Robert A. Shellie

Principles and applications of comprehensive two-dimensional gas chromatography

Abstract This issue of Trends in Analytical Chemistry celebrates 50 years of gas chromatography (GC) — the greatest enabling technology for chemical analysis of volatile compounds. However, what may be considered the most powerful separation tool in GC — comprehensive two-dimensional gas chromatography (GC×GC) — is a development born of the 1990s. It was first described and almost fully established in the last decade of the twentieth century. The coming decades can be expected to see it flourish into a major operating mode of GC, when applications and fundamental principles will be further expanded, and, most importantly, its universal acceptance will be unquestioned. This article describes why the pioneers of GC×GC have so much faith in the new opportunities afforded by this exciting technology.

Gas chromatographic technologies for the analysis of essential oils.

Essential oil analysis has basically had one technical goal: to achieve the best possible separation performance by using the most effective, available technology of the day. The result achieved from this may then be used to answer the research or industrial analysis questions which necessitated the analysis. This may be for comparative purposes, where one oil is contrasted with other(s) for quality control or investigation of adulteration, to discover new components, or to characterise the chemical classes of compounds present. Clearly, today the analyst turns to chromatography as the provider of separation and then may supplement that with mass spectrometry to aid identification. The power of GC-MS means that advances in both the separation technique, and improvements in mass spectrometry detection - along with improved data handling tools - will immediately be relevant to the essential oil area. This present review outlines the developmental nature of instrumental approaches to essential oil analysis using gas chromatography. Mass spectrometry will be included to the extent that it represents the hyphenation of choice for most analysts when analysing essential oils. Thus single-column and multi-dimensional analysis will be covered, as will sample handling or introduction techniques prior to the analysis step, where these techniques provide some measure of separation. The recent demonstration of comprehensive gas chromatography will be discussed as the potentially most powerful separation method for essential oils. This brief review is not intended to be a comprehensive dissertation on the field of essential oil analysis since that would require sufficient space to occupy a book in its own right. Rather, it will outline selected considerations and developments, to help explain where new technology has been applied to advantage in this field.

Towards high capacity latex-coated porous polymer monoliths as ion-exchange stationary phases

The preparation of high capacity agglomerated monolithic ion-exchangers for capillary ion chromatography is described. Post-modification of reactive monoliths was investigated as an alternative to co-polymerisation of a suitable functional monomer with an overarching goal of increasing ion-exchange capacity. Direct sulfonation of poly styrene-co-divinyl benzene monoliths using concentrated sulfuric acid or chlorosulfonic acid was unsuccessful even for monoliths containing as low as 8% crosslinker. In contrast, chemical transformation of reactive monoliths containing glycidyl methacrylate was used to increase the ion-exchange capacity by up to more than thirty-fold with ion exchange capacities of 14-29 microequiv g(-1) achieved. Three different reactions were considered, including reaction with 4-hydroxybenenesulfonic acid under basic conditions; reaction with thiobenzoic acid followed by transformation to a reactive thiol and the subsequent oxidation to the sulfonic acid; and direct sulfonation with sodium sulfite. Of these, the reaction with sodium sulfite resulted in the most significant increase in the capacity and the best separation performance. In the isocratic mode separation efficiencies of over 13,500 plates m(-1) were observed (for iodate). The separation of seven inorganic anions was also demonstrated using a hydroxide gradient.

Identification of homemade inorganic explosives by ion chromatographic analysis of post-blast residues

Anions and cations of interest for the post-blast identification of homemade inorganic explosives were separated and detected by ion chromatographic (IC) methods. The ionic analytes used for identification of explosives in this study comprised 18 anions (acetate, benzoate, bromate, carbonate, chlorate, chloride, chlorite, chromate, cyanate, fluoride, formate, nitrate, nitrite, perchlorate, phosphate, sulfate, thiocyanate and thiosulfate) and 12 cations (ammonium, barium(II), calcium(II), chromium(III), ethylammonium, magnesium(II), manganese(II), methylammonium, potassium(I), sodium(I), strontium(II), and zinc(II)). Two IC separations are presented, using suppressed IC on a Dionex AS20 column with potassium hydroxide as eluent for anions, and non-suppressed IC for cations using a Dionex SCS 1 column with oxalic acid/acetonitrile as eluent. Conductivity detection was used in both cases. Detection limits for anions were in the range 2-27.4ppb, and for cations were in the range 13-115ppb. These methods allowed the explosive residue ions to be identified and separated from background ions likely to be present in the environment. Linearity (over a calibration range of 0.05-50ppm) was evaluated for both methods, with r(2) values ranging from 0.9889 to 1.000. Reproducibility over 10 consecutive injections of a 5ppm standard ranged from 0.01 to 0.22% relative standard deviation (RSD) for retention time and 0.29 to 2.16%RSD for peak area. The anion and cation separations were performed simultaneously by using two Dionex ICS-2000 chromatographs served by a single autoinjector. The efficacy of the developed methods was demonstrated by analysis of residue samples taken from witness plates and soils collected following the controlled detonation of a series of different inorganic homemade explosives. The results obtained were also confirmed by parallel analysis of the same samples by capillary electrophoresis (CE) with excellent agreement being obtained.

Comprehensive two-dimensional gas chromatography-mass spectrometry analysis of Pelargonium graveolens essential oil using rapid scanning quadrupole mass spectrometry

The analysis of Pelargonium graveolens essential oil is reported using comprehensive two-dimensional gas chromatography with quadrupole mass spectrometric detection (GC×GC-qMS). A spectral acquisition rate of 20 Hz was achieved by using a reduced mass scan range of 188 u. 65 components were identified within the two-dimensional separation space based upon retention index and mass spectral matching with literature data. Very high quality mass spectra were obtained, which facilitated accurate library matching, without the need for background correction. High efficiency in the short, fast-GC second dimension column was achieved by applying the principles of vacuum-GC. It is demonstrated that facile determination of the key citronellol : geraniol ratio and the amount of 10-epi-γ-eudesmol in geranium essential oils using GC×GC will be possible. The method should be generally suited to the analysis of similar essential oil samples.

