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February / March 2012
Figure 4: Chromatograms showing separation of a lubricant product methanol extract using different column coatings. The red box indicates the bulk of nonpolar hydrocarbon oil matrix, blue indicates bulk of polar anti-oxidant and anti-ware additive components.
preparation required to achieve good analysis. A more detailed discussion of this type analysis has been published by Ragonse et al [7]. In the current work the elution order of palmitic (C16:0) methyl ester as first major peak of soya methyl ester and docosane (C22 n-alkane) as the last major peak of diesel respectively, was investigated. The advantages of using a highly polar column in this analysis can be clearly seen from Figure 3. On a non polar HP5 phase, C16:0 elutes after C19, but as polarity is increased C22 elutes more quickly relative to C16:0. So that on a wax phase C16:0 elutes after C22, on IL61 after C23, on IL76 after C24 and after C26 on IL100. Contribution of saturated alkanes after C22 is low and very low after C26, interferences from high boiling mono and poly aromatics are also low at these carbon numbers.
Figure 5: Flow modulated comprehensive GCxGC chromatogram of B20 biodiesel. Retention time in the second dimension is highlighted by arrows.
An alternative application utilising the same principles as discussed above is the analysis of methanol extracts of fully formulated lubricating oils. As previously described, lubricants are complex mixtures of hydrocarbons and additional components designed to carry out specific functions. These additives are typically polar in nature finding application as surfactants, anti- oxidants and anti-wear compounds. Base oil and additives co-elute using HP5 columns as shown in Figure 4. The increased polarity of the wax column improves the situation significantly but not fully. Separation is only achieved using the more polar IL76 and IL100 ionic liquid phases. This separation of the analytes of interest allows for a number of improvements: more accurate quantitation - less baseline noise due to low levels of hydrocarbon matrix, greater ease of chromatogram overlaying – for observation of changes between samples, and additional chemical information (well retained components with good peak shape are likely to possess significant polarity).
Highly polar ionic liquid columns therefore present improvements for the analysis of polar components in hydrocarbon matrices. The improved maximum column temperatures when compared to traditional polar phases such as polyethyleneglycol and even 1,2,3-tris(2-cyanoethoxy)propane (TCEP) increase the boiling range of materials amenable to analysis and allow for the study of lubricants and lubricant extracts.
Figure 6: Graphical representation of analyte retention in the second dimension from Figure 5.
Many discussions of the analysis of fatty-acid methyl esters (FAMES) using ionic liquid coated GC columns can be found in the literature [3-7]. In addition to the analysis of
difficult to separate isomers, the high polarity enables the bulk of the hydrocarbon envelope to be eluted before the methyl esters reducing the amount of sample
Analysis in two dimensions
One of the major drawbacks of comprehensive GCxGC where both columns are in the same oven is that the maximum possible temperature is limited by the polar
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