Isotopic separation | Article about Isotopic separation by The Free Dictionary Isotopic separation | Article about Isotopic separation by The Free Dictionary [′ī·sə‚tōp ‚sep·ə¦rā·shən] (nucleonics) The physical separation of different stable isotopes of an element from one another. Isotope Separation the separation of pure isotopes from mixtures of isotopes of a given element or the enrichment of a mixture with individual isotopes. Isotope separation is an important problem of great scientific and practical significance. Attempts to separate isotopes were made from the time of their discovery until the 1930’s primarily with the aim of detecting isotopes of stable elements and measuring their mass and isotopic composition. It proved possible to separate only small (tracer) quantities of some elements insignificantly enriched with isotopes. Fundamental research was begun in the 1930’s on atomic nuclei, nuclear reactions, interactions between particles and nuclei, and related areas. The reliability of experimental data and the interpretation of the results obtained to a large extent depended on the purity and available amount of the isotope in question.

However, the preparation of even milligram quantities of isotopes proved to be a complex task. Only small quantities of enriched mixtures of isotopes, mainly of the light elements, were isolated. Deuterium was the only isotope to be produced on an industrial scale. Further development of the technology of isotope separation was stimulated in 1939 by the discovery of the fission of 235U under the action of neutrons. This opened up possibilities for the peaceful and military uses of atomic energy. The production of large quantities of uranium isotopes and other elements, which are necessary components of “nuclear fuels” and materials in nuclear technology, became an important task at this time. Huge plants were constructed for this purpose. There are a number of methods of isotope separation. All are based on the differences in the properties of isotopes and their compounds resulting from the differences in mass of their atoms. The relative differences in mass of the isotopes are fairly small for most elements. Therein lies the complexity of the task. The effectiveness of isotope separation is characterized by the separation factor α.

For a mixture of two isotopes, α = [C’/ (1 - C’)]/[C”/(1 – C”)], where C’ and (1 - C”) are the relative contents of the light and heavy isotopes, respectively, in the enriched mixture and C” and (1 – C”) are the corresponding quantities in the starting mixture. For most methods, a is only slightly greater than unity, and therefore to obtain a high final isotopic concentration requires multiple repetitions of the single stage of isotope separation. Only electromagnetic separation is characterized by an a value of 10–1,000 per separation cycle. Selection of the method of isotope separation depends on such considerations as the properties of the substance to be separated, the required degree of separation, the desired isotope quantity, and the cost of the process (at high volumes of isotope production). Gaseous diffusion. In the gaseous diffusion method, a gaseous compound of the element being separated is “pumped” at the rather low pressure of ~ 0.1 N/m2 (~ 10~3 mm Hg) through a porous membrane containing up to 106 openings per cm2.

Light molecules penetrate the membrane faster than heavy molecules, since the velocities of the molecules are inversely proportional to the square root of their molecular weights. Consequently, the gas is enriched in the light component on one side of the membrane and in the heavy component on the other side. If the difference in molecular weights is very small, this process must be repeated thousands of times. The number of stages of separation n is determined by the relationship q = α“, where q is the required degree of separation. The operation of huge gaseous diffusion plants for the production of U from gaseous UF6 (α ~ 1.0043) is based on this method. Production of the required 235U concentration makes it necessary to perform about 4,000 individual stages of separation. Diffusion in a vapor stream (countercurrent mass diffusion). In this method, the isotope separation is conducted in a cylindrical vessel (column), divided vertically by a diaphragm. The diaphragm contains about 103 openings per cm3. The gaseous isotope mixture moves against a stream of secondary vapor.

Owing to the concentration gradients of the gas and vapor within the cylinder cross section and the greater coefficient of diffusion for the light molecules, the gas, which has passed through the stream of vapor into the left-hand part of the cylinder, is enriched by the light isotope. The enriched portion is withdrawn from the upper end of the cylinder with the bulk of the vapor, and the gas remaining in the right-hand half moves along the diaphragm and is withdrawn from the apparatus. Vapor that has penetrated into the right-hand part is condensed. The isotopes of neon, argon, carbon, krypton, and sulfur are separated on a laboratory scale (up to 1 kg) using separators consisting of several dozen sequentially connected diffusion columns with vaporizing liquids, such as mercury or xylene. Thermal diffusion. A thermal diffusion separation column consists of two vertical concentric pipes heated to different temperatures. The mixture to be separated is introduced into the space between the pipes. The temperature drop A T between the surfaces of the pipes generates a diffusion flux, which leads to a difference in the concentration of the isotopes in the cross section of the column. At the same time, the temperature drop leads to the generation of vertical convection flows of gas.

Consequently, the lighter isotopes concentrate near the heated inner pipe and move upward. The separation coefficient α = 1 + γ (ΔT/T), where γ is the thermal diffusion constant, which is dependent on the relative difference in mass of the isotopes, and T = (T1 + T2)/2. The thermal diffusion method makes it possible to separate isotopes in both the gas and liquid phases. The variety of isotopes that may be separated by this method is greater than the separations possible by gaseous diffusion or diffusion in a stream of vapor. However, α is small for the liquid phase. The simplicity of this method and the absence of vacuum pumps, among other features, make it a convenient method to use when separating isotopes under laboratory conditions. This method has yielded He containing 0.2 percent 3He (the natural abundance of this isotope is 1.5 X 10 5 percent), as well as the isotopes l80, 15N, 13C, 20Ne,22Ne, 35C1, 84Kr, and 86Kr in concentrations greater than 99.5 percent.

