibrants than that for direct ion production from surfaces by energetic particles this is especially important for the analysis of surfaces, interfaces, and unknown bulk materials. 

ReflEXAES can be used for near-surface structural analysis of a wide variety of samples for which no other technique is appropriate. As with EXAES, ReflEXAES is particularly suited for studying the local atomic structure around particular atomic species in non-crystalline environments. It is, however, also widely used for the analysis of nanocrystalline materials and for studying the initial stages of crystallization at surfaces or interfaces. ReflEXAES was first proposed by Barchewitz [4.135], and after several papers in the early nineteen-eighties [4.136, 4.168-4.170] it became an established (although rather exotic) characterization technique. Most synchrotron radiation sources now have beam-lines dedicated to ReflEXAES experiments. 

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. 

In spite of the development of physicochemical techniques for surface analysis, spectroscopic methods applicable to the study of bonding between adsorbed metal ion species and substrate are limited, especially those applicable to in situ measurement at interfaces between solid and aqueous phases (1,2). In previous papers, we showed that emission Mossbauer measurement is useful in clarifying the chemical bonding environment of dilute metal ions adsorbed on magnetic metal oxide surfaces (3,1 ) ... 

The chemistry of surfaces or interfaces is becoming increasingly important in a variety of scientific disciplines, including the environmental sciences. In the case of atmospheric particles, modern methods of surface chemical analysis are providing valuable insights into the chemical speclation, mechanisms of formation, sources, reactivity, and potential toxicity of pollutant species. 

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