Anal. Chem. 2006, 78, 3985
Gas Chromatography Gary A. Eiceman*
Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003-0001 Jorge Gardea-Torresdey
Department of Chemistry, University of Texas, El Paso, El Paso, Texas 79968 Frank Dorman
Restek Corporation, Bellefonte, PA 16682, and Chemistry Department, Juniata College, 1700 Moore Street, Huntingdon, Pennsylvania 16652 Ed Overton, A. Bhushan, and H. P. Dharmasena
Louisiana State University, Baton Rouge, Louisiana 70803 Review Contents Reviews, Books, and General Interest Columns Principles and Technology General Information Stationary Phases Fundamental Characterizations High-Speed and Portable Gas Chromatography General Information Microfabrication of Components Portable Gas Chromatographs Multidimensional Gas Chromatography Gas Chromatographic Detectors Spectroscopic Detectors Mass Detectors Emerging Detectors Data Processing and Analysis Summary Literature Cited
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This review of the fundamental developments in gas chromatography (GC) includes articles published from 2004 and 2005, and emphasis is given to those developments that are thought particularly significant for basic advances in GC. Any measure of activity for GC during this and recent past decades has shown that applications of GC easily eclipse that devoted to fundamental studies in GC, and this is seen in the number of articles published. Such references are excluded from or restricted in this review since emphasis is given to basic developments. Nonetheless, laboratory activity in GC was seen again as vibrant and innovative for technology and methodology and little or none of this is evident in the discussion below. Instead, aspects of retention, resolution, and detection and the development of these topics are treated here in a selective manner. Not every article of merit could be discussed or cited, and numerous fine works were not referenced here for sake of space. 10.1021/ac060638e CCC: $33.50 Published on Web 00/00/0000
© 2006 American Chemical Society
REVIEWS, BOOKS, AND GENERAL INTEREST Books or proceedings in all aspects of GC peaked in number in the mid-1970s, and there has been a steady slow decline since with only several volumes per year published in the present and recent past review cycles. Nonetheless, some significant and valuable monographs were released during the past two years and include Modern Practice of Gas Chromatography, 4th ed. (A1) and a broad discussion of chromatography with significant sections on GC (A2). These new texts supplement existing fine volumes (A3, A4) for both experienced and prospective investigators in GC and should form the core of the holdings in GC of a private or shared library. Several texts on specialized topics in GC were also published and include a welcomed fresh treatment of inverse gas chromatography (A5) and a detailed discussion on the chemistry underlying response in electron capture detectors (A6), which was last described in detail in the early 1980s. Proceedings were released from the 27th International Symposium on Capillary Chromatography (A7) and provide a compact sampling of those topics in GC today which are the center of interest and development. As a compliment to the texts noted above, several reviews were published and provide worthy discussion and treatment of the fundamental topics in GC. In one review on the theoretical principles and methods of determination of partition coefficients, analysis of retention theory, imperfections in measurements, or errors in determinations were all treated (A8). Though the discussion is centered on packed column technology, there is broad value in the presentation and the principles can be extended to contemporary column technology. Another useful review of fundamentals for GC was found in a discussion of gas-solid adsorption isotherms (A9). The methods of data analysis are described as are the interpretations of experimental findings for both inverse gas chromatography and reversed-flow GC. A broad treatment of instrumentation in GC was seen as a current primer on the condition and limits of technology and practice (A10). This Analytical Chemistry, Vol. 78, No. 12, June 15, 2006 3985
