Hydrogen isotopes standard

SMOW. standard mean ocean water (a standard for oxygen and hydrogen isotopes)... 

There is a range of standards for hydrogen isotopes. The primary reference standard, the zero point of the 5-scale, is V-SMOW, which is virtnally identical in isotopic composition with the earlier defined SMOW, being a hypothetical water sample orginally defined by Craig (1961b). 

Table 8 Standardized Rates of Hydrogen Isotope Exchange... 

The hydrogen-isotope exchange reactions have already been discussed in previous sections. For various aromatic and heteroaromatic substrates, standard reaction rates in protodedeuteration and in nitration have been obtained. A plot of these shows a wide scatter. This means that there is no simple relation between these two measures for the susceptibility to electrophilic attack. No single reactivity index can be used as a measure to derive a unique order of the susceptibility of individual ring positions towards electrophilic attack (75TL1395). 

Various isotope standards are used for reporting isotopic compositions the composition of each of the standards has been defined as 0 per mil. Oxygen and hydrogen isotopic compositions are commonly reported relative to standard mean ocean water (SMOW or V-SMOW) (17). 

Carbon and hydrogen measurements were made on a Varian MAT 250 triple collecting MS and sulfur and nitrogen on a Nuclide RMS 6-60 dual collecting MS. Atmospheric N, Chicago Pee Dee belemnite, SMOW and Canyon Diablo troilite were used for isotope standard Analytical errors for isotope ratjp measurements were as follows 8 C 0.05 per mil, SD 3 per mil, 5 S 0.4 per mil, 5 N 0.4 per mil, and for elemental analyses, C 0.5%, H l%, S 0.5%, N 0.5%. 

Table 7.6 Internationally accepted stable isotope standards for hydrogen, carbon, oxygen, nitrogen, and sulfur.

Stable oxygen and hydrogen isotopic ratios are normally reported relative to the standard mean ocean water (SMOW) standard (Craig, 1961b) or the equivalent Vienna-SMOW (V-SMOW) standard. Carbon, nitrogen, and sulfur stable isotope ratios are reported relative to the Pee Dee Belemnite (PDB) or Vienna-PDB (VPDB), ambient air (AIR), and Canyon Diablo Troilite (CDT) standards, respectively, as defined later. The use of the V before SMOW or PDB indicates that the measurements were calibrated on normalized per mil scales (Coplen, 1996). 

We report here measurements of the stable hydrogen Isotopic composition of methane and water, the stable carbon Isotopic composition of methane and carbon dioxide, and ancillary parameters from several freshwater environments and from a few locations within the Tampa Bay estuary. The stable Isotopic compositions determined In this study are reported as a parts-per-mll ( /oo) deviation (5) from a standard with a known stable isotopic ratio. The definition of the 6 value Is ... 

Oxygen-isotopic ratios were determined by the CO2 equilibrium method of Epstein and Mayeda (1953). The hydrogen-isotopic ratios of water were determined by the method of Friedman and Woodcock (1957). The data are reported in the usual delta notation with SMOW as the standard (Craig, 1961). Our analyses of I.A.E.A. SLAP was —424.6%o 5D and —55.95%o... 

CSIA yields data of the isotopic composition of a single compound relative to an international standard that is usually expressed as delta notation (S) values in parts per thousand (%o) according to Eq. 1. The most common ones, the carbon and hydrogen isotopic compositions (R), are reported as and relative to Vienna Pee Dee Belemnite standard (V-PDB) and Vienna Standard Mean Ocean Water (V-SMOW), respectively [20] ... 

Kinetic isotope effects provide useful information concerning the transition state structure of E2 reactions but they suffer from some serious shortcomings. Not all elements exist in more than one stable isotopic form and isotope effects for heavier atoms are extremely small and cannot be measured using presently available experimental techniques. Moreover, even carbon and hydrogen isotope effects can give ambiguous results. An alternative approach makes use of the linear free energy relationship concept, namely the relation of rate or equilibrium constant of an unknown reaction to those of known standard reactions. 

Hydrogen isotopes are measured in parts per thousand reladve to the SMOW standard and are calculated in an analogous manner to that for oxygen isotopes (see Eqn [7.1]) and expressed as 5D /oo. Precision is between 1 and 2 / . 6D values for the SLAP standard reladve to SMOW are -428 /m. D/H ratios are usually measured on H2 gas which is produced from the reduction of water at high temperatures. 

D Delta notation of stable hydrogen isotopes, which is used for stable isotopic data, and is equal to [(/ sampie - standard)/ standard)) 1000, where R is the ratio of D (for deuterium, the name given to H) to H. 

For p d l the quantity exp(ip d) would be equal to unity and one would recover the standard cross section. The average < exp(ip-d) > depends on the initial momentum p of the proton and its orientation relative to H-H vector d. For p perpendicular to d, exp(ip-d)j = 1, but for p 11 d, the < exp(ip-d) >n terms are strongly reduced if there is a large zero-point contribution to the momentum distribution n(p). The oscillations in exp(ip-d) are effectively averaged out by the large intrinsic zero-point momentum spread in of the hydrogen isotopes, which typically amounts to A = 4 for protons (see Fig. 22.6). 

A comparison of hydrogen, oxygen, and carbon isotopic standards... 

FIGURE 15.2 Potential variation in the hydrogen isotopic composition of a range of different materials. Reprinted from T.B. Coplen et al. [16], with permission from Coplen. SLAP, Standard Light Antarctic Precipitation GISP, Greenland Ice Sheet Precipitation VSMOW, Vienna Standard Mean Ocean Water NBS, National Bureau of Standards NGS, natural gas standards. 

Vienna Standard Mean Ocean Water is the current international standard for water isotopes. Naturally occurring water is almost completely composed of the neutron-less hydrogen isotope protium. Only 155 ppm include deuterium ( H or D), a hydrogen isotope with one neutron, and less than 20 parts per quintillion include tritium ( H or T), which has two. 

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