Tal Alon (1,2) and Aviv Amirav (1,2)
1. School of Chemistry, Tel Aviv University, Tel Aviv 69978 Israel.
2. Aviv Analytical LTD, 3 Haarad Street, Tel Aviv 69107 Israel.
Table 1 presents selected values found on the black plot in figure 2, at various points which are compared to real GC-MS measurements. The NIST library mass spectra were searched by the NIST library and naturally gave 999 matching factors while as shown in Table 1 their identification probabilities were not 100% due to high matching factors of competing non-Dimethoate compounds.
A similar 'rise and fall' pattern can be seen in the identification probability provided for real measured spectra (right column of table 1) and both software simulated and measured data shows improved identification at higher Cold-EI level of relative abundance (better than Standard EI).
3.2 – Eicosane
Figure 5 shows the results of molecular ion enrichment manipulation (gray line) and MI + fragments enrichment manipulation (dark line) for Eicosane. The different manipulated mass spectra were tested by the NIST MS library and the identification probabilities were plotted vs the relative abundance of the molecular ion. The same rise and fall behavior shown above for Dimethoate is demonstrated here as well, showing that a bigger molecular ion relative abundance produces better identification results but only up to a certain point from which the identification probability decreases. This graph shows the important contribution of the fragments enhancement, which decrease this downward slope and postponed it. The mass spectrum which was most similar to Cold-EI in terms of the relative abundance is already on the downwards slope of the black line due to the highly enhanced molecular ion and high mass fragments, but NIST still identifies it better (higher probability) than it does the standard EI mass spectrum.
As can be seen in table 2 below, the downward slope provided by enhanced molecular ion in Cold-EI mass spectra in reality is much more moderate, giving the Cold-EI mass spectrum an impressive identification probability of 80.2% vs the 35.9% of Standard EI.
|Table 2 – Some interesting simulation and measurement numbers obtained from the black plot line in figure 5. In both real and simulated data, the identification is better when a Cold-EI mass spectrum is used instead of standard EI mass spectrum.|
4. Discussion and Conclusions