Thermal diffusion was used on an industrial scale in the USA for the preliminary enrichment of 235U prior to the final separation of this isotope in an electromagnetic installation. The thermal diffusion plant consisted of 2,142 columns measuring 15 m high. Distillation (fractional distillation). Since, as a rule, liquid isotopes have different saturated vapor pressures, for example, P1 and p2, and different boiling points, it is possible to separate isotopes by fractional distillation. Distilling columns with a large number of separation stages are used; a depends on the ratio P1/P2and its value decreases with increasing molecular weight and temperature. For this reason the process is most effective at low temperatures. Distillation has been used in the preparation of isotopes of light elements, such as 10B, “B, 18O, 15N, and 13C, as well as in the production of hundreds of tons of heavy water per year. Isotope exchange. Isotope separation can also be carried out by chemical reactions in which the isotopes of the element undergoing separation change places.

Thus, for example, if hydrogen chloride HC1 is brought into contact with hydrogen bromide HBr, both of which have the same initial content of deuterium D, the exchange reaction will result in a D content of HC1 that is several times higher than that of HBr. The use of several stages leads to the production of hydrogen, nitrogen, sulfur, oxygen, carbon, and lithium highly enriched in the individual isotopes. Centrifugation. In a centrifuge rotating with a high peripheral speed (100 m/sec), the heavier molecules are concentrated near the periphery and the lighter molecules near the rotor. The stream of vapor in the outer part with the heavy isotope is directed downward, and the flow in the inner part with the light isotope is directed upward. Connecting several centrifuges in a cascade leads to the desired isotope enrichment. During centrifugation, a depends on the difference in atomic mass of the isotopes being separated rather than on the ratio of the masses. For this reason, centrifugation is suitable for separating the isotopes and heavy elements. Owing to the existence of improved centrifuges, this method has found applications in the industrial separation of the isotopes of uranium and other heavy elements. Electrolysis.

During the electrolysis of water or of aqueous solutions of electrolytes, the hydrogen evolved at the cathode contains less deuterium than the initial water. As a result, the concentration of deuterium increases in the electrolytic cell. This method has been used for the industrial production of heavy water. The separation of other isotopes of light elements (lithium, potassium) by the electrolysis of their chlorides is performed only in laboratory quantities. Electromagnetic process. In the electromagnetic method, the substance whose isotopes are to be separated is placed in a crucible of an ion source and is evaporated and ionized. The ions are extracted from the ionization chamber by a strong electric field, and combined into a beam of ions; they then enter a vacuum separation chamber placed in a magnetic field H, oriented perpendicularly to the motion of the ions. Under the influence of the magnetic field, the ions travel in circular paths with radii of curvature that are proportional to the square root of the ratio of the ion mass M to its charge e. This leads to differences in the radii of the paths of the heavy and light ions.

It is therefore possible to collect ions of various isotopes in collectors located in the focal plane of the installation. The productivity of electromagnetic installations is determined by the magnitude of the ion current and the effectiveness of collecting the ions. In large installations the ion current varies from tens to hundreds of mA, which makes it possible to isolate up to several grams of isotopes per day (total of all isotopes). The productivity is lower in laboratory separators by a factor of 10–100. The electromagnetic method is characterized by large a and by the possibility of the simultaneous separation of all isotopes of a given element. Usually, at large industrial installations a ~ 10–100 per stage, and in laboratory installations a is 10 to 100 times greater. In most cases, a single electromagnetic stage of separation is adequate; the repeated separation of previously enriched isotopic materials to obtain isotopes of extremely high purity is rarely done. The principal drawbacks of this method are relatively low productivity, high operating cost, and considerable losses of the separated material. The electromagnetic method has made it possible to prepare kilogram quantities of 235U for the first time. The electromagnetic plant in Oak Ridge, Tenn.

(USA), included 5,184 separation chambers called calutrons. Owing to their high universality and flexibility, electromagnetic installations are used for the separation of about 50 elements of the periodic system in quantities ranging from several milligrams to hundreds of grams and are the principal source of isotopes for scientific research and some practical applications. Laboratory separators, like the large electromagnetic separation installations for industrial isotope production, have also found numerous applications. The laboratory separators are used in the preparation of radioactive isotopes, which are necessary for nuclear spectroscopy and for studies of the interaction between ions and solids (in ion implantation as well as for other purposes). Other methods. In addition to the methods listed above, there are a number of other methods whose applications are limited or which are in the process of being developed or improved. These methods include preparation of 3 He based on the phenomenon of superfluidity of 4 He; separation by diffusion in a supersonic gas stream, which expands in a space with a decrease in pressure; chromatographic separation based on differences in the adsorption rates of isotopes; and biological separation methods. Summary. The methods of isotope separation possess certain features that determine the areas of their mostefficient application.

In the separation of light elements with mass numbers of about 40, distillation, isotope exchange, and electrolysis are the most economical and effective. Diffusion, centrifugation, and the electromagnetic method are used in the separation of the isotopes of heavy elements. Gaseous diffusion and centrifugation can, however, be used, if gaseouscompounds of these elements are available. Since such compounds are scarce, the real potential of these methods is limitedThermal diffusion permits the separation of isotopes in both the gas and liquid states, but a is small for the separation ofisotopes in the liquid phase. The electromagnetic method is characterized by high a, but it has low productivity and is used mainly in the production of isotopes on a moderate scale. In order to provide for the scientific study and practical uses of isotopes, the State Fund of Stable Isotopes was created in the USSR. The fund contains reserves of isotopes of almost all the elements. Considerable quantities of deuterium, 10B, 13C, 15N, l8O, 23Ne, and other isotopes are regularly produced.

The production of various chemical compounds labeled with stable isotopes has also been set up. REFERENCES Basic tomic theory The word "atom" comes from the Greek a-tomos, meaning "un-cut-able" Throughout most of history the idea that matter is composed of minute particles had languished as a philosophical abstraction known as atomism , and no clear relation between these "atoms" and the chemical "elements" had been established. This began to change in the early 1800's when the development of balances that permitted reasonably precise measurements of the weight changes associated with chemical reactions ushered in a new and fruitful era of experimental chemistry. This resulted in the recognition of several laws of chemical change that laid the groundwork for the atomic theory of matter: conservation of mass . Laws of chemical change Recall that a "law", in the context of science, is just a relationship, discovered through experimentation, that is sufficiently well established to be regarded as beyond question for most practical purposes. Because it is the nature of scientists to question the "unquestionable", it occasionally happens that exceptions do arise, in which case the law must undergo appropriate modification. Conservation of mass-energy This is usually considered the most fundamental of law of nature.

