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43


column 2. Although the EI chromatogram shows higher background and column bleed than the FI and PI chromatograms, the chromatograms are otherwise identical.


EIC chromatograms To identify cholestane isomers (C27


H48


) in


the crude oil, we created an extracted ion current (EIC) chromatogram (referred to as ‘Selected Ion Chromatogram’ in the GC Image software) for the exact mass for the molecular ion (m/z 372.3756). However, fragmentation in the EI mass spectra is problematic [18] because of interference from the heavy isotope peaks for the fragment ions from higher cholestane homologues. For example, loss of a methyl group from 14-methyl cholestane (C28


H50 Figure 2. 2D Chromatograms for GCxGC-HRTOFMS analysis of a standard crude oil sample.


produces a fragment ion in the EI mass spectrum with the composition C27


H47 13 ) . The C isotope peak for this fragment is found


at m/z 372.3712, which requires a resolving power of over 200,000 to distinguish it from the cholestane molecular ion C27


H48 +• .


However, this resolving power is not available for mass spectrometers that can be operated at the high speeds required for GCxGC.


As shown in Figure 1, field ionisation and photoionisation of cholestanes do not exhibit fragmentation to C27


H47


+. Therefore,


Figure 3. (A) Electron ionisation EIC chromatograms for C27 12C26


13C1 H47 for C27H48 (D) Photoionisation EIC chromatogram for C27H48 H48 + isotope peak from the methyl loss fragment from C28


(m/z 372.3756) showing artifacts from the H50


do not exhibit methyl-loss fragments and are therefore free of artifacts.


Both the PI and FI mass spectra show abundant molecular ions with drastically reduced fragmentation. Little or no fragmentation is observed for the FI mass spectra, but the PI mass spectra show a characteristic C16


H26 sterane fragment[16] at m/z 218.204.


Figure 4. Twenty isomers of cholestane identified in the field ionisation GCxGC-HRTOFMS analysis of acrude oil sample.


The summed EIC chromatograms for the homologous series Cn


H2n-6 (n=19-30, Figure 5) in the PI data show


androstane, sterane and related compounds. The EIC chromatogram for the common C16


H26 fragment


(Figure 6B) shows high selectivity but with slightly less sensitivity due to the lower relative abundance of the fragment ion.


The 2D total-ion chromatograms for GCxGC- HRTOFMS of the standard crude oil sample using EI, FI and PI are shown in Figure 2. The x-axis represents separation by boiling point on column 1 and the y-axis represents separation by polarity on


the FI and PI EIC chromatograms for m/z 372.3756 do not exhibit any interference from isotope peaks of fragment ions of higher homologues. Figure 3 compares the electron ionisation EIC chromatogram for m/z 372.3756 with the FI and PI EIC chromatograms for the same m/z. Although


. (B) Field ionisation EIC chromatogram


from the same sample, (C) EI mass spectrum from one of the methyl cholestanes circled in (A) and from the same sample. The PI and FI EIC chromatograms


the EI data show artifact peaks from 13


C1 12C26 H37 +, these artifacts are absent in the


FI and PI data. Twenty isomers of cholestane can be clearly identified in the artifact-free FI and PI data (Figure 4).


Figure 5. Summed EIC chromatograms for the homologous series Cn


H2n-6 (n=19-30, mass tolerance +/- 0.01Da).


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