Gas Chromatography - Analytical Chemistry (ACS Publications)

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Anal. Chem. 2000, 72, 137R-144R

Gas Chromatography Gary A. Eiceman*

Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003-0001 Herbert H. Hill, Jr.

Department of Chemistry, Washington State University, Pullman, Washington 99164 Jorge Gardea-Torresdey

Department of Chemistry, University of Texas, El Paso, El Paso, Texas 79968 Review Contents Reviews, Books, and General Interest Solid Adsorbents and Supports Natural Adsorbents Synthetic Adsorbents Liquid Phases Synthetic Organic Phases Chiral Phases and Natural Phases Chromatographic Theory Columns and Column Technology Multidimensional Gas Chromatography Data Processing and Quantitative Aspects Analysis of Peak Shape Pattern Recognition and Artificial Intelligence Quantitative Aspects High-Speed Gas Chromatography Detectors Literature Cited

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This review of the fundamental developments in gas chromatography (GC) includes articles published from 1998 and 1999 and an occasional citation prior to 1998. The literature was reviewed principally using CA Selects for Gas Chromatography from the Chemical Abstracts Service, and some significant articles from late 1999 may be missing from the review. In addition, the online capability SciSearch Database (Institute for Scientific Information) was used as a second means to scan articles within certain topics. As with the prior recent reviews, emphasis below has been given to the identification and discussion of selected developments and the review should not be regarded as a comprehensive literature search. Most trends that were recognized during the last review cycle persisted during the past two years. These include advances in both the foundations and the practice of two-dimensional GC and high-speed GC. The computational-based methods of extracting information from chromatograms was also an active area, and there is a sense that this facet of treating data may have passed out of the introductory stages. Also, there has been a growing understanding that a unified model for retention time prediction may soon arise, but no such model yet exists. As may be expected with a mature technique, changes have been gradual in the various topics treated below. No dramatic advances in resolution or in 10.1021/a10000054 CCC: $19.00 Published on Web 04/25/2000

© 2000 American Chemical Society

principles of separation were disclosed or suggested, as many may hope with so well-established and helpful a technique. REVIEWS, BOOKS, AND GENERAL INTEREST Review articles on GC appeared during this review cycle and included a broad discussion of the trends in GC developments and applications (A1). The conclusion from the review, principally with articles from the mid-1990s, was that gas chromatography remains a healthy and growing measurement technique with expanding influence. This is consistent with the findings from this review where the vitality of GC is found in helpful and innovative applications with increasing measure. Still, as in prior reviews, fundamental developments constitute a small component (∼20% or less) of all publications in GC. One review, which should be useful with a growing trend in stationary-phase offerings, was the discussion of methods to characterize the chromatographic properties of stationary phases (A2). This reminder is welcome in view of the growing selection of stationary phases and after a decade where most separations were completed on only a few stationary phases, as available in bonded phase capillary columns. Another review, from past technology, was given (A3) for porous layer open tubular (PLOT) capillary columns. Once seen as means for providing excellent resolution with sample sizes greater than available in thin-film capillary columns, PLOT columns were situated in performance between packed and capillary columns. These properties are balanced against some limitations and the authors suggest that the value of PLOT columns for future users remains unclear; nonetheless, this review may serve as a point of reference for speculation on the future possibilities of PLOT columns. In common GC experiments, the carrier gas is intended only for mass transport. A review (with 145 references) on the role of carrier gases on the separation process (A4) demonstrates that carrier gas interactions are integral to the chromatographic process. This may be regarded as inadvertent by some, yet others intentionally modify the carrier to obtain specialized effects. One such gas, ammonia, was the subject of a review on interactive gases in GC (A5). Examples were presented and discussed where ammonia, blended at low levels in the carrier gas, has improved peak symmetry and detection limits. The subject of pyrolysis GC, long seen as a means of probing the chemical composition of nonvolatile materials using GC, was Analytical Chemistry, Vol. 72, No. 12, June 15, 2000 137R

