Contents 9 Isotope ratio measurements

Abundance sensitivity becomes critical when measuring extreme isotope ratios (>100 000) or when neighbouring elements are present in the sample at high concentrations. In ICP-MS this occurs for isotope ratio measurements of elements such as U and Th (extreme ratios) or boron isotope ratio measurements in the presence of high carbon contents (tailing of onto "B). Specially designed filters (electrostatic filter, quadrupole lenses) can increase the abundance sensitivity up to 2 x 10 (see Chapter 2, section 2.2.1 for further details). In the case of boron, a simple matrix separation can help. In the majority of isotope ratio measurements however abundance sensitivity effects are negligible. 

The reverse isotope dilution technique can be applied for accurate determination of the Mg contents in a sample, sample, on applying equation 7, by measuring the isotope ratio of a selected pair of stable isotopes, in a weighed mixmre of the sample with an isotopically enriched CRM. The average atomic masses m and the isotopic ratios R of Mg in the enriched CRM and in nature are known. The method was applied for determination of Mg in plant material using a CRM isotopically enriched with Mg, measuring with an ICP/MS instruments. ... 

Natural oxygen is 99.8% l60, 0.2% lsO, and 0.04%, 70. Small variations in isotope ratios in H20 in the ocean, the atmosphere, and rain and snow depend on local temperature. The warmer the air, the more enriched it is in H2I80. The lsO content of snow preserved in glaciers provides a record of air temperatures for 250 000 years in 2-km-long cores drilled from ice sheets in Greenland and Antarctica. Depth in the ice pack has been correlated with age in years by a variety of measurements. 

In this work we report organic and elemental carbon data, l C content and 13c/12c isotopic ratios for samples obtained in a larger set of measurements in the POLTERCAIST (POLlutant-TERpene CAnopy Interaction STudy) taken at the Walker Branch experimental site, Oak Ridge National Laboratory, Oak Ridge, TN, during a summer period (July, 1983) and a late winter period (March,... 

Sample analysis by thermal ionization mass spectrometry (TIMS) results in measurement of isotopic ratios of minerals. Total mineral content of samples is then determined by one of two methods. One approach is to use flame atomic absorption spectrophotometry (AAS) to determine total mineral content of samples. Since AAS does not have the same level of precision as TIMS a sufficient number of replicates is analyzed for a mineral content determination with a CV of within 1%. Alternatively if a mineral has 3 or more isotopes and fractionation corrections are not made the following procedure may be used. An individual is fed one isotope and another isotope is added to the sample prior to analysis to determine the total mineral content of the sample by dilution of the second isotope. In this way both the amount of the isotope fed which is recovered in the feces and the total mineral content of the sample can be determined simultaneously. If fractionation corrections are to be made a mineral must have at least four isotopes. Details of these procedures will be reported separately. 

When isotope dilution is used to determine total mineral content, two ratios are measured. The following two equations are used to determine the two unknowns, (1) the total natural mineral content of the sample ( ) and (2) the amount of isotope spike ( N) recovered in feces ... 

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