BTEX Determination in Gasoline Containing Ethanol by Single Oven MDGC

Introduction

The determination of BTEX in gasoline is usually performed in accordance with the standard test method ASTM D3606 using gas chromatography. However, particularly in the presence of ethanol, co-elutions are observed in one-dimensional chromatography. In this application note a method is described applying multidimensional GC to overcome the separation problem.

Multidimensional GC

In multidimensional GC two columns of different properties are coupled to each other. In this way, compounds that co-elute on one stationary phase can be separated on another column, as only this part of the chromatogram is sent to the second column and detector. Separation of other compounds in the sample remains unchanged and they are monitored at the first detector.

By using the recently developed Multi-Deans Switching technique the outlet pressure for the first column remains unchanged even when switching is performed. This enables an easy multiple heart-cut method set-up without any retention time shifts in the monitor chromatogram.

Instrumentation

GC-2010 with MDGC switching package and two FID-2010

Experimental Conditions

Table 1: Switching time interval.

Results

Figure 1 shows a chromatogram of a gasoline sample containing ethanol and BTEX monitored on the first FID (no cuts).

Figure 1

If the same sample is measured in cut mode, the following chromatogram is obtained on the 2nd FID (Figure 2).

Figure 2

To obtain a good chromatogram on the 2nd FID it is important that retention time shifts do not occur on the 1st chromatogram, otherwise definition of the correct cut times is very difficult.

By using the recently developed Multi-Deans Switching technique the outlet pressure for the first column remains unchanged even when switching is performed. This enables an easy multiple heart-cut method set-up without any retention time shifts in the monitor chromatogram.

Figure 3 shows a comparison of the monitor chromatogram on the 1st FID with (purple) and without (black) cutting to the 2nd FID. It is clear that the retention times for the gasoline peaks are the same, regardless of whether cutting occurred or not.

Figure 3

Conclusions

The multidimensional GC method is well suited to the analysis of BTEX and ethanol in gasoline samples. Co-elutions observed in one-dimensional GC can be overcome using a newly developed Multi-Deans Switching device in the column oven, which transfers selected cuts to a second separation column and second FID without any retention time shifts in the first dimension.

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