Isotopic peaks elemental composition

One can get an enormous amount of information from studying the region of the molecular ion in a mass spectrum. The mass of M+ is the molecular mass of the analyte. The ratio of the isotopic peaks (see below) allows one to roughly establish the elemental composition, while accurate mass measurements using high resolution mass spectrometry give exact elemental composition. The relative intensity of the M+ peak... 

The exact mass of an ion (4 to 6 decimal points) reliably defines its elemental and isotopic composition, while the method is called high resolution mass spectrometry. The measurements are conducted manually or automatically (computerized). Manual measurements are based on the parallel acquisition of the peak of interest with the closest peak of an ion with the known composition. Any compound with an intense ion peak with m/z value in the region +10% may serve as a marker. The most widespread markers are perfluorokerosene, perfluorotributylamine, and other polyfluorinated compounds. The use of these compounds is based on their volatility, as well as on the fact that fluorine is a monoisotopic element. In the spectra of these compounds intense ion peaks randomly cover all the range between m/z 19 and M+. ... 

Determination of the Elemental Composition of Ions on the Basis of Isotopic Peaks... 

Establishing the elemental composition based on the isotopic peaks may be problematic if, for example, the sample contains impurities with the masses in the region of the molecular ion cluster. In the El mass spectra of amines, alcohols, acids, and some other classes of organic compounds there is often a peak of [M + H]+ ion. It distorts the isotopic picture. It is worth mentioning as well that in real experimental conditions the peak intensity may vary slightly in each... 

Unknown 10. Establish the elemental composition of the molecules on the basis of the intensities of the isotopic peaks. In the majority of cases the spectra are represented as m/z value (intensity to the base peak in the spectrum, %). The molecular peak is the first in the row. 

Using the intensities of the isotope peaks try to identify the elemental composition of all the ions (where it is possible). Calculate the unsaturation degree of these ions. 

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. 

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. 

Isotopic patterns provide a prime source of such additional information. Combining the information from accurate mass data and experimental peak intensities with calculated isotopic patterns allows to significantly reduce the number of potential elemental compositions of a particular ion. [31] Otherwise, even at an extremely high mass accuracy of 1 ppm the elemental composition of peptides, for example, can only be uniquely identified up to about 800 u, i.e., an error of less than 0.8 mmu is required even if only C, H, N, O and S are allowed. [27,32,33]... 

It has been pointed out that routine accurate mass measurements are conducted at resolutions which are too low to separate isobaric isotopic compositions in most cases. Unfortunately, coverage of multiple isotopic compositions under the same signal distort the peak shape. This effect causes a loss of mass accuracy when elemental compositions have to be determined from such multicomponent peaks, e.g., if the monoisotopic peak is too weak as the case with many transition metals. The observed decrease in mass accuracy is not dramatic and the loss of mass accuracy is counterbalanced by the information derived from the isotopic pattern. However, it can be observed that mass accuracy decreases, e.g., from 2-3 mmu on monoisotopic peaks to about 4—7 mmu on multicomponent signals. 

The molecular ion peak directly provides valuable information on the analyte. Provided the peak being of sufficient intensity, in addition to mere molecular mass, the accurate mass can reveal the molecular formula of the analyte, and the isotopic pattern may be used to derive limits of elemental composition (Chaps. 3.2 and 3.3). Unfortunately, the peak of highest m/z in a mass spectrum must not necessarily represent the molecular ion of the analyte. This is often the case with El spectra either as a result of rapidly fragmenting molecular ions or due to thermal decomposition of the sample (Chaps. 6.9 and 6.10.3)... 

Although convenient at first sight, the lack of fragment ion peaks in FI spectra also means a lack of structural information. If more than an estimate of the elemental composition based on the isotopic pattern is desired, collision-induced dissociation (CID, Chap. 2.12.1) can deliver fragment ions for structure elucidation. Fortunately, the fragmentation pathways of IVT ions in CID are the same as in EI-MS (Chap. 6). 

