Spectral line narrowing techniques usefulness

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

Manganese may be determined with good sensitivity by flame AAS or AFS, the former technique being very widely used in environmental analysis. The detection limit at 279.5 nm by AAS in a lean air-acetylene flame is about 10 ng ml-1, which is quite adequate for most environmental analyses. A narrow spectral bandpass should be used, and care taken to make sure that the 279.5 nm line is being used rather than one of the adjacent lines at 279.8 or 280.1 nm. The detection limit at 403.1 nm by flame AES using a nitrous oxide-acetylene flame is usually slightly better, at around 5 ng ml-1, than that obtained by flame AAS. 

The solute molecule is dissolved in the liquid crystal solvent at low concentration. A variety of nematic solvents are available, some of which are nematic at room temperature. Representative high-resolution proton NMR spectra are given in Figure 1. Because the solvent order depends on composition and temperature, it is important that temperature and composition gradients at the NMR probe be minimized if the narrow line widths of a few hertz are to be obtained. The spectra of Figure 1 show the rapid increase of spectral complexity with the number of nuclei. The spectra become almost continuous and uninterpretable at about 10 spins. Simplified proton NMR spectra can be obtained by partial deuterium substitution and decoupling.6 This has been described for cyclohexane, but has not been used extensively. Proton double resonance is also a useful experimental technique for the identification of spectral lines.6... 

The technique is applicable to proteins and nucleic acids in their solution state. A few protein structures have been solved by using solid-state NMR, which involves spinning the sample at thousands of revolutions per second. This gives narrow spectral lines that are similar to those obtained from proteins in solution, but the instruments and data analysis are much more complex than for solution-state NMR spectroscopy. 

Each hollow cathode emits the spectrum of metal used in the cathode. For this reason, a different HCL must be used for each different element to be determined. This is an inconvenience in practice and is the primary factor that makes AAS a technique for determining only one element at a time. The handicap is more than offset, however, by the advantage of the narrowness of the spectral lines and the specificity that results from these narrow lines. 

Densitometers use a variety of optical systems. The optics utilizes either a slit at a fixed position with a mechanism to slowly move the spectral plate across the slit, or a slit that moves across the spectral line with the photographic plate in a fixed position. Either technique permits scanning the spectral line. Slits must be sufficiently narrow so that, when centered on the image of the spectral line, no light enters the slit from alongside the line. Some densitometer slits are adjustable for length and width. Others are of fixed width, but a series of fixed slits is available for interchange. 

One of the greatest advantages of AAS, namely, its specifity, is based on the use of element-specific radiation sources that emit the spectrum of the analyte element in the form of very narrow spectral lines. While the quality of an instrument in other spectrometric techniques frequently depends on the resolution of the monochromator or on its spectral bandpass (the range of radiation that passes through the exit slit), these factors are not of primary importance in AAS. If the element-specific radiation is modulated and the amplifier tuned to the same frequency, AAS is selective and free of spectral interferences caused by overlapping of atomic lines of different elements (see Sec. 1.6). 

The narrow spectral line of a DL enables isotope selective analysis. For light and heavy elements (such as Li and U) the isotope shifts in spectral lines are often larger than the Doppler widths of the lines, in this case isotopically selective measurements are possible using simple Doppler-limited spectroseopy - DLAAS or laser induced fluorescence (LIF). For example, and ratios have been measured by Doppler-limited optogalvanic. spectroscopy in a hollow cathode discharge. DLAAS and LIF techniques have been combined with laser ablation for the selective detection of uranium isotopes in solid samples. This approach can be fruitful for development of a compact analytical instrument for rapid monitoring of nuclear wastes. 

One well-known and often used technique employed in conjunction with MAS is proton decoupling, where the influence of protons surrounding the measured species, usually carbon or lithium, is inactivated or saturated by a 90° pulse train at the proton frequency, a so-called broad band decoupling. The resulting spectral line is then free from proton dipolar interactions and is considerably narrowed. Another advantage of this technique is that the heteronucleus directly bonded to a proton will be affected much more than other sites that are not, therefore facilitating spectral assignments. 

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