type of review is helpful in accessing where advances in GC can be made and was published in a new journal devoted to expert reviews. One of the most welcome developments this review cycle was the appearance of reviews on two-dimensional GC signaling the coalescence of a core of experience and the stabilization of methods with this powerful approach to GC separations (A11A17). In one review, emphasis was given to time-of-flight mass spectrometers as detectors for two-dimensional GC (or GC×GC) where fast acquisition rates and peak shape allow signal processing for the identification of large numbers of substances. Though emphasis in the review was on applications, the summary provides a broad contemporary measure of GC×GC as a separations technique. In a sign of maturation, applications are appearing in significant numbers for two-dimensional GC with emphasis given to polyhalogenated aromatic hydrocarbons in environmental samples, flavors, and fragrances, fatty acid Me esters and essential oils, and hydrocarbons in petroleum samples (A14), some of the most favored early uses of GC. Another sign of maturation in multidimensional GC is the growing attention given to processing data, and this can be seen with the discussion of chemometrics to extract information for extended evaluations of experimental findings (A16). Another helpful broad overview of GC×GC is found in a specialized journal and is worth the effort to locate and study (A17). These reviews, as a collection, offer an expansive discussion of two-dimensional GC spanning principles, practices, molecular interpretations, and applications. Further discussion about GC×GC can be found later in this review. Reviews were found concerning one of the most demanding separations in GC, direct enantiomeric separation of optically active components with chiral stationary phases (A18, A19). Though this field is decades old, progress and advances are occurring and necessitate reviews. Particularly useful is the organization of one review where methods are grouped by the type or class of chiral phase. Though enantiomeric GC is likely to be seen as a specialized topic within the whole body of work in GC separations, contemporary reviews provide perspective on the developments and status. Another specialized topic, liquidcrystalline stationary phases, was reviewed with fresh discussion from the basics of separations to examples of uses (A20). COLUMNS PRINCIPLES AND TECHNOLOGY General Information. As noted in the previous two review cycles, most separations in GC are performed using capillary columns containing one of a very few number of stationary phases (B1, B2). While this may be seen as positive in terms of the efficiencies these phases exhibit, the limited number of selections is certainly also a limitation since the stationary phase has potentially the largest impact on a separation. Over the course of the last two years, however, there seems to be increased activity in research on and publications about new stationary phases. Several new commercially available phase chemistries have been introduced and directed toward application specific separations. Additionally, there has been an increase in reported successful investigations toward possible column format changes, led most notably by Professor Richard Sacks (B3), who tragically passed away in February 2006. This paper demonstrates the first reported publication of an efficient liquid phase-coated square-channel 3986
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microfabricated GC column for possible use in separations with miniaturized and low-power technologies. Both Professor Sacks and his research will truly be missed in the scientific community. Investigations of alternative column formats were also reported (B4) as was the use of monolayer-protected gold nanoparticles in square-channel microfabricated columns and silica-packed microfabricated columns (B5). While the use of microfabricated columns has attractive possiblities, an alternative is nonconventional temperature control with common fused-silica columns (B6). This transfers the burden of innovation from column design to control of heating with columns that already exhibit high efficiency and are readily available. There was also a report on using hightemperature carborane-coated Zylon fibers as packing materials inside capillary tubing. (B7). This was one of many papers over the last two years to investigate the analytical utility of carborane/ siloxane copolymers, as opposed to siloxane-only polymers for high-temperature GC work. While this is not a new concept, there is seemingly a resurgence in interest with extended-temperature analyses. Stationary Phases. Most new studies with stationary phases in capillary GC are related to the use of derivitzed cyclodextrin molecules as selectors for molecules with specific configurations. While these molecules can allow for a high degree of selectivity, they are not routinely used in separations due to limitations in thermal stability. As a result, specific or tailored phases typically are synthesized for the separation of certain isomers (B8-B12). Schurig and co-workers have also published work on the use of liner dextrins for similar isomeric separations (B13). Liquid crystals as GC stationary phases may also suffer from similar thermal limitations as derivitized cyclodextrins, but offer a level of selectivity for isomeric separations unavailable with conventional siloxane-based polymers and merit development (B14, B15). These phases tend to have narrower operational ranges than derivitized-dextrin chemistries and are generally selective over an