The next higher mass peak, M+l, provides information on elemental composition. Table 22-1 lists the natural abundance of several isotopes. For carbon, 98.93% of atoms are, 2C and 1.07% are l3C. Nearly all hydrogen is H, with 0.012% 2H. Applying the factors in Table 22-2 to C Hm, the intensity of the M+l peak should be... 

Each part of this problem is quite long and best worked by groups of students.) Peak intensities of the molecular ion region are listed in parts (aMg) and shown in the figure below. Identify which peak represents the molecular ion, suggest a composition for it. and calculate the expected isotopic peak intensities. Restrict your attention to elements in Table 22-1. 

In LC-MS analysis, identification of lipids requires the use of accurate mass measurements for the determination of elemental composition, combined with MS" experiments to get information of the molecular fragments. In addition, in LC-MS, one metabolite may produce multiple peaks due to the presence of isotopes, adducts, and neutral loss fragments. Therefore, ion annotation is crucial in order to recognize a group of ions likely to originate from the same... 

These isotopes are responsible for the peaks in the mass spectrum appearing as isotopic clusters that are characteristic of the elemental composition. They provide important analytical data. Indeed, even without exact mass measurement, the possibilities for elemental composition determination can often be restricted by using isotopic abundance data. For example, the fragments CioH2o and C x 11 i 2O2, both with a nominal mass of 140 u, produce peaks at mass 141 with 11 and 8.8 %, respectively, of the abundance at mass 140 u. This is the result of a different statistical probability of having 13C isotopes. These two elemental compositions can thus be distinguished in a mass analysis. 

Because most elements consist of a mixture of stable isotopes (Table 2.3), isotope peaks are observed in mass spectra. For a given elemental composition, the isotope pattern can be predicted using a computer program. The equidistant peaks in an isotope pattern, i.e., at one m/z unit for a single-charge ion, represent a series of ions with relative abundances, that should closely agree with the theoretically predicted values. 

Even with a lower-resolution mass spectrometer we can get hints about the elemental composition. Molecular ions (M) have higher weight peaks at M+1, M+2 due to minor isotopes, which can tip you off to their presence. Common examples are shown below (with the intensity of the taller peak set at 100%). Hydrogen, nitrogen, oxygen, fluorine, and iodine have no significant isotope peaks (<0.4%). The presence of chlorine or bromine is important to identify in the mass spectrum. 

This analysis can be extended to the other small peaks in the mass spectrum, and one will find some ion resulting from fragmentation of the complexes during ionization. We will not go into detail here, but rather conclude that the careful analysis of the exact mass and the isotope pattern provides information on the elemental composition and with it on the stoichiometry of the complex (e.g. 2 2... 

The composition of the Earth was determined both by the chemical composition of the solar nebula, from which the Sun and planets formed, and by the nature of the physical processes that concentrated materials to form planets. The bulk elemental and isotopic composition of the nebula is believed or usually assumed to be identical to that of the Sun. The few exceptions to this include elements and isotopes such as lithium and deuterium that are destroyed in the bulk of the Sun s interior by nuclear reactions. The composition of the Sun as determined by optical spectroscopy is similar to the majority of stars in our galaxy and, accordingly, the relative abundances of elements in the Sun are referred to as "cosmic abundances". Although the cosmic abundance pattern is commonly seen in other stars, there are dramatic exceptions, such as stars composed of iron or solid nuclear matter, as is the case with neutron stars. The best estimation of solar abundances is based on data from optical spectroscopy and meteorite studies and in some cases extrapolation and nuclear theory. The measured solar abundances are listed in Fig. 2-1 and Table 2-1. It is believed to be accurate to about 10% for the majority of elements. The major features of the solar abundance distribution are a strong decrease in the abundance of heavier elements, a large deficiency of Li, Be, and B, and a broad abundance peak centered near Fe. The factor of 10 higher... 

A molecular compound (which consists of an immense population of individual molecules) gives a mass spectrum on which appears several molecular peaks (isotopic cluster at M, M - -1, M - - 2, etc.), whose intensities reflect elemental and isotopic composition of the compound. 

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