treated in an introduction (A6) and a review with extensive bibliography for analysis of synthetic polymers (A7). These should be valuable resources for novice and experienced users alike, regardless of interests with specific polymers. Several reviews or general discussions of uses of GC were published in the review cycle and are included here as a gauge of the level of maturity of GC principles and technology: explosives (A8), steroids (A9), and pheromones (A10). A book appeared on chiral chromatography (A11) and one on general chromatographic principles (A12) with treatment of GC in both books. SOLID ADSORBENTS AND SUPPORTS Discussions in this section focus on reports where solid adsorbents are described or characterized with emphasis on new or modified materials. As in prior reviews, inverse gas chromatography (IGC) was a prominent method for exploring surface structure and interactions between solid surfaces and probe solutes. Natural Adsorbents. A new method was applied to analyze the catalytic fibrous carbon microtexture using adsorption of halogenated benzene derivatives (B1). In another report, the heat of adsorption (infinite dilution) of coal was measured by GC (B2). This study gave a reasonable explanation for the nonlinear elution of sorbates of low molecular weight. Others studied the adsorption of organic compounds differing in geometry and electronic distribution on carbon fibers (B3). Three types of active carbons were evaluated for their adsorption properties of volatile organic compounds (B4). More recently, the same investigators reported a new method of calculating Kovats retention indexes for substances chromatographed on two types of active carbons (B5). Microporous silica gel with differing mechanical properties was characterized for adsorptive properties toward refrigerating fluids (B6). In another report, silica and mullite powders were prepared by the sol-gel method and their surface characteristics were studied by IGC (B7). Also, IGC was used to study the physicochemical properties of silica gel modified with selected titanates (B8). Reports on three other natural adsorbents and supports, including alumina, clays, and cellulose, were given during this review period. The gas-solid chromatographic separation of hydrogen isotopes, with palladium-modified alumina and Kieselguhr, was evaluated (B9). For the same Pd loading, alumina was found to be more efficient than Kieselguhr in separating the isotopes of hydrogen gas. Heats of adsorption for hydrocarbons on Laponite-RD clay were determined by two GC methods (B10). The first method was an extension of packed-column gas-solid chromatography. The second method employed a novel wallcoated open tubular (WCOT) column that was prepared from clay aqueous suspensions. The WCOT method provided the same thermodynamic results, as did the conventional packed-column method, while offering some definite improvements in the measurement process and data quality. Another study reported the wetting transition in alkane liquids on silanized diatomaceous GC supports (B11). The investigators found a new version of the wetting transition model that takes into account the pore structure of the diatomaceous support and the surface tensions of the stationary liquid and support. A report also appeared on the 138R

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use of IGC, at finite concentrations, for the evaluation of the surface properties of hematite samples obtained by the heat treatment of goethite (B12). It was shown, from the distribution of the adsorption sites of alkanes, that the surface of hematite undergoes significant changes when heated to 500 °C. Others reported the use of IGC to examine the adsorption behavior of cellulose surfaces from elution characteristics of different adsorbates (B13) and to evaluate the physicochemical modifications of partially esterified cellulose (B14). In this study, it was found that the attaching of alkyl chains to the surface of cellulose reduced acidic character and hydrophilicity; the degree of substitution required for such an effect depended on the chain length. Synthetic Adsorbents. IGC was utilized to explore properties of several synthetic materials, and reports have included the following: the adsorption isotherm and heat of adsorption of pyridine on poly(imide)siloxane (B15); the determination of acidbase properties of solid surfaces (B16); the determination of solubility parameters of polystyrene (B17); the characterization of lignocellulosic fibers modified with maleic anhydride (B18); the evaluation of surface properties of modified carbon blacks obtained by vacuum pyrolysis of different used rubbers (B19); and the characterization of bleached-modified eucalypt kraft pulp fibers (B20). Additionally, thermodynamic and surface properties, solvent interaction, blend transition behavior, surface energies, and solubility parameters were all characterized using IGC for several new polymeric materials (B21-B32). Others studied the influence of temperature on the polarity of new porous polymer beads (B33). In another work (B34), three types of porous polymers containing different functional groups were synthesized as chromatographic materials for GC. The influence of functional groups in the copolymer skeletons on their selectivities was studied. Synthetic inorganic materials were the subject of several reports. Some examples of new materials include the following: zinc sulfate crystalline hydrates (B35); polybutadiene-coated zirconia (B36); titanium, lead, and iron oxides (B37); and platinum-rhodium bimetallic catalysts (B38). In another report (B39), the synthesis of a fullerene-containing stationary phase by chemically linking C60 to an aminopropylpolysiloxane was described. This new stationary phase was used in GC for the separation of polychlorinated biphenyl (PCB) isomers with different degrees of substitution in the ortho position. This stationary phase showed increased affinity with increased planarity of the PCB molecule. LIQUID PHASES Various types of liquid phases have been described during this review cycle and include synthetic organic phases, chiral and natural phases, and metal-based phases. Synthetic Organic Phases. Several types of liquid crystalline phases were reported as possible GC stationary phases. The retention behavior and separation properties of mixed liquid crystalline and resorcarene phases were evaluated using disubstituted benzene isomers (C1). The mixed phases showed that they have a “synergistic effect” in their separation properties. A new naphthalene-containing side-chain liquid crystalline polysiloxane stationary phase for GC was synthesized (C2). This phase