It is also a good example of a law that had to be modified; it was known simply as Conservation of Mass until Einstein showed that energy and mass are interchangeable. However, the older term is perfectly acceptable within the field of ordinary chemistry in which energy changes are too small to have a measurable effect on mass relations. Within the context of chemistry, conservation of mass can be thought of as "conservation of atoms". Chemical change just shuffles them around into new arrangements. Mass conservation had special significance in understanding chemical changes involving gases, which were for some time not always regarded as real matter at all. (Owing to their very small densities, carrying out actual weight measurements on gases is quite difficult to do, and was far beyond the capabilities of the early experimenters.) Thus when magnesium metal is burned in air, the weight of the solid product always exceeds that of the original metal, implying that the process is one in which the metal combines with what might have been thought to be a "weightless" component of the air, which we now know to be oxygen. More importantly, as we will see later, this experimental result tells us something very important about the mass of the oxygen atom relative to that of the magnesium atom.

Law of definite proportions This law is also known as the law of constant composition. It states that the proportion by weight of the element present in any pure substance is always the same. This enables us to generalize the relationship we illustrated above. Problem Example 1 How many kilograms of metallic magnesium could theoretically be obtained by decomposing 0.400 kg of magnesium oxide into its elements? Solution: The mass ratio of Mg to O in this compound is 1/1.66 = 0.602, so 0.400 kg of the oxide contains (0.400 kg) x 0.602 = 0.241 kg of Mg. The fact that we are concerned with the reverse of the reaction cited above is irrelevant. The laws of definite and of multiple proportions, along with conservation of mass, are known collectively as the laws of chemical composition .2857 4 Next, identify the smallest weight ratio (.0714 here), and, in the rightmost column, divide each of the values in column 3 by this smallest ratio. What you get are the “small whole numbers” that illustrate the law of simple multiple proportions.

Note that, in some cases, the values in column 4 will not all initially work out to integer numbers. Suppose, for example, that the bottom row refers to some compound other than ammonium chloride, and that the numbers in column 4 (reading from top to bottom), work out to {2 - 1- 1.33}. If you recognize that the last number corresponds to 4/3, then multiplying each value by 3 yields the whole numbers {6 3 4} — not quite as “small”, but at least”whole” as the law requires. How Dalton's interpretation of the laws of chemical change established the atomic theory The idea that matter is composed of tiny "atoms" of some kind had been around for at least 2000 years. Dalton's accomplishment was to identify atoms with actual chemical elements.

If Nobel prizes had existed in the early 1800's, the English schoolteacher/meteorologist/chemist John Dalton (1766-1844) would certainly have won one for showing how the experimental information available at that time, as embodied in the laws of chemical change that we have just described, are fully consistent with the hypothesis that atoms are the smallest units of chemical identity. These points of Dalton's atomic theory provided satisfactory explanations of all the laws of chemical change noted above: Explanation of the law of conservation of mass This is really a consequence of "conservation of atoms" which are presumed to be indestructible by chemical means. In chemical reactions, the atoms are simply rearranged, but never destroyed. Explanation of the law of constant composition If compounds are made up of definite numbers of atoms, each of which has its own characteristic mass, then the relative mass of each element in a compound must always be the same. Thus the elements must always be present in a pure sample of a compound in the same proportions by mass. Explanation of the law of multiple proportions A given set of elements can usually form two or more compounds in which the numbers of atoms of some of the elements are different.

Because these numbers must be integers (you can't have "half" an atom!), the mass of one element combined with a fixed mass of any other elements in any two such compounds can differ only by integer numbers. Thus, for the series of nitrogen-hydrogen compounds cited in the Problem Example above, we have the following relations: Compound (AFM), an exquisitely sensitive electromechanical device in which the distance between the tip of a submicroscopic wire probe and the surface directly below it is recorded as the probe moves along a surface to which atoms are adsorbed. Right: Schematic diagram of an atomic force microscope using beam deflection detection. As the cantilever is displaced as it is drawn across a surface, so too will the reflection of the laser beam be displaced on the surface of the photodiode. (Wikimedia Commons) The general principle of the AFM is quite simple, but its realization in an actual device can appear somewhat intimidating! This highly specialized atomic force microscope is one of several similar devices described at the Argonne National Laboratory site → which carries a positive electric charge whose value identifies each element and is known as the atomic number of the element.

• Almost all of the volume of an atom consists of empty space in which electrons, the fundamental carriers of negative electric charge, reside. The extremely small mass of the electron (1/1840 the mass of the hydrogen nucleus) causes it to behave as a quantum particle , which means that its location at any moment cannot be specified; the best we can do is describe its behavior in terms of the probability of its manifesting itself at any point in space. It is common (but somewhat misleading) to describe the volume of space in which the electrons of an atom have a significant probability of being found as the electron cloud. The latter has no definite outer boundary, so neither does the atom. The radius of an atom must be defined arbitrarily, such as the boundary in which the electron can be found with 95% probability. Atomic radii are typically 30-300 pm. The nucleus is itself composed of two kinds of particles. Protons are the carriers of positive electric charge in the nucleus; the proton charge is exactly the same as the electron charge, but of opposite sign.

This means that in any [electrically neutral] atom, the number of protons in the nucleus (often referred to as the nuclear charge) is balanced by the same number of electrons outside the nucleus. Because the electrons of an atom are in contact with the outside world, it is possible for one or more electrons to be lost, or some new ones to be added. The resulting electrically-charged atom is called an ion The other nuclear particle is the neutron . As its name implies, this particle carries no electrical charge. Its mass is almost the same as that of the proton. Most nuclei contain roughly equal numbers of neutrons and protons, so we can say that these two particles together account for almost all the mass of the atom. Atomic number What single parameter uniquely characterizes the atom of a given element? It is not the atom's relative mass, as we will see in the section on isotopes below. It is, rather, the number of protons in the nucleus, which we call the atomic number and denote by the symbol Z. Each proton carries an electric charge of +1, so the atomic number also specifies the electric charge of the nucleus. In the neutral atom, the Z protons within the nucleus are balanced by Z electrons outside it.

