Isotope peak

By experimentally determining the ratio of abundances of C and isotope peaks for CO2 dissolved in sea water at various temperatures, a graph can be drawn relating the solubility of CO2 compared with that of CO2 (the ratio described above). On extracting the CO2 from sediment containing the shells (calcium carbonate) of dead sea creatures by addition of acid, a ratio (R) of abundances of CO2 to CO2 can be measured. If this value is read from the graph, a temperature T is extrapolated, indicating the temperature of the sea at the time the sediment was laid down. Such experiments have shown that 10,000 years ago the temperature of the Mediterranean was much as it is now. 

In a mass spectrum, the ratios of isotopes give a pattern of isotopic peaks that is characteristic of a given element. For example, the mass spectrum of any corn ound containin carbon, hydrogen, nitrogen, and oxygen will show patterns of peaks due to the, 7C, 7N, gO, gO, and... 

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. 

Isotope peaks can be very informative in GC/MS analysis. Generally for interpretation, one focuses on the monoisotopic peak. The monoisotopic... 

Adamantanes (Ci0H)5), C6-C18 n-alkylbromides (CH2)4Br (with an isotope peak at m/z 137)... 

You have calculated the resolution required to separate the molecular ion of atomio oomposition C284H432N84O79S7 from the isotopic peak containing one atom. Carry out a similar exerolse to oaloulate the resolution required to separate the equivalent Ions when they eaoh have (a) 5 charges, (b) 7 charges, and (c) 10 charges. Compare the values obtained. 

ESI mass spectra of mixtures are difficult to interpret, because each component produces ions with many different charge states. The most direct and reliable method to solve this problem is to use high-resolution MS and calculate the charge states by measuring the spacing of the isotope peaks. ESI mass spectrometry of (polymeric) mixtures with broad molecular weight distribution benefits from a prior separation that reduces the polydispersity of the analyte. 

The molecular peak of the spiro[adamantine-2,2 -(l,3,4)-thiadiazolidine]-3, 4 -dicarboxylate 29 appears with 99% intensity and shows four distinct fragmentations <1999T12783>. The molecular formulas of the fragments were confirmed by the intensities of 13C and (34S+13C2) isotope peaks. The two major fragmentations involve cleavage of the weak C-S bonds and afford the base peak at mjz 208 (100%) corresponding to and a peak at m/z 239... 

Table 9.12 Intensities of isotope peaks (relative to the parent peak) for combinations of bromine and chlorine...

Rule 3). The size of the M + 2 peak indicates the absence of sulphur and halogens and the empirical formula C9H10O2 given in Beynon s tables best fits the isotope peak ratios. The number of saturated sites is 9 + 1 - 5 = 5, i.e. one ring and four double bonds. 

The mass spectrum of CTC, shown in Figure 14 (42), is characterized by a reasonably intense molecular ion at m/e 478 with the concomitant isotope peak at P+2 representing one chlorine atom in the ring system. Although it has been suggested that this chlorine atom be employed as a tracer via the isotope ratio for detection of species containing... 

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... 

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

Figures 5.19b and 5.19c represent the mass spectra of compounds with single chlorine and bromine atoms, respectively. Roughly, in the case of chlorine the ratio of intensities of A and A + 2 peaks is 3 to 1, and in case of bromine, 1 to 1. One should take into account that the presence of several atoms of A + 2 elements in the molecule results in the appearance of intense peaks M + 4, M + 6, etc., that is, the presence of two or more atoms of A + 2 elements in an ion again gives a unique ratio of isotopic peaks in the...

Therefore, the relative intensities of the peaks in the multiplet will be 9 24 22 8 1, or after normalization, 37.5 100 92 33 4. This example demonstrates that the intensity of the M peak may be notably lower than that of the isotopic peaks. 

Formally oxygen is also an A + 2 element. However, the natural abundance of the lsO isotope is only 0.2% of that of the main isotope ieO. That is why reliable calculation of the number of oxygen atoms based on the intensity of the isotopic peaks is hardly possible. Nevertheless, it is quite possible to estimate this number. Thus, if the intensity of the M + 2 peak in the mass spectrum of a sample with a low number of carbon atoms is higher than 0.5% relative to M+, this compound may contain one or more oxygen atoms. 

The presence of one carbon atom in a molecule of carbon dioxide results in registration of the molecular ion peak of m/z 44 and of the A + 1 isotopic peak of m/z 45. The intensity of the latter is 1.1 % of that of M+. It appears due to the presence of 13C02 molecules. An increase in the number of carbon atoms in a molecule leads to an increase of the intensity of the M + 1 ion peak to 1.1 n% of M+, where n is the number of carbon atoms in the molecule. To calculate the number of carbon atoms in a molecule using a mass spectrum one should divide the intensity of the M + 1 peak as a percentage of M by 1.1. The result defines the maximum possible number of carbon atoms. One should remember that calculations may be more complicated if an [M — H]+ ion peak is present. 

A large number of carbon atoms in a molecule leads to an increase in the possibility of the simultaneous presence of two or more 13C atoms, resulting in a higher intensity of the isotopic peaks [M + 2], [M + 3], etc. (Table 5.5). For each atom of another element... 

The theoretic intensity of isotopic peaks due to the presence of several 13C isotopes may be easily calculated for any molecule. The intensity of the A + 2 peak (as a percentage of the A peak) for a compound with n carbon atoms may be established by the following formula ... 

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 9. Estimate the number of carbon atoms in the following compounds using 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. 

Unknown 12. Calculate the intensities of the molecular ion isotopic peaks for the compounds listed below ... 

---