Interfaces Using Laser Light

Laser-aided interfaces find applications in fundamental studies in physical chemistry (see Section 4.2.1). However, they can also be used in the monitoring of environmental matrices as well as chemical and biochemical reactions (MA)LDI-MS is the most prominent example (Section 4.2.2). Moreover, laser beams can be introduced into canonical ion sources such as ESI to enable efficient transfer of microscale aliquots of solid or liquid matrices to the ionization zone. Along these lines, Cheng et al. [143] used electrospray-assisted laser desorption/ionization (ELDI) to monitor epoxidation of chalcone in ethanol, chelation of ethylenediaminetetraacetic acid with copper and nickel ions in aqueous solution, chelation of 1,10-phenanthroline with iron(II) in methanol, and 

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The enhancement can even be pushed further by 3 orders of magnitude by using laser light, which s in resonance with an electronic transition of the substances. This technique is called surface enhanced resonance Raman scattering ( resonant SERS ). Applications of SERS include not only the detection of molecules, but also the investigation of the structure and function of large biomolecules as well as the analysis of chemical processes at interfaces. SERS is particularly well suited for sandwich-type immunoassays and hybridization assays. 

Interfacial Viscosity. In a clean system in which two pure liquids produce an interface, the viscosity of the interface should be the same as the bulk solution viscosity. However, surfactant or impurity adsorption at an interface can cause a resistance to fiow to occur that can be measured as the interfacial shear viscosity. This viscosity is defined as the ratio between the shear stress and the shear rate in the plane of the interface (12), Methods used to make these measurements include a viscous traction surface viscometer (J2), droplet-droplet coalescence (J3), the rotating ring viscometer (14), and surface laser light scattering (9). 

Localized reactions at solid/liquid interfaces driven by focused laser light can be phenomenologically divided into deposition and dissolution processes as well as substrate modification. They can be used either for analytical or preparative purposes. An example is a direct writing procedure for the preparation of microstructures on macroscopic substrates without any masking technique. 

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. 

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