enlarged variety of compounds in that range of temperatures. Armstrong et al. reported further on the use of ionic liquids for GC separations (B16-B18). These materials have been shown as useful for chiral separations as well as for general purpose use. They represent the newest category of materials used for GC stationary phases and exhibit strong retention selectivity not found in siloxane-type phases. Recent modifications have extended the operating temperatures to 350 °C and are commercially available from Advanced Separation Technologies Inc. Additionally, there have been reports on new commercially available siloxanes for separations of volatile compounds (B19), chlorinated dioxin and furan separations (B20), and cis- and transfatty acid methyl esters (B21). This measure of high activity with the introduction of new stationary phases is unlike the previous decade where choices narrowed when bonded-phase capillary columns were introduced to the chromatographic marketplace, and several new stationary phases were introduced this past review cycle. These tend to cluster into two categories: those with low bleed and high inertness and those with selectivity for either polar or more functionalized target compounds. Both Supelco and Restek have introduced new column lines based on chemistry that results in lower bleed. Supelco’s SLB column line is designed for GC/MS separations and uses silarylene chemis-
tries (B22). The Restek Rxi column line utilizes chemistry and deactivation improvements to lower bleed levels and improve inertness to reactive compounds (B23). Functionalized phases were also introduced as the Restek Rtx440 for use as a general purpose column (B24), which exhibits improved retention for polar and aromatic compounds. A phase with high polarity known as the BPX-90 (SGE) provides improved bleed and thermal stability over other high-cyano-contaning polymers due to the use of arylene backbone stabilizers. (B25) The underlying message in these events is that column performance and efficiencies of the first generation of bonded-phase capillary met many but not all expectations and that requirements for additional selectivity, resolution, or stability have existed and are now being addressed. Finally, there was a report on a new modification to bentonite as a packed column phase, which demonstrates that interesting and innovative research persists in an area that has been considered mature and possibly dormant (B26). Packed columns still remain the format of choice when separation of highly volatile compounds is required, and the versatility offered by such separations is welcome and should be retained as useful tools amid the advances in the improved column formats and stationary phases for capillary columns. There were also a few application-related publications where various stationary phases were evaluated for analytical performance, and these, while not fitting into the scope of this review, still warrant comment because of the value of expansive or global evaluations of numerous phases or of specific separations. Fishman et al. reported on the use of most 5% diphenyl capillary columns for the separation of the chlorinated dibenzo-p-dioxins and dibenzofurans (B27), and the performance of both diphenyldimethylpolysiloxanes and arylene-modified polysiloxanes was compared. Korytar and co-workers evaluated several different column chemistries for the separation of the environmentally important polybrominated diphenyl ethers (B28), where congener distributions were evaluated in commercial mixtures that are commonly used as flame retardants. Bjorklund has also investigated the separation of these compounds on many commercially available phases (B29). These studies are valuable as a whole since many of these columns are considered equivalent; instead, unique selectivity is shown for sensitive congener separations and columns are not interchangeable. This demonstrates the need for additional work in the understanding of fundamental characterization of column chemistries and the resulting separation from even minor differences in the stationary phases. Perhaps such works when seen as a sum may stimulate refined questions, studies, and understanding of structure and retention in GC. Fundamental Characterizations. Characterization of the mechanisms of retention is vital to the understanding of separations in GC columns, and studies of the past review cycle may be fit into three categories: (1) tabulations of retention of various compounds on different stationary phases, so that a best phase can later be chosen for another separation, (2) structure-based predictions developed from empirical information that attempts to relate retention to various physical properties of both stationary phase, and target analyte, and finally, (3) thermodynamic and or molecular modeling approaches that may predict and optimize separation based upon physical chemical principles.
