Anal. Chem. 2010, 82, 4775–4785
Gas Chromatography Frank L. Dorman* Department of Biochemistry and Molecular Biology, Penn State University, University Park, Pennsylvania 16802 Joshua J. Whiting LECO-ARD, Saint Joseph, Michigan 49085 Jack W. Cochran Restek Corporation, Bellefonte, Pennsylvania 16823 Jorge Gardea-Torresdey Department of Chemistry, University of Texas, El Paso, El Paso, Texas 19968 Review Contents General Interest and Reviews Texts Review Articles Columns Principles and Technology General Information Stationary Phases Fundamental Characterizations Portable and Microfabricated GC Technology Development Microfabrication Developments Comprehensive Two-Dimensional Gas Chromatography (GC × GC) Scope Reviews Instrumentation Data Handling Theory Biological Clinical and Forensics Environmental Flavor and Fragrance Food and Beverage Metabolomics Petrochemical Gas Chromatography Detectors Improvement in GC Detector Techniques GC Detector Improvement through Software Development Development of GC New Detectors Literature Cited
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GENERAL INTEREST AND REVIEWS This review of the fundamental developments in gas chromatography (GC) includes articles published following the previous review (1) in 2008 up to March, 2010. Emphasis is given to developments which are considered significant as far as basic advances. This accounts for a much smaller amount of publications, as the majority of papers deals with applications of the GC technique and, as such, do not necessarily represent advances in * To whom correspondence should be addressed. 10.1021/ac101156h 2010 American Chemical Society Published on Web 05/26/2010
the fundamentals. A recent search of the literature had slightly more that 800 publications per year that have the broad subject area of “gas chromatography”. Most of these publications are applied separations which use GC as the separation technique, so this review really focuses on the small subset that are viewed by the authors as improvements in the technique or the understanding of the technique. Despite the large numbers of publications in other separation techniques (HPLC, CE, etc), GC is arguably still the best separation tool that is in common use for the compounds which are amenable to the technique. Possibly, the greatest expansion of this technique that attracts interest from researchers is comprehensive two-dimensional GC (GC × GC). Increasing interest from the commercial field has led to additional instrumentation and improvements in ease of use following the previous review. While this technique is still not common in analytical laboratories, it does account for a significant amount of the publications and academic research; thus, it occupies a rather extensive portion of this review. Texts. Several texts were published in this period, some of which were revised editions. Of particular note are three first edition texts: “Comprehensive Two Dimensional GC”, edited by Ramos (2); “Quantification in LC and GC: A Practical Guide to Good Chromatographic Data”, edited by Kuss and Kromidas (3); and “Ionic Liquids in Chemical Analysis”, edited by Koel (4). Also revised during this period was McNair’s text on “Basic Gas Chromatography” (5), which is a text that is often used in the teaching of the GC technique to students. Finally, for the GC/ MS technique, Hubschmann has revised and updated the Handbook of GC/MS text (6). Review Articles. During the time period of this review, approximately 40 review articles were published on various aspects of GC. Many were, again, application driven, or were focused on detection strategies or increasing the instrumental throughput. There were a surprising number of articles, however, that were more focused on the GC separation itself or on expanding the range of compounds which can be analyzed. Often referred to as Analytical Chemistry, Vol. 82, No. 12, June 15, 2010
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the “Achilles heel” of GC, the injection technique can be a large source of frustration when analyzing thermally labile compounds. Large volume injection (LVI) methods have been well reviewed (7) including direct sample introduction and through oven transfer adsorption desorption (TOTAD) techniques. Also considered an “injection” technique, static headspace extraction GC (SHE-GC) was reviewed (8). Methods of extending the working range of volatility for this technique are addressed, and good coverage of the fundamentals is also summarized. Fast GC was also reviewed (9) with particular attention being paid to theory and influence of various band-broadening mechanisms which can potentially limit this technique. It was demonstrated that peak widths of 1 ms are readily achievable in GC if extra-column band broadening is controlled. A few reviews concerning the role and understanding of the stationary phase in GC separations were published. The first describes the various retention models used in prediction of retention times based on thermodynamic parameters (10). This also covers the use of thermodynamic modeling to various column arrangements with simultaneous temperature and pressure programming. Also discussed is extension of these techniques to fast GC and GC × GC. Most of the stationary phase work published still continues to be nonpolysiloxane materials, and a review of molten salt and ionic liquids as stationary phases for organic compound analysis includes a discussion as to how stationary phases should be selected by potential