has high efficiency in the separation of isomers of polychlorinated dibenzo-p-dioxins, polynuclear aromatic hydrocarbons, and pesticides. Also, other liquid crystalline phases were evaluated for separation of isomeric compounds of polynuclear aromatic hydrocarbons (C3) and of 2,3,7,8-tetrachlorodibenzodioxins (C4). Stationary phases using crown ethers were also reported (C5C11), and one mixed crown ether-cyclodextrin phase was synthesized for capillary GC (C5). This phase showed excellent selectivity for the separation of enantiomers and positional isomers. The investigators determined that the combined effect between the special caves of β-cyclodextrin and crown ether exerted a significant influence in the separation. Others found that a mixed crown ether-cyclodextrin phase had synergistic effects in the separation of isomers (C6). A similar synergistic effect was reported for a phase mixed with crown ether and o-methyl-pphenylenebis(p-heptoxy benzoate) (MPBHpB) (C7). Some of the separations revealed positive or negative synergistic effects, which depended on the temperature, the mixing ratio, and the linear velocity of the carrier gas. Other phases containing polysiloxanes with pendant hand-basket-type calixarene were prepared (C8). In this report, the mechanism(s) of specific selectivity for position isomers based on the calix[4]crown ether ring, the molecular size of the solute, and its shape were discussed. Two new phases containing catenary crown ether (C9) and dibenzo-24-crown-8 (C10) were reported. Other phases based on 4,13-diaza-18-crown-6 were used for capillary column GC (C11). These phases containing the diaza macrocycle were selective for hydroxyl-containing organic compounds. New polysiloxane stationary phases containing different functional groups were developed (C12-C19). Polysiloxane phases containing calixarenes (C12, C13), [60]fulleropyrrolidines (C14, C15) and methyl, phenyl, trifluoropropyl, and cyanoalkyl groups (C16, C17) were studied. Others reported a new phase that consisted of 50% methyl-50% phenyl polysiloxane, which was prepared by using an in situ process (C18). This phase showed unique elution orders of PCB congeners and the sum of the McReynolds constants was 5 times lower than on conventional equimolar methyl-phenyl phases. In another study, a new phase (Optima-δ-3) with a methyl-phenyl-polysiloxane basis, was also used for the separation of PCB congeners (C19). In addition, a review with 30 references on the application of silicones as GC stationary phases was given (C20). Others evaluated the new phase, N-methyl-2-piperidone, for the measurement of activity coefficients for 36 organic compounds (C21). In another report, activity coefficients for refrigerants were evaluated with a polyol ester oil stationary phase (C22). Two new stationary phases containing olefinic groups (C23) and R-(3-chloroazobenzene-4-oxy)methylnaphthalene (C24) were characterized. The olefinic stationary phase showed that the presence of the electronwithdrawing chlorine atom in polychloroprene contributed to its ability to act as a hydrogen bond donor acid. In another study, water vapor was added to the carrier gas to study the effect of moisture on the retention properties of a poly(ethylene glycol) GC stationary phase (C25). In this study, a dramatic increase in hydrogen bonding was observed toward alcohols and carboxylic acids and the Kovats index for methanol was found to increase by 351 units. Chiral Phases and Natural Phases. As in the past review, the most common phases in this section were based on cyclo-

