Isotope analyses silicon

S., and Clayton R. N. (2000a) Toward complete isotopic analysis of individual presolar silicon carbide grains C, N, Si, Sr, Zr, Mo, and Ba in single grains of type X. In Ijunar Planet. Sci. XXXI, 1917. The Lunar and Planetary Institute, Houston (CD-ROM). 

Silicon occurs very frequently in trace analysis. Silicones get into the analysis through derivatization (silylation), partly through clean-up (joint grease), but more frequently through bleeding from the septum or the column (septa of autosampler vials, silicone phases). The typical isotope pattern of all silicone masses can be recognized rapidly and excluded from further evaluation measures (Figure 3.42). 

During the late 1960s and early 1970s, neutron activation analysis provided a new way to measure bulk chemical composition. Neutron activation analysis utilizes (n,y) reactions to identify elements. A sample is placed in a nuclear reactor where thermal neutrons are captured by atoms in the sample and become radioactive. When they decay, the radioactive isotopes emit characteristic y-rays that are measured to determine abundances. Approximately 35 elements are routinely measured by neutron activation analysis. A number of others produce radioactive isotopes that emit y-rays, but their half-lives are too short to be useful. Unfortunately, silicon is one of these elements. Other elements do not produce y-ray-emitting isotopes when irradiated with neutrons. There are two methods of using neutron activation to determine bulk compositions, instrumental neutron activation analysis (INAA) and radiochemical neutron activation analysis (RNAA). 

Of course nowadays exact mass measurement could also distinguish these two molecules, as could a variety of other instrumental techniques. The analysis of isotopic clusters is most useful for detecting the presence of halogens, sulfur, and silicon, all of which have abundant isotopes of two atomic weight units higher, thus leading to relatively large M + 2 peaks. 

Molecular Cl(H)Si=S (126) was also formed in an argon matrix in a photochemically induced reaction of SiS with HC1. From the isotopic splittings (H/D and 35C1/37C1) of the IR absorptions the Cs structure of the species with silicon as the central atom is deduced. By a normal coordinate analysis a value of 4.83 mdynA-1 is obtained for the SiS force constant, a value which was confirmed by ab initio SCF calculations of the IR spectrum51. 

It has been found by mass spectrometric analysis that in nature the relative abundances of the various isotopic atoms of silicon are 92.23% 28Si, 4.67% 29Si, and 3.10% 30Si. Calculate the atomic mass of silicon from this information and from the nuclidic masses. 

The mass spectra of 7, obtained from the NADPH/02/P-450 oxidation of 5, and 8 are compared in Table IV. Three isotopic species are expected for the allylic hydroxylation of 5. Oxidation with removal of hydrogen will produce 7-d3and 7a-d3. Extensive controls have demonstrated that 7-d3 and 7a-d3 are indistinguishable by mass spectrometry because of rapid 1,3-migration of the siloxy group upon electron impact. Oxidation with deuterium removal leads to the production of 7-d2 (Scheme 4). By this analysis, the isotope effect is simply the ratio of 5-d3 (and 5a-d3) to 5-d2 in the oxidized sample. Deconvolution of the parent region (with appropriate correction for carbon and silicon... 

An attractive, although tentative, alternative would be an alkyl-substituted silylsilylene formed from the polymer chain. Two thermodynamically reasonable routes to such intermediates are possible. The first route (equation 4) involves 1,1-elimination to produce the silylsilylene directly. This route has a precedent in organosilane thermal processes (78, 79). The second route (equations 5a and 5b) involves rearrangement from a silene produced by the disproportionation (46, 80, 81) of two silyl radicals caused by bond homolysis. This type of rearrangement has also been described in the literature (82). The postulated silylsilylenes are also attractive intermediates to explain the rebonding of silicon to carbon atoms other than those in the original a positions (CH insertion), which is obvious from the mass spectral analysis of gaseous products from the laser ablation of isotopically labeled poly(di-n-hexylsilane). 

M. Croset and D. Dieumegard, Quantitative secondary ion mass spectrometry analysis of oxygen isotopes and other light elements in silicon oxide fdms, Corros. Sci. 16, 703, 1976. 

The interpretation of SSMS data falls into two distinct areas — element Identification and estimates of quantity. The criteria used in our laboratory for positive elemental identification are the presence of the doubly ionized species and the Identification of the Isotopic pattern when possible. Quantitation will be discussed later. Last, the Instrument source must be cleaned regularly to avoid memory problems. Our approach to the memory problem is to have a complete set of source parts for each matrix. A set of parts are dedicated for silicon analyses, another for gallium arsenide, etc. These parts and the source Itself are cleaned on a regular and frequent basis. When these factors are under control, SSMS has proved to be a reliable, reproducible technique for the bulk analysis of trace impurities. 

Silicon consists predominantly of Si (92.23%) together with 4.67% Si and 3.10% Si. No other isotopes are stable. The Si isotope (like the proton) has a nuclear spin I =, and is being increasingly used in nmr spectroscopy. Si, formed by neutron irradiation of Si, has ti 2.62 h it can be detected by its characteristic activity ( max 1-48 MeV) and is very useful for the quantitative analysis of Si by neutron activation. The radioisotope with the longest half-life ( 172 y) is the soft emitter Si ( max... 

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