Isotopic Patterns of Molecules

The basic technique, how to calculate relative abundance of isotope clusters, has been given in the literature [4], Computer programs that calculate the isotopic patterns of molecules are available [5-7]. 

From the information in Table 12.4, the isotopic patterns of compounds constituted from carbon and chlorine can be easily calculated. We focus on carbon tetrachloride CCI4. We are constructing now a generating function for the abundance of the respective isotopes, the abundance generating function. 

We attach the relative abundance associated with a certain mass number to a dummy variable set to the power of this respective mass number. So the abundance generating function of the isotopic pattern of carbon is 

If a chlorine ion is analyzed in a mass spectrometer, peaks are seen at the mass numbers 35 and 37. The relative intensity of the peaks at mass number n is just the coefficient attached to t . The pattern that appears for carbon tetrachloride is a convolution of the abundance generating functions for both carbon and chlorine. 

We form now the abundance generating function for the isotopic pattern of carbon tetrachloride  

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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 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]... 

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... 

Isotopic Patterns of Large Molecules at Sufficient Resolution... 

A first step towards the analysis of isotopic patterns is the isotopic analysis of molecule parts, which must also have, in case of natural origin, intramolecular isotope correlations in this case, pure compounds in sufficient amounts must be available. The performance of the molecule degradation depends on its stmcture, but in many cases a simple hydrolysis will be of practical value. Examples are compiled in Tables 6.9 and 6.10. They represent typical groups of compounds, as esters and glycosides, which can easily be hydrolysed, or a few other compounds which can be split in two parts by a suitable other method, and which can subsequently be converted into CO2. 

Mass spectra have been used principally for the determination of molecular weights and deduction of the detailed composition of the molecule from the isotopic patterns of the various ions. Structural information can also be obtained from a detailed analysis of the fragmentation pattern. The clusters which have been studied by mass spectrometric methods have largely been carbonyl, 7r-cyclopentadienyl, and carbonyl hydride clusters, and references to these studies have been included in Tables II, VIII, and III, respectively. 

Fig. 3.5 ESI-MS spectra of Se-cystamine and Se-ethionine. Molecular information from ESI-MS. The total molecular masses of two Se compounds are detected. Structural information (two Se in the molecule) is additionally available without any further analytical efforts due to appearance of the isotope pattern of two (Se-cystamine = SeCM) or one (Se-ethionine = SeE) Se atom(s) in the molecule, each scanned around the total molecular mass. (From Michalke et al. 1999 and Lindemann and Hintelmann 2002 both reproduced with permission from Springer.)...

The general procedure to find out the isotopic pattern of a molecule is straightforward. 

Some halogens. Cl and Br, and metals, e.g., Sn, Pt, and Pd, significantly alter the isotope patterns of small molecules... 

MS has a unique advantage in that the two exchange mechanisms can be readily distinguished in the isotope patterns of the deuterated peptides (see also Sections 1.3.1 and 3.2.2). In the EX2 mechanism, the local structure will unfold and refold many times until an exchange event happens. Therefore, the deuterium labeling on each backbone amide site of a peptide will be independent to one another. As a result, the distribution of deuterium across the molecules will be uncorrelated and result in a unimodal isotope distribution. 

Although a spectral feature reveals the mass of an ion, the value is not necessarily accurate enough for rehable identification of the corresponding molecule. In low-resolution mass spectra, the isotopic pattern of an ion may be buried within a single spectral feature. The measurement of m/z may result in an observation closer to the average mass than the exact mass of the ion (as discussed in Section 5.3.2). Even in the case of a well-resolved isotopic pattern, mass measurement may still be affected by the width and shape of the spectral feature. Deconvolution of spectral features contaminated with multiple ions of similar mass is important for accurate molecular identification but beyond the scope of this book. Here we will focus on the determination of accurate masses of single-component samples. 

When applying ESI-MS for the characterization of polymers, a high spectral resolution is beneficial. This allows the isotope pattern of multiply ionized peaks to be resolved. Even after a separation by, for example, SEC, copolymers may stiU give rise to very complex ESI-MS spectra, because many different molecules may elute at the same retention time,. ain, high-resolution MS is desirable. MS-MS is also an interesting option. 

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. 

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