Moseley searched for a measurable property of each element that increases linearly with atomic number. He found this in a class of X-rays emitted by an element when it is bombarded with electrons. The frequencies of these X-rays are unique to each element, and they increase uniformly in successive elements. Mosely found that the square roots of these frequencies give a straight line when plotted against Z; this enabled him to sort the elements in order of increasing atomic number. Atomic numbers were first worked out in 1913 by Henry Moseley , a young member of Rutherford's research group in Manchester. You can think of the atomic number as a kind of serial number of an element, commencing at 1 for hydrogen and increasing by one for each successive element. The chemical name of the element and its symbol are uniquely tied to the atomic number; thus the symbol "Sr" stands for strontium, whose atoms all have Z = 38. Mass number This is just the sum of the numbers of protons and neutrons in the nucleus. It is sometimes represented by the symbol A, so A = Z + N in which Z is the atomic number and N is the neutron number The term nuclide simply refers to any particular kind of nucleus.

For example, a nucleus of atomic number 7 is a nuclide of nitrogen. Any nuclide is characterized by the pair of numbers (Z ,A). The element symbol depends on Z alone, so the symbol 26Mg is used to specify the mass-26 nuclide of manganese, whose name implies Z=12. A more explicit way of denoting a particular kind of nucleus is to add the atomic number as a subscript. Of course, this is somewhat redundant, since the symbol Mg always implies Z=12, but it is sometimes a convenience when discussing several nuclides. Know the difference between mass number and atomic mass! (TylerDeWitt, 9 min) Because it is not always easy to display a subscript directly beneath a superscript, it is not uncommon to use constructions such as 12Mg26 , which will often be our practice in this document when it is necessary to show both Z and A explicitly. Isotopes Two nuclides having the same atomic number but different mass numbers are known as isotopes .

Most elements occur in nature as mixtures of isotopes, but twenty-three of them (including beryllium and fluorine, shown in the table) are monoisotopic. For example, there are three natural isotopes of magnesium: 24Mg (79% of all Mg atoms), 25Mg (10%), and 26Mg (11%); all three are present in all compounds of magnesium in about these same proportions. Approximately 290 isotopes occur in nature. The best place to find out about the isotopes of individual elements is this page at the Lawrence Berkeley site , which covers both the natural isotopes and the artificially-produced ones. The two heavy isotopes of hydrogen are especially important— so much so that they have names and symbols of their own: Deuterium accounts for only about 15 out of every one million atoms of hydrogen. Tritium, which is radioactive, is even less abundant. All the tritium on the earth is a by-product of the decay of other radioactive elements.

6 Relative atomic masses: the atomic weight scale For historical reasons, the term atomic weight has a special meaning in Chemistry; it does not refer to the actual "weight" of an atom, which would be expressed in grams or kg. Atomic weights, sometimes called relative weights, are more properly known as relative atomic masses, and being ratios, are dimensionless. See this Wikipedia article for a full discussion of these and related terms. Please note that although the terms mass and weight have different meanings, the differences between their values are so small as to be insignificant for most practical purposes, so the terms atomic weight and atomic mass can be used interchangeably. Atoms are of course far too small to be weighed directly; weight measurements can only be made on the massive (but unknown) numbers of atoms that are observed in chemical reactions. The early combining-weight experiments of Dalton and others established that hydrogen is the lightest of the atoms, but the crude nature of the measurements and uncertainties about the formulas of many compounds made it difficult to develop a reliable scale of the relative weights of atoms. Even the most exacting weight measurements we can make today are subject to experimental uncertainties that limit the precision to four significant figures at best.

Atomic weights are average relative masses In the earlier discussion of relative weights of atoms, we explained how Dalton assigned a relative weight of unity to hydrogen, the lightest element, and used combining weights to estimate the relative weights of the others he studied. Later on, when it was recognized that more elements form simple compounds with oxygen, this element was used to define the atomic weight scale. Selecting O = 16 made it possible to retain values very close to those already assigned on the H=1 scale. Finally, in 1961, carbon became the defining element of the atomic weight scale. But because, by this time, the existence of isotopes was known, it was decided to base the scale on one particular isotope of carbon, C-12, whose relative mass is defined as exactly 12.000. Because almost 99% of all carbon atoms on the earth consist of 6C12, atomic weights of elements on the current scale are almost identical to those on the older O=16 scale. Most elements possess more than one stable isotope in proportions that are unique to each particular element. For this reason, atomic weights are really weighted averages of the relative masses of each that are found on earth.

Atomic weights are the ratios of the average mass of the atoms of an element to the mass of an identical number of 6C12 atoms. You can visualize the atomic weight scale as a long line of numbers that runs from 1 to around 280. The beginning of the scale looks like this: Atomic weights (relative atomic masses) of the first ten elements The red vertical lines beneath each element symbol indicate where that element is located on the atomic weight scale. Of these ten elements, only two, beryllium and fluorine, have a single isotope. The other eight atomic weights are weighted averages of the relative masses of the multiple isotopes that these (and most) elements possess. This is especially noticeable in the case of boron, whose average relative mass falls between 10 and 11. (Historically, observations of cases such as these led to the very concept of isotopes.) For many elements, one particular isotope so dominates the natural mixture that the others have little effect on the average mass. For example, 99.99 percent of hydrogen atoms consist of 1H1, whereas 1H2, the other stable isotope, amounts to only 0.01 percent.

