Aviv Amirav, Tel Aviv University and Aviv Analytical Ltd.
- Response uniformity: The response uniformity of the HES is not good even at 300ºC and for example its cholesterol peak height is only ~6% of that of the n-C16H34. Clearly the HES response is reduced with the sample compounds size (as with other EI ion sources) and 300ºC is required and even such high ion source temperature is not enough. On the other hand, Classical EI-SMB exhibits uniform compound independent response which is important for quantitation of unknowns and for the evaluation of chemical reaction yields.
- Signal strength: The HES exhibits very high signal for 1 ng relatively volatile compounds such as n-C16H34 (the first to elute compound among the four). Furthermore, the total ion count (TIC) signal to noise for the n-C16H34 is approaching 10,000 which is impressive. However, while the HES TIC sensitivity is very high for the volatile n-C16H34 its TIC signal to noise ratio for cholesterol (and n-C32H66) is only ~200. On the other hand, in Classical EI-SMB the cholesterol TIC signal to noise ratio is ~1600 and it is similarly high for all compounds including for the sample impurities. A one way to compare EI-HES and Classical EI-SMB is the case of OFN which serves for GC-MS sensitivity specifications. The test mixture included 10 pg OFN (on-column amount) that was detected with the 5977B-HES with S/N (RMS) = 4800 using RSIM on m/z = 272 +- 0.3 and S/N (RMS) = 12600 while using m/z=272+-0.05. In Classical EI-SMB the S/N was infinite in both RMS and PTP using m/z = 272 +-0.3 since the noise was zero (no single ions noise).
3.2 Classical EI-SMB Mass Spectra
The classical EI-SMB mass spectrum is the red upper mass spectrum in Figure 2 while the library mass spectrum is the bottom blue one. The left side of the figure shows that hexadecane is rated as #1 in the NIST library hit list with matching and reversed matching factors of 915 and identification probability of 40.7% which is considered as high for hydrocarbons since they all share similar fragmentation patterns. Clearly the Classical EI-SMB mass spectrum is visibly similar to that of the NIST library EI mass spectrum and thus the Classical EI-SMB ion source provides classical EI mass spectra and as a result with it there is no need for any additional EI ion source.
Table 1. A comparison of NIST library based sample identification parameters by EI with the HES ion source and by Classical EI-SMB.
We note that the identification of the two amides in the Classical EI-SMB mass chromatogram by the NIST library as hexdecanamide and octadecanamide is of importance. These compounds were not found at all in standard EI with the HES even at the estimated ~20 pg on-column amounts, probably since they are reactive with the ion source metallic surfaces. The origin of these amides impurities is unknown and probably they emerge from the solvent that we used. The importance of this observation is that it demonstrates another benefit of Classical EI-SMB that uses contact-free fly-through ion source and thus any compound degradation on the standard EI in sources such as HES is inherently eliminated in it. Thus, Classical EI-SMB actually extends the range of compounds that are amenable for GC-MS analysis to include underivatized fatty acids, amides and low levels of alcohols and steroids.
While both Classical EI-SMB and Cold EI mass spectra exhibit some molecular ion the much more abundant Cold EI molecular ion facilitates better provision of its elemental formula with the TAMI software that is based on the conversion of experimental isotope abundance and the measured mass with the limited quadrupole mass accuracy (http://www.avivanalytical.com/Isotope-Abundance.aspx). We note that about 33% of the NIST library compounds do not show molecular ions in classical EI and for those with molecular weight greater than 400 amu over 50% do not exhibit molecular ions and thus for these compounds the Cold EI provision of trustworthy molecular ions is critical for their proper identification. In addition, Cold EI often improves the NIST library identification probabilities as described in details in T. Alon and A. Amirav "How Enhanced Molecular Ions in Cold EI Improve Compound Identification by the NIST Library" Rapid. Commun. Mass. Spectrom. 29, 2287-2292 (2015). One example is dotriacontane (n-C32H66). As shown in table 1 this compound failed to be identified by the NIST library with its EI-HES mass spectrum while in Classical EI-SMB it was #3 in the NIST library hit list via its 1% molecular ion abundance. In contrast, in Cold EI due to its combination of dominant molecular ion and availability of all the fragment ions dotriacontane was identified by the NIST library as #1 in the hit list with impressive 73% identification probability.