Quantitative ESR

Quantitative ESR measurements confirmed that almost all of the total quantity of copper is present as [Cu(RS)] complex during the reaction (65). The kinetic data were consistent with a rate law which is zeroth-order in cysteine concentration ... 

Development of Dosimeter Materials. - 4.2.1 Quantitative ESR and Intensity Standards of Mn1+ and CuS04.5H20. An ESR spectrometer consists of vacuum tube amplifier and a Klystron oscillator with thermal noise and frequency drift. Hence, an inherent standard of Mn2+ was used in ESR dating of carbonate stalactites,8 and patented as a standard in ESR radiation dosimetry in 1980.102 The standard sample of MgO with Mn2+ is frequently used for calibration of -factor and the magnetic field as well as for radiation dosimetry.103... 

For example, reactions of xanthine oxidase have been shown to occur by both one-and two-electron mechanisms with oxygen [157,158] and benzoquinone [159] as electron acceptors. Production of superoxide from one-electron donation to oxygen has been demonstrated by rapid-freeze ESR studies [160]. The immobilized free radical species that was detected was identified as Oj by comparison with spectra obtained in chemical systems. However, the fraction of one-electron transfer that occurs depends [157] on a number of factors, including oxygen concentration, pH, and the concentration of the electron donor. The situation with benzoquinone is similarly complex quantitative ESR studies [159] have shown that the extent of one-electron reduction depends upon the concentration of benzoquinone if xanthine is used as donor, but not if NADH is used. In addition, with NADH the reaction is very pH dependent. The apparent Michaelis constant for benzoquinone is much smaller with xanthine than with NADH. Because of the complexities of the xanthine oxidase system, it would appear that data from studies involving acceptors other than oxygen or benzoquinone must be analyzed carefully if reliable conclusions are to be drawn regarding the reaction mechanism. 

It may be clear that a quantitative determination of paramagnetic species is rather comphcated and requires a great deal of experimental care. Therefore, absolute determination of paramagnetic entities, especially transition metal ions, can only be done within 10 to 20% accuracy and explains why the number of quantitative ESR studies in molecular sieve science is rather limited [12,16]. 

Quantitative ESR was used in early studies to determine the amount of transition metal ions in frozen solutions of metallo-proteins [3], to obtain the yields of free radicals in solids [5] and for the quantitative analysis of metals in aqueous solutions [6]. 

The accuracy that can be achieved in quantitative ESR depends on the instrument used to record the spectra, and on the numerical processing to extract the numbers from the experimental data. To reduce the measurement errors several precautions have been recommended, see Appendix F in [10] ... 

Additional recommendations of value for the non-specialist to improve accuracy can be found in an early review [13], The largest source of uncertainty was attributed to differences in the microwave field strength even with identical microwave power input for different samples. The uncertainty can be reduced by using similar materials for the reference as for the sample, both contained in sample tubes of the same diameter and wall thickness, to minimize differences in the cavity Q factor between the measurements. A modern guide to quantitative ESR by the same authors is in preparation [15]. 

The free radical yields of the potential ESR-dosimeter materials in Table 9.2 are in several cases not known. In other cases G-value data obtained in independent studies of a specific substance disagree, indicating that the techniques of quantitative ESR spectrometry were not generally established at the time the measurements were done [4],... 

Experimental studies are scarce, but an analysis of the frequency dependence on the sensitivity in fact suggests that frequencies lower than X-band might give higher sensitivity in quantitative ESR [55]. Measurements at low frequency are not an alternative in chnical apphcations, however, where even the dosimeters for X-band (5-5 mm) in many cases are too large. 

The retention volume for the green component, XXII, was identical to that of [Cu(py)2Cl2]. The formation of this product was confirmed by quantitative ESR measurements which were consistent with the overall reaction shown in equation (115). 

The electronic absorption spectra of p-type-doped polypyrrole have been interpreted in terms of polarons and bipolarons for the first time [112]. Electronic absorption spectra of doped polypyrrole in the range from visible to NIR show three bands due to intragap transitions at low dopant contents and two bands at high dopant contents [113]. The three bands are attributed to polarons and the two bands to bipolarons [112], because a polaron is expected to have three intragap transitions and a bipolaron is expected to have two intragap transitions (see Section 4.4.2). A quantitative ESR study [114] has confirmed the interpretation of the electronic absorption experiments. This rule of thumb for band assignment has so far also been applied to other conducting polymers. 

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