LYCOS RETRIEVER 
Isotope: Elements
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Advanced methods employed for the isotopic charaterization of materials include time-of-flight secondary ion mass spectrometry (ToF-SIMS), conventional SIMS, accelerator mass spectrometry (AMS), isotope ratio mass spectrometry (IRMS), and low-level counting (LLC) by highly-discriminated detection of natural radioactivity. Nuclear measurement techniques can measure differences between chemically-identical materials, help to discriminate sources of materials, and be used in isotope dilution methods to definitively characterize SRMs for elemental and chemical abundances
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Of the 83 elements which occur naturally in significant quantities on Earth, 20 are found as a single isotope (mononuclidic), and the others as admixtures containing from 2 to 10 isotopes. Isotopic composition is mainly determined by mass spectroscopy. See ... Mass spectroscope.
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Because all of the isotopes of an element have the same atomic number, the atomic number is often left off the isotope notation. Another way of naming isotopes uses the name of the element followed by the isotope’s mass number. For example, carbon-14 can be
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The Neptune offers high precision isotope measurements for almost all elements in the periodic table. The instrument is typically used for Pb, Nd, Hf, U, Th, Fe, Cr and Mo isotope analyses.
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A technique similar to radioisotopic labelling is radiometric dating: using the known half-life of an unstable element, one can calculate the amount of time that has elapsed since a known level of isotope existed. The most widely known example is radiocarbon dating used to determine the age of carbonaceous materials.
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Other stable isotope techniques rely on adding trace amounts of compounds that are artificially enriched in the rare (heavy) isotope of the element of interest. These are referred to as isotope tracer techniques. For example, without isotopes, measuring independently the processes of microbial production of ammonium (NH4+) through mineralization and the consumption of NH4+ through immobilization and nitrification was not possible, because all these processes occur simultaneously.By adding 15NH4+ to soil and monitoring the rate at which it is 'diluted' by the more abundant 14NH4+ , one gets a measure of the rate of mineralization of soil organic matter, a rate that is independent of nitrification and immobilization (the NH4+ -consuming processes). By adding 15N as NH4+ or NO3 - and monitoring both 15N and 14N in the soil, it is possible to quantify each of these microbial transformations, in situ.
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