Spectroscopy limitations

Workman, J. and Mark, H., Chemometrics in Spectroscopy Limitations in Analytical Accuracy - Part 1 Horwitz s Trumpet, Spectroscopy 21(9), 18-24 (2006). 

Information concerning the tertiary structure of the proteins has been obtained from fluorometry, proton magnetic resonance spectroscopy, limited proteolysis, and X-ray analysis of protein crystals. 

The FIM images do not contain information about the chemical identity of the atoms. By combining with tunneling spectroscopy, limited chemical information can be extracted from STM images. 

Direct spectroscopic observation of the postulated diradical intermediates has not been possible so far. Thus, multiphoton infrared laser excitation of tetramethyldioxetane in the gas phase failed to detect diradical intermediates with lifetimes greater than about 5 nsec.Picosecond spectroscopy limited the lifetime of a diradical intermediate, if formed, to less than about lOpsec in the 264-nm pulsed photolysis of tetramethyldioxetane in acetonitrile, using a mode-locked neodymium... 

T. Osterlund, D.J. Beussman, K. Jule-nius, P.H. Poon, and S. Linse, Domain identification of hormone-sensitive lipase by circular dichroism and fluorescence spectroscopy, limited proteolysis, and mass spectroscopy, J. Biol. Chem., 1999, 274, 15382-15388. 

Skill 18.1 Includes demonstrating knowledge of the basic principles used in spectroscopy, limited to UV, visible, infrared, and mass spectroscopy... 

For molecules the line densities are much higher, and often the rotational structure can only be resolved by sub-Doppler spectroscopy. Limiting the collimation angle of the molecular beam below 2 x 10 rad, the residual Doppler width can be reduced to values below 500 kHz. Such high-resolution spectra with linewidths of less than 150 kHz could be, for instance, achieved in a molecular iodine beam since the residual Doppler width of the heavy I2 molecules, which is proportional to is already below this value for a collimation ratio 6 < 4 x 10 [400]. At... 

Alkenylfluorophosphazene copolymers, quantitative reactivity studies of copolymerization reactions, 390-394 Alkoxyalkoxy side groups, synthesis of n-silylphosphoranimines, 311-322 Alkyl-group polymers, deprotonation-substitution reactions, 248-249 Allylboration polymerization, polycyclo-diborazane synthesis, 407-408,409, 410 Alucone polymers characterization ceramics, 177 polymers, 173-178 examples, 166,167/ solid-state NMR spectroscopy limitations, 177,179/ quadrupolar broadening, 177-180 status, 180-181 syntheses ceramics, 170-174 polymers, 166,168-170 synthetic route, 166... 

X-ray spectroscopy Analytical method by which a sample is irradiated with X-rays, characteristic radiation being emitted after scattering from the specimen. The detection limits for various elements are of the ordering cm. ... 

Raman microscopy is more developed than its IR counterpart. There are several reasons for this. First, the diffraction limit for focusing a visible beam is about 10 times smaller than an IR beam. Second, Raman spectroscopy can be done in a backscattering geometry, whereas IR is best done in transmission. A microscope is most easily adapted to a backscattermg geometry, but it is possible to do it in transmission. 

The third important source for infonnation on modem Raman spectroscopy are the books cataloguing the proceedings of the International Conference on Raman Spectroscopy (ICORS) [37]. ICORS is held every two years at various international locations and feahires hundreds of contributions from leading research groups covering all areas of Raman spectroscopy. Although the published presentations are quite limited in lengdi, they each contain references to the more substantial works and collectively provide an excellent overview of current trends m Raman spectroscopy. A snapshot or brief sununary of the 1998 conference appears at the end of this chapter. 

Nonnal spontaneous Raman scahering suffers from lack of frequency precision and thus good spectral subtractions are not possible. Another limitation to this technique is that high resolution experiments are often difficult to perfomi [39]. These shortcomings have been circumvented by the development of Fourier transfomi (FT) Raman spectroscopy [40]. FT Raman spectroscopy employs a long wavelength laser to achieve viable interferometry. 

Plenary 15. B Scluader et al, e-mail address beriilrard.scluader uni-essen.de (NIR-FTRS). A review of the use of Raman spectroscopy in medical diagnostics. Its possibilities, limitations and expectations. Emphasizes the need for a library of reference spectra and the applications of advanced analysis (chemometry) for comparing patient/library spectra. 

Closs G L and Forbes M D E 1991 EPR spectroscopy of electron spin polarized biradicals in liquid solutions. Technique, spectral simulation, scope and limitations J. Phys. Chem. 95 1924-33... 

Figure Bl.22.6. Raman spectra in the C-H stretching region from 2-butanol (left frame) and 2-butanethiol (right), each either as bulk liquid (top traces) or adsorbed on a rough silver electrode surface (bottom). An analysis of the relative intensities of the different vibrational modes led to tire proposed adsorption structures depicted in the corresponding panels [53], This example illustrates the usefiilness of Raman spectroscopy for the detennination of adsorption geometries, but also points to its main limitation, namely the need to use rough silver surfaces to achieve adequate signal-to-noise levels.

Ultraviolet photoelectron spectroscopy (UPS) [2, 3 and 4, 6] differs from XPS in that UV light (He I, 21.2 eV He II, 40.8 eV) is used instead of x-rays. At these low excitmg energies, photoemission is limited to valence electrons. 

These limitations have recently been eliminated using solid-state sources of femtosecond pulses. Most of the femtosecond dye laser teclmology that was in wide use in the late 1980s [11] has been rendered obsolete by tliree teclmical developments the self-mode-locked Ti-sapphire oscillator [23, 24, 25, 26 and 27], the chirped-pulse, solid-state amplifier (CPA) [28, 29, 30 and 31], and the non-collinearly pumped optical parametric amplifier (OPA) [32, 33 and 34]- Moreover, although a number of investigators still construct home-built systems with narrowly chosen capabilities, it is now possible to obtain versatile, nearly state-of-the-art apparatus of the type described below Ifom commercial sources. Just as home-built NMR spectrometers capable of multidimensional or solid-state spectroscopies were still being home built in the late 1970s and now are almost exclusively based on commercially prepared apparatus, it is reasonable to expect that ultrafast spectroscopy in the next decade will be conducted almost exclusively with apparatus ifom conmiercial sources based around entirely solid-state systems. 

Microwave studies in molecular beams are usually limited to studying the ground vibrational state of the complex. For complexes made up of two molecules (as opposed to atoms), the intennolecular vibrations are usually of relatively low amplitude (though there are some notable exceptions to this, such as the ammonia dimer). Under these circumstances, the methods of classical microwave spectroscopy can be used to detennine the stmcture of the complex. The principal quantities obtained from a microwave spectmm are the rotational constants of the complex, which are conventionally designated A, B and C in decreasing order of magnitude there is one rotational constant 5 for a linear complex, two constants (A and B or B and C) for a complex that is a symmetric top and tliree constants (A, B and C) for an... 

As described above, classical infrared spectroscopy using grating spectrometers and gas cells provided some valuable infonnation in the early days of cluster spectroscopy, but is of limited scope. However, tire advent of tunable infrared lasers in tire 1980s opened up tire field and made rotationally resolved infrared spectra accessible for a wide range of species. As for microwave spectroscopy, tunable infrared laser spectroscopy has been applied botli in gas cells and in molecular beams. In a gas cell, tire increased sensitivity of laser spectroscopy makes it possible to work at much lower pressures, so tliat strong monomer absorjDtions are less troublesome. 

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