dextrin and cyclodextrin derivatives (C26, C27). The separation mechanism of isomers and enantiomers of branched C10-C12 phenylalkanes on GC stationary phases containing modified cyclodextrins was studied (C26). In this study, the shape selectivity factors of modified cyclodextrins indicated no inclusion of the considered solutes in cyclodextrin cavities. Thus, the authors concluded that the enantioselective interactions most probably occur on the outer sphere of cyclodextrins. Others reported that the size, polarity, and aromatic properties of the substituted group at the 3-position of heptakis-modified cyclodextrins greatly influenced the chromatographic properties and separation ability of the stationary phases (C27). The introduction of an aromatic group or a group containing a double bond may bring π-π interactions between the host and guest molecules and, therefore, increase the separation ability of the modified cyclodextrins for substituted benzene isomers. Gas chromatography on polysiloxane-anchored cyclodextrin derivatives has been used to separate the enantioners and isomers of different classes of compounds, such as PCBs, benzosuberan, chlorofluorocarbons, amino acid derivatives, chiral organochlorines, and volatile organic compounds. However, these studies are not cited in this review because they do not discuss the fundamental mechanisms of the interactions of the solute with the stationary cyclodextrin phase. A new polymeric Pirkle-type chiral stationary phase with a silicon backbone and substituted side chains of benzamide for GC was reported (C28). This phase has a good selectivity for enantioners due to the π-π interactions, and this phase expands the use of Pirkle phases into the field of open tubular column chromatography. New transition metal complex stationary phases for GC were reported (C29). In this study, the polymeric phases were obtained via polycondensation followed by polymerization of the corresponding dichloro- or dimethoxysilanes. Both the phases were modified by bonding transition metal chlorides with cyano (copper or cobalt) and thiol groups (nickel or cobalt). The phases were examined to determine their application to the analysis of ethers, thioethers, and ketones. The fundamental separation principle of these phases is due to the presence of lone electron pairs on oxygen or sulfur atoms, which should be capable of specific interacting with the electron-withdrawing center of the liquid stationary phase. In another study, the retention of RohrschneiderMcReynolds standards and selected chiral alcohols and ketones was measured in order to determine the thermodynamic selectivity parameters of new stationary phases containing tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] derivatives of praseodymium(III), europium(III), erbium(III), dysprosium(III), and ytterbium(III) dissolved in poly(dimethylsiloxane) (C30). In this study, the determined values of thermodynamic enantioselectivity were correlated with the molecular structure of the solutes studied. The decrease of the ionic radius of lanthanides induced greater increase of complexation efficiency for the alcohols than for the ketone coordination complexes. The selectivity of the studied stationary phases was found to follow a common trend which was rationalized in terms of opposing electronic and steric effects of the Lewis acid-base interactions between the selected alcohols, ketones, and lanthanide chelates. Analytical Chemistry, Vol. 72, No. 12, June 15, 2000

