Lanthanide determination element abundances

By the time our sun had formed, countless stars had already completed their life cycles. The clot of gas that produced the sun was a mixture of primordial hydrogen and heavier elements. These heavier elements were essential to producing the inner planets, satellites, asteroids, and other objects in the solar system which cannot be constructed from hydrogen and helium. The bulk of the heavy elements in the solar system is in the outer portions of the sun itself, which contains more than 99.87 percent of all the mass of the solar system. (The outer portions of the sun do not mix with the core, where nuclear reactions destroy heavy elements.) The abundances of yttrium and the lanthanides in the sun s atmosphere have been determined spectroscopically and are believed known with medium accuracy (Ross and Aller, 1976). Pieces of the Earth, the Moon, and meteorites, all of which condensed from the same batch of material as the sun, have been analyzed chemically to determine their abundances. 

Anodier stone that has found wide cultural use as a carving medium in many early societies is steatite or soapstone, a very soft metamorphic rock related to chlorite and talc. In this laboratory, steatite (actually chlorite) from quarries near Tepe Yahya (Iran) was characterized by observation of the ratios of the relative intensities of basal-plane x-ray diffraction peaks after it was found that NAA-determined trace element concentrations varied wildly within a given specimen. Another technique that has been used involves the determination by NAA of a number of lanthanide elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb and Lu) and the taxonomy of their abundances relative to each other — in other words, true pattern recognition , when plotted as ratios to the levels of the same elements in a standard reference chondrite Although this technique found successful provenience application... 

In this section, we discuss the question of the bulk planetary abundances of the rare earth elements. Central to the problem of planetary abundance determinations is the assumption that the composition of the original solar nebula, for the non-gaseous elements, is given by the composition of the Cl meteorites. It is accordingly of interest to see what evidence is available from the planets, and how it relates to the primordial nebula values. In the previous section, we have seen that although the moon is enriched in the lanthanides relative to those in the primordial solar nebula by about 2.5 times, the pattern is probably parallel to that of Cl. The evidence for an apparent depletion in the heavy lanthanides is readily explicable as a consequence of early lunar magma ocean crystallisation of phases such as olivine and orthopyroxene, which selectively accept Gd-Lu. 

Stable isotope dilution mass-spectrometry (MSID) is the most accurate technique for determining lanthanide abundances in geochemical materials.. The superior quality of the method may be attributed principally to the inherent sensitivity of mass-spectrometers, and to the use of the ideal internal standard, namely, an artificially enriched isotope of each element to be determined. The utilization of isotopic internal standards virtually eliminates such potential analytical problems as quantitative recovery and instrument calibration. The sensitivity of the mass spectrometer is such that the lower limit of measurable abundance is usually controlled by the purity of the reagents used in preparing the sample for analysis. 

Inasmuch as the method depends upon mixing natural and synthetic isotopic compositions, MSID is applicable theoretically to all polyisotopic elements for which an adequate ion current can be generated. Thus all the even Z lanthanides, Ce, Nd, Sm, Gd, Dy, Er and Yb, and the odd Z lanthanides La, Eu, and Lu may be analyzed by this technique. Isotope dilution, theoretically, can be used in determining abundances of the monoisotopic rare earths. Sc, Y, Pr, Tb, Ho, and Tm, by utilizing artificial radioactive (unstable) isotopes this will not be pursued further in the present chapter. 

Subscript N values may be obtained from isotope abundance data (e.g. see table) subscript S values, for the spike, are generally determined empirically. Typically, the amount of the lanthanide in the unknown is given in ppm by weight so that a factor consisting of the ratio of the atomic weight of normal and spike element must be applied. Hence ... 

Geochemical studies often require determination of all the lanthanides (and often yttrium as well) in various geological matrices such as rocks, sediments, coal, etc. In most geological matrices, the light rare earths (e.g.. La and Ce) are much more abundant than the heavier rare earths (e.g., Tb and Tm). Therefore, the analytical technique for this application should have multi-element capabilities, large dynamic range, and limited susceptibility to interferences. This problem was not easily fulfilled before the advent of ICP-AES, which provided the first analytical technique with the potential to cover the whole suite of rare earths in a reasonable time. 

Representative spectra from a lanthanide and an actinide are shown in figs. 21 and 22. The most abundant analyte peaks are from monatomic ions (M" ), and these are observed at sensitivities of 10 -10 count s per mg in solution. Ion count rates as low as 2counts s can be distinguished from the background, so the detection limits for most elements are of the order of 10-100 ng/ . At present, these powers of detection are superior to those obtainable with any other common multi-element technique. Atomic absorption spectrometry with electrothermal vaporization does provide detection limits in a similar range but is generally used only for single-element determinations. 

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