Interpretation isotope patterns

All evidence of isotope patterns in the product ion spectrum of the m/z 304 parent (Fig. 19, bottom panel) disappears because the selected parent is monoisotopic—CiqHisNOF Cli. Justification for the absence of isotope patterns is identical to that of interpretation of the m/z 300 product ion spectrum. The detailed explanation of this excellent example serves to illustrate the care one needs to exercise in interpreting isotope patterns (or the lack thereof) in MS-MS spectra. At the same time, it demonstrates the structure-indicating power of careful interpretation. 

With the recent development of the combined quadrupole TOP tandem instrument geometry, MS-MS product ion spectra are starting to appear in the literature with isotope patterns reflecting the natural abundance of the involved elements. Considering the discussion above, how does this occur Note that our selection of parent ions for CID in the experiments described above was done under unit resolution conditions. With a tandem instrument composed of a quadrupole and a TOP mass analyzer, the resolution of the first mass analyzer can be set to substantially less than unit resolution, such that the entire isotope pattern can be selected as the parent. This approach preserves the isotope pattern information while providing MS-MS product ion information. This new consideration must be added while interpreting isotope patterns in MS-MS spectra—namely to identify on what kind of instrument the experiment is being done, and how the instrument is set up. 

Qualitatively, the spark source mass spectrum is relatively simple and easy to interpret. Most instrumentation has been designed to operate with a mass resolution Al/dM of about 1500. For example, at mass M= 60 a difference of 0.04 amu can be resolved. This is sufficient for the separation of most hydrocarbons from metals of the same nominal mass and for precise mass determinations to identify most species. Each exposure, as described earlier and shown in Figure 2, covers the mass range from Be to U, with the elemental isotopic patterns clearly resolved for positive identification. 

In order to successfully interpret a mass spectrum, we have to know about the isotopic masses and their relation to the atomic weights of the elements, about isotopic abundances and the isotopic patterns resulting therefrom and finally, about high-resolution and accurate mass measurements. These issues are closely related to each other, offer a wealth of analytical information, and are valid for any type of mass spectrometer and any ionization method employed. (The kinetic aspect of isotopic substitution are discussed in Chap. 2.9.)... 

Even if the analyte is chemically perfectly pure it represents a mixture of different isotopic compositions, provided it is not composed of monoisotopic elements only. Therefore, a mass spectrum is normally composed of superimpositions of the mass spectra of all isotopic species involved. [11] The isotopic distribution or isotopic pattern of molecules containing one chlorine or bromine atom is listed in Table 3.1. But what about molecules containing two or more di-isotopic or even polyisotopic elements While it may seem, at the first glance, to complicate the interpretation of mass spectra, isotopic patterns are in fact an ideal source of analytical information. 

Note Mass spectrometers usually are delivered with the software for calculating isotopic distributions. Such programs are also offered as internet-based or shareware solutions. While such software is freely accessible, it is still necessary to obtain a thorough understanding of isotopic patterns as a prerequisite for the interpreting mass spectra. 

Note The assignment of empirical formulae from accurate mass measurements always must be in accordance with the experimentally observed and the calculated isotopic pattern. Contradictions strongly point towards erroneous interpretation of the mass spectrum. 

The platinum has a complicated isotope pattern as shown. The spectrum corresponding to the peak A in the HPLC trace can be interpreted easily as corresponding to cisplatin. Indeed, its molecular weight calculated from the first isotopes of both platinum and chlorine is 194 + (2 x 35) + (2 x 17) = 298 u. Adding 23 u for a sodium ion adduct leads to the observed m/z 321 peak, accompanied by the expected isotopes. 

NMR is the only method which yields complete isotopic patterns, an absolutely non-adulterable characteristic of a natural product. Due to the fact that these patterns can more and more be interpreted by biochemical correlations ]33], they are actually the most potent base for the assignment of a compound to its climatic origin and authenticity. 

The interpretation of SSMS data falls into two distinct areas — element Identification and estimates of quantity. The criteria used in our laboratory for positive elemental identification are the presence of the doubly ionized species and the Identification of the Isotopic pattern when possible. Quantitation will be discussed later. Last, the Instrument source must be cleaned regularly to avoid memory problems. Our approach to the memory problem is to have a complete set of source parts for each matrix. A set of parts are dedicated for silicon analyses, another for gallium arsenide, etc. These parts and the source Itself are cleaned on a regular and frequent basis. When these factors are under control, SSMS has proved to be a reliable, reproducible technique for the bulk analysis of trace impurities. 

Mass spectrometry is the most important way of determining the molecular formula. Characteristic patterns arising from different isotopes aid the interpretation. Fragmentation patterns can give information about the structural arrangement and are also usefiil for fingerprinting. 

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