Comprehensive Two-Dimensional Gas Chromatography with Mass Spectrometry: Toward a Super-Resolved Separation Technique

A data interpretation and processing approach for improved compound identification and data presentation in comprehensive two-dimensional gas chromatography (GC×GC) is described. A footprint peak of a compound in 2D space can be represented by a centroid or peak apex, similar to the data-reduced histogram spectra used in mass spectrometry. The workflow was demonstrated on data from GC×GC-TOFMS. Peaks in a modulated chromatogram were initially detected by conventional chromatographic integration, followed by a curve-fitting approach, which interpolated high-precision, absolute retention times for all modulated peaks. First dimension retention time (1tR) was obtained by using an exponentially modified Gaussian (EMG) fitting model for near-Gaussian distributed subpeaks, polynomial fitting for highly asymmetrical peaks, and parabolic fitting for under-sampled peaks, which allows determination of a precise 1tR, considering the dwell-time arising from modulation and 2tR. Area summation of the modulated peaks belonging to the same compound was then performed to yield the total peak area. Each compound in the GC×GC-MS result was then represented by its position at the intersecting coordinates, (1tR, 2tR), in the 2D separation plane, having a height of the same magnitude as the total component summed area. This results in a novel and uncluttered GC×GC output convention based on the scripted total ion chromatogram (TIC) data with precise 1tR, 2tR, and area. Comparison between the contour plots from the scripted and conventional TIC revealed improved data presentation, accompanied by an apparent enhanced resolution. The described approach was applied to the identification of 177 aroma compounds from peaches as indicators of fruit quality

Novel molecular materials and computational strategies with gas chromatography

The ability to detect, identify and quantify many types of compounds in complex matrices is paramount for many disciplines and application areas such as proteomics, environmental, pharmaceutical, medical, petrochemical, food and beverages. Existing detection technologies, such as high resolution mass spectroscopy, may fail to adequately analyse compounds of interest in multi‐component samples; isomers or compound identity may be incorrectly reported, e.g. false positive identification of illicit drugs, biomarkers or synthetic compounds due to the presence of interferences or by‐products. Chromatographic methods offer an effective solution where target analytes in complex matrices are separated from the non‐targets allowing improved identification of many compounds with minimised signal interference in a single analysis. Identification and quantification of well separated isomers can also be achieved. Among different separation techniques, comprehensive two dimensional gas chromatography (GC×GC) has been realised as a high resolution analysis technique allowing the separation of hundreds of analytes based on their differing boiling points and/or polarities. A simple concept may be that components in samples should be separated as much as possible, permitting unambiguous analysis of target analytes in GC×GC. Within this simple concept, a tremendous amount of time, consumables and energy needs to be spent on the optimisation process since the GC×GC method incorporates two dissimilar column geometries, and choice of stationary phases leading to many variables that must be considered when performing separation such as relative column dimensions, types of stationary phases, temperature programs and carrier gas flow. Thousands of experiments may need to be performed in order to obtain an effective chromatographic outcome for each sample. Understanding the impact of these variables on separation mechanisms is thus important to design an effective optimisation processes, and to reduce valuable resources. Recently, there has been an increasing interest in developing new methodologies for improved GC×GC analysis, utilising novel materials such as ionic liquids (IL) as stationary phases with high polarity and good thermal stability. Besides the polar/nonpolar interactions, additional hydrogen bond basicity is also obtained with these phases, due to the customisable functionalities of IL allowing introduction of acid moieties onto the IL stationary phase molecules which broadens selectivity in GC and finally offers improved overall separation quality (orthogonality) in GC×GC. In this thesis, new theoretical concepts and approaches to direct column selection and to aid optimisation of experimental conditions in GC×GC were established according to linear solvation energy relationship (LSER) and molecular modelling. Relevant computational software was developed according to the established approaches in order to simulate GC×GC results for individual experimental investigation covering a wide range of compounds such as fatty acid methyl esters, hydrocarbons in petroleum, alcohols, aldehydes, terpenes, polychlorinated naphthalenes and polychlorinated biphenyls. These simulated results were evaluated by comparison with experimental results. The theories, methods and results presented in this thesis will, in the future, allow further targeted developments of novel experimental design, effective stationary phase material selection, an understanding of separation mechanisms with IL stationary phases in GC×GC, and the possibility to design tuned stationary phases that further improve separation performance. These principles could also equally well apply to heart‐cut multidimensional GC operation