users (11). A review of the use of metal-containing stationary phases was also published (12). These materials are useful for separation of compounds through a complexation mechanism. Examples of chiral separations were addressed. Finally, several reviews on the GC × GC technique were published. The use of this technique for the analysis of complex metabolomic samples was reviewed (13). This is a very challenging analysis in biological fluid, and the increase in peak capacity of the GC × GC technique may lend considerable benefit to these separations. Further, coupling the GC × GC to a time-of-flight mass spectrometer (TOFMS) adds another dimension of separation and can be a very powerful analytical technique. Two other general reviews of the GC × GC technique also summarize the state of the art of this technique and show numerous relevant applications. The first includes an industrial perspective as to the benefit of GC × GC in semiroutine analyses at Dow (14). The second also includes a discussion of the prediction of separation based on thermodynamic properties and attempts to simplify the column selection process (15). COLUMNS PRINCIPLES AND TECHNOLOGY General Information. While the GC column is, of course, the only part of the entire GC system that physically separates compounds from each other, only a small percentage of publications deal with the understanding of this process or the development of new materials. While this may have been a very active area of research years ago, it certainly is viewed as “mature” in the present day, and as a result, most publications that fit into the category of column advancements are very niche in application. Professor Klaus Unger was quoted at the HPLC 2008 meeting as saying that “we should put our efforts towards gaining selectivity, not towards the mad rush to increase efficiency”. He was, of course, discussing the area of small4776
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particle based HPLC separations, but his comments could easily apply to GC as well. Most practicing chromatographers have little interest or knowledge in the mechanisms which lead to separation and often rely on detection techniques to overcome chromatographic coelutions. While this may give acceptable results in some cases, there is still need for development of new materials which would give additional selectivities and/ or allow GC to extend further into the analysis of reactive nonvolatile compounds. Stationary Phases. Clearly, ionic liquids continue to represent the highest percentage of papers published using new materials as GC stationary phases. To date, these materials have not had the efficiency of the polysiloxanes, but they do offer unique selectivities. The use of ionic liquids has been reported in the analysis of biodiesel blends (16), another recent topic of interest! This paper uses one of the commercially available columns which is based on the chemistry developed at Dan Armstrong’s group. The comparison to more standard poly(ethylene glycol) (PEG) column separations demonstrates the ionic liquid’s benefit of separation of the FAME’s from the less-retained saturates. Other researchers have reported on an ionic liquid bonded polysiloxane stationary phase, which they claim has a “high separation efficiency” of 3200 plates/m (17). Finally, the development of a new triflate ionic liquid for GC × GC was reported (18). The comparison to more conventional PEG phases as second-dimension columns was discussed. The ionic liquid column was stated to have significantly larger selectivity for several of the test compounds. Second dimension column orthogonality is a very important issue for GC × GC separations, and ionic liquids may have a role here, even if their efficiencies are not as high as more routine phases. The separation of polycyclic aromatic hydrocarbons (PAH’s) has been of increased interest. Several commercial companies have reportedly developed columns specifically for this application. Most notable, Agilent Technologies (www.agilent.com) and Varian (www.varian.com) have marketed a specific column for this separation. Restek Corporation (www.restek.com) has also reported on the use of molecular modeling techniques to develop a new material that is designed for this separation (19). Additional work in the area of modified cyclodextrines was still an area of research, and one paper (20) discussed the use of a maltooctaose derivative as a stationary phase for the separation of enantiomers. Another one (21) used a common permethylated β-cyclodextrine diluted in a variety of liquid phases in order to determine the role of the polymer type on retention. This work used PEG and also SE-30 and SE-54 as liquid phases. Fundamental Characterizations. During this review period, very little work was published on the modeling or characterizations of stationary phases. Typically, there have been a few papers on thermodynamic modeling and also quantitative structure property relationship (QSPR) approaches. There were two papers published, both using QSPR models for retention prediction of environmentally important compounds. In the first (22), the retention indices of 168 pesticides were used to construct a QSPR model. The second paper (23) used a QSPR approach to determine four optimal descriptors which then let the authors predict retention on 18 different GC columns for all 209 PCB
Figure 1. Number of citations by year of Terry, Jerman, and Angel’s 1979 paper describing the first etched silicon GC system.