Similarly, oxygen is dominated by 8O16 (over 99.7 percent) to the near exclusion of its two other isotopes. Atomic weights are listed in tables found in every chemistry textbook; you can't do much quantitative chemistry without them! The "standard" values are updated every few years as better data becomes available. You will notice that the precisions of these atomic weights, as indicated by the number of significant figures, vary considerably. Atomic weights of the 26 elements having a single stable isotope (monoisotopic elements) are the most precisely known. Two of these, boron and fluorine, appear in the above table. Owing to geochemical isotopic fractionation (discussed farther on), there is always some uncertainty in averaging the atomic weights of elements with two or more stable isotopes. Industrial processes associated mainly with nuclear energy and weapons production require the isolation or concentration of particular isotopes. When the by-product elements or compounds from which these isotopes have been depleted eventually get distributed in the environment or sold into the commercial marketplace, their atomic weights can vary from "official" values. This has occurred, for example, with lithium, whose isotope Li-6 has been used to produce hydrogen bombs.

Naturally-occurring radioactive elements (all elements heavier than 82Pb) all gradually decay into lighter elements, most of which are themselves subject to radioactive decay. These radioactive decay chains eventually terminate in a stable element, the most common of which is one of the three stable isotopes of lead. Subsequent geochemical processes can cause lead ore bodies from such sources to mix with "primeval" Pb (derived from the cosmic dust that formed the solar system), leading to a range of possible average atomic weights. For these reasons, lead, with a listed average atomic weight of 207.2, has the least-certain mass of any stable element. Weighing atoms: mass spectrometry A major breakthrough in Chemistry occurred in 1913 when J.J. Thompson directed a beam of ionized neon atoms through both a magnetic field and an electrostatic field. Using a photographic plate as a detector, he found that the beam split into two parts, and suggested that these showed the existence of two isotopes of neon, now known to be Ne-20 and Ne-22.

This, combined with the finding made a year earlier by Wilhelm Wien that the degree of deflection of a particle in these fields is proportional to the ratio of its electric charge to its mass, opened the way to characterizing these otherwise invisible particles. Thompson’s student F.W. Aston improved the apparatus, developing the first functional mass spectrometer , and he went on to identify 220 of the 287 isotopes found in nature; this won him a Nobel prize in 1921. His work revealed that the mass numbers of all isotopes are nearly integers (that is, integer multiples of the mass number 1 of the protons and neutrons that make up the nucleus. Mass spectrometry begins with the injection of a vaporized sample into an ionization chamber where an electrical discharge causes it to become ionized. An accelerating voltage propels the ions through an electrostatic field that allows only those ions having a fixed velocity (that is, a given charge) to pass between the poles of a magnet. The magnetic field deflects the ions by an amount proportional to the charge-to-mass ratios. The separated ion beams are detected and their relative strengths are analyzed by a computer that displays the resulting mass spectrum.

In modern devices, a computer also controls the accelerating voltage and electromagnet current so as to being successive ion beams into focus on the detector. Neutral atoms, having no charge, cannot be accelerated along a path so as to form a beam, nor can they be deflected. They can, however, be made to acquire electric charges by directing an electron beam at them, and this was one of the major innovations by Aston that made mass spectrometry practical. The mass spectrometer has become one of the most widely used laboratory instruments. Mass spectrometry is now mostly used to identify molecules. Ionization usually breaks a molecule up into fragments having different charge-to-mass ratios, each molecule resulting in a unique "fingerprint" of particles whose origin can be deduced by a jigsaw puzzle-like reconstruction. For many years, "mass-spec" had been limited to small molecules, but with the development of novel ways of creating ions from molecules, it has now become a major tool for analyzing materials and large biomolecules, including proteins . The mass spectrum of magnesium shows that it consists of three isotopes of masses 24 through 26. The height of each peak shows the abundance of each isotope.

University lecture on MS (UC-Irvine, 55 min) Isotopic mixtures and abundances Only 26 of the elements that occur on the Earth exist as a single isotope; these are said to be monoisotopic . The remaining elements consist of mixtures of between two and ten isotopes. The total number of natural isotopes is 339; of these, 254 are stable, while the remainder are radioactive, meaning that they decay into stable isotopes. Recalling that a given isotope (also known as a nuclide ) is composed of protons and neutrons, each having a mass number of unity, it should be apparent that the mass number of a given nuclide will be an integer, as seen in the mass spectrum of magnesium above. It also follows that the relative atomic masses (“atomic weights”) of monoisotopic elements will be very close to integers, while those of other elements, being weighted averages, can have any value. Problem Example 4 Estimate the average atomic weight of magnesium from the isotopic abundance data shown in the above mass spectrometry plot.

Solution: We just take the weighted average of the mass numbers: (0.7899 × 24) + (0.1000 × 25) + (0.1101 × 26) = 24.32 Note: The measured atomic weight of Mg (24.305) is slightly smaller than this because atomic masses of nuclear components are not strictly additive, as will be explained further below. When there are only two significantly abundant isotopes, you can estimate the relative abundances from the mass numbers and the average atomic weight. The following is a favorite exam problem: Problem Example 5 The average atomic weight of chlorine is 35.45 and the element has two stable isotopes 17Cl35 and 17Cl37. Estimate the relative abundances of these two isotopes. Solution: Here you finally get to put your high-school algebra to work! If we let x represent the fraction of Cl35, then (1-x) gives the fraction of Cl37.

The weighted average atomic weight is then 35x + 37(1-x) = 35.45 Solving for x gives 2x = 1.55, x = 0.775, so the abundances are 77.5% Cl35 and 22.5% Cl37. Problem Example 6 Elemental chlorine, Cl2, is made up of the two isotopes mentioned in the previous example. How many peaks would you expect to observe in the mass spectrum of Cl2? Solution: The mass spectrometer will detect a peak for each possible combination of the two isotopes in dichlorine: 35Cl-35Cl, 35Cl-37Cl, and 37Cl-37Cl. Calculate percent isotopic abundance from average atomic weight (MrB, 6½ min) Isotope effects The chemical behavior of an element is governed by the number and arrangement of its electrons in relation to its nuclear charge (atomic number). Because these quantities are identical for all isotopes of a given element, they are generally considered to exhibit identical chemical properties.

