Isotopes heavy

Natural Isotopic Abundances. The relative abundances of natural isotopes produce peaks one or more mass units larger than the parent ion (Table 7.75a). For a compound C H O N, a formula allows one to calculate the percent of the heavy isotope contributions from a monoisotopic peak, Pto the Pm + 1 peak ... 

Compounds that contain chlorine, bromine, sulfur, or silicon are usually apparent from prominent peaks at masses 2, 4, 6, and so on, units larger than the nominal mass of the parent or fragment ion. Eor example, when one chlorine atom is present, the P + 2 mass peak will be about one-third the intensity of the parent peak. When one bromine atom is present, the P + 2 mass peak will be about the same intensity as the parent peak. The abundance of heavy isotopes is treated in terms of the binomial expansion (a -I- h) , where a is the relative abundance of the light isotope, b is the relative abundance of the heavy isotope, and m is the number of atoms of the particular element present in the molecule. If two bromine atoms are present, the binomial expansion is... 

If this is multiplied by 100, it becomes atom%, a percentage that relates the proportion of heavy isotope to the sum of the heavy and light isotopes. 

In addition to H2, D2, and molecular tritium [100028-17-8] the following isotopic mixtures exist HD [13983-20-5] HT [14885-60-0] and DT [14885-61-1]. Table 5 Hsts the vapor pressures of normal H2, D2, and T2 at the respective boiling points and triple points. As the molecular weight of the isotope increases, the triple point and boiling point temperatures also increase. Other physical constants also differ for the heavy isotopes. A 98% ortho—25/q deuterium mixture (the low temperature form) has the following critical properties = 1.650 MPa(16.28 atm), = 38.26 K, 17 = 60.3 cm/mol3... 

Birds and animals are tagged so that their wanderings and ultimate fate can be followed. To do the same for nitrogen fertilizer, we use the heavy isotope of nitrogen, as a tag or label . This is a safe isotope to use because it is not radioactive. The of the label is taken up by the crop, incorporated in the soil s... 

Another widely used approach to the elucidation of metabolic sequences is to feed cells a substrate or metabolic intermediate labeled with a particular isotopic form of an element that can be traced. Two sorts of isotopes are useful in this regard radioactive isotopes, such as and stable heavy isotopes, such as or (Table 18.3). Because the chemical behavior of isotopically labeled compounds is rarely distinguishable from that of their unlabeled counterparts, isotopes provide reliable tags for observing metabolic changes. The metabolic fate of a radioactively labeled substance can be traced by determining the presence and position of the radioactive atoms in intermediates derived from the labeled compound (Figure 18.13). 

Properties of Radioactive and Stable as Tracers in Metabolic Studies Heavy Isotopes Used... 

Heavy isotopes endow the compounds in which they appear with slightly greater masses than their unlabeled counterparts. These compounds can be separated and quantitated by mass spectrometry (or density gradient centrifugation, if they are macromolecules). For example, O was used in separate experiments as a tracer of the fate of the oxygen atoms in water and carbon dioxide to determine whether the atmospheric oxygen produced in photosynthesis arose from HgO, COg, or both ... 

The oxygen in water is primarily (99.8%) l60, but water enriched with the heavy isotope, 80 is also available. When an aldehyde or ketone is dissolved in 180-enriched water, the isotopic label becomes incorporated into the carbonyl group. Explain. 

As you can see from Figure 2.5, the neutron-to-proton ratio required for stability varies with atomic number. For light elements (Z < 20), this ratio is close to 1. For example, the isotopes C, N, and are stable. As atomic number increases, the ratio increases the belt of stability shifts to higher numbers of neutrons. With very heavy isotopes such as 2j Pb, the stable neutron-to-proton ratio is about 1.5 ... 

As an energy source, nuclear fusion possesses several additional advantages over nuclear fission. In particular, light isotopes suitable for fusion are far more abundant than the heavy isotopes required for fission. You can calculate, for example (Problem 73), that the fusion of only 2 X 10-9 % of the deuterium ( H) in seawater would meet the total annual energy requirements of the world. 

Evaporitic sulfur has a range of sulfur isotopic composition from +10%o to +30%o, while sedimentary sulfur is depleted in the heavy isotope and has a range of isotopic composition of about —40%o to +10%o. Most of this variation reflects systematic changes with geological age. The source fractions of a river water can be estimated from an isotopic mass balance ... 

The global atmospheric circulation acts as an enormous filtration system, which depletes high-latitude precipitation of heavy isotope-bearing water molecules. Because of this system, measurements of the stable isotopic composition of the ice sheets and of ocean-floor sediments reveal very important paleo-environmental information (see Sections 18.2.2,18.3.2, and 18.3.3). Here we examine this filtration system at a physical level. This system was first understood by a great Danish geochemist named Willi Dansgaard (Dansgaard, 1964). 

H2 0 vapor, and Oq moles of HDO vapor. Write nj to signify either of the heavy-isotope-bearing vapors (n or nig). Define the isotopic ratio of the vapor as Rv/ = npn (,. If the condensation of the vapor to form liquid water or solid ice is an equilibrium process, and this condensation is quickly removed from the air mass as precipitation, the corresponding isotopic ratio in the precipitation (RpPis proportional to that of the vapor... 

The concentrations of the heavy isotope must be low, which is true for experiments using natural levels of abundance, or the reaction must be near the initial starting point. 

Today a good understanding of transition state structure can be obtained through a combination of experimental measurements of kinetic isotope effects (KIE) and computational chemistry methods (Schramm, 1998). The basis for the KIE approach is that incorporation of a heavy isotope, at a specific atom in a substrate molecule, will affect the enzymatic reaction rate to an extent that is correlated with the change in bond vibrational environment for that atom, in going from the ground state to the... 

The ratio of the quantum partition functions (Eq. (4-29)) for two different isotopes can be obtained directly through free energy perturbation (FEP) theory by perturbing the mass from the light isotope to the heavy isotope. Consequently, only one simulation of a given isotopic reaction is performed, while the ratio of the partition function, i.e., the KIE, to a different isotopic reaction, is obtained by FEP. This is conceptually and practically an entirely different approach than that used previously [23]. 

Both the 12C/13C primary KIE and the 14N/15N secondary KIE have been determined (Table 4-2) [19, 20], with the immediate adjacent atoms about the isotopic substitution site quantized as well. To our knowledge, we are not aware of any such simulations prior to our work for a condensed phase reaction with converged secondary heavy isotope effects. This demonstrates the applicability and accuracy of the PI-FEP/UM method. 

Ans. Hydrogen occurs as diatomic molecules, and it would be easy to separate H , H H, and 2H , but not the individual atoms. But there would be relatively little "H, since in abundance the heavy isotope accounts for only 0.015% of naturally occurring hydrogen atoms. 

Dissolve one vial of heavy isotope, cleavable ICAT reagent (Applied Biosystems) in 20 pi acetonitrile (use a fume hood for handling solvents). Dissolve a second vial containing the normal isotope ICAT compound in 20 pi acetonitrile. Vortex mix each vial to dissolve. 

Add 100 pg of the control protein solution to one vial of dissolved normal isotope ICAT reagent. Mix to dissolve. Add 100 pg of the test protein solution to one vial of dissolved heavy isotope ICAT reagent. Mix to dissolve. 

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