Mass-anomalous fractionation

The CO + OH CO2 + H reaction has been reported as showing mass independent fractionation [11]. This term, in the strict sense of "mass-independence" used earlier means that the slope of a O/ O fractionation plotted versus the corresponding 0/ 0 fractionation would be about unity, rather than having the conventional mass-dependent value of 0.52. However, what is frequently meant by MIF of oxygen instead is that the quantity A O = S O — 0.525 0 is different from zero. The number of experiments needed to determine is only one, instead of the number needed to determine the slope of a 3-isotope and is commonly used in the literature. Indeed, sometimes the data are such that a three-isotope plot cannot be obtained from the available data. To avoid possible confusion one might term reactions that have a non zero value for A O but for which the vs. slope has not been measured, as being mass-anomalous fractionated, MAF. However, the term MIF is in widespread usage, and includes both the strictly MIF and any mass anomalous fractionation (MAF) reactions. 

Oxygen consists of three stable isotopes 0, " O and " O. For most applications only the ratio between the more abundant Lso-topes, 0 and O, is measured. If all fractionation processes in the environmental system were purely mass-dependent, measurements of 0/ 0, would be redundant, as they could be predicted from the 0/ 0 ratio. However, it has recently been ob.served that photochemical exchange between O2, Oi, and CO, in the stratosphere involves mass-independent fractionation among the oxygen isotopes (1990 Thiemens et al., 1993a, b Thiemens, 1999). Thereby O2 becomes anomalously depleted, while CO2 becomes anomalously enriched. Because of this, measurements of 0/ 0 in atmospheric O2 and/or CO2 provide an independent piece of information. Conveniently, one may define an O anomaly tracer (A 0) (Thiemens et n/., 1995b)... 

All of the isotope fractionation effects discussed so far are mass-dependent. This means that there is a linear relationship between the extent of fractionation observed and the mass difference between the isotopes considered. Some processes, however, give rise to an apparently aberrant behavior, whereby one or more isotopes display an additional effect on top of the well-understood mass-dependent fractionation. This apparently aberrant behavior is referred to as mass-independent or anomalous isotope fractionation. This effect is illustrated in Figure 1.8, where the extent of isotope fractionation experimentally observed for methylation of inorganic Sn using methylcobalamine is plotted as a function of the mass difference between the Sn isotope considered and the reference isotope Sn. When this reaction proceeds in the dark, only mass-dependent fractionation is observed for all isotopes. However, under UV radiation, the two odd-numbered isotopes of Sn studied show a considerably more pronounced fractionation effect... 

Stable isotope analysis of Earth, Moon, and meteorite samples provides important information concerning the origin of the solar system. 8lsO values of terrestrial and lunar materials support the old idea that earth and moon are closely related. On the other hand three isotope plots for oxygen fractionation in certain meteoric inclusions are anomalous. They show unexpected isotope fractionations which are approximately mass independent. This observation, difficult to understand and initially thought to have important cosmological implications, has been resolved in a series of careful experimental and theoretical studies of isotope fractionation in unimolecular kinetic processes. This important geochemical problem is treated in some detail in Chapter 14. 

Fig. 14.4 Oxygen isotopic composition of atmospheric species measured to date (After Thiemens, M., Ann. Rev. Earth Planet. Sci. 34, 217 (2006)). For these data, m, Equation 14.31 is approximately (0.7 < m < 0.9) indicating for these materials the fractionation is better described as anomalous rather than mass independent ...

Figure 1. Example of compositionally resolved bimodal and monomodal distributions of aerosols. The ordinate gives the percent of the species found in the given size fraction of the impactor. The mode near 0.3 xm is the accumulation mode , and that above 8 xm is the coarse mode The minimum of mass between 1 and 2 xm is typical the chlorine distribution is anomalous. Chlorine is in fact a coarse-mode marine aerosol that has lost its larger particles during transport from the ocean to Davis, California, a distance of roughly 100 km. (Reproduced with permission from reference 15. Copyright 1988.)...

High precision determination of Rb/ Rb isotope ratios by MC-ICP-MS (Isoprobe, Micromass) is advantageous in comparison to MC-TIMS (VG Sector 54, Micromass) due to an unproved precision, and fraction capability problems can be easily considered using admixed Zr for mass bias correction via the Zr/ °Zr isotope ratio. Rb isotopic composition in geological materials (basalt, granite and greywacke) and synthetic samples (in house mica standard, NBS 607 K-fsp standard) have been examined with a reproducibility of Rb/ Rb measurements of 0.02-0.05 % (2 O ). No anomalous isotope fractionation was observed for the samples investigated. 

T. Rockmann, et ah. The origin of the anomalous or "mass-independent" oxygen isotope fractionation in tropospheric N2O, Geophys. Res. Lett. 28 (3) (2001) 503-506. 

Convincing evidence of recycling of volatiles to the mantle and their incorporation into diamonds has recently been reported from sulfur isotope studies of sulfide inclusions within diamond (Farquhar et al., 2002). SIMS analysis of sulfide inclusions has revealed anomalous mass-indepen-dently fractionated sulfur isotopic compositions from the Orapa kimberlite, which are explained as a consequence of the recycling of surface sulfur produced through photolytic chemistry in the Archean atmosphere. Such studies need to be extended to other diamond populations and combined with C-N isotope studies on the host diamonds. 

Degradation of the sample and related problems, such as the concentration effect and anomalous flow, are the more important problems in the fractionation of UHMM polymers. The critical point in the characterization of the UHMM polymers is the fractionation in the SEC columns. For a successful fractionation of UHMM macromolecules, one must use specifically designed SEC columns with large particle sizes and ultralarge pore sizes. Furthermore, many aspects of the experimental protocol, such as flow rate and sample concentration, which is not critical in the usual molar mass range, become determining with UHMM polymers. A successful characterization of UHMM polymers requires optimization of the experimental protocol. Each step of the experimental protocol should be performed methodically to achieve the absence of sample degradation and reliable results. 

With UHMM HA sample, only F-FFF fractionation is effective. It has been demonstrated that if the average of the HA sample is higher than ca. 2-3 MDa, the molar mass estimated by a SEC-MALS system is systematically underestimated due to shear degradation and anomalous elution in SEC columns [274]. The F-FFF fractionation technique encompasses a number of separation methods characterized by the transverse compression induced by an external field orthogonal to a laminar parabolic flow in a very thin flat channel. Because of the peculiar nature of the field, F-FFF is the most extensively employed sub-technique for the analysis of biological macromolecules. 

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