However, it turns out that the mass differences between different isotopes can give rise to very slight differences in their physical behavior that can, in turn affect their chemical behavior as well. These isotope effects are most evident in the lighter elements, in which small differences in neutron number lead to greater differences in atomic mass. Wikipedia article on heavy water Thus no element is more subject to isotope effects than hydrogen: an atom of "heavy hydrogen" 1H2 (also known as deuterium and often given the symbol D) has twice the mass of an atom of 1H2. When this isotope is combined with oxygen, the resulting "heavy water" D2O exhibits noticeably different physical and chemical properties: it melts at 3.8° C and boils at 101.4° C. D2O apparently interferes with cell division in organisms; mammals given only heavy water typically die in about a week. When two or more elements whose atoms contain multiple isotopes are present in a molecule, numerous isotopic modifications become possible.

For example, the two stable isotopes of hydrogen and of oxygen (O16 and O18) give rise to combinations such as H2O18, HDO16, etc., all of which are readily identifiable in the infrared spectra of water vapor. The amount of the rare isotopes of oxygen and hydrogen in water varies enough from place to place that it is now possible to determine the age and source of a particular water sample with some precision. These differences are reflected in the H and O isotopic profiles of organisms. Thus the isotopic analysis of human hair can be a useful tool for crime investigations and anthropology research. See also this Microbe Forensics page , and this general resource on water isotopes. Isotopic fractionation Isotope effects manifest themselves in both physical and chemical changes. In general, Chemical bonds involving lighter isotopes tend to be more readily broken, so reactions that depend on the rupture of such bonds will lead to products that are slightly enriched in the lighter isotopes. Phase changes such as vaporization similarly favor the lighter isotopes. Thus when water vapor condenses in clouds to form rain, the heavier water isotopes become slightly more concentrted in the liquid phase.

These two effects give rise to isotopic fractionation as chemical substances move through the environment — or on a much smaller scale, through the various metabolic processes that occur in organisms. Over time, this leads to changes in the isotopic signatures of elements in different realms of the world that can reveal information that would otherwise be hidden. The degree of isotopic fractionation depends on the temperature. This fact has been put to practical use to estimate the global average temperature in past times by measuring the degree of enrichment of the heavier isotopes of oxygen in glacial ice cores and also in ancient sediments containing the shells of microorganisms. Two interesting discussions of isotopic fractionation from Wesleyan U: Atomic weights, molecular weights and formula weights Molecules are composed of atoms, so a molecular weight is just the sum of the atomic weights of the elements it contains. Problem Example 7 What is the molecular weight of sulfuric acid, H2SO4? Solution: The atomic weights of hydrogen and of oxygen are 1.01 and 16.00, respectively (you should have these common values memorized.) From a table, you can find that the atomic weight of sulfur is 32.06.

Adding everything up, we have (2 x 1.01) + 32.06 + (4 x 16.00) = 98.08 Because some solids are not made up of discrete molecules (sodium chloride, NaCl, and silica, SiO2 are common examples), the term formula weight is often used in place of molecular weight. In general, the terms molecular weight and formula weight are interchangeable. 7 Atomic masses and the atomic mass unit Here again is the beginning of the atomic weight scale that you saw above: You understand by now that atomic weights are relative weights, based on a scale defined by 6C12 = 12. But what is the absolute weight of an atom, expressed in grams or kilograms? In other words, what actual mass does each unit on the atomic weight scale represent? The answer is 1.66053886 × 10–27 kg. This quantity (whose value you do not need to memorize) is known as the unified atomic mass unit , denoted by the abbreviation u. (Some older texts leave off the "unified" part, and call it the amu.

Wikipedia article on the unified atomic mass unit The unified atomic mass unit is defined as 1/12 of the mass of one atom of carbon-12. Why such a hard-to-remember number? Well, that's just how Nature sometimes does things. Fortunately, you don't need to memorize this value, because you can easily calculate its value from Avogadro's number, NA, which you are expected to know: 1 u = 1/NA gram = 1 ÷ (1000 NA) Kg ... but more about that in the later lesson on moles . Masses of the subatomic particles Atoms are composed of protons, neutrons, and electrons, whose properties are shown below: particle Two very important points you should note from this table: The mass of the electron is negligible compared to that of the two nuclear particles; The proton and neutron have masses that are almost, but not exactly, identical. Nuclear masses As we mentioned in one of the problem examples above, the mass of a nucleus is always slightly different from the masses of the nucleons (protons and neutrons) of which it is composed.

The difference, known as the mass defect, is related to the energy associated with the formation of the nucleus through Einstein's famous formula e = mc2. This is the one instance in chemistry in which conservation of mass-energy, rather than of mass alone, must be taken into account. But there is no need for you to be concerned with this in this part of the course. For all practical purposes, until you come to the section of the course on nuclear chemistry, you can consider that the proton and neutron have masses of about 1 u, and that the mass of an atom (in u) is just the sum of the neutron and proton numbers. What you should be able to do Make sure you thoroughly understand the following essential ideas which have been presented above. It is especially important that you know the precise meanings of all the highlighted terms in the context of this topic. Give a chemical definition of element © 2004-2016 by Stephen Lower - last modified 2016-05-13 For information about this Web site or to contact the author, please see the Chem1 Virtual Textbook home page.

