The relative abundance of different elements

So far we have considered only the overall metallicity of DLAs as measured by the [Zn/H] ratio. However, the relative abundances of different elements offer additional insights into the chemical evolution of this population of galaxies, as we shall now see. This aspect of the work has really blossomed with the advent of efficient echelle spectrographs on both... 

In the other study. X-ray fluorescence spectroscopy was used to analyze trace element concentrations by observing dusts on 37 ram diameter cellulose acetate filters (20). Twenty-three elutriator and twenty-three area samples from 10 different bales of cotton were analyzed. The average fraction of total dust accounted for by the elements analyzed was 14.4% amd 7.6% for vertical elutriator and area samples, respectively. Although the variation in absolute quantity of atn element was high, the relative abundance of an element was consistent for measurements within a bale. Averaged over all the samples analyzed, calcium was the most abundant element detected (3.6%), followed by silicon (2.9%), potassium (2.7%), iron (1.1%), aluminum (1.1%), sulfur (1.0%), chlorine (0.8%) and phosphorous (0.6%). Other elements detected in smaller aunounts included titanium, manganese, nickel, copper, zinc, bromine, rubidium, strontium, barium, mercury amd lead. 

Isotopes can be divided into two fundamental kinds, stable and unstable (radioactive) species. The number of stable isotopes is about 300 whilst over 1,200 unstable ones have been discovered so far. The term stable is relative, depending on the detection limits of radioactive decay times, hi the range of atomic numbers from 1 (H) to 83 (Bi), stable nuclides of all masses except 5 and 8 are known. Only 21 elements are pure elements, in the sense that they have only one stable isotope. AU other elements are mixtures of at least two isotopes. The relative abundance of different isotopes of an element may vary substantially. In copper, for example, Cu accounts for 69% and Cu for 31% of all copper nuclei. For the light elements, however, one isotope is predominant, the others being present only in trace amounts. 

The four elements—magnesium, silicon, iron, and oxygen—contribute more than 90% by mass to the bulk Earth. As stated above, magnesium, silicon, and iron have approximately similar relative abundances (in atoms) in the Sun, in chondritic meteorites, and probably also in the whole Earth. On a finer scale there are, however, small but distinct differences in the relative abundances of these elements in chondritic meteorites. Figure 10 shows the various groups of chondritic meteorites in an Mg/Si versus Al/Si plot. As discussed before, the Earth s mantle has Al/Si and Al/Mg ratios within the range of... 

Isotopic Studies. In this analysis mode the mass spectrometer Is used to determine the relative abundance of different Isotopes of the same element. SIMS Is the only common surface analysis technique that can distinguish the Isotopes of both the light and heavy elements. This mode Is advantageously used In mechanistic studies using Isotopically labelled samples. By using different Isotopes of the same element experiments can be performed without concern for the differences In chemical effects between the species. 

Enrichment, Isotopic—An isotopic separation process by which the relative abundances of the isotopes of a given element are altered, thus producing a form of the element that has been enriched in one or more isotopes and depleted in others. In uranium enrichment, the percentage of uranium-235 in natural uranium can be increased from 0.7% to >90% in a gaseous diffusion process based on the different thermal velocities of the constituents of natural uranium (234U, 235U, 238U) in the molecular form UF6. 

Figure 12.17a shows lithophile element abundances, and Figure 12.17b shows sid-erophile and chalcophile element abundances in CM chondrites, normalized to Cl chondrites. Illustrated for comparison are the abundances in CO chondrites, which are the anhydrous carbonaceous chondrite group most closely allied to CM chondrites. As in other chondrites, the greatest differences are in volatile elements. The volatile and moderately volatile elements in CM chondrites are present at 50-60% of the abundances of the refractory elements. The volatile elements are primarily located in the matrix, and the matrix comprises 50-60% of CM chondrites. This implies that the matrix has essentially Cl abundances of all elements, while the chondrules and refractory inclusions have Cl relative abundances of refractory elements but are highly depleted in the volatile elements. The sloping transition in the region of moderately volatile elements indicates... 

It should be emphasized that solar abundance ratios are used here only as a convenient referenoe point. The LMC is known to have a total heavy element abundance that is approximately two to three times less than solar (van Genderen, van Driel, and Greidanus 1986 Dufour 1984). The abundances of Sc, Sr, and Ba in the LMC are not known because of the difficulty in detecting lines of these elements in objects. They are probably not solar however, unless the history of nucleosynthesis in the Large Cloud is completely different from that in our Galaxy, the relative abundances of the s-process elements with respect to each other and to Fe should not differ greatly from those of the sun. 

Elements are formed by three different mechanisms in stars, as elucidated by Burbidge et al. [82]. In brief, these are the s process, involving the capture of slow neutrons by nuclei the r process, involving the capture of rapid neutrons and the p process, which leads to neutron-deficient nuclides that are generally less abundant than those generated by the s and r processes. The relative abundances of the various isotopes of a given element reveal which processes, or combinations thereof, are involved in their creation in the nuclear reactions that power stars. Measurements of isotopic and elemental abundances in stars thus serve to test theories of stellar processes. 

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