Isotopic ions

Precise measurement of isotope ratios can be obtained by comparing the yields of isotopic ions desorbing from a sample placed on a strongly heated filament that is generally made from platinum, tantalum, rhenium, or tungsten. 

Isotopic ion. Any ion containing one or more of the less abundant naturally occurring isotopes of the elements that make up its structure. 

Isotopic molecular ion. A molecular ion containing one or more of the less abundant naturally occurring isotopes of the atoms that make up the molecular structure. Thus, for ethyl bromide there exist molecular isotope ions such as CCHjBi, C2H4DBi , C2H5 Bi, C2H5 Bi, etc. 

Different isotopes differ in their atomic masses. The intensities of the signals from different isotopic ions allow isotopic abundances to be determined with high accuracy. Mass spectrometry reveals that the isotopic abundances in elemental samples from different sources have slightly different values. Isotopic ratios vary because isotopes with different masses have slightly different properties for example, they move at slightly different speeds. These differences have tiny effects at the level of parts per ten thousand (0.0001). The effects are too small to appear as variations In the elemental molar masses. Nevertheless, high-precision mass spectrometry can measure relative abundances of isotopes to around 1 part in 100,000. 

For the confirmatory procedure, it is recommended that the sponsor develop spectral data based on at least three structurally specific ions that completely define the marker residue molecule. These ions may or may not include the molecular ion. The use of water loss and isotopic ions is usually unacceptable and CVM concurrence should be sought when water loss ions or isotopic ions are selected for the confirmatory analysis. The proposed fragment ion structures should be consistent with the fragmentation pattern, and justification for specificity of selected ions or scan range should be included. All confirmation criteria should be specified in the standard operating procedure. 

Spectral data based on at least three structurally specific ions fhaf completely define the parent molecule (may or may not include the molecular ion), or more if nonspecific ions are included. Use of water loss and isotopic ions is discouraged. 

Q Categorial variable characterizing chemical entities (species) being investigated, e.g., elements, isotopes, ions, compounds ... 

Example From the tribromomethane spectrum (Fig. 3.9) a mass difference of 79 u is calculated between m/z 250 and m/z 171, which belongs to Br, thus identifying the process as a loss of a bromine radical. Starting from the CH r2 Br isotopic ion at m/z 252 would yield the same information if Br was used for the calculation. Use of Br would be misleading and suggest the loss of H2Br. 

As we have already seen, the isotopic mass also is the exact mass of an isotope. The isotopic mass is very close but not equal to the nominal mass of that isotope (Table 3.1). Accordingly, the calculated exact mass of a molecule or of a mono-isotopic ion equals its monoisotopic mass (Chap. 3.1.4). The isotope C represents the only exception from non-integer isotopic masses, because the unified atomic mass [u] is defined as of the mass of one atom of nuclide C. 

It is a chemical ionization-free interface (unless operated intentionally) with accurate reproduction of the expected isotope ion abundances. 

Figure 9.24 b compares the isotope ion distribution of natural 40Ca+ and doped 44Ca+ in a cross section of a root of a Norway spruce. These measurements were carried out by SIMS using 69Ga+ primary ions (lOkeV, 0.1 nA, Ionoptika) with a lateral resolution < 0.1 pm.80 The investigations can be used to study the mechanism of mineral element uptake and kinetics of transport processes in plants as a function of time. About 20 min after the start of the tracer experiment a barrier was observed for the transport of enriched isotope 44Ca+ in the middle of the root.80... 

An ion containing a less abundant combination of isotopes, also included under P.I.D., is not classified separately because identification is usually more simple from the more abundant isotopic combination. The mass number and relative abundance of isotopic ions can be calculated from the accompanying table. It might be argued that classification of these could be useful where the more abundant isotopic combination is obscured by another ion of nominally identical mass. This, however, will be an unusual circumstance and can be overcome by careful use of the table or by the use of exact empirical structure determination through high resolution techniques. 

Fig. 5. Observed (solid line) and simulated (dashed line) isotopic ion distributions for the (M + BPh4)+ ion in the positive ion FABMS of [Au6Pt(PPh3)7](BPh4)2 (149).

Uranium in nature may be measured either radiometrically or chemically because the main isotope - 238U - has a very long half life (i.e., relatively few of its radioactive atoms decay in a year). Its isotopes in water and urine samples usually are at low concentrations, for which popular analytical methods are (1) radiochemical purification plus alpha-particle spectral analysis, (2) neutron activation analysis, (3) fluorimetry, and (4) mass spectrometry. The radiochemical analysis method is similar in principle to that of the measurement of plutonium isotopes in water samples (Experiments 15 and 16). Mass spectrometric measurement involves ionization of the individual atoms of the uranium analyte, separation of the ions by isotopic mass, and measurement of the number of separated isotopic ions (see Chapter 17 of Radioanalytical Chemistry text). 

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