Bromine common isotopes

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. 

You must be careful when studying molecules containing chlorine or bromine atoms, since these elements have two commonly occurring isotopes. Chlorine has isotopes of 35 (relative abundance = 75.77%) and 37 (relative abundance = 24.23%) bromine has isotopes of 79 (relative abundance = 50.5%) and 81 (relative abundance = 49.5%). When these elements are present, take special care not to confuse the molecular ion peak with a peak corresponding to the molecular ion with a heavier halogen isotope present. 

For four of the elements—oxygen, sulfur, chlorine, and bromine—the principal heavier isotope is two mass units greater than the most common isotope. 

Molecules containing chlorine or bromine can readily be identified as these halogens have two common isotopes in a known ratio. 

Table 1.1 Isotopic abundance of common elements. Interesting to note is that chlorine and bromine have t /o naturally intense isotopes.

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. 

Mass spectrometers determine atomic and molecular isotope ratios. Table 2 lists the relative isotopic abundance of elements commonly encountered in pharmaceutical analysis [3,4]. The values in Table 2 have been empirically determined and refinements in the values are necessary as atomic mass measurements improve, but for this discussion any inaccuracies in the table are insignificant. For some elements there are only two naturally occurring isotopes. For example, if you were to randomly sample carbon atoms in nature, 99% of the time you would find 12C, and roughly 1% of the time a 13C would turn up. Other elements, such as chlorine and bromine, have elemental isotope ratios that are not as heavily... 

An equally useful technique, and the one most commonly used, is to add a scavenger, such as iodine, bromine or oxygen. If the concentrations of these are kept sufficiently low, they will not interfere with hot reactions but they will scavenge thermal tritium atoms and the thermal addition products of olefins. They will also suppress the concentration of radicals produced by the over-all radiation to the system during isotope production. Examples of scavenger action are given in Table 4. 

Table 2.3 shows the elements commonly found in organic compounds and the relative abnndances of their isotopes. Of these elements, only fluorine, phosphorus, and iodine are monoisotopic. Most elements are mixtures of two or more stable isotopes, differing in mass by 1 or 2 Da. Most elements include one major isotope (greater than 90 percent relative abundance), but chlorine and bromine have two rather abundant isotopes separated by 2 Da. 

Absolute shielding data for the quadrupolar halogen nuclei are not yet available. Spin rotation interaction constants have however been determined for GIF [153 154 155], HCl [156], CICN [157], HBr [158] and HI [159]. In order to make use of these data to establish an "absolute" shielding scale for the magnetic chlorine, bromine and iodine isotopes it would be necessary to determine the halogen chemical shift of the gaseous compounds relative to a common reference such as an aqueous sodium halide solution. The experimental problems in such measurements are however by no means trivial. 

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