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41


GCxGC with Soft Ionisation and High-Resolution Mass Spectrometry Applied to Petroleum Biomarker Analysis


Robert B. Cody*, Masaaki Ubukata, Yoshihisa Ueda and A. John Dane on behalf of JEOL (USA) Inc and JEOL Ltd.


Abstract


Soft ionisation (chemical ionisation, field ionisation and photoionisation) methods are used in combination with comprehensive two-dimensional gas chromatography (GCxGC) coupled with high-resolution time-of-flight mass spectrometry (HRTOFMS). These techniques provide molecular weight and elemental composition information that is often absent from electron ionisation mass spectra and can simplify the extracted ion current (EIC) chromatograms in GCxGC-HRTOFMS analysis of complex mixtures. Examples are presented of field ionisation and photoionisation for biomarker identification in crude oil.


Introduction


Comprehensive two-dimensional gas chromatography (GCxGC) [1-3] coupled with high-resolution time-of-flight mass spectrometry (HRTOFMS) is a powerful tool for the analysis of complex mixtures [4-7]. By using two columns with different stationary phases, GCxGC can achieve separations that cannot be attained with one-dimensional gas chromatography while HRTOFMS provides an additional dimension of separation by providing accurate masses for each mass-to-charge ratio.


Electron ionisation (EI) is widely used to produce mass spectra characterised by fragment ions that can be searched against mass spectral databases for compound identification. However, fragmentation adds an additional level of complexity in interpreting complex mixtures, such as crude oil. Furthermore, molecular ions can be weak or absent in the EI mass spectra of many compounds (e.g. alkanes, fatty acid esters, alcohols). Although the mass spectral databases contain entries for hundreds of thousands of compounds, these represent only a small fraction of all compounds that can be analysed by gas chromatography.


Soft ionisation methods suitable for GC-MS and GCxGC-MS include chemical ionisation (CI), photoionisation (PI) and field ionisation


(FI). Each has its respective strengths and weaknesses.


Chemical ionisation is the most common soft ionisation method for GC-MS. This technique can be very sensitive for trace analysis and can be used for electron capture negative ion formation. However, CI requires reagent gases so ionisation, adduct formation and fragmentation are gas- and compound-dependent.


Field ionisation [8] [9] is the softest ionisation method for GCxGC-MS. For this technique a high voltage is applied to the emitter, which is a wire coated with tiny carbon ‘whiskers’. As the vaporised sample passes close to the tip of the ‘whiskers’ on the emitter, it experiences a high electric field, and electrons tunnel from the sample to the emitter, producing a positive ion. FI produces almost exclusively molecular ions for ‘difficult’ compound classed like alkanes and alcohols, and fragmentation is typically weak or absent. If a sample is deposited directly on the field emitter, direct probe analysis is possible. This type of experiment is referred to as field desorption (FD) [10] to distinguish it from FI, in which the sample is introduced in the gas phase. FI/FD emitters are fragile and expensive, but they can be reused hundreds of times. Also, FI is about an order of magnitude less sensitive than


EI. Modern FI/FD sources on time-of-flight mass spectrometers are quite reliable and do not suffer from the high-voltage arcing problems that plagued FI/FD sources on earlier generation magnetic sector mass spectrometers. JEOL offers a combination EI/FI/FD source that permits switching between ionisation modes by using a probe and vacuum interlock to easily exchange the EI repeller and the FI or FD emitter.


Vacuum photoionisation [11-13] with a deuterium lamp ionises compounds with ionisation energies lower than the photon energy (10.8 eV). PI is simple to use with the JEOL combination EI/PI source by turning on the lamp and turning off the EI filament current. PI is more sensitive than FI for certain compound classes (e.g. PAH’s), but less sensitive for others (e.g. alkanes). It also produces slightly more fragmentation than FI but simpler spectra than CI.


These methods can be combined with high-resolution exact-mass data to simplify the processing of complex GCxGC data (e.g. petroleum type analysis), provide clear molecular weight and elemental composition that can be combined with EI data to identify unknowns, and provide selective ionisation of target compounds. Although GCxGC-HRTOFMS with soft ionisation is applicable to any GCxGC


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