Tal Alon and Aviv Amirav, Tel Aviv University and Aviv Analytical
Electron Ionization (EI) mass spectra contain a vast amount of information that can be considered during an identification procedure. It might be surprising to know that most of us examine and utilize only a part of that information and as a result decrease the probability of a correct identification and in many cases cannot identify the analytes at all. It might also be surprising to know that it is rather easy to consider more of the data and improve identification results.
- The mass spectrum of the analyzed compound is not in the library's database (and this is obviously the case for the majority of new synthetic compounds)
- It often fails in cases of acute noise and co-elution, two cases in which the ions in the mass spectrum originate from several different sources that are not the analyte itself
- It fails when the fragmentation pattern is too generic and can be related to a class of compounds.
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In order to better exploit the extra information found in mass spectra, including isotope related information, we developed the Tal-Aviv Molecule Identifier (TAMI) software.
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In such a case it is rather easy to perform an independent analysis of the isotopomeric pattern and look for molecules with theoretical patterns that are similar. The isotopomeric pattern is calculated from the elemental composition of any compound and therefore no library of patterns is needed and such an analysis encompasses all possible compounds. Figure 4 shows the results provided by the TAMI software after the analysis of the experimentally measured isotope pattern of TATP. It has found that the most resembling theoretical (calculated) pattern is that of C9H18O6 which is the correct elemental formula of TATP.
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Alternatively, the same procedure can increase the probability of successful library identifications as shown in figure 5 where the Dodecane's probability of identification is increased from 42% to 80% thanks to the similarity of the measured isotopomeric pattern to that of Dodecane's (0.197% difference).
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Quadrupole Mass Accuracy
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If we measure a mass that is closer to the sides of this distribution, for example when we analyzed C22H40 at m/z 304.3. The filter is even more effective, as it filters out more of the otherwise relevant identification candidates.
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With this combination the Library's results are automatically scrutinized and if the identification appears to be wrong the molecular ions are analyzed with the abovementioned independent tools with good results.