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CHROMATOGRAPHIC THEORY In the past reviews, major themes for this section were based on the following: (1) understanding chromatographic behavior for a solute using only molecular structure and stationary-phase properties or (2) associating thermodynamic parameters with efficiency, resolution, or retention. These topics continued to be vibrant though easily overlooked areas of GC. For example, over one hundred references, most of which involved basic investigations, can be found on retention alone in GC. In contrast, the number of reports involving attempts to link molecular structures to retention times or indexes was dramatically lessened in comparison to past review cycles. Some advances suggest that the last stage of accomplishment, i.e., a unified prediction scheme for retention, are nearly ready to be prepared. A summary work from four decades of study (D1) was presented as a unified system for predicting retention for various stationary phases and temperatures, but only under isothermal conditions. While valuable, isothermal conditions offer limitations in general applications of GC where temperature programming (TPGC) and pressure programming are commonly practiced. Fortunately, the influence on retention from pressure effects and carrier gas have been described and modeled and should be useful for continued refinements on predictive capabilities (D2, D3). Indeed, detailed and automatic prediction of retention times with programmed pressure isothermal GC has been reported (D4). When temperature programs were used in GC, the percent standard deviation for a homologous series was measured as 0.10.6 depending upon the experimental methods. A particularly notable work reopened the discussion of the meaning and validity of a retention index for temperature-programmed separations (D5). The misuse of retention indexes with TPGC was emphasized and should be carefully heeded by anyone wishing to make basic measurements that can be reliably shared by chromatographers. An alternate view to this was published with an attempt made to provide meaning and interpretation to retention indexes with TPGC (D6). Retention indexes based upon n-alkanes have been the subject of general criticism with polar stationary phases, and an alternative reference system was proposed for polycyclic aromatic hydrocarbons compounds and polychlorobiphenyls (D7). Another long-recognized component of GC behavior, i.e., linear plots for the logarithm of retention time (or adjusted retention time) versus carbon number, was questioned and shown to be flawed (D8, D9). As noted above, few studies were reported on quantitative structure-retention modeling; a few were given and included studies for polycyclic aromatic hydrocarbons (D10) and for alkylbenzenes (D11). These studies disclosed which molecular interactions were responsible for retention behavior as has been approached during the past decade or more. A new approach to this subject was seen with the prediction of retention indexes of alkanes, cyclic alkanes, alkenes, alcohols, esters, ketones, and ethers using artificial neural networks (D12). This work was a preliminary exploration where the network parameters were explored for 184 chemicals. Still, the findings suggest a promising means to discover structure-retention principles when properly managed within experiments. Special emphasis can be given to the stationary phase and merits mention here. In one work, the indicators of polarity for phases were explored using principal component analysis (D13). 140R

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Three principal components accounted for over 99% of data variance from 30 stationary phases and 8 possible indicators. The associated polarity indicators were divisible into two groups including (1) the sum of the first five McReynolds constants, Kovats’ coefficients, and Castello’s ∆C values and (2) Snyder’s selectivity parameters. Another significant work regarding stationary phases was the finding that temperature influenced the measured polarities and strong correlations existed between temperature and polarity (D14). COLUMNS AND COLUMN TECHNOLOGY The emphasis in this section is toward new GC columns and column technologies. Investigations of adsorption mechanisms have been included in the section on Chromatographic Theory. A tandem GC column system for the isomer and enantiomer selective separation of 18 toxaphenes was described (E1). In this technique, separations were achieved using a steep temperature program rate before a selected isothermal temperature. A highprecision selectivity tuning was also achieved using a seriescoupled (tandem) ensemble of a nonpolar and a polar capillary GC column with adjustable pressure at the junction point between the columns (E2). A change in the junction pressure produced a differential change in the holdup times for the two columns, and this resulted in a change in the fractional contribution that each column makes to the overall separation selectivity. In this study, an electronic pressure controller was used to obtain greater control and improved tuning precision. Others reported the modulation and manipulation of GC bands by using novel thermal means (E3). In this report, a small tube incorporating an internal sleeve cooled cryogenically, placed over a capillary GC column, could be moved backward and forward over the column to permit collection and remobilization of focused bands. Results demonstrated that peaks could be fully accumulated just prior to a detector and then rapidly flushed into the detector, allowing considerable increase in peak height as the peak width diminished. A novel dual-capillary GC column technology combining the advantages of PLOT and nonpolar narrow-bore WCOT columns was developed for the determination of C2-C9 hydrocarbons in air (E4). A refocusing step was not required due to the resolving power of the PLOT column for C2 and C3 hydrocarbons. The dual-column technology was superior to single-column systems due to better resolution of low molecular weight components. A silicon microchannel GC system was fabricated (E5) and involved the use of both wet and plasma etching. This microchannel system was coated with stationary phases and was successfully evaluated for the GC separation of organic compounds. A new type of capillary PLOT column consisting of a hydrophobic silica layer on a fused-silica capillary was tested for the separation of halocarbons (E6). Unlike alumina PLOT columns, this new silica PLOT column did not dehydrohalogenate labile halocarbons. Other reports have included a novel method to deactivate metal (stainless steel) capillaries for GC using perhydropolysilazane (E7), the development of a high-pressure GC microcolumn packed with 5-µmparticle-diameter silica particles with a variety of bonded groups (E8), and the development and characterization of three GC columns for the in situ separation of various gas components expected to be present in Titan’s atmosphere (E9).

