6. SPECIAL TECHNIQUES FOR SELECTIVE COMPOUND
6.4. Strategies for Olefin Analysis
Analysis of olefins is important in the petroleum industry due to their possible adverse effects on the quality of petroleum products. However, the complicated
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mixture containing olefins produced from catalytic and coking processes such as FCC (discussed above) presents a serious analytical challenge for any method, including ‘‘classical’’ GC–MS. These materials span the same boiling range as paraffins and cycloparaffins (naphthenes) of the same carbon number and have an exponentially larger number of isomers than the paraffins, which makes them difficult to separate by even the highest-resolution GC columns. In addition, ole- fins have similar mass spectral patterns as cycloparaffins under EI conditions, making them difficult to differentiate from each other. High-resolution MS will not work because the exact masses of these species are the same as those of corresponding cyclic saturate species.
Olefins bind preferentially to silver ions. Supercritical fluid chromatogra- phy with silver-impregnated columns has therefore been employed to separate olefins from saturates and aromatics in the same sample [85]. Once the olefins have been isolated, they can be analyzed by conventional GC–MS to determine the distribution of various carbon numbers and isomers. In the analysis, the aro- matics, olefins, and saturates are resolved by SFC as three separate peaks. With careful switching between the two SFC columns to elute one fraction at a time through the septum into an unmodified GC–MS injection port, the fraction is trapped cryogenically on the GC column head. The trapped fraction is analyzed by GC–MS prior to subsequent fractions being introduced onto the injector [86].
Figure 14 shows the GC–MS profiles of the three different SFC fractions. All analyses were performed on one 10-àl injection and data taken into the same data file with all three instruments under the control of the MS data station.
Olefins do exhibit selective chemistry, and many derivatization procedures have been attempted on these either prior to analysis or in the ionization chamber to achieve olefin selectivity. Acetone CI gives preferential acetylation at the dou- ble bond, which can be used to differentiate olefins from cycloparaffins [87].
Classically, olefins are derivatized prior to analysis as part of the sample prepara- tion procedure. Some of these methods are possible in a complex mixture such as gasoline, most notably the bis-alkylthiolation of the double bond using a re- agent such as dimethyldisulfide [88,89]. This interesting technique has the advan- tage of adding two methyl sulfide groups into a molecule, possibly pushing the GC retention time out of that observed for the rest of the sample. The fragmenta- tion pattern is indicative of starting double bond position, an advantage over hydrogenation. Hydrogenation of olefins can be easily carried out by exposing the gasoline to hydrogen gas in the presence of a palladium (Pd) catalyst. Figure 15 shows the results from an experiment where 20 mg 1% Pd on C powder was put onto the frit in the injection port of a GC, and then hydrogen used as the carrier gas to hydrogenate the olefins on-line. With on-line hydrogenation, the olefinic components in the 112-Da ion current trace disappear, leaving cyclopar- affins unchanged. The alkane distributions shown in the bottom two 114-Da ion
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Figure 14 Supercritical fluid chromatography–gas chromatography–mass spectroscopy chromatograms. Top chromatogram shows the SFC-FID chromatogram, bottom chromato- gram shows the GC–MS TIC. Gasoline, 10àl is injected into a column-switching super- critical fluid chromatograph [85] that elutes the aromatic, saturate, and olefin fractions as three separate peaks under computer control. The SFC effluent is split between the FID detector and a capillary, which pierces the septum of an unmodified GC–MS injection port. The injection port is set at 5000 : 1 split to release the 100% CO2SFC mobile phase.
During a SFC peak, the split is turned off, and the sample cryogenically trapped on the GC column at⫺40°C (the CO2passes through the GC column). When the SFC peak is over, the split is turned back on so helium flows through the GC column, and the GC temperature ramped to acquire the GC–MS data on that specific SFC cut. When the fast GC–MS run is over and the gas chromatograph cool, the supercritical fluid chromatograph is allowed to elute the next peak. All data are acquired in the same run into the same data file, with the SFC, GC and MS all controlled by the MS data system. Conditions: Dionex (Sunnyvale, CA) supercritical fluid chromatograph, Varian 3400 gas chromatograph (Var- ian, Palo Alto, CA) with 20 m⫻0.10 mm ID DB-1 column (0.10-àm film thickness), Finnigan SSQ710 mass spectrometer in 70-eV EI mode scanning 20 scans/sec in full- scan mode.
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current traces are the same with and without hydrogenation. This procedure serves to identify olefin GC peaks by observing their disappearance, but destroys all information about the location of the double bond.