4 May / June 2019
High-resolution MS coupled to Photoionisation-GC×GC for Petrochemicals Characterisation
by Author: Anupam Giri, SABIC, GTC-EUR Analytical Technology, Plasticslaan 1, 4612PX, Bergen op Zoom, The Netherlands. Email:
Anupam.GIRI@
sabic.com Tel: +31 164293212 Web:
www.sabic.com
The trend in shifting focus from petrochemical refining to the downstream industry brings new analytical challenges for advanced characterisation of feedstocks, including structural elucidation of hydrocarbons, heteronate impurities and functional biomarkers in petroleum products. Comprehensive GC (GC×GC) as a separation technique coupled with high resolution (HR) MS is almost invariably required to tackle such complex matrices. Soft ionisation coupled to GC×GC-HRMS can add several other dimensions on top of GC×GC separation for in-depth elucidation. A new generation soft-ionisation technique, photoionisation (PI) has shown to enhance analyte speciation by providing superior data on both molecular ions and structurally significant fragment ions at low-energy PI mass spectra.
Introduction
The analysis of hydrocarbon classes as well as heteroatom impurities in different feedstock has long been an established practice in chemical industries as this information is used as input feed vector to calculate the cracking value of the feedstock, processability evaluation and also as indication for predicting possible fouling. The majority of those low-to-middle distillate feedstock is easily amenable via GC using classic gas or liquid sample introduction techniques. However, the sheer number of compounds in such distillate petrochemical samples quickly exceeds the available peak capacity of single-column GC techniques [1]. Consequently, the use of multidimensional separation techniques is essential. Despite the superior separation achieved by GC×GC, the identification of individual compounds in complex petrochemicals remains challenging where the majority of compounds in a hydrocarbon class displays similar skeleton-spectra at conventional EI (70 eV) ionisation energies. Moreover, new generation feedstock, like pyrolysis oil from mixed plastic waste, often brings complexities, as those are highly abundant with heavily branched alkanes, alkenes, olefins and diolefins together with numerous groups of heteroatom (O, N, S, and Cl etc.) impurities.
Although mass spectral fragmentation is useful for molecule identification via
Figure 1: GC × GC-TOF/MS contour plots of pyrolysis oil sample for EI and PI revealing selectivity towards different ion sources as visualised by 2D plots using reverse phase column configuration (1 retention time; 2
tR tR : second dimension retention time).
comparison of fragmentation patterns to spectral libraries, for many chemical classes, particularly for hydrocarbons, identification of compounds with isomeric structure or alkyl series is often ambiguous due to lack of molecular ion information [2]. One of the major approaches to ensure high yields of molecular ions is lowering ionisation energies (or soft ionisation) by providing minimal excess internal energy to the compounds resulting in limited or no fragmentation. Among several soft-ionisation techniques available for GC-MS like field ionisation (FI), chemical ionisation (CI), cold EI, variable ionisation [3], increasingly popular studies support the view that photoionisation (PI) may become a major universal and standard soft ionisation technique [4]. The enhanced sensitivity and selectivity stemming from the dramatic reduction in fragmentation at low energies
in PI greatly increases the number of compounds identified, permitting effective comparisons and chemical fingerprinting of petrochemicals. High resolution and tandem mass spectrometry are very often and effectively used in conjunction with gas chromatography as mass spectrometers with high resolving power becoming more and more available, affordable and popular. The correct usage of three virtues: resolving power, sensitivity and high data acquisition speed creates the opportunity for novel GC×GC methods with improved reliability and lower detection and quantitation limits for both targeted and non-targeted analyses.
In this study, the application of photoionisation (PI) coupled with GC×GC- HRTOF-MS was evaluated. This soft- ionisation technology was shown to enhance analyte speciation by providing superior
: first dimension
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