The Chem1 Virtual Textbook home page is at This work is licensed under a Creative Commons Attribution-Share Alike 3.0 License . Chem1 Atoms, Elements and the Nucleus covers Basic atomic theory for a course in General Chemistry. It is part of the General Chemistry Virtual Textbook, a free, online reference textbook for General Chemistry by Stephen Lower of Simon Fraser University . This chapter covers the following topics: atoms & elements, laws of chemical change, Dalton's atomic theory, combining and relative weights of atoms, nuclei & nuclides, atomic weight scale,atomic masses. It can be accessed directly at . This material is directed mainly at the first-year college level, but much of it is also suitable for high-school students. It is licensed under a Creative Commons Attribution 3.0 Unported License. Chem1 Virtual Textbook home & main menu: ; Download the Chem1 Virtual Textbook from or from the Archive at Electromagnetic isotope separation | Article about Electromagnetic isotope separation by The Free Dictionary Electromagnetic isotope separation | Article about Electromagnetic isotope separation by The Free Dictionary Also found in: Acronyms , Wikipedia .

isotope separation [′ī·sə‚tōp ‚sep·ə¦rā·shən] (nucleonics) The physical separation of different stable isotopes of an element from one another. Isotope Separation the separation of pure isotopes from mixtures of isotopes of a given element or the enrichment of a mixture with individual isotopes. Isotope separation is an important problem of great scientific and practical significance. Attempts to separate isotopes were made from the time of their discovery until the 1930’s primarily with the aim of detecting isotopes of stable elements and measuring their mass and isotopic composition. It proved possible to separate only small (tracer) quantities of some elements insignificantly enriched with isotopes. Fundamental research was begun in the 1930’s on atomic nuclei, nuclear reactions, interactions between particles and nuclei, and related areas. The reliability of experimental data and the interpretation of the results obtained to a large extent depended on the purity and available amount of the isotope in question. However, the preparation of even milligram quantities of isotopes proved to be a complex task. Only small quantities of enriched mixtures of isotopes, mainly of the light elements, were isolated.

Deuterium was the only isotope to be produced on an industrial scale. Further development of the technology of isotope separation was stimulated in 1939 by the discovery of the fission of 235U under the action of neutrons. This opened up possibilities for the peaceful and military uses of atomic energy. The production of large quantities of uranium isotopes and other elements, which are necessary components of “nuclear fuels” and materials in nuclear technology, became an important task at this time. Huge plants were constructed for this purpose. There are a number of methods of isotope separation. All are based on the differences in the properties of isotopes and their compounds resulting from the differences in mass of their atoms. The relative differences in mass of the isotopes are fairly small for most elements. Therein lies the complexity of the task. The effectiveness of isotope separation is characterized by the separation factor α.

For a mixture of two isotopes, α = [C’/ (1 - C’)]/[C”/(1 – C”)], where C’ and (1 - C”) are the relative contents of the light and heavy isotopes, respectively, in the enriched mixture and C” and (1 – C”) are the corresponding quantities in the starting mixture. For most methods, a is only slightly greater than unity, and therefore to obtain a high final isotopic concentration requires multiple repetitions of the single stage of isotope separation. Only electromagnetic separation is characterized by an a value of 10–1,000 per separation cycle. Selection of the method of isotope separation depends on such considerations as the properties of the substance to be separated, the required degree of separation, the desired isotope quantity, and the cost of the process (at high volumes of isotope production). Gaseous diffusion. In the gaseous diffusion method, a gaseous compound of the element being separated is “pumped” at the rather low pressure of ~ 0.1 N/m2 (~ 10~3 mm Hg) through a porous membrane containing up to 106 openings per cm2.

Light molecules penetrate the membrane faster than heavy molecules, since the velocities of the molecules are inversely proportional to the square root of their molecular weights. Consequently, the gas is enriched in the light component on one side of the membrane and in the heavy component on the other side. If the difference in molecular weights is very small, this process must be repeated thousands of times. The number of stages of separation n is determined by the relationship q = α“, where q is the required degree of separation. The operation of huge gaseous diffusion plants for the production of U from gaseous UF6 (α ~ 1.0043) is based on this method. Production of the required 235U concentration makes it necessary to perform about 4,000 individual stages of separation. Diffusion in a vapor stream (countercurrent mass diffusion). In this method, the isotope separation is conducted in a cylindrical vessel (column), divided vertically by a diaphragm. The diaphragm contains about 103 openings per cm3. The gaseous isotope mixture moves against a stream of secondary vapor.

Owing to the concentration gradients of the gas and vapor within the cylinder cross section and the greater coefficient of diffusion for the light molecules, the gas, which has passed through the stream of vapor into the left-hand part of the cylinder, is enriched by the light isotope. The enriched portion is withdrawn from the upper end of the cylinder with the bulk of the vapor, and the gas remaining in the right-hand half moves along the diaphragm and is withdrawn from the apparatus. Vapor that has penetrated into the right-hand part is condensed. The isotopes of neon, argon, carbon, krypton, and sulfur are separated on a laboratory scale (up to 1 kg) using separators consisting of several dozen sequentially connected diffusion columns with vaporizing liquids, such as mercury or xylene. Thermal diffusion. A thermal diffusion separation column consists of two vertical concentric pipes heated to different temperatures. The mixture to be separated is introduced into the space between the pipes. The temperature drop A T between the surfaces of the pipes generates a diffusion flux, which leads to a difference in the concentration of the isotopes in the cross section of the column. At the same time, the temperature drop leads to the generation of vertical convection flows of gas.

Consequently, the lighter isotopes concentrate near the heated inner pipe and move upward. The separation coefficient α = 1 + γ (ΔT/T), where γ is the thermal diffusion constant, which is dependent on the relative difference in mass of the isotopes, and T = (T1 + T2)/2. The thermal diffusion method makes it possible to separate isotopes in both the gas and liquid phases. The variety of isotopes that may be separated by this method is greater than the separations possible by gaseous diffusion or diffusion in a stream of vapor. However, α is small for the liquid phase. The simplicity of this method and the absence of vacuum pumps, among other features, make it a convenient method to use when separating isotopes under laboratory conditions. This method has yielded He containing 0.2 percent 3He (the natural abundance of this isotope is 1.5 X 10 5 percent), as well as the isotopes l80, 15N, 13C, 20Ne,22Ne, 35C1, 84Kr, and 86Kr in concentrations greater than 99.5 percent.

