10
Analytical Instrumentation
The Rapid Determination of Benzene and Toluene in Finished Gasolines Containing Ethanol Using Gas Chromatography
A. Tipler, PerkinElmer® , Shelton, Connecticut, USA
The determination of benzene and toluene in gasoline is an important application in the petrochemical industry to monitor and ensure that levels of these compounds meet regulatory requirements. Gasoline is a highly complex sample matrix and, in this instance, we want to just see the benzene and toluene content in among the hundreds of other components present. Traditionally, this analysis has been performed by gas chromatography using packed columns in a mechanical valve backflush configuration as described in ASTM method D3606-07. A non-polar precolumn allows the volatile fraction of an injected gasoline sample (which contains benzene and toluene) to elute into a second column which is polar. The heavier sample fraction, which was left in the precolumn, is removed by backflushing the precolumn out to vent. The polar analytical column retains the benzene and toluene more than the volatile hydrocarbons that also eluted into this column and so enables these aromatic compounds to be fully separated from all other components in the gasoline. A polar internal standard is added to the gasoline prior to analysis to improve the quantitative precision. Such methods have been used for many years but the relatively recent introduction of high concentrations of ethanol in modern gasolines has caused peak-coelution issues on the polar analytical column. Alternative column sets have been developed to improve the chromatography but these have extended the analysis time.
This article describes a similar approach to this analysis as used in the ASTM method but using high resolution capillary columns. Interference effects from ethanol have been totally eliminated and the chromatography is complete within 4 minutes while still meeting the performance requirements of ASTM method D3606-07.
Analytical Approach
Figure 1 shows a diagram of the system used for this analysis. The packed columns were replaced with narrow-bore capillary columns. A split/splitless injector was used to introduce the sample into a non-polar precolumn. A SwaferTM
system was used Figure 1: Analytical system used for the determination of benzene and toluene in gasoline
to manage the transfer from the precolumn to a polar analytical column and to control the backflush process. Although D-3606 specifies the use of a thermal conductivity detector (TCD), in this work a flame ionization detector (FID) was used as it was more suited to high-resolution capillary chromatography and did not give baseline drift as column pressures were changed during the backflushing process. A fused silica restrictor was connected between one of the S-Swafer outlet ports and the same detector as connected to the analytical column. This provided two benefits: the gas flow rate through the first column could now be higher than through the analytical column giving flexibility in the method and, because the chromatography on the precolumn is complete before the first peak elutes from the analytical column, the detector could record both chromatograms in the same run. This makes set up much easier and provides extra confidence in the results because the precolumn chromatography is now visible in all the runs.
Figure 2 shows a chromatogram of a standard mixture without backflushing applied. The backflush point is easily identified – just after the last peak of interest has eluted from the precolumn.
Figure 3 shows the final chromatography with backflushing applied of a sample of 87-octane gasoline. 2-butanol internal standard is added to the gasoline. Even though there are hundreds of components in this sample, the method is able to eliminate all of these potential interferences from the benzene, toluene and 2-butanol internal standard peaks in just four minutes. The total analytical cycle time which includes autosampler loading and equilibration times is approximately 5.4 minutes.
The peak shapes are very symmetrical, the resolution between all the peaks is very high and the baseline is flat – these are all attributes needed for a rugged and reliable method.
Figure 2: Chromatogram of a standard mixture without backflushing. Annual Buyers’ Guide 2011
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