congeners. These approaches should allow for improved column selection and also potential identification of congeners that may not be in a particular user’s calibration standards. PORTABLE AND MICROFABRICATED GC TECHNOLOGY DEVELOPMENT Analytical samples are often at risk for sample contamination, decomposition, degradation, and loss during storage and transport from the collection site to the laboratory for analysis. This has resulted in a growing trend toward efforts to bring the lab to the sample when possible. Significant efforts have been invested to develop and test portable instrumentation. There have been several recent reports of the development and use of portable GC systems including work done at the University of Michigan on a portable GC with a chemiresistor array detector (24). The system was designed for rapid, trace analysis of complex mixtures. It consists of a small, multistage preconcentrator; two series-coupled columns with fast, independent temperature programming capabilities; pressure control at the junction point between the columns; and an array of chemiresistors for detection. A new commercially available portable GC with detection provided by a toroidal ion trap mass spectrometer has been developed and described by researchers at Brigham Young University and Torion Technologies (www.torion.com) (25). The system consists of a SPME injection system, a low-thermal mass GC, and a miniature toroidal ion trap mass analyzer housed in a Pelican case. The entire system including pumps and batteries weighs about 13 kg. Two other application studies included the use of portable GC systems for the analysis of acetaldehyde in tobacco smoke (26) and the clinical measurement of volatile sulfur compounds (VSCs) which can be a cause of oral malodor (27). Microfabrication Developments. Thirty years after the publication of the seminal paper by Terry, Jerman, and Angell in 1979 (28), which described the first GC system composed largely of microfabricated components, interest in separation systems facilitated by microfabricated devices continues to grow as micromachining capabilities mature. Evidence of this is shown in Figure 1 which shows the number of citations of the Angell paper as a function of publication year. The promise of this research continues to be reduced system size, low-power requirements, enhanced performance, and the promise of batch manufacturing to reduce costs. In practice, significantly enhanced
performance continues to elude researchers, with the best performing microfabricated columns performing on par with comparable commercially coated fused silica columns. These performance challenges were largely predicted by Golay in 1981 (29) when he described the end effects of high aspect ratio columns. As novel geometries and fabrication techniques evolve, the potential exists for greater improvement in these areas. Even within these current limitations, interest remains high due to the promise of reduced manufacturing costs by utilizing similar batch manufacturing techniques now common in the semiconductor industry for the manufacturing of microprocessors, memory, etc. However, an important consideration, as the number of researchers developing new stationary phase coatings, coating techniques, column designs, and methods of fabrication increases, is that a standardized column evaluation method should be developed to directly compare their performance and test column reproducibility. The size and power benefits are both obvious and connected; as one reduces the mass of components, the power required to heat and cool individual components decreases as well. Several GC systems have been designed and manufactured using microfabricated components to achieve this end. Examples include the Canary line from Defiant Technologies (www.defiant-tech.com) which integrates microfabricated GC columns, preconcentrators, and detectors for multiple applications; SLS micro’s GCM series (www.slsmt.com) which integrates a microfabricated column, detector, and sample loop along with the electronics into a PDA sized fluidics package; and Thermo Scientific’s C2 V-200 micro GC (www.c2v.nl) which integrates a microfabricated inlet and detectors with conventional wall coated open tubular fused silica columns. Two recent developments include the introduction of an “intelligent” preconcentrator by Defiant Technologies that “onthe-fly” determines the appropriate sampling time and the acquisition of Concept to Volume (C2 V) by Thermo Scientific. Several researchers continue to work on the development of new systems based on microfabricated components; one recent report describes a new portable GC system utilizing a micro