The Use of GCxGC-TOFMS and Classifications for the Quantitative Determination of Different Compound Classes

Introduction

One of the important challenges facing hydrocarbon analysts is the accurate determination of the amounts of different classes of compounds in complex samples. For example a typical aliphatic hydrocarbon sample may contain linear hydrocarbons ranging over twenty or more carbon numbers, as well as a plethora of branched and cyclic hydrocarbon isomers over a similar range of carbon numbers. One dimensional gas chromatographic (GC) analysis is inadequate for this task, even when using long specialized columns, and Time-of-Flight Mass
Spectrometry (TOFMS), as can be seen in Figure 1.

Figure 1. GC-TOFMS Analysis of the Complex Aliphatic Hydrocarbon Sample 1.

The sample complexity precludes accurate assessment of all the different classes of hydrocarbon components, even when using advanced techniques such as GC-TOFMS and the powerful deconvolution algorithm available through the ChromaTOF^® software package. Comprehensive two-dimensional gas chromatography (GCxGC) coupled to Time-of-Flight mass spectrometry (TOFMS) can play a significant role in handling samples where complexity is a key issue. The increased peak capacity of GCxGC, coupled with the powerful deconvolution software available in the ChromaTOF software package used to operate LECO Pegasus^® and TruTOF^® systems, allows the co-elution always present in complex hydrocarbon samples to be minimized. Where it does occur, the software handles this in such a way that compound identification and quantitation are not compromised. In addition, the Classification feature provides a powerful tool for differentiation and quantitation of compound classes.

Experimental Conditions

Samples Two samples (1 and 2) were used for analysis which contained linear, branched and cyclic alkanes over a significant range of carbon numbers. No aromatic components were present in the samples. Both samples contained over 1000 components. Analysis Conditions The correct choice of columns is a prerequisite for successful GCxGC analysis of complex hydrocarbon samples. The column set should provide good separation of all the components, it should be thermally robust with the ability to handle the elevated temperatures needed for successful chromatography of the less volatile components, and it should make good use of the total chromatographic space of the analysis. This is of particular importance in the samples used in this analysis. Because of the sample complexity and the extremely large number of branched and cyclic hydrocarbons present in the samples it was found that the customary "non-polar" – "polar" column combination did not provide suitable component separation. The final column set used and the conditions for the analyses are shown in Table 1.

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