Molecular ion and isotopic peaks

Table 8.6 in Section 8.5 can be used to determine what the ratio of the intensities of the molecular ion and isotopic peaks should be when more than one chlorine or bromine is present in the same molecule. The mass spectra of dichloromethane (Fig. 8.51), dibromomethane (Fig. 8.52), and l-bromo-2-chloroethane (Fig. 8.53) are included here to illustrate some of the combinations of halogens listed in Table 8.6. 

Unfortunately, it is not always possible to take advantage of these characteristic patterns to identify halogen compounds. Frequently the molecular ion peaks are too weak to permit accurate measurement of the ratio of the intensities of the molecular ion and isotopic peaks. However, it is often possible to make such a comparison on certain fragment ion peaks in the mass spectrum of a halogen compound. The mass spectrum of 1-bromohexane (Fig. 8.47) may be used to illustrate this method. The presence of bromine can be determined using the fragment ion peaks at m/e values of 135 and 137. 

Unfortunately, it is not always possible to take advantage of these characteristic patterns to identify halogen compounds. Frequently, the molecular ion peaks are too weak to permit accurate measurement of the ratio of the intensities of the molecular ion and isotopic peaks. However, it is often... 

These results show that the reproducibility for determining the peaks of the molecular ion and its isotope by ordinary GC-MS is satisfactory and applicable for practical use. It is expected that this method of isotope analysis can also be applied to organic compounds which have a wide range of molecular weight. 

For example, the presence of bromine can be determined easily, because bromine causes a pattern of molecular ion peaks and isotope peaks that is easily identified. If we identify the mass of the molecular ion peak as M and the mass of the isotope peak that is two mass units heavier than the molecular ion as M -t- 2, then the ratio of the intensities of the M and M+2 peaks will be approximately one to one when bromine is present (see Chapter 8, Section 8.5, for more details). When chlorine is present, the ratio of the intensities of the M and M + 2 peaks will be approximately three to one. These ratios reflect the natural abundances of the common isotopes of these elements. Thus, isotope ratio studies in mass spectrometry can be used to determine the molecular formula of a substance. 

The example of ethane can illustrate the determination of a molecular formula from a comparison of the intensities of mass spectral peaks of the molecular ion and the ions bearing heavier isotopes. Ethane, C2H6, has a molecular weight of 30 when it contains the most common isotopes of carbon and hydrogen. Its molecular ion peak should appear at a position in the spectrum corresponding to a mass of 30. Occasionally, however, a sample of ethane yields a molecule in which one of the carbon atoms is a heavy isotope of carbon, This molecule would appear in the mass spectrum at a mass of 31. The relative abundance of in nature is 1.08% of the atoms. In the tremendous number of molecules in a sample of ethane gas, either of the carbon atoms of ethane will turn out to be a atom 1.08% of the time. Since there are two carbon atoms in ethane, a molecule of mass 31 will turn up (2 x 1.08) or 2.16% of the time. Thus, we would expect to observe a peak of mass 31 with an intensity of 2.16% of the molecular ion peak intensity. This mass 31 peak is called the M+ peak, since its mass is one unit higher than that of the molecular ion. 

Most abundant ion mass the mass that corresponds to the most abundant peak in the isotopic cluster of the ion of a given empirical formula. Average mass the mass of an ion for a given empirical formula calculated with the atomic weight of each element (e.g., C = 12.01115 and H = 1.00797) that is, the average of the isotopic masses of each element, weighted for isotopic abundance. The average mass represents the centroid of the distribution of the isotopic peaks of the molecular ion and is used by chemists in stoichiometric calculations. 

GC-NCI-MS and GC-EI-MS are the approaches more frequently used for PBDE analyses. Mass spectra strongly depend on the type of ionization used. NCI mass spectra of all PBDEs were dominated by the bromine ion [Br] and did not show any molecular ion. In contrast, El provided better structural information, giving the molecular ions and the sequential losses of bromine atoms. For NCI-MS experiments, the two ions corresponding to ra/z = 79 and 81 ([Br]") were monitored, whereas for El-MS experiments, the two most-abundant isotope peaks for each level of bromination, corresponding to the molecular cluster for mono- to tri-BDEs and [M-Br2] for tetra- to hepta-BDEs, were selected. 

One example is the high resolution ESI-FT-ICR MS analysis of a mixture of verapamil and peptide A. Abbreviated segment of the mass spectrum of verapamil and peptide A (Fig. 10.1) shows the molecular ion peak of peptide A, as well as its accompanying isotope peaks (Fig. 10.3). The following valnes were obtained for the monoisotopic molecular ion and the isotopic molecular ions using an external calibration (calculated mass mass error in ppm) (829.5393 0.1 ppm), (830.5423 1.3 ppm), (831.5450 1.0 ppm), and (832.5476 1.2 ppm). Similar results were obtained for verapamil (data not shown). 

LOD, defined here as the lowest quantity of analyte injected on-column to be detected with respect to peak identification criteria of S/B > 3, plus verification of the isotope abundance ratio for at least two isotopologs of the molecular ion and retention time consistency in both GC dimensions, was 0.5 pg for 2,3,7,8-TeCDD (Figure 11.24). This represents an improvement by a factor of 5-10 compared with conventional GC-TOFMS but is an order of magnitude higher than for GC-HRMS instruments (typically 0.04pg for 2,3,7,8-TeCDD). 

A metal M forms a carbonyl M (CO) which contains 18.40% of carbon and in the mass spectrometer shows a molecular ion, the isotopic pattern of which shows its most intense peak at m/z = 652. The carbonyl reacts with triphenyl-phosphine to produce two compounds A, which has a metal— phosphorus ratio of 1 1, and B, for which the corresponding ratio is 2 1. A reacts with bromine to form a single product C, whereas B with bromine produces C and D. D, which is identical with the sole product from the reaction of M (CO) with bromine, reacts with triphenylphosphine to produce E, which is isomeric with C. C and D, but not E, have a axis of symmetry. All the compounds are diamagnetic. 

Reaction of a complex chloride of A, K ACl, with copper and carbon monoxide formed D, a carbonyl chloride of A. In the mass spectrometer D showed a molecular ion, the isotopic pattern of which showed its most intense peak at m/e = 362. The cracking pattern showed a stepwise loss of five carbon monoxide groups. 

Molecular ion Dominant base peak in phenyl thiocyanate. Odd mass for odd number of N atoms in the molecule. Characteristic isotope peak at [M+2] " and [Frag+2] for S-containing fragments (per S atom 4.4% relative to M" "). 

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