electromechanical system (MEMS) enabled miniaturized GC for the subparts per billion detection and monitoring of aromatic volatiles (30). Zampolli and others at the CNR-IMM Institute for Microelectronics and Microsystems in Bologna, Italy, describe a system consisting of a micromachined preconcentrator, a 50 cm × 800 µm × 1 mm microfabricated column packed with 80-100 mesh carbograph 2 + 0.2% carbowax particles, and metal oxide (MOX) gas sensors as a detector. The demonstrated system shows separation of BTEX compounds in about 10 min. Another report by Nishino and others at Shimadzu describes the development of a prototype instrument built around a microfabricated column 8.5-17 m in length generating 35 000 theoretical plates with a flame ionization detector (FID) (31). There remain several research programs focused on the development of new MEMS GC systems including Sandia National Laboratories and the University of Michigan WIMS Center. Sandia National Laboratories has had an ongoing development program in the area of microfabricated GC systems since 1996, primarily geared toward the rapid, portable, low-power detection of chemical weapons, explosives, and more recently toxic industrial chemicals. Recent advances include the development of masssensitive microfabricated preconcentrators (32), a new selective Analytical Chemistry, Vol. 82, No. 12, June 15, 2010
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and sensitive chemiresistor GC detector (33), and the demonstration of the first consumable free comprehensive two-dimensional GC system (GC × GC) consisting of a pair of microfabricated columns and a microfabricated detector (34). The mass-sensitive preconcentrator consists of a thin film resistive heater on the surface of a MEMS pivot plate resonator (PPR). The PPR is Lorentz force actuated, and the frequency of the resonator shifts with mass loading. When sufficient sample has been collected for analysis by the downstream microfabricated gas analyzer, the film is heated and the sample is desorbed. Under partial funding from the Defense Advanced Research Projects Agency Micro Gas Analyzer (DARPA-MGA) program, Sandia developed a sensitive chemiresistor utilizing a conjugated molecule linked gold nanoparticles in a sol-gel matrix. The chemiresistor demonstrated significant selectivity for phosphonates (CWA simulants) and sensitivity. The GC × GC utilized a pair of microfabricated GC columns. The first column was coated with polydimethyl siloxane and was 90 cm in length; the second column was 30 cm in length and was coated with polyethylene glycol. The modulator was a stop-flow design based on the work of Richard Sacks, and detection was provided by a nanoelectromechanical systems (NEMS) cantilever resonator developed by the Roukes group at Caltech. The system demonstrated a separation of ∼29 components in less than 8 s. The NSF funded Wireless Integrated MicroSystems (WIMS) Engineering Research Center (ERC) (www.wimserc.org) is in the final year of NSF funding for the center and is transitioning from an ERC to the WIMS institute. The center has been developing several parallel MEMS GC based systems for multiple applications, such as breath analysis (35), explosives detection (36), extraterrestrial exploration (37), and indoor air quality monitoring (38). They have developed new gas phase detectors such as nanoscale chemiresistor arrays (39) and microdischarge detectors (40). The WIMS center has also demonstrated the first thermally modulated GC × GC system utilizing microfabricated columns (41). While these efforts continue, much of the research being reported has focused on the development of microfabricated components such as preconcentrators, columns, and detectors. In the area of preconcentrators, several groups are exploring the use of carbon nanotubes (CNTs) as a sorbent material. CNTs demonstrate excellent adsorption and desorption characteristics due to low mass transfer resistance because of the nonporous nature of the material (42, 43). Other materials investigated for use as sorbents in microfabricated preconcentrators include the use of microporous activated carbon (44), chemically polymerized polypyrrole (45), as sorbent materials, and inkjet printing of polymer sorbents prior to anodic bonding (46). In the area of column development, there have been several reported advances. Zareian-Jahromi and Agah at Virginia Tech have recently reported revisiting the concept of multicapillary columns (MCCs) (47). MCCs are a concept first operatively demonstrated by researchers in Novosibirsk, Russia, in the 1980′s and exist now as a commercial product sold by Multichrom, Ltd. (www.mcc-chrom.com). The principal concept is to get around the column capacity limitations and pressure restrictions of microbore columns using a bundle of identical microbore columns in parallel. This allows a fraction of each injection to be separated on each column preventing overloading and enables the overall 4778
