ABSTRACT

Comprehensive two-dimensional gas chromatography is a powerful technique capable of separating compounds in a retention plane according to their volatility and polarity. The orthogonal nature of this separation technique is ensured by using two columns having different retention mechanisms so the physical and chemical-physical properties of the analytes, such as boiling point and shape/polarity, act independently during the separation in the two columns.  This feature is particularly advantageous for complex mixtures containing families of homologous compounds since the results are aligned into the retention plane in very organized and recognizable patterns, making their identification much easier and reliable. 

In this work developed by Thermo Fisher Scientific, some petrochemical samples have been analyzed for group characterization.  The capabilities of HyperChrom Data System (chromatography software from Thermo Scientific), have been shown for qualitative and quantitative group-type analysis for complex hydrocarbons mixtures. 

                                                                                                                                                                             INTRODUCTION

Complex hydrocarbon mixtures are very suitable samples for exploiting not only the highest separation power of the GCxGC technique, but also its capability to generate chromatograms with highly organized patterns on the two dimensional retention plane [1].

The oil industry is taking great advantage in using this technique, especially regarding group characterization, rather than the detailed analysis of each component of the mixture. If the total number of compounds contained in an oil fraction is enormous, typically the contents of homologous classes are limited, and this group-type characterization is fundamental for correlating the product composition with its physical and chemical properties [2].

The compounds which are part of homologous families are usually aligned to form straight lines according to boiling point along the first retention axis and to polarity along the second retention axis. In particular, within the same carbon number, branched homologues show a typical “roof tile” pattern [1], which becomes more evident for classes featuring higher retention in the second dimension.

Such types of homologous compound clustering greatly aid in the qualitative interpretation of the chromatogram, since the 2D plot can give an immediate perception of the sample composition. In a similar fashion, the quantitative evaluation of the relative contents of different groups also becomes simpler when the chemical classes are well separated into the 2D retention plane. In addition, quantitative results obtained with the FID have already proven to be as good as conventional GC [3].   For this purpose, the HyperChrom Data System has been used, and results are illustrated for the analysis of different petrochemical fractions (See figure below).

EXPERIMENTAL

Thermo Scientific GC instrumentation, provided with a CO₂ Dual-Jets modulator [4], has been used for the analyses. The modulator is entirely controlled by the GC, allowing for the setting and storing of the modulation time parameter as part of the GC method. Moreover, the embedded electronics allow synchronization of the modulation with the acquisition frequency.

The GCXGC was configured with the SSL injector and the FID as detector with an acquisition frequency up to 300 Hz. Thermo Scientific’s AI3000 autosampler has been used for automating sample injections.

The HyperChrom Data System has been used for acquisition and data handling for group-type analysis. The calculation of the relative contents of different groups was achieved by graphically enclosing the homologous compounds in the same box. This operation is made easier by the intrinsic capability of the GCxGC technique to group homologous compounds in organized patterns, which can be easily identified. In practice, a component table is built up with all of the groups of interest by drawing a box around each group. A total peak area is then automatically calculated by summing the area of all the peaks found within the same given box and the relative content of each group is expressed as Area %. 

Two column sets were used for the majority of the work:

A) 1D column: 30m x 0.32mm i.d., 0.25mm df; 2D column: BPX-50 0.8m x 0.1mm i.d., 0.1mm df

B) 1D column: 30m x 0.32mm i.d., 0.25mm df; 2D column: DB-17 1m x 0.1mm i.d., 0.2mm df. 

A prototype of Fast Flame Photometric Detector (FFPD) has also been used for sulfur selective response, with a sampling rate of 200Hz.

RESULTS AND DISCUSSION

Different oil fractions were analyzed using the different column sets, choosing the second dimension column in order to get the proper groups separation [5].  The HyperChrom Data System allowed for the compilation of a component table by grouping compounds belonging to homologous families. As an example, the analysis of a Light Cycle Oil is reported in the figure below, which indicates the identification of some groups.

The possibility to use a selective detector as the Flame Photometric Detector also permits to highlight and confirm the presence of sulfur compounds, such as thiophenes and benzothiophenes.  If grouping is done in a more detailed way, as shown for paraffines and naftenes according to the carbon number, further grouping, according to the family, is still possible. The relative content of each group is calculated by the data system as area percentage, as shown by the generated report.

Conclusion

Comprehensive two-dimensional GC provides in one run much more information than conventional gas chromatography for the characterization of complex hydrocarbons mixture. The capability of the technique to generate an orthogonal separation on two different columns allows the homologous compounds to be grouped in bands on the retention plane, according to their volatility along the first dimension and polarity along the second dimension. This makes the group identification much easier, giving an at-a-glance, immediate qualitative information regarding the sample composition. Moreover, the HyperChrom data system provides a valid and easy-to-use tool for quantitative calculation, permitting a comprehensive characterization of petrochemical fractions.