Application of comprehensive two‐dimensional gas chromatography (GC×GC) to the enantioselective analysis of essential oils

A new method for the enantioselective analysis of essential oils is described, using comprehensive two-dimensional gas chromatography (GC×GC). The column set comprised a primary column containing a cyclodextrin derivative as a chiral selector, and a secondary column containing a polyethylene glycol stationary phase. A cryogenic modulation system was used to achieve the GC×GC experiment. The enantiomeric compositions of a number of monoterpene hydrocarbons and oxygenated monoterpenes in Australian tea tree (Melaleuca alternifolia), including -thujene, sabinene, -pinene, -phellandrene, limonene, trans-sabinene hydrate, cis-sabinene hydrate, linalool, terpinen-4-ol, and -terpineol are reported. The GC×GC resolution advantage is shown to improve the efficiency of enantioselective essential oil analyses. In a single temperature programmed analysis, the individual antipodes of optically active components can be separated, and are effectively free from matrix interferences.

Characterization and comparison of tea tree and lavender oils by using comprehensive gas chromatography.

The essential oils from French lavender (Lavandula angustifolia) and tea tree (Melaleuca alternifolia) were separated by the two-dimensional GC technique of comprehensive gas chromatography. A coupled column combination of non-polar (5% phenyl equivalent) and polyethylene glycol phase columns was used to provide the desired resolution performance. By using a range of known standards, some of the peaks in lavender oil can be assigned. Some of these also occur in tea tree oil; however, from our knowledge of the major constituents in this oil and their relative retention behaviour, most of the major peaks may be tentatively assigned within the 2-dimensional separation space. There appear to be elution patterns within the 2-D space which should be useful in correlating retention with chemical and structural properties of the components, although this will require further evaluation. A range of coeluting peaks, which may not be so readily separated by using a single column capillary GC analysis, are resolved in the experiment described.

Identification of Inorganic Improvised Explosive Devices Using Sequential Injection Capillary Electrophoresis and Contactless Conductivity Detection

A simple sequential injection capillary electrophoresis (SI-CE) instrument with capacitively coupled contactless conductivity detection (C(4)D) has been developed for the rapid separation of anions relevant to the identification of inorganic improvised explosive devices (IEDs). Four of the most common explosive tracer ions, nitrate, perchlorate, chlorate, and azide, and the most common background ions, chloride, sulfate, thiocyanate, fluoride, phosphate, and carbonate, were chosen for investigation. Using a separation electrolyte comprising 50 mM tris(hydroxymethyl)aminomethane, 50 mM cyclohexyl-2-aminoethanesulfonic acid, pH 8.9 and 0.05% poly(ethyleneimine) (PEI) in a hexadimethrine bromide (HDMB)-coated capillary it was possible to partially separate all 10 ions within 90 s. The combination of two cationic polymer additives (PEI and HDMB) was necessary to achieve adequate selectivity with a sufficiently stable electroosmotic flow (EOF), which was not possible with only one polymer. Careful optimization of variables affecting the speed of separation and injection timing allowed a further reduction of separation time to 55 s while maintaining adequate efficiency and resolution. Software control makes high sample throughput possible (60 samples/h), with very high repeatability of migration times [0.63-2.07% relative standard deviation (RSD) for 240 injections]. The separation speed does not compromise sensitivity, with limits of detection ranging from 23 to 50 μg·L(-1) for all the explosive residues considered, which is 10× lower than those achieved by indirect absorbance detection and 2× lower than those achieved by C(4)D using portable benchtop instrumentation. The combination of automation, high sample throughput, high confidence of peak identification, and low limits of detection makes this methodology ideal for the rapid identification of inorganic IED residues.

Forensic Identification of Inorganic Explosives by Ion Chromatography

Abstract While there is a great deal of published literature describing methods for the analysis of organic explosives by various methods, there are comparatively few publications concerning the analysis of explosives based on inorganic chemicals. This mini‐review examines the literature that is concerned with the analysis of such explosives, commonly referred to as improvised explosive devices, using ion chromatography. Pertinent details from the reviewed publications are used to highlight the strengths and weaknesses of these methods and several comparisons are also made to other analytical techniques. The general utility and limitations of the reported research for the analysis of improvised explosive devices are discussed and future directions are proposed.

Interactive Use of Linear Retention Indices on Polar and Apolar Columns with an MS-Library for Reliable Characterization of Australian Tea Tree and Other Melaleuca sp. Oils

Abstract Australian tea tree oil from Melaleuca alternifolia and M. linariifolia is well known for exhibiting antibacterial, antifungal, and general antiseptic properties. Hence characterization of tea tree oil is important, as this leads to a better understanding of the important components contributing to the observed properties of the oil. GC/MS is widely used for this task. However, reliable identification of oil components is not always possible using mass spectral data only. Retention times or retention indices are often used to support the MS data. The present study investigated a number of different tea tree and other Melaleuca sp. oil samples, and reports qualitative and quantitative data for each sample. Characterization of the individual oil components was performed using a private MS library, which incorporates an interactive linear retention index filter. The method described in this report increases the ease of characterization of individual oil components, and allows a greater number to be reliably assigned. More than 80% of the components in the samples studied are now reliably identified.