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flow restriction to be very small. The challenges remain in manufacturing uniform channels, both in length and phase volume ratio, distribution of the injection plug equally to all columns, and dead volume at the inlet and outlet of the system. Problems in any of these areas will result in band broadening. Agah’s group has demonstrated some success using a MEMS approach to address these problems. Ali and Agah have also demonstrated MEMS based semipacked columns (48). These columns used microfabricated posts in the column channels to decrease diffusion distances and column capacity relative to more traditional WCOT columns but still demonstrate significantly reduced pressure drops relative to traditional packed columns. Zareian-Jahromi and Agah have also reported a novel coating technique using a monolayer protected gold stationary phase (49). There are several other reports of novel column fabrication development. Radadia and Masel at the University of Illinois describe work on an all silicon column using a gold diffusion eutectic bonding process to bond a silicon lid instead of a Pyrex lid to the silicon channels (50) and a partially buried microcolumn with an interesting cross section (51). Lewis and Milton describe a microfabricated planar glass column with a circular cross section manufactured in two hemispherical halves and bonded using epoxy (52). Sun and Chen at the Chinese Academy of Sciences report the development of a silicon/Pyrex microfabricated 100 µm × 100 µm × 6 m column generating 4850 theoretical plates (53). There are also reports of new column coatings which have also been developed. Researchers from both the University of Washington/Lawrence Livermore National Laboratories (54) and University of Tokyo (55) describe the use of CNTs as stationary phases. Nakai et al. from the University of Tokyo also reports on the use of a functionalized parylene (56) as a stationary phase. A report by researchers at the WIMS center at the University of Michigan highlighted an often overlooked aspect of microfabricated columns: column reproducibility. The study compared the performance of several (2-8) columns with different preparations to develop a column coating strategy (57). There have been several advances in the area of microfabricated and miniaturized GC detectors including mass analyzers, ion mobility spectrometers (IMS), optical sensors, and microcantilever (MC) arrays. Malcom and Finlay at Micorsaic Systems, Ltd. and the Imperial College describe a microengineered quadrupole mass filter consisting of microfabricated components to fabricate a quadrupole mass filter with dimensions of 35 mm × 6 mm × 1.5 mm (58). Researchers at the University of Cordoba describe a system that couples a multicapillary column with a miniaturized IMS for rapid GC × IMS separations (59). Researchers from the University of Missouri and ICx Nomadics have reported on the use of a optofluidic ring resonator (OFRR) sensor for on-column detection (60-62). The OFRR is a thin walled fused silica capillary. The inner wall of the capillary is coated with a thin polymer film. The circular cross-section of the capillary forms an optical ring resonator where circulating waveguide modes are supported by total internal reflection of light along the curved inner and outer boundary. The evanescent field extends into the core and is sensitive to the refractive index change induced by the interaction between the analyte and the stationary phase. Long and Sepaniak at the University of Tennessee report on the use of
MC arrays for gas phase sensing (63). MC arrays have similar response mechanisms to quartz crystal microbalances and surface acoustic wave sensors; however, they demonstrate better mass sensitivity, smaller dimensions, and decreased fabrication costs. COMPREHENSIVE TWO-DIMENSIONAL GAS CHROMATOGRAPHY (GC × GC) Scope. This GC × GC review covers literature published mostly from 2009 until the end of April 2010, when it was compiled, but unlike GC × GC, it is not comprehensive because of the large number of papers found during the search. The number of papers published in the review period was around 100, about a 10% increase over the last review done for Analytical Chemistry across a similar time period (64). Only select articles of the 100 or so are covered, broken down into distinct topic lines as seen below. Reviews. Cortes