- The molecular ions of hydrocarbons are weak and their relative abundances fall exponentially with the number of carbon atoms as shown in our previous note. In addition, the molecular ion is further reduced in branched isomers and this reduction can be by an additional one order of magnitude. Consequently, the intensity of reconstructed (extracted) molecular ions (RSIM) mass chromatograms is weak and noisy and often with insufficient intensity.
- The relative abundance of the molecular ions is strongly dependent on the isomer branching and carbon number (size) and thus isomer distributions via the molecular ions does not provide appropriate quantitative information on the relative abundance of isomers.
- In view of the complexity of fuels and oils compositions and the weakness of the molecular ions and high mass fragments in standard EI mass spectra, isomer structural mass spectral information is obscured and cannot be properly exposed and used.
- While the molecular ions are weak and the mass spectra are dominated by low mass fragments, high mass fragments are also often more abundant than the weak molecular ions. Thus, high mass spectral fragments of branched isomers of bigger hydrocarbons have fragments with M-15, M-29 and M-43 and these fragments have C13 isotopomers with M-14, M-28 and M-42 that have the same mass as smaller size hydrocarbons. As a result, the mass chromatograms of a given hydrocarbon with standard EI are contaminated by interference from other hydrocarbons and cannot be trusted to provide accurate isomer distributions.
|Figure 1. Standard EI (upper) and Cold EI (bottom) mass spectra of n-C16H34 in the m/z=150-250 mass spectral range.|
|Figure 2. Standard EI (upper) and Cold EI (bottom) mass spectra of a highly branched isoprenoid C16H34 compound in the m/z=150-250 mass spectral range as obtained from a fuel-like complex hydrocarbon mixture. (Click to Enlarge Image)|
- The intensity of the molecular ion in standard EI is too weak hence its mass chromatogram has poor quality and insufficient signal to noise ratio for any quantification. In contrast, the Cold EI ion statistics is at least two orders of magnitude better than that of standard EI. Note that the TIC numbers should be ignored as these experiments were performed using two different systems with x12 higher gain in standard EI relative to Cold-EI.
- The mass chromatogram of standard EI is highly contaminated by other non C16H34 hydrocarbons that elute after the peak of linear chain n-C16H34. This later elution implies that the isomer distribution in the appropriate elution time window could be contaminated by isomers of other bigger hydrocarbons.
- While not shown in Figure 3, the ratio of the mass chromatogram peak of n-C16H34/iso-C16H34 was bigger in standard EI by a factor of almost four than in Cold EI, as a result of the highly non uniform relative abundance of the molecular ions in standard EI.