Sensitivity enhancement methods modulation

Electron Spin Echo Envelope Modulation (ESEEM) and pulse Electron Nuclear Double Resonance (ENDOR) experiments are considered to be two cornerstones of pulse EPR spectroscopy. These techniques are typically used to obtain the static spin Hamiltonian parameters of powders, frozen solutions, and single crystals. The development of new methods based on these two effects is mainly driven by the need for higher resolution, and therefore, a more accurate estimation of the magnetic parameters. In this chapter, we describe the inner workings of ESEEM and pulse ENDOR experiments as well as the latest developments aimed at resolution and sensitivity enhancement. The advantages and limitations of these techniques are demonstrated through examples found in the literature, with an emphasis on systems of biological relevance. 

Sensitivity and complexity represent challenges for ATR spectroscopy of catalytic solid liquid interfaces. The spectra of the solid liquid interface recorded by ATR can comprise signals from dissolved species, adsorbed species, reactants, reaction intermediates, products, and spectators. It is difficult to discriminate between the various species, and it is therefore often necessary to apply additional specialized techniques. If the system under investigation responds reversibly to a periodic stimulation such as a concentration modulation, then a PSD can be applied, which markedly enhances sensitivity. Furthermore, the method discriminates between species that are affected by the stimulation and those that are not, and it therefore introduces some selectivity. This capability is useful for discrimination between spectator species and those relevant to the catalysis. As with any vibrational spectroscopy, the task of identification of a species on the basis of its vibrational spectrum can be difficult, possibly requiring an assist from quantum chemical calculations. 

A number of spectroscopic techniques, as well as many of the tools of surface science, are used to study photoelectrochemical systems. The objective may be to monitor changes in the structure or chemical composition of the surface. Alternatively, the aim may be to probe the bulk properties of the semiconductor. Modulation techniques are particularly important, since they greatly enhance the sensitivity of spectroscopic measnrements. It is not possible to give an exhanstive survey of all these methods in this chapter. Instead, we will emphasise the complementary nature of different techniques and the way that they can be used to achieve a deeper understanding of the physical and chemical processes taking place in photoelectrochemical systems. The reader is referred to a number of reviews and textbooks for a more detailed account of fundamental aspects of semiconductor electrochemistry (Morrison, 1980 Pleskov and Gurevich, 1986 Hamnett, 1987 Finklea, 1988 Sato, 1998 Peter, 1999, Memming, 2001). 

The introduction of in-situ infrared spectroscopy to electrochemistry has revolutionised the study of metal/electrolyte interfaces. Modnlation or sampling techniques are applied in order to enhance sensitivity and to separate snrface species from volume species. Methods such as EMIRS (electrochemicaUy modulated IR spectroscopy) and SNIFTIRS (subtractively normalised interfacial Fonrier Transform infrared spectroscopy) have been employed to study electrocatalytic electrodes, for example. There have been surprisingly few studies of the semiconductor/electrolyte interface by infrared spectroscopy. This because up to now little emphasis has been placed on the molecnlar electrochemistry of electrode reactions at semiconductors because the description of charge transfer at semiconductor/electrolyte interfaces is derived from solid-state physics. However, the evident need to identify the chemical identity of snrface species should lead to an increase in the application of in-situ FTIR. 

One of the most commonly applied IR techniques developed to overcome these problems is the external reflectance technique. In this method, the shong solvent absorption is minimized by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The sensitivity problem, that is, the enhancement of the signal/noise ratio in the case of external reflectance techniques is solved by various approaches. These are, for instance, electrochemically modulated infrared spectroscopy (EMIRS), in situ FTIR (which use potential modulation), and polarization modulation infrared reflection absorption spectroscopy (PM-IRAS, FTIR) [86,117-123]. 

Steroids are hormones that are known to modulate DNA expression and protein expression. Adrenal hormones (e.g., aldosterone and cortisol) participate in the control of metabolism and of salt and water homeostasis. The other class of steroid hormones, gonadal hormones (e.g., estradiol, progesterone, and testosterone), influence mammalian sexual development and function. Several steroids also modulate neurotransmitter action, and some athletes have misused anabolic steroids to enhance their physical strength and performance. GC/MS has been used in conjunction with El or Cl as an ionization method for steroids to determine molecular mass, elemental composition, and length of the side chain of a steroid hydrocarbon. Although these techniques are highly sensitive, they require multistep derivatization of these thermally labile compounds. FAB-MS has proven to be a useful technique to analyze underivatized urinary... 

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