et al. reviewed the achievements in GC × GC across 2007 until October 2008 citing 121 contributions from that period (65). They noted how GC × GC work has shifted from instrumentation development to practical applications and listed those for petroleum, polymer, pharmaceutical, flavor and fragrance, metabolomics, and environmental. Interestingly, they commented that mass spectrometry (MS) is the detector of choice, now and in the future, for GC × GC, with time-of-flight (TOF) MS outpacing the quadrupole due to having the faster acquisition rates to define ultra narrow peaks from GC × GC but predicted that the use of differential flow modulation (DFM) will increase (as an alternative to cryogenically modulated systems); DFM is not inherently compatible with the pumping capacity of most MS systems, with the supersonic molecular beam approach of Amirav being an exception (66). DFM should be more appropriate for field and process instruments that employ alternate detectors such as flame ionization detection (FID). Although we may disagree on predictions for DFM, we concur on one statement in their review; “beer is a highly popular alcoholic beverage around the world.” In conclusion, Cortes puts emphasis on the need for improved quantification and qualitative data analysis software and suggests that computational chemistry methods combined with GC × GC chromatogram structure and retention data for known compounds can yield identifications for unknown components. Hamilton specifically reviewed the use of GC × GC to study the atmosphere (67). GC × GC, especially with MS, is ideally suited for this research given the complex array of natural and manmade emissions and oxidation products present in the atmosphere. An important observation by the author was the need for sophisticated data handling approaches, including image processing, to deal with the wealth of information generated using GC × GC/MS. Instrumentation. Research devoted specifically to modulation hardware was relatively light during the review period, although the group of Shellie contributed a substantial piece on designing flexible, pulsed flow modulation systems (68). Drawbacks of flow modulation setups, versus the cryogenic approach, include pneumatic complexity, multiple connecting pieces, baseline instabilities, and lack of flexibility for changing modulation times. In particular, inability to vary the modulation time is problematic for flow modulated systems due to the potential for wrap-around. Shellie developed a dynamic flow model and employed a postfirstdimension-column restrictor that offsets some of these disadvantages, including allowing different modulation times. That benefit
can now be added to those already realized in flow modulation: less expensive hardware and no cryogenic fluid use. Pizzutti et al. derived a better compressed air modulator (69) from an earlier model (70) to address developing countries’ needs for low-cost systems. While the volatility range is limited for this air modulator, with breakthrough shown for tetradecane, lower volatility pesticides ranging from Trifluralin to Deltamethrin were successfully analyzed in grapes with GC × GC-ECD. Begnaud et al. modified the Marriott longitudinally modulated cryogenic system (LMCS) to temperature program the cooling chamber in conjunction with the GC oven, which reportedly increases the modulation efficiency across a wide range of compound volatilities while simultaneously reducing cryogenic fluid use (71). The use of the LMCS as the heart of a switchable multidimensional/GC × GC system was described and then tested with compounds important to essential oil analysis and lavender oil (72). A microfluidic Deans switch placed after a primary GC column, upstream of the LMCS, allowed selection of either a longer second column for heart-cutting work or a shorter column for GC × GC. The system was flexible enough to allow switching from heart-cut to GC × GC multiple times in one analytical run. Although not demonstrated, the system is proposed to be especially effective for heart-cut olfactory work, where broader peaks are necessary for sensory perception. Klee and Blumberg performed flow modulation of methanedoped carrier gas, which allowed direct observation/calculation of second dimension hold-up times in GC × GC (73). Subsequently, they calculated/visualized retention factors for alkanes and diesel components in both columns for GC × GC. Unfortunately, for cryofocusing modulator users, this approach will not work since methane cannot be trapped with current systems. The tendency for those users to employ stationary phase bleed from the first column as a way to calculate hold-up