Sensitivity of ESR

The sensitivity of esr spectroscopy for detection of radicals is very high. Under favorable conditions, a concentration of radicals as low as 10 12M can be detected readily. Identification of simple hydrocarbon radicals often is possible by analysis of the fine structure in their spectra, which arises from spin-spin splittings involving those protons that are reasonably close to the... 

Delocalisation onto oxygen stabilizes radicals considerably. An important example is the ascorbate radical (Scheme 1.3) formed by electron-loss from the ascorbate anion, or electron-capture by dehydroascorbate. This is remarkably stable, and is characterized by an ESR doublet (1.7 G) which is quite distinctive. Because of the high sensitivity of ESR spectroscopy, and the fact that opaque samples can be used, ascorbate radical intermediates have been widely studied (Liu et al., 1988a). The most probable structure is shown in Scheme 1.3 but this is still a matter of some controversy (Liu et al., 1988a). A key factor in the formation of ascorbate radicals is that ascorbate anions... 

The sensitivity of ESR spectroscopy is such that, in ancient bone, what we see may be a relict protein structure far beyond the limits of extractive techniques. The stability of the signal and the correspondence between ancient and modem bone suggests that the signal has not been caused by recent intrusions or a free amino acid. The alanine signal may be produced by an alanine moiety on the end of a protein chain that is denatured by the y-irradiation. Presumably, heating the modem samples accelerates some chemical reaction that would otherwise take many years to complete this... 

For a given value of B, the energies of Am/ = 1 transitions between the nuclear sublevels of a given electronic spin state are much lower than those between the electronic spin components. Information on the amplitude of the wave function of the electron whose spin is responsible for the ESR spectrum at different lattice sites in the vicinity of the centre was obtained by Feher [17] by monitoring the ESR spectrum as a function of the frequencies in the nuclear frequency range, and this technique was called electron nuclear double resonance (ENDOR). Improvements in the sensitivity of ESR can be obtained using optical or electrical detection methods [47]. 

The spectral sensitivity of ESR depends on a variety of factors, but with a response time of 1 sec, as few as 10 spins ( 10 moles) can be detected with currently available spectrometers. This sort of sensitivity suggests that ESR spectroscopy would be useful for trace analysis. A minimum detectable concentration is perhaps 10 M in samples with very small dielectric loss. For qualitative measurement in aqueous solutions, 10" M is more reasonable, while for quantitative measurements the sample concentration should be greater than about 10 M. Unfortunately, ESR spectra are more applicable to qualitative and semiquan-... 

The heterogeneous structure of the swollen B. mori silk fibroin membrane was clarified from the complicated ESR spectra of the spin-labeled silk fibroin membranes. Spin-label ESR methods are useful for dynamic analysis of silk fibroin because of the inherent high sensitivity of ESR observation and the wide detectable range of the motion of the spin-labeled site, from 10 to lO " s. The hydroxy group of the Tyr side-chain is active and thus it can be labeled with nitroxide radical compounds as shown in Fig. 31. ... 

The possibility of a radical path in a reaction is often suggested by an unexpected racemization or an unusual reactivity order (see Section 1.5.4). In an effort to prove that the mechanism involves radical participation, it might appear that direct observation of the radicals by ESR would be the method of choice. Generally, however, one wishes to demonstrate not only the presence of radicals, but that the major path of the reaction proceeds via a radical route. The extreme sensitivity of ESR may make detection of radicals relatively straightforward, but making quantitative estimates to prove that a radical is a loop species is another matter. Nevertheless, direct ESR detection of an intermediate is a quite useful approach. 

Isotopic substitution The sensitivity of ESR-dosimeters in the low-dose range can in some cases be increased by isotope exchange of the materials. Samples enriched in certain isotopes hke or Li often have narrower line-widths, and accordingly stronger amplitudes than those of the same material with natural isotope abundance. An approach to increase the sensitivity of ammonium formate for X- and y-irradiation by recrystalhsation in D2O has been described [52]. 

In this article, I have summarized applications of ESR spectroscopy to the radical polymerization of vinyl and diene compounds, with particular emphasis put on the advantages of the special cavities designed for enhancing the sensitivity of ESR spectrometers. These cavities allow determinations of the conformations of propagating radicals as well as the propagation rate constants for several monomers under conditions similar to usual radical polymerizations. However, I am afraid that the reliability of the kinetic data obtained with them is not yet high enough for precise determination of the rate constants because of experimental errors involved in the measurement of radical concentrations and polymerization rates. It is desirable that the ESR spectrometer be made about 10 times more sensitive for really reliable data... 

More generally, the higher sensitivity of ESR experiments can be used for the detection of NMR frequencies by applying both resonant mw and resonant radio frequency (if) irradiation to the spin system. Such electron nuclear double-resonanee (ENDOR) experiments are discussed in Chapter 2. 

Table 5.58 lists the main characteristics of ESRI. The basic requirement for an ESRI experiment is that a species having unpaired electrons be present in sufficient concentrations this is in contrast to MRI where the ubiquitous proton can be used to study most materials. Fortunately, the high sensitivity of ESR compared with NMR (arising from the... 

Fundamental understanding of the mechanism of degradation of polymers by high-energy radiation has been based mainly on structural cban ol erved in the polymers, and to a much smaller extent on measurements of small molecule products. ESR has been used since 1960 to observe radicals produced in irradiated polymers, and hence to provide evidence for intermediate species in the radiolysis. However, recent improvements in the stability and sensitivity of ESR spectrometers and in computer manipulation of the spectra have enhanced the use of this technique. 

Hyde, J. S. and W. K. Subczynski. 1984. Simulation of ESR spectra of the oxygen-sensitive spin-label probe CTPO. J. Magn. Reson. 56 125-130. 

ESR spectroscopy, used in the direct detection or spin trapping modes, is a sensitive method for the detection of polymer fragments and for determining the degradation mechanism. Recent applications for the study of stability in ionomer membranes used as proton exchange membranes in fuel cells demonstrate the capability of ESR to detect details that cannot be obtained by other methods. 

With ESR spectroscopy, open-shell species can be observed and characterized as long as their total spin differs from zero. With variable-temperature ESR spectroscopy, it is possible to deduce whether the observed multiplicity is a thermally populated excited state or is the ground state [69]. From such experiments, the T-S splittings of a variety of biscarbene and bisnitrenes have been determined. ESR spectroscopy is very sensitive to paramagnetic species, and because it does not see any singlet impurities or by-products, it is relatively easy to pick out the desired signals. At the same time, analysis of ESR spectra is not trivial and special simulations are required for their interpretation. 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

Time-resolved laser flash ESR spectroscopy generates radicals with nonequilibrium spin populations and causes spectra with unusual signal directions and intensities. The signals may show absorption, emission, or both and be enhanced as much as 100-fold. Deviations from Boltzmann intensities, first noted in 1963, are known as chemically induced dynamic electron polarization (CIDEP). Because the splitting pattern of the intermediate remains unaffected, the CIDEP enhancement facilitates the detection of short-lived radicals. A related technique, fluorescence detected magnetic resonance (FDMR) offers improved time resolution and its sensitivity exceeds that of ESR. The FDMR experiment probes short-lived radical ion pairs, which form reaction products in electronically excited states that decay radiatively. ... 

ESR provides a rapid method for assessing the sensitivity of a particular polymer structure to radiation. We have shown in several systems that there is a good correlation between G(R-) and the G values for permanent change in... 

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