Development of a Novel Mass Spectrometric ... - Jacobs University
Development of a Novel Mass Spectrometric ... - Jacobs University
Development of a Novel Mass Spectrometric ... - Jacobs University
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Introduction<br />
review <strong>of</strong> the work done in this field will be reported. Research efforts have<br />
investigated the potential <strong>of</strong> various ionisation methods to create intact product<br />
ions representing the neutral composition <strong>of</strong> complex mixture. It follows that the<br />
choice <strong>of</strong> ionisation method plays a key role towards a rational detailed analysis <strong>of</strong><br />
crude oil samples.<br />
1.3.1 EI and CI<br />
Traditionally low-energy EI and CI ionisation were used for petroleum analysis. 44-<br />
48 EI (70 eV) produces extensive fragmentation <strong>of</strong> the ionised hydrocarbon<br />
molecules. Nevertheless characteristic fragment ions <strong>of</strong> petroleum were developed<br />
to provide valuable crude oil assay for light or medium hydrocarbons. However,<br />
this method can’t be used to identify each type <strong>of</strong> hydrocarbons as well as it is not<br />
accurate if sample contains olefins or heteroatom containing compounds. 49 The<br />
obtained molecular weight information becomes rather complicated due to<br />
fragmentation and difficulties in identifying molecular ions. Also since molecules<br />
are brought to gas phase by thermal vaporisation that defines EI and CI methods,<br />
high boiling point molecules <strong>of</strong> hydrocarbons can’t be detected thereby.<br />
1.3.2 ESI, DESI and MALDI<br />
Other s<strong>of</strong>t ionisation techniques such as ESI, APCI, APPI and MALDI have also<br />
taken part in the analysis <strong>of</strong> individual or complex mixtures <strong>of</strong> hydrocarbons. For<br />
numerous applications in mass spectrometry ESI is widely used as an ionisation<br />
method. It has been used to evaporate and ionise polar compounds <strong>of</strong> petroleum<br />
containing functional groups with nitrogen or oxygen atom. 41,50-56 The technique<br />
can successfully vaporise and ionise hydrocarbon analytes and produce<br />
pseudomolecular ions without fragmentation. The molecules in positive molecular<br />
ion formation are either protonated (basic compounds) or deprotonated (acidic<br />
compounds). With ESI FT ICR combination allows a compact mass spectral<br />
display for visual resolution <strong>of</strong> up to thousands <strong>of</strong> peaks. However polar<br />
hydrocarbons are only a small portion <strong>of</strong> petroleum (10%). ESI is ‘blind’ to other<br />
major nonpolar hydrocarbon fraction <strong>of</strong> petroleum especially saturate and aromatic<br />
11