time was discouraged upon proving that stationary phase bleed was retained (versus methane) by the second dimension column. However, the authors’ setup did not consider an independently temperatureprogrammed secondary oven that could be positively offset versus a primary column oven. Primary and secondary column temperatures were the same in their experiments. Tobias et al. gave the first report on the coupling of GC × GC to combustion isotope ratio MS and used their system to demonstrate the analysis of steroids relevant to sports performance enhancement (74). They also suggested applications for food authentication and environmental pollutant tracing, where sample complexity may make one-dimensional GC insufficient. Data Handling. Given the high data density of GC × GC, especially when TOFMS is used, the sample complexity that predetermines GC × GC use for an application, and the special needs of the metabolomics community in particular, it is not surprising to discover reports on data handling outside of what is already provided by an instrument vendor. Vial et al. used dynamic time warping to align second dimension peaks in GC × GC chromatograms, followed by multivariate analysis to distinguish three types of tobacco (75). Through additional interpretation, marker compounds for a tobacco could be identified via the collected mass spectrometry data. An imaging process technique borrowed from the proteomics field was used to compare fruit aromas represented by contour plots after analysis by headspace Analytical Chemistry, Vol. 82, No. 12, June 15, 2010
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solid phase microextraction (SPME) GC × GC/MS (76). Image profiles created by the process can be used for statistical analysis and suggesting sample origin. While seemingly very powerful, the authors noted that any mass spectrometry data collected during GC × GC is “off-line” to the imaging software, a distinct disadvantage compared to other GC × GC data processing software. Almstetter et al. used GC × GC-TOFMS vendor software that included automated baseline correction, peak finding, spectral deconvolution, and library searching prior to applying retention time normalization and data alignment to compare two strains of Escherichia coli (77). They exported peak lists from the vendor software and aligned peaks with a model fitted to known derivatized fatty acids in the samples, followed by principal component analysis to achieve metabolic fingerprinting. Theory. Blumberg and Klee took a critical look at definitions for multidimensional separations, including GC × GC, discussing Giddings concepts and proposing a new definition that includes only “analytical separations” (78). Seeley et al. developed a simplified solvation parameter model to predict GC × GC compound retention behavior on a variety of stationary phase combinations using literature values for solute and stationary phase descriptors (79). They saw excellent agreement of the model and experimental GC × GC data and suggested the model could be used to design optimal column setups. Biological. Kalinova´ et al. investigated the male wing gland secretions of a bumblebee parasite, the wax moth Aphomia sociella, with SPME GC × GC-TOFMS (80). Bumblebees are important pollinators in greenhouses and are being considered for open agricultural field pollination given the recent decline of the domesticated honeybee. Chemically understanding gland secretions may allow the creation of effective lures for bumblebee parasite control. GC × GC-TOFMS, in conjunction with GCelectroantennographic detection (EAD) and GC-FT-IR, identified several compounds as potential sex pheromones for the wax moth. With perhaps the most provocative title in this review, Irresistible Bouquet of Death, only topped by the keywords, Carcass Attractiveness, Kalinova´ et al. again used SPME with GC × GC-TOFMS and GC-EAD, this time to study how burying beetles are attracted to dead mice (81). GC × GC-TOFMS allowed the identification of sulfur-containing volatile organic chemicals evolved after death that are likely stimulants for carrion location by the beetles. Although a very interesting read, this is one paper not to be studied over lunch. Clinical and Forensics. GC × GC-TOFMS was used to analyze opiates and benzodiazepines in human serum after solid phase extraction and derivatization (82). Excellent second dimension separations from the higher-concentration matrix interferences were achieved for the subject compounds on the 50% phenyltype column. A limit of quantification of approximately 5 ng/mL was noted for Flunitrazepam in serum, significant since it is necessary to detect therapeutic levels at