MULTIDIMENSIONAL GAS CHROMATOGRAPHY As in prior review cycles, applications of multidimensional GC lead all reports with special interests in separations of natural products and biomedical samples. The evidence for advances in this topic with the formulation of a systematic methodology, first recognized in the last review, was largely untreated during this past review cycle. A few exceptions to this included a description of 2D GC for resolution of PCB mixtures (F1) where predicted separations were compared to actual separations. In addition, software tools for quantitative 2D GC were developed and tested (F2). Detection limits were improved with 2D GC (versus ordinary capillary GC methods) by 18×, and the relative standard deviation in determinations was 0.9%. Both of these terms suggest that 2D GC is worthy of improved attention and development. Finally, the principles and formulas for efficiencies with the peak isolation step were described (F3). Automation and hardware was a minor theme, much like the last review, though several important developments have occurred. A new way of handling and processing migrating solutes in capillary columns was described (F4), and a fully automated tandem gas chromatography system was created (F5). Finally, a robust and reliable thermal modulator was demonstrated (F6). The thermal modulator is the device that passes effluent from a first capillary column multiple sharp chemical pulses suitable for high-speed chromatography on a second column. These are critical devices, and disclosure of such a unit is a welcome development for further advances of 2D GC. DATA PROCESSING AND QUANTITATIVE ASPECTS The use of artificial intelligence and pattern recognition methods, such as neural networks and multivariate analysis, has altered the value of GC experiments. Computational tools have changed how chromatograms are viewed and how much information may be gleaned from a chromatogram. This was an active area during the past two years and continues to be a growth area as seen since ca. 1995. Analysis of Peak Shape. A small but important thread of investigations occurred in the treatment of peak shape as a means of obtaining from the chromatogram details that are not easily seen by inspection. These relied largely on deconvolution methods and were noteworthy in the use of mass spectral information to assist peak recognition rather than analysis of peak shape alone (G1, G2). In one instance (G1), the number of substances seen in the chromatogram of a mixture was increased 2-fold after processing the GC/MS data by deconvolution. In another work based upon peak shape or properties, methods were developed to rank peak significance in complex chromatograms through information theory (G3). Such methods should prove useful in distinguishing useful versus trivial information in chromatograms. Pattern Recognition and Artificial Intelligence. The use of neural networks and principal component analysis, among others, has now been demonstrated widely in handling complex GC results. Most of the reports in 1998-1999 concern results from specific applications and only a few are noted here. For example, chromatographic information was extracted from GC-ECD patterns of chlorobiphenyls (G4) accumulated in sea mammels and illustrated a degree of accumulation dependent upon age and gender. Others used GC chromatograms to classify alcoholic

