Carbon Isotopic Patterns

The occurrence of the elements carbon, nitrogen, and oxygen manifests itself in the isotope patterns occurring for all molecular or fragment ions. For small numbers of carbon atoms, the... 

Partial mass spectra showing the isotope patterns in the molecular ion regions for ions containing carbon and (a) only one chlorine atom, (b) only one bromine atom, and (c) one chlorine and one bromine atom. The isotope patterns are quite different from each other. Note how the halogen isotope ratios appear very clearly as 3 1 for chlorine in (a), 1 1 for bromine in (b), and 3 4 1 for chlorine and bromine in (c). If the numbers of halogens were not known, the pattern could be used in a reverse sense to decide their number. 

The isotope patterns for two simple organometallic compounds in the molecular ion region (a) dimethylmercury and (b) dimethylplatinum. The seven isotopes of mercury show clearly and appear quite different from the six isotopes of platinum. Since there are only two carbon atoms, the contribution from C is negligible. 

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. 

Fig. 3.2. Calculated isotopic patterns for carbon. Note the steadily expanding width of the pattern as X+2, X+3, X+4,... become visible. At about C90 the X-i-1 peak reaches the same intensity as the X peak. At higher carbon number it becomes the base peak of the pattern.

Example The isotopic pattern related to the elemental composition of ethyl propyl thioether, C5H12S, (Chap. 6.12) is shown below. The contributions of S and C to the M+1 and of S and C2 to the Mh-2 signal are indicated (Fig. 3.6). If the M+1 peak resulted from C alone, it would indicate rather the presence of 6 carbon atoms, which in turn would require an M+2 intensity of only 0.1% instead of the observed 4.6 %. The introduction of Si to explain the isotopic pattern would still fit the M+2 intensity with comparatively low accuracy. For M+1, the situation would be quite different. As Si alone demands 5.1 % at M+1, there would be no or 1 carbon maximum allowed to explain the observed M+1 intensity. 

Example The presence or absence of the polyisotopic element tin (Table 3.1) can readily be detected from its characteristic isotopic pattern. In case of tetrabutyltin, Ci6H3gSn, the lowest mass isotopic composition is CieHse Sn, 340 u. Due to the 16 carbon atoms, the isotopic abundance is about 17.5 %. This is superimposed on the isotopic pattern of elemental Sn, which becomes especially... 

Example Fullerene soots as obtained by the Huffman-Kratschmer synthesis of fullerenes can be characterized by positive- as well as negative-ion LDI. [115] The LDI-TOF spectrum of such a sample exhibits fullerene molecular ion signals well beyond m/z 3000 among these, Ceo and 070" are clearly preferred (Fig. 10.11). Furthermore, such samples provide experimental carbon-only isotopic patterns over a wide mass range (Chap. 3.2.1). 

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. 

The mass determination of ionic species (atomic or polyatomic ions) in mass spectrometry is always a comparative measurement, which means the mass of an ionic species is determined with respect to reference masses of elements (or substances) used for mass calibration. The reference mass is thus acquired from the mass unit (m = In = 1/12) of the mass of the neutral carbon isotope (m = 1.66 X 10 kg). A mass calibration is easy to perform in solid-state mass spectrometry if the sample contains carbon (using carbon cluster ions with whole masses, as discussed above). The so-called doublet method was apphed formerly, e.g., ions and doubly charged Mg + forming a doublet at the same nominal mass number 12 were considered, where they are slightly displaced with respect to one another. The doublet method is no longer of relevance in modern inorganic mass spectrometry. Orientation in the mass spectra can be carried out via the matrix, minor and trace elements after mass calibration and by comparing the measured isotopic pattern of elements with theoretical values. 

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