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Radiocarbon dating

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Radiocarbon dating (or simply carbon dating) is a radiometric dating technique that uses the decay of carbon-14 (14
C) to estimate the age of organic materials, such as wood and leather, up to about 58,000 to 62,000 years.^[1] Carbon dating was presented to the world by Willard Libby in 1949, for which he was awarded the Nobel Prize in Chemistry. Since its introduction it has been used to date many items, including samples of the Dead Sea Scrolls, the Shroud of Turin, enough Egyptian artifacts to supply a chronology of Dynastic Egypt,^[2] and Ötzi the Iceman.^[3]

isotope

n. form of a chemical element which has the same atomic number as the other forms but a different atomic weight (Chemistry)

The Earth's atmosphere contains various isotopes of carbon, roughly in constant proportions. These include the main stable isotope (12
C) and an unstable isotope (14
C). Through photosynthesis, plants absorb both forms from carbon dioxide in the atmosphere. When an organism dies, it contains the standard ratio of 14
C to 12
C, but as the 14
C decays with no possibility of replenishment, the proportion of carbon 14 decreases at a known constant rate. The time taken for it to reduce by half is known as the half-life of 14
C. The measurement of the remaining proportion of 14
C in organic matter thus gives an estimate of its age (a raw radiocarbon age).^[4] However, over time there are small fluctuations in the ratio of 14
C to 12
C in the atmosphere, fluctuations that have been noted in natural records of the past, such as sequences of tree rings and cave deposits. These records allow fine-tuning, or "calibration", of the raw radiocarbon age, to give a more accurate estimate of the calendar date of the material.

One of the most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites.

Mass spectrometry

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Mass spectrometry (MS) is an analytical technique that produces spectra (singular spectrum) of the masses of the molecules comprising a sample of material. The spectra are used to determine the elemental composition of a sample, the masses of particles and of molecules, and to elucidate the chemical structures of molecules, such as peptides and other chemical compounds. Mass spectrometry works by ionizing chemical compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios.^[1]

In a typical MS procedure, a sample, which may be solid, liquid, or gas, is ionized. The ions are separated according to their mass-to-charge ratio.^[1] The ions are detected by a mechanism capable of detecting charged particles. Signal processing results are displayed as spectra of the relative abundance of ions as a function of the mass-to-charge ratio. The atoms or molecules can be identified by correlating known masses to the identified masses or through a characteristic fragmentation pattern.

A mass spectrometer consists of three components: an ion source, a mass analyzer, and a detector.^[2] The ionizer converts a portion of the sample into ions. There are a wide variety of ionization techniques, depending on the phase (solid, liquid, gas) of the sample and the efficiency of various ionization mechanisms for the unknown species. An extraction system removes ions from the sample, which are then trajected through the mass analyzer and onto the detector. The differences in masses of the fragments allows the mass analyzer to sort the ions by their mass-to-charge ratio. The detector measures the value of an indicator quantity and thus provides data for calculating the abundances of each ion present. Some detectors also give spatial information, e.g. a multichannel plate.

Mass spectrometry has both qualitative and quantitative uses. These include identifying unknown compounds, determining the isotopic composition of elements in a molecule, and determining the structure of a compound by observing its fragmentation. Other uses include quantifying the amount of a compound in a sample or studying the fundamentals of gas phase ion chemistry (the chemistry of ions and neutrals in a vacuum). MS is now in very common use in analytical laboratories that study physical, chemical, or biological properties of a great variety of compounds.

As an analytical technique it possesses distinct advantages such as: 1. Increased sensitivity over most other analytical techniques because the analyzer, as a mass-charge filter, reduces background interference 2. Excellent specificity from characteristic fragmentation patterns to identify unknowns or confirm the presence of suspected compounds. 3. Information about molecular weight. 4. Information about the isotopic abundance of elements. 5. Temporally resolved chemical data.

A few of the disadvantages of the method is that often fails to distinguish between optical and geometrical isomers and the positions of substituent in o-, m- and p- positions in an aromatic ring. Also, its scope is limited in identifying hydrocarbons that produce similar fragmented ions.^[3]