Thermal diffusion was used on an industrial scale in the USA for the preliminary enrichment of 235U prior to the final separation of this isotope in an electromagnetic installation. The thermal diffusion plant consisted of 2,142 columns measuring 15 m high. Distillation (fractional distillation). Since, as a rule, liquid isotopes have different saturated vapor pressures, for example, P1 and p2, and different boiling points, it is possible to separate isotopes by fractional distillation. Distilling columns with a large number of separation stages are used; a depends on the ratio P1/P2and its value decreases with increasing molecular weight and temperature. For this reason the process is most effective at low temperatures. Distillation has been used in the preparation of isotopes of light elements, such as 10B, “B, 18O, 15N, and 13C, as well as in the production of hundreds of tons of heavy water per year. Isotope exchange. Isotope separation can also be carried out by chemical reactions in which the isotopes of the element undergoing separation change places.

Thus, for example, if hydrogen chloride HC1 is brought into contact with hydrogen bromide HBr, both of which have the same initial content of deuterium D, the exchange reaction will result in a D content of HC1 that is several times higher than that of HBr. The use of several stages leads to the production of hydrogen, nitrogen, sulfur, oxygen, carbon, and lithium highly enriched in the individual isotopes. Centrifugation. In a centrifuge rotating with a high peripheral speed (100 m/sec), the heavier molecules are concentrated near the periphery and the lighter molecules near the rotor. The stream of vapor in the outer part with the heavy isotope is directed downward, and the flow in the inner part with the light isotope is directed upward. Connecting several centrifuges in a cascade leads to the desired isotope enrichment. During centrifugation, a depends on the difference in atomic mass of the isotopes being separated rather than on the ratio of the masses. For this reason, centrifugation is suitable for separating the isotopes and heavy elements. Owing to the existence of improved centrifuges, this method has found applications in the industrial separation of the isotopes of uranium and other heavy elements. Electrolysis.

During the electrolysis of water or of aqueous solutions of electrolytes, the hydrogen evolved at the cathode contains less deuterium than the initial water. As a result, the concentration of deuterium increases in the electrolytic cell. This method has been used for the industrial production of heavy water. The separation of other isotopes of light elements (lithium, potassium) by the electrolysis of their chlorides is performed only in laboratory quantities. Electromagnetic process. In the electromagnetic method, the substance whose isotopes are to be separated is placed in a crucible of an ion source and is evaporated and ionized. The ions are extracted from the ionization chamber by a strong electric field, and combined into a beam of ions; they then enter a vacuum separation chamber placed in a magnetic field H, oriented perpendicularly to the motion of the ions. Under the influence of the magnetic field, the ions travel in circular paths with radii of curvature that are proportional to the square root of the ratio of the ion mass M to its charge e. This leads to differences in the radii of the paths of the heavy and light ions.

It is therefore possible to collect ions of various isotopes in collectors located in the focal plane of the installation. The productivity of electromagnetic installations is determined by the magnitude of the ion current and the effectiveness of collecting the ions. In large installations the ion current varies from tens to hundreds of mA, which makes it possible to isolate up to several grams of isotopes per day (total of all isotopes). The productivity is lower in laboratory separators by a factor of 10–100. The electromagnetic method is characterized by large a and by the possibility of the simultaneous separation of all isotopes of a given element. Usually, at large industrial installations a ~ 10–100 per stage, and in laboratory installations a is 10 to 100 times greater. In most cases, a single electromagnetic stage of separation is adequate; the repeated separation of previously enriched isotopic materials to obtain isotopes of extremely high purity is rarely done. The principal drawbacks of this method are relatively low productivity, high operating cost, and considerable losses of the separated material. The electromagnetic method has made it possible to prepare kilogram quantities of 235U for the first time. The electromagnetic plant in Oak Ridge, Tenn.

(USA), included 5,184 separation chambers called calutrons. Owing to their high universality and flexibility, electromagnetic installations are used for the separation of about 50 elements of the periodic system in quantities ranging from several milligrams to hundreds of grams and are the principal source of isotopes for scientific research and some practical applications. Laboratory separators, like the large electromagnetic separation installations for industrial isotope production, have also found numerous applications. The laboratory separators are used in the preparation of radioactive isotopes, which are necessary for nuclear spectroscopy and for studies of the interaction between ions and solids (in ion implantation as well as for other purposes). Other methods. In addition to the methods listed above, there are a number of other methods whose applications are limited or which are in the process of being developed or improved. These methods include preparation of 3 He based on the phenomenon of superfluidity of 4 He; separation by diffusion in a supersonic gas stream, which expands in a space with a decrease in pressure; chromatographic separation based on differences in the adsorption rates of isotopes; and biological separation methods. Summary. The methods of isotope separation possess certain features that determine the areas of their mostefficient application.

In the separation of light elements with mass numbers of about 40, distillation, isotope exchange, and electrolysis are the most economical and effective. Diffusion, centrifugation, and the electromagnetic method are used in the separation of the isotopes of heavy elements. Gaseous diffusion and centrifugation can, however, be used, if gaseouscompounds of these elements are available. Since such compounds are scarce, the real potential of these methods is limitedThermal diffusion permits the separation of isotopes in both the gas and liquid states, but a is small for the separation ofisotopes in the liquid phase. The electromagnetic method is characterized by high a, but it has low productivity and is used mainly in the production of isotopes on a moderate scale. In order to provide for the scientific study and practical uses of isotopes, the State Fund of Stable Isotopes was created in the USSR. The fund contains reserves of isotopes of almost all the elements. Considerable quantities of deuterium, 10B, 13C, 15N, l8O, 23Ne, and other isotopes are regularly produced.

The production of various chemical compounds labeled with stable isotopes has also been set up. REFERENCES

Question: The process of increasing the abundance of a particular isotope in a mixture of isotopes is known by what name?
Answer: Enrichment