beverages (G5) by neural networks and identify fuel spills by pattern recognition (G6). These are noteworthy to suggest that advanced data processing using the new computation tools is now a nearly routine means of extracting information from the GC experiment. These methods are also valuable for exploring basic chromatographic processes and principle component analysis was used to delineate the important parameters governing retention index in a GC separation (G7). The conclusion: boiling point is most influential in retention. Quantitative Aspects. A few reports concerned fundamental facets of quantitative determinations by GC. For example, the integration of peaks to obtain area values is highly dependent upon setting a baseline, and a method suitable to imitate human judgment was demonstrated (G8). Signal-to-noise ratios were improved using a longitudinally modulated cryogenic system (G9) assisting limits of detection. Quantitative performance of GC/MS was evaluated using multiple perdeuterated PAH with internal standardization methods (G10). A single internal standard provided worse accuracies than when multiple standards were employed. The quantitative aspects of GC×GC were explored (G11) in which the foundations were established for data processing to gain low error in retention time. HIGH-SPEED GAS CHROMATOGRAPHY The later part of the 1990s saw increased emphasis on speed and the efficient use of time. From on-line shopping to the review of scientific manuscripts, the obsession is to accomplish tasks in the shortest amount of time possible. The separation of chemically complex environmental, biological, and industrial mixtures by chromatography was no exception with a drive to decrease process time. Modifiers such as “high-speed”, “fast”, “rapid”, and “ultrafast” were commonly seen associated with the gas chromatographic separation process during the past two years. While the concept and theory of high-speed gas chromatography (HSGC) has been well-appreciated for more that 30 years, recent instrumental and engineering advances have enabled a routine practice of HSGC (H1). Also, strategies for the optimization of speed have been revisited and applied to different column types such as micropacked capillary, megabore, and multicapillary (H2). To increase the range of components that can be separated in a single chromatographic run, considerable effort has been placed on the development of rapid, temperature-programmed methods for high-speed gas chromatography. With a temperature program rate of 50 °C/min, normal alkanes up to C19 can be separated in 178 s with a peak capacity of 168 (H3). In addition to increasing peak capacity with temperature-programmed operation, selectivity in HSGC was controlled by programming flow (H4). By placing a series of columns of different polarities in tandem and adjusting the flow through each column, selectivity was programmed during the chromatographic run. The application of chemometrics to HSGC was also investigated and found to enhance the information possible from fast GC separations (H5). The reduction of analysis time for GC/MS has been one of the primary goals of HSGC during the past two years. A review of the HSGC/MS literature, describing possibilities and limitations of conventional GC/MS for HSGC, was reported (H6). The most exciting development, however, has been in the use of HSGC with time-of-flight mass spectrometry (TOFMS) (H7), which allows the Analytical Chemistry, Vol. 72, No. 12, June 15, 2000

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mass spectrum of each peak to be obtained as it elutes from the chromatograph. One pragmatic complication with interfacing a HSGC to a mass spectrometer is that the exit of the column is at vacuum conditions. Investigation of high-speed, vacuum-outlet GC has led to a better understanding of the influences that the vacuum outlet has on chromatography (H8). Related to interfacing HSGC with mass spectrometry, supersonic molecular beams were reported to be efficient for the introduction of laser-desorbed samples from a HSGC into a quadrupole MS (H9). Advances made in high-speed GC during the past two years have occurred in all areas of the method. Cryogenic focusing methods, laser ablation methods, and novel injector designs have provided improved sample introduction methods. Column design evaluation has helped to provide a clear understanding of the relationships of column capacity, speed, and efficiency. Standard detection methods such as photoionization detection continued to be of practical value. However, it is the potential of HSGC/ TOFMS that has captured the imagination of the chromatographic community, which is expecting a myriad of practical applications from this technique soon. DETECTORS Next to speed and resolving power, the major advantage of gas chromatography as a analytical separation technique is the wide variety of sensitive and selective detectors with which it can be interfaced. During the past two years, novel applications of GC separations with a multiplicity of detection methods have continued to be developed at a rate similar to those of previous years. The electron capture detector (ECD), the photoionization detector (PID), and the flame ionization detector (FID) were still the dominant detection methods of choice for the development of new analytical applications for organic compounds. Both mass spectrometry (MS) and ion mobility spectrometry (IMS) were used for qualitative analysis after GC. Various ionization methods such as helium microwave plasma ionization, field ionization, and electron capture negative ionization were reported for GC/MS. Atomic fluorescence spectrometry (AFS), atomic absorption spectrometry (AAS), inductively coupled plasma (ICP) spectrometry, ICP-mass spectrometry, the atomic emission detector (AED), and microwave-induced plasmas have been used for novel detection of compounds containing metal atoms. Other previously reported detectors, which have been used with GC during the past two years in novel analytical methods, include the Fourier transform infrared absorption (FT-IR), the electrolytic conductivity detector, the microwave-induced plasma atomic emission detector, the piezoelectric detector, the “electronic nose”, the flame photometric detector (FPD), the nitrogen selective detector, the phosphorus selective detector, the helium ionization detector (HID), and the thermal conductivity (TCD) detector. As in other years, combinations of detectors with different response factors served to provide qualitative information. The ability of these multichannel detection methods to identify compounds that were not well separated was enhanced by the use of a new data analytical method called “window target-testing factor analysis” (WTTFA) (I1). Although the flame ionization detector was one of the first GC detectors developed