Continuing to lead in characterizing numerous commercially available columns, Poole and co-workers have taken the first and second approaches listed above (B30, B31). In their research, even recently introduced columns such as the HP-88 and the Rtx440 referenced in the previous section, have been characterized. The core of understanding of a separation is when retention and structure can be described, and GC×GC has been suggested as a tool of new opportunity to advance this understanding and to explore difficult to isolate processes (B32). These may include chemical decompositions, molecular interconversions, various nonlinear chromatographic effects, and kinetically limited events, and GC×GC was seen as a tool for another level of chromatographic investigations. Finally, some studies were published with modeling of thermodynamics of solution. These tended to be directed toward a narrow target group of molecules. The modeling of the separation of a series of esters on several different phases was described (B33). Additionally, studies were shown for the modeling of a series of dialkyl sulfides on metallomesogenic stationary phases (B34) and the solvation enthalpies and heat capacities of four phases with 6-12 carbon normal alkanes (B35). The subject of thermodynamic modeling has become a relatively quiet region of GC investigations though some advances have been made with the use of chemometric methods of principal component analysis and neural networks, as described below in the section on Data Processing and Analysis. In summary, research conducted on new stationary phases in GC was reported at a level not seen in recent cycles with much involving noncyclodextrin materials. These phases should expand capabilities in separations for improved ranges of substances, and a rapid transition from research to marketplace has occurred. This is seen here as beneficial for the subject of GC separations since the chemistry of stationary phases exerts a major influence in the separation event. The lack of many fundamental characterization publications is a shortcoming, and the long-held dream of convenient, user-friendly, general predictive and interpretative tools for retention remains unavailable. At present, only specialists with narrow boundaries are able to make predictions of retention, separation, or efficiency. In this sense, chromatography remains an empirical science. HIGH-SPEED AND PORTABLE GAS CHROMATOGRAPHY General Information. Interest in high-speed gas chromatography has been focused during this past review cycle in three areas. These include the following: (1) demonstrations of highspeed GC for increased efficiency in the analysis of samples with minimum demand on valuable analytical and laboratory resources, (2) developments of rapid injection techniques and other instrument parameters to exploit the full capability of the separating efficiency of short, narrow-bore capillary columns with hydrogen carrier gas and fast temperature programming, and (3) clarification of the need for microfabricated and small, very fast, GC-based analyzers in on-site determinations of substances associated with monitoring for defense and homeland security. An example of this last focus, which is influencing current interests in high-speed GC is a Department of Defense requirement for an ultralow power micro gas analyzer capable of detecting target compounds within 4 s at sub-ppb levels. Clearly, a project like this will require the Analytical Chemistry, Vol. 78, No. 12, June 15, 2006
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innovation and integration of fast GC methods with microfabrication manufacturing. High-speed gas chromatography and portable gas chromatography (small and microfabricated GC systems and components) are not necessarily synonymous, though substantial areas of overlap frequently exist. The usefulness of portable GC applications comes largely from the short time delay between sample collection and sample analysis. Thus, the need for fast on-site GC analysis is at least implied in these field analytical situations. More importantly, the applications where quick answers are needed almost always require that the sample collection and analysis functions be integrated and rapid. Elimination of sample transport and storage are attractions and motivations for small, on-site instruments. However, these benefits are substantially lost without the high-speed separations and detection needed to complete the transformation of a sample into useful or reliable chemical information. Current on-site GC analyzers are significantly smaller and more compact than laboratory instruments although size, weight, and power consumption are often serious limitations to true portability since the units may contain conventionally manufactured components. Microfabricated injectors, columns, detectors, and integrated electronics, as well as hybrid integration of these components, offer opportunity for true portability in an analytically powerful GC analyzer. Moreover, microfabrication offers the promise of achieving the performance of conventional GC analyzers in an instrument package that is at least 2 orders of magnitude smaller and greater than 1 order of magnitude faster than current portable GC instruments. However, such potential reduction in size and increase in speed have not yet been fully realized. The domain of high-speed or fast GC can span a range of topics from comprehensive 2D gas chromatography using open tubular or capillary column separations that are fast in both dimensions to the other extreme employing