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Chemical Analysis Basics
Published in Thomas J. Bruno, Paris D.N. Svoronos, CRC Handbook of Basic Tables for Chemical Analysis, 2020
Thomas J. Bruno, Paris D.N. Svoronos
A vibrational spectroscopic method that arises from the inelastic scattering of monochromatic radiation by molecules that undergo a change in polarizability during the vibration. This is in contrast to IR spectrophotometry, in which a change in the dipole moment occurs during the vibration. When radiation (typically light from a laser in the visible, near-IR, or near-UV range) is scattered, a small fraction of the scattered radiation is observed to have a different frequency (the Raman effect). The variations of Raman spectroscopy are used to locate functional groups or chemical bonds in molecules. There are several variations in the approach to Raman spectroscopy. In resonance Raman spectroscopy, the excitation wavelength is matched to an electronic transition of the molecule, enhancing the vibrational modes. In coherent anti-Stokes Raman spectroscopy (CARS), two laser beams are used to generate a coherent anti-Stokes frequency beam. In surface-enhanced Raman spectroscopy (SERS), surface plasmons (a quantum of plasma oscillation) on a silver or gold colloid on a surface (such as a mirror) are excited by the laser, resulting in an increase in the electric fields surrounding the metal.
Sensors and Transducers
Published in David C. Swanson, ®, 2011
The technical challenge with Raman spectroscopy is to filter out the laser frequency and capture the very weak Stokes backscatter in a high SNR spectrometer. The noise in a spectrometer is called “dark current” and is found by reading the detector output with no light entering the spectrometer. The spectrometer signal can be summed over time to reduce the electronic background noise as the square-root of the number of spectra summed. The amplitude of the Raman spectra is typically in arbitrary units (a.u.) because of the difficulty in obtaining physical units, so the spectra are usually compared relatively, sometimes even using spectral subtraction to emphasize some chemical reaction. Some of the molecular modes are even weaker than others due to the nature of the molecular motion and some modes are not Raman active at all. It depends on the symmetry of the molecule among other things such that some modes are best seen using MAS or FTIR and others are best seen using Raman, while some other modes are extremely difficult to see in either spectrographic method. Using a laser frequency close to a molecular absorption line enhances the molecular vibration and the Raman backscatter and is called “resonance Raman spectroscopy.” From the signal processor’s point of view, the Raman spectrum is just a spectrum, where the peaks are to be detected and associated with particular target analytes.
Analytical Chemistry
Published in W. M. Haynes, David R. Lide, Thomas J. Bruno, CRC Handbook of Chemistry and Physics, 2016
W. M. Haynes, David R. Lide, Thomas J. Bruno
radiation by molecules that undergo a change in polarizability during the vibration. This is in contrast to infrared spectrophotometry, in which a change in the dipole moment occurs during the vibration. When radiation (typically light from a laser in the visible, near-infrared, or near-ultraviolet range) is scattered, a small fraction of the scattered radiation is observed to have a different frequency (the Raman effect). The variations of Raman spectroscopy are used to locate functional groups or chemical bonds in molecules. There are several variations in the approach to Raman spectroscopy. In resonance Raman spectroscopy, the excitation wavelength is matched to an electronic transition of the molecule, enhancing the vibrational modes. In coherent antiStokes Raman spectroscopy (CARS), two laser beams are used to generate a coherent anti-Stokes frequency beam. In surfaceenhanced Raman spectroscopy (SERS), surface plasmons (a quantum of plasma oscillation) on a silver or gold colloid on a surface (such as a mirror) are excited by the laser, resulting in an increase in the electric fields surrounding the metal. Atomic Absorption Spectroscopy (AAS) and Atomic Emission Spectroscopy (AES): Two related spectroscopic methods applied primarily to the analysis of inorganic compounds. Atomic absorption procedures use the absorption of optical radiation (light) by free atoms in the gaseous state. The light can be produced by a hollow cathode lamp, an electrodeless discharge lamp, or a deuterium lamp. The light is absorbed by the analyte during an electronic transition, the wavelength of which corresponds to only one element in the analyte, and the width of an absorption line is of the order of only a few picometers. This method can be used for the quantitative determination (on the basis of a calibration curve) of approximately 70 different elements in solution or directly in solid samples. Atomic emission spectroscopy (AES) uses the light emitted by a vaporized sample in a flame, plasma, arc, spark, or laser, at a particular wavelength, to determine the atomic spectrum (for determination of the elemental composition) and to determine the quantity of an element in a sample. The wavelength of the atomic spectral line gives the identity of the element while the intensity of the emitted light is proportional to the number of atoms of the element. No single source, as described above, is optimal for a given sample, and it is the choice of source that distinguishes the various techniques.
Electron paramagnetic resonance of globin proteins – a successful match between spectroscopic development and protein research
Published in Molecular Physics, 2018
Sabine Van Doorslaer, Bert Cuypers
By selected point mutations in the haem-pocket region, the properties of globins can be enormously influenced. Recently, Azarov et al. attracted a lot of public attention by showing that the E7Gln mutant of neuroglobin can act as a ligand-trap antidote for carbon-monoxide poisoning [104,105]. Its high affinity for CO is in this case obtained by eliminating the competition with the E7His distal ligand via the mutation. This E7Gln mutant, as well as the E7Val variant, exhibits distal ligation of the E11Lys at high pH as follows from EPR [58] and resonance Raman spectroscopy [106]. The Lys-Fe(III)-His ligation mode with its high gz value is typical of a type-I ferric state (Table 2). This type of ligation has been observed before in ferric cytochromes with similar EPR values [107]. A recent study showed that the E7Ala mutant of Ngb rapidly binds H2S and converts it efficiently to oxidised products, while ferric wild-type Ngb is only slowly reduced by H2S [87]. A clear difference can be seen in the EPR parameters of the HS−-ligated ferric forms of wild-type Ngb and its E7Ala variant (Table 3), indicating quite different stabilisations of the distal ligand, which directly influence the possible reaction pathways as confirmed by other spectroscopies [87].
Compact turnkey system for multi-contact diode lasers for portable spectroscopic applications
Published in Instrumentation Science & Technology, 2023
Bernd Sumpf, Lucas Wittenbecher, Thomas Filler, Daniel Bandke, Maria Krichler, André Müller, Kay Sowoidnich, Arnim Ginolas, Ulrike Winterwerber, Martin Maiwald
Potential applications with multi-contact diode lasers may include the implementation of a DBR RW laser diode with heater elements for spectral tuning as required by, e.g., sequentially shifted Raman spectroscopy[6], a dual wavelength Y-branch DBR-RW laser in alternating operation for SERDS or parallel operation for THz generation, or a frequency-doubled 976 nm diode laser for resonance Raman spectroscopy at 488 nm.