GCxGC-FID for Qualitative and Quantitative Analysis of Perfumes

Sep 01, 2010
Volume 23, Issue 8, pg 430–438

Comprehensive two-dimensional gas chromatography (GC×GC) is a powerful and sensitive analytical technique, which has been extensively used for both the qualitative and quantitative analysis of complex samples. However, its potential as a quantitative technique has not been full explored, but the use of multivariate chemometric techniques instead of conventional peak integration strategies is emerging as an important alternative for this task. The use of GC×GC–FID for the quantification of either individual chemical components or complex formulation ingredients on perfumes and similar materials using this approach is described. A procedure based on the application of a novel chemometric algorithm — multivariate curve resolution (MCR) — was applied to quantify one of the ingredients in a perfume-like formulation, rosemary oil, using GC×GC–FID data. The combination of GC×GC–FID and MCR allowed accurate and precise quantification of rosemary oil on this formulation, with root mean squared error of prediction of 0.46% for concentrations in the range between 4–12% v/v.

Comprehensive two-dimensional gas chromatography (GC×GC) was introduced by J.B. Phillips and co-workers in 1991 and has become increasingly popular in the 21st century. GC×GC is a powerful alternative to conventional GC due to its ability to simultaneously enhance the separation capacity and significantly increase the sensitivity and detectability using an instrument which, in essence, is a modified standard gas chromatograph.

An early report from Dallüge and co-workers,1 where about 30000 single peaks could be detected on a single GC×GC–TOF-MS chromatogram from cigarette smoke, had a considerable impact on the recognition of the technique and drew the attention both from end-users and researchers involved in separation science. However, the more striking aspect of graphical representations of GC×GC Chromatograms is the presence of spatially ordered groups of peaks resulting from structurally-related compunds, which are easily discerend upon visual inspection of these chromatograms. The presence of chromatographic structure depends both on the orthogonality of the stationary phases on the column set used and, of course, on the existence of groups of these related compounds in the sample.2 Most of the applications of GC×GC described to date are focused on its application for qualitative analysis, ranging from systematic and comprehensive identification of eluates using GC×GC–TOF-MS,3 or –qMS systems,4 to more complex applications on pattern recognition, fingerprinting and classification of complex samples.5

However, there are comparatively few quantitative applications of GC×GC systems reported. Paradoxically, one of the possible reasons may be the remarkable ability of GC×GC to provide qualitative data related both to sample composition and identity, even using non-MS detection. In particular where peak structure is clearly visible, some users are certainly enticed to focus their efforts on these qualitative applications. Of course, there are other and more solid reasons for the restricted use of GC×GC for quantitative analysis at present.

The general principle for quantification of discrete eluates on GC×GC is essentially the same as in conventional chromatography: ideally, the mass of analyte or its concentration on the sample is proportional to the "volume" of the corresponding three-dimensional peak on the 1tR × 2tR plane (in fact, the sum of the individual peak areas for each 2D narrow peak resulting from the modulation of the chromatographic band emerging from the 1D column),6 provided some requirements are met. The mass transfer from the 1st to 2nd dimension columns through the modulation device should be complete, or at least quantitative (which is true for most of the modulator designs presently in use). Also, since base widths of properly modulated 2D peaks are on the hundreds of milliseconds range, both detector and its corresponding signal conditioning electronics should allow data acquisition fast enough to provide a number of digitized points: enough to achieve reliable integration (at least 10 points for each peak, corresponding to typical acquisition rates of 100 Hz or more).7 The integration of individual 2D peaks can be performed using standard chromatographic software, and the individual 2D peak areas can be manually compiled and added. This was usual in early reports, such as the work of Truong et al.8 where a newly proposed GC×GC system was validated, among others, through analytical curves for sterols and 5-α-cholestane.

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