millisecond separations of a few compounds. Separations using capillary columns, that were thought fast compared to those with packed columns, are now standard practice. The current convention for chromatography is the laboratory GC performing separations with 10-30-m columns having inner diameters of roughly 0.18-0.32 mm and using average carrier gas velocities between 30 and 100 cm/s with temperature program rates generally around 50-60 °C/min. Systems that perform somewhat equivalent separations in less than 10% of this time can be considered a fast GC system. Finally, fast GC using narrow-bore capillary columns is not necessarily high-resolution GC. Some of the fastest chromatograms have peaks capacities on the order of 10-20, but when the separation occurs in tens to hundreds of milliseconds, they certainly qualify as fast GC. Fundamental advances in high-speed gas chromatography developments can be categorized as improvements in (a) mass transport efficiency in the column, (b) column heating efficiency allowing higher temperature programming rates, (c) sample introduction systems designed to load narrower bands onto the column, (d) multidimensional techniques, and (e) improvements in extracolumn instrumental performance. Often, compromises are made between resolution and resolution rate (peak capacity per unit time). The goal of fast GC development is the production of the highest peak capacity per unit time, within the constraint that the resolution of the system must be adequate 3988
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to solve an analytical problem. The use of ultra-narrow-bore, 50µm-i.d. capillary columns has been reported for very fast separation of highly complex mixtures of fuels (C1). These researchers used a 5 m by 50 µm column with a thin 0.05-µm film to achieve highresolution separations of complex hydrocarbon blends in ∼3 min. Traditionally, comprehensive two-dimensional GC relies on a fairly slow first-dimension separation with very rapid separations using thermally modulated injections onto a short seconddimension column. In a variation of this, a short first-dimension column was employed to lower the elution temperature and thus allow analysis of thermally labile compounds (C2). A complication with this and other fast GC analyses is the reduced peak capacities and resolution compared to slower analysis on the same column. One way to overcome this limitation is through the use of selective detection with fast separations. Researchers developed a compact laser ionization detector that provided selectivity for aromatic compounds when compared to saturated compounds with detection limits in the range of 1 pg (C3). Microfabrication of Components. Interesting developments have been made in the field of microfabricated GC systems including reports on the use of microelectromechanical system (MEMS)-based columns, injectors, and detectors; these, when integrated as a system, contain the promise of very rapid analysis in a miniature system. Because of the constraints imposed by MEMS-based manufacturing, most column developments have been focused on rectangular cross section columns. Researchers at the University of Michigan (C4) have achieved 1000-2800 plates/m using 3-m-long, 150-µm-wide, and 240-µm-tall columns. These columns, which were fabricated by deep reactive ion etching, are arranged in a square spiral pattern on a 3.2-cm square silicon substrate. Isothermal separation of 20 compounds (methanol to butyl acetate/chlorobenzene) was performed in 4 min by using hydrogen carrier gas and in 10 min by using air carrier gas. When a temperature program was employed, both the separation time and the peak capacity showed significant improvements over isothermal separations. The microfabricated column can be interfaced with emerging detectors, for example, with a microfabricated differential mobility spectrometer where both positive and negative ions are characterized in a single detector (C5). The separation time in these columns is currently limited by their temperature programming rate (10-30 °C/min). Golay and Spangler (C6, C7) have suggested that a high aspect ratio for a rectangular cross section GC columns offers the potential for high resolution, governed by the narrow dimension, and high carrier flow rates governed by the wide dimension. Researchers at Louisiana State University (C8) are building high aspect ratio metal rectangular cross section columns. These columns are 50 µm wide × 600 µm tall channels of 0.5-2-m length and are fabricated with electroplated nickel using the LiGA technique. Six to twelve carbon alkanes have been separated in under 1 s. Discussion of this technology has been centered on the ability to coat these very narrow columns and the effects of the flow dynamics on the column’s height equivalent to a theoretical plate associated with the square ends of rectangular columns (C9). Chromatographic separation of 15 compounds in under 5 s was shown possible with a diaphragm valve-based rapid injection system and conventional short capillary columns with very high
carrier flow velocities (C10). The diaphragm valve provides small injection pulses which, when passed through a 2-m, 180-µmdiameter capillary column, gives the spectra of separated components in ∼5 s. This method is suitable for separating simple mixtures; extension of the method to complex mixtures yielded partially separated compounds (overlapping peaks) and postanalysis processing using chemometrics was needed. Since the total analysis time of a high-speed GC separation can be very small (
