Molecules isotopic patterns

Simple molecular weight changes relative to the parent molecule may indicate the gain or loss of the equivalent of a methylene group (i.e. the loss of a methyl group replaced by a proton), oxidation, and hydration/dehydration or hydrolysis to list only a few possibihties. The formation of adduct ions, for example, sodium, potassium, or acetonitrile at +23, +39, and +43 Da, respectively, can provide the means for the preliminary identification of the molecular ion of a molecule. Isotope patterns, for example, from Cl, and even can also be diagnosti-... 

Large Molecules - Isotopic Patterns at Sufficient Resolution... 

A typical SSIMS spectrum of an organic molecule adsorbed on a surface is that of thiophene on ruthenium at 95 K, shown in Eig. 3.14 (from the study of Cocco and Tatarchuk [3.28]). Exposure was 0.5 Langmuir only (i.e. 5 x 10 torr s = 37 Pa s), and the principal positive ion peaks are those from ruthenium, consisting of a series of seven isotopic peaks around 102 amu. Ruthenium-thiophene complex fragments are, however, found at ca. 186 and 160 amu each has the same complicated isotopic pattern, indicating that interaction between the metal and the thiophene occurred even at 95 K. In addition, thiophene and protonated thiophene peaks are observed at 84 and 85 amu, respectively, with the implication that no dissociation of the thiophene had occurred. The smaller masses are those of hydrocarbon fragments of different chain length. 

The highest mass peaks observed in the mass spectra of alkyl chlorides may correspond to the loss of HX or X (loss of HI is seldom observed), depending on the structure of the molecule. In order to deduce the molecular ion, add the mass of X or HX to the mass at which the highest mass peak is readily observed. (Note that higher mass ions having the isotope pattern of X may be present... 

The isotopic patterns of or atoms present in the analyte molecule... 

Even if the analyte is chemically perfectly pure it represents a mixture of different isotopic compositions, provided it is not composed of monoisotopic elements only. Therefore, a mass spectrum is normally composed of superimpositions of the mass spectra of all isotopic species involved. [11] The isotopic distribution or isotopic pattern of molecules containing one chlorine or bromine atom is listed in Table 3.1. But what about molecules containing two or more di-isotopic or even polyisotopic elements While it may seem, at the first glance, to complicate the interpretation of mass spectra, isotopic patterns are in fact an ideal source of analytical information. 

The calculation of isotopic patterns as just shown for the carbon-only molecule Qo can be done analogously for any X-rl element. Furthermore, the application of this scheme is not restricted to molecular ions, but can also be used for fragment ions. Nevertheless, care has be taken to assure that the presumed isotopic peak is not partially or even completely due to a different fragment ion, e.g., an ion containing one hydrogen more than the presumed X-rl composition. 

So far we have treated the X-f1 and the X-f2 elements separately, which is not how they are encountered in most analytes. The combination of C, H, N and O with the halogens F, Cl, Br and I covers a large fraction of the molecules one usually has to deal with. When regarding H, O and N as X elements, which is a valid approximation for not too large molecules, the construction of isotopic patterns can be conveniently accomplished. By use of the isotopic abundance tables of the elements or of tables of frequent combinations of these as provided in this chapter or... 

The calculation of isotopic patterns of molecules of several 10 u is not a trivial task, because slight variations in the relative abundances of the isotopes encountered gain relevance and may shift the most abundant mass and the average mass up or down by 1 u. In a similar fashion the algorithm and the number of iterations employed to perform the actual calculation affect the final result. [16]... 

Although isotopic patterns and deconvolutions may be calculated manually (6), to achieve full potential a computer is virtually a necessity. Manual calculations often omit the 1.1% contribution for for molecules with large ligands, thirty carbons are not unusual, and these would give a 33% contribution to the m/e value, greater than that from the nominal mass by one mass unit. 

Isotope patterns. From the natural abundance of 79Br and 81 Br, predict the relative amounts of CH79Br3, CH79Br28lBr, CH79Br81Br2, and CHM Brv As explained in Exercise 22-C, the fraction of each isotopic molecule comes from the expansion of (a + b)3, where a is the abundance of 79Br and b is the abundance of 81Br. Note that... 

I 3 Isotope patterns. (Caution This problem could lead to serious brain injury.) For an element with three isotopes with abundances a, b, and c, the distribution of isotopes in a molecule with n atoms is based on the expansion of (a + b + c)". Predict what the mass spectrum of Si2 will look like. 

Dissection of the chemical structure of jamaicamides A-C led to the speculation that these metabolites derive from a mixture of polyketides (nine acetate units), amino acids (t-Ala and p-Ala), and the S-methyl group of methionine. To map out the biosynthetic subunits of these molecules, isotopically labeled precursors were supplied to I. majuscula JHB, and the labeling patterns discerned by NMR spectroscopy (Figure 6.12). From these experiments, insights were gained into the biochemical transformations that produce the jamaicamides, especially the mechanism of formation of the vinyl chloride group [157]. 

The isotopic pattern may complicate the molecular weight assignment on the other hand, it will also provide valuable reference for recognizing the type and number of element in a molecule. The characteristic patterns resulting from multiple isotopic contributions of the chlorine, bromine, and sulfur isotopes are shown in Table 7-5. One example is illustrated in the CI-MS spectrum of mometasone furoate (Figure 7-2), displaying prominent... 

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