Measurement units molecular peaks

The latter expression of specific activity is used almost always for high specific activity tritiated compounds, and often for carbon-14-labeled compounds. Measures of radioactivity per molar unit can be calculated from mass spectrometry data ". In this analysis, the distribution of isotopic species (e.g., Hq, Hj,. ..) is determined by measuring the relevant peak intensities in the molecular ion envelope and correcting them for naturally occurring isotopes (e.g., C, present in the molecule this can be done manually or by use of readily available computer algorithms. The contribution of each isotopic species to the total can be used to calculate the average number of isotopic atoms per molecule and thence, from the molar specific activity of the pure isotope, the molar specific activity of the compound. 

The peak width increases with injection volume. Therefore this parameter has to be fixed for comparative measurements. It has become the custom to inject low molecular weight test samples in very small volumes at very high concentrations, occasionally even as pure compounds. This extreme is not recommended as it is more important to inject a constant sample amount, reproducibly, in a precisely kept volume. Typical GPC injections are between 50 and 200 /a1. It is better to inject a larger volume of a lower concentration polymer solution. GPC units are often not designed for injection volumes lower... 

The conformational mobility of a chromophoric main-chain polymer is often connected to its electronic structure. Therefore, changes in the UV-visible absorption spectra and/or chiroptical properties are spectroscopically observable as thermo-, solvato-, piezo-, or electrochromisms. It is widely reported that o-conjugating polysilanes exhibit these phenomena remarkably clearly.34 However, their structural origins were controversial until recently, since limited information was available on the correlation between the conformational properties of the main chain, electronic state, and (chir)optical characteristics. In 1996, we reported that in various polysilanes in tetrahydrofuran (THF) at 30°C, the main-chain peak intensity per silicon repeat unit, e (Si repeat unit)-1 dm3 cm-1, increases exponentially as the viscosity index, a, increases.41 Although conventional viscometric measurements often requires a wide range of low-dispersity molecular-weight polymer samples, a size exclusion chromatography (SEC) machine equipped with a viscometric detector can afford... 

Resolution (or resolving power) plays an important role in mass spectrometry for applications requiring the characterization of very similar chemical species. The ability to detect and accurately measure the m/z ratio of a particular ion depends directly on the resolving power of the mass analyzer. For example, if a sample contains two isobaric compounds (i.e., having the same nominal molecular mass but different elemental formulae) the difference in the exact masses of the molecular ions will be much less than 1 m/z unit. Any mass analyzer possessing a nominal resolving power (e.g., RP< 1000) will register only one peak in the mass spectrum of such a binary mixture. Attempts to measure the... 

Spectra of G2-OH and G3-OH indicate the presence of dendrimers corresponding to the molecular weight of the ideal dendrimer structure (Mjs) 3273.05 for G2-OH (Fig. 9a) and 6941.8 for G3-OH (Fig. 9c). Some lower masses corresponding to the loss of one or several 115 mass units (Mis-(nx 115)) are also evident these are due to missing arm (-CH2CH2CONHCH2CH2OH) defects. The observation of an M]s-(4 x 115)-60 peak in G2-OH is due to four missing arms and the formation of an intramolecular loop (Mjg - 4 arms -I-1 loop). These defects have also been observed by electrospray (ES)-MS measurements [116]. The mass spectrum of G4-OH with or without Cu + is much more complicated than for the lower generations but a complete discussion is available in the primary literature [98]. 

With the Chromalytics Model MC-2 mass chromatograph, a sample is introduced into the unit, split into two portions, collected onto traps, and then analyzed simultaneously with two gas density detectors as shown in Figure 1. The peak height or area ratios from each detector are measured and the molecular weights calculated from Equation 3. 

The same precision as discussed above can be extended about 50 mass units by using N2 (molecular weight 28) and perfluoropropane (molecular weight 188) compared with C02 and SF6. For example, with a standard deviation in K of 0.5, a mass error standard deviation of 1 mass unit would be 300 instead of 250. Since the measurement of detector response is a function of the recorder (peak heights), integrator system (for areas), columns (absorption sites), electronics, temperature, etc., the overall precision of molecular weight measurement should be further improved in the future. 

Considerable evidence exits of the survival of Zintl ions in the liquid alloy. Neutron diffraction measurements [5], as well as molecular dynamics simulations [6, 7], give structure factors and radial distribution functions in agreement with the existence of a superstructure which has many features in common with a disordered network of tetrahedra. Resistivity plots against Pb concentration [8] show sharp maxima at 50% Pb in K-Pb, Rb-Pb and Cs-Pb. However, for Li-Pb and Na-Pb the maximum occurs at 20% Pb, and an additional shoulder appears at 50% Pb for Na-Pb. This means that Zintl ion formation is a well-established process in the K, Rb and Cs cases, whereas in the Li-Pb liquid alloy only Li4Pb units (octet complex) seem to be formed. The Na-Pb alloy is then a transition case, showing coexistence of Na4Pb clusters and (Pb4)4- ions and the predominance of each one of them near the appropiate stoichiometric composition. Measurements of other physical properties like density, specific heat, and thermodynamic stability show similar features (peaks) as a function of composition, and support also the change of stoichiometry from the octet complex to the Zintl clusters between Li-Pb and K-Pb [8]. 

The minimum requirement for the organic chemist is the ability to record the molecular weight of the compound under examination to the nearest whole number. Thus, the spectrum should show a peak at, say, mass 400, which is distinguishable from a peak at mass 399 or at mass 401. In order to select possible molecular formulas by measuring isotope peak intensities (see Section 1.5.2.1), adjacent peaks must be cleanly separated. Arbitrarily, the valley between two such peaks should not be more than 10% of the height of the larger peak. This degree of resolution is termed unit resolution and can be obtained up to a mass of approximately 3000 Da on readily available unit resolution instruments. 

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