| 释义 |
isotope|ˈaɪsətəʊp|
[f. iso- + Gr. τόπ-ος place.]
1. A variety of a chemical element (strictly, of one particular element) which is distinguished from the other varieties of the element by a different mass number but shares the same atomic number and chemical properties (and so occupies the same position in the periodic table); freq. used to denote any individual variety without reference to identity of atomic numbers (see quot. 1947 and cf. nuclide).
1913F. Soddy in Nature 4 Dec. 400/1 The same algebraic sum of the positive and negative charges in the nucleus, when the arithmetical sum is different, gives what I call ‘isotopes’ or ‘isotopic elements’, because they occupy the same place in the periodic table. They are chemically identical, and save only as regards the relatively few physical properties which depend upon atomic mass directly, physically identical also. 1915Rep. Brit. Assoc. Adv. Sci. 1914 301 Sir E. Rutherford (replying) said that the chemical inseparability of certain isotopes was, indeed, derived from experiments with small quantities, but the methods used were very delicate. 1923Glasgow Herald 7 Apr. 7 Another entirely revolutionary factor in chemical theory has been the discovery that the majority of the chemical elements are mixtures of isotopes. 1927N. V. Sidgwick Electronic Theory of Valency i. 11 What Soddy has called an isotope—an element of the same atomic number but a different atomic weight. Ibid., The one element of which the isotopes can be obtained in any quantity in a state of approximate purity..is lead. 1942J. D. Stranathan ‘Particles’ of Mod. Physics v. 205 Hertz, employing gaseous diffusion at low pressure through a special porous material, succeeded in producing a real separation for the isotopes of Ne. Ibid. 207 By this method the isotopes H2, Li7, C13 and N15 have been produced in significant concentration. 1947Amer. Jrnl. Physics XV. 356/2 There is at present no word in the English language to express the concept of a particular species of atom, differing from all others in the constitution of its nucleus... In recent years the word isotope has come into use for this purpose, less by design than by default. According to its definition [by Soddy], this general usage is incorrect, since isotope properly refers to a species of a particular and designated element, and emphasizes its relationship to other isotopes of that element. It is analogous to such words as brother and colleague. 1959New Biol. XXX. 96 Radioactive isotopes behave chemically in identical fashion with the normal isotopes, but can be distinguished by means of their radioactivity. 1962Nature 19 May 621/2 There are about 300 naturally occurring isotopes, and several hundred additional isotopes can be prepared artificially. 1963D. W. & E. E. Humphries tr. Termier's Erosion & Sedimentation i. 4 The isotopes O16, O17, and O18 of oxygen are in different proportions in atmospheric and sea water, the latter being rich in heavy isotopes as a result of differential evaporation. 1966McGraw-Hill Encycl. Sci. & Technol. VII. 291/2, 20 elements possess no isotopes; each of these consists of one type of atom only. 1966C. R. Tottle Sci. Engin. Materials i. 23 Hydrogen has three isotopes, the two heavier ones being known as deuterium (originally called heavy hydrogen), with one neutron per atom, and tritium, with two neutrons per atom.
b. In the usage of biologists and biochemists: a less common, usually radioactive, isotope of an element as used in tracer or other studies, in contradistinction to the common, naturally occurring isotope of the element; freq. without the.
1945Jrnl. Biol. Chem. CLIX. 697 Isotopic analyses were usually made on these inorganic salts, thus avoiding the dilution of the isotope [sc. C13] by non-isotopic carbon. 1954A. White et al. Princ. Biochem. xiv. 307 By degradation of the product and determination of the distribution of isotope among its atoms, one may often procure information as to the mechanism of the transformation. 1954Cantarow & Schepartz Biochem. xiv. 352 The assay of stable isotopes is based upon the difference in atomic weight between the normal element and the isotope used. 1971Mazur & Harrow Textbk. Biochem. (ed. 10) iv. 99 Synthesis of physiologically important compounds, containing isotopes in place of the normally occurring elements, furnishes the worker with a ‘tag’ that can easily be followed. 1972Science 2 June 1032/1 Swiss albino mice were labeled with 45Ca by injection with 10 µc of isotope 1 day after birth.
2. Special combs.: isotope dilution, a diminution of the concentration of one isotope (or isotopically labelled compound) by the addition or presence of another isotope of the same element (or of the unlabelled compound); esp. as a technique for measuring the amount of an element or compound in a system by introducing a known amount of a different isotope (or a labelled compound) and then measuring its concentration in a sample withdrawn from the system after mixing; isotope effect, a variation in some physical or chemical characteristic between one isotope of an element and another; isotope shift, a small difference in the wavelength of corresponding spectral lines of different isotopes of an element owing to the different masses and charge distributions of their nuclei.
1940Rittenberg & Foster in Jrnl. Biol. Chem. CXXXIII. 737 (heading) A new procedure for quantitative analysis by isotope dilution. Ibid. 744 A new method (isotope dilution procedure) for the analysis of complex mixtures is described. 1956Nature 28 Jan. 159/1 Isotope-dilution analysis, a technique which is now being increasingly applied in geology, agriculture, metallurgy and biology. 1971Metabolism XX. 1099 (heading) Kinetics of potassium distribution in man using isotope dilution and whole-body counting. 1972Chemico-Biol. Interactions IV. 103 This finding suggested that the decreases in specific activity were a result of isotope dilution rather than inhibition of protein synthesis.
1923Science 31 Aug. 164/2 (heading) The vibrational isotope effect in the band spectrum of boron nitride. 1956Nature 28 Jan. 159/2 Most properties of solids depend on the atomic masses, and the study of isotope effects can be a powerful means of testing the validity of theories. 1967A. H. Cottrell Introd. Metall. xxiv. 503 The importance of the lattice ions in superconductivity is shown by the isotope effect. Different isotopes of mercury (also tin) are found to have different critical temperatures, proportional to M-½, where M is the atomic mass of the isotope concerned. 1968A. White et al. Princ. Biochem. (ed. 4) xiii. 288 Distinct isotope effects, e.g. differential rates of reaction between normal and tritiated compounds, have been observed.
1932Physical Rev. XLII. 350 It appears that this determination fits in with the magnitude of the isotope shift. 1958Oxf. Mag. 8 May 418/1 Work on high resolution optical spectroscopy is now mainly directed towards the study of ‘isotope shifts’. 1971Physics Bull. Oct. 583/2 Isotope shifts occur when the energy levels of different isotopes are slightly shifted with respect to one another, resulting in an apparent splitting of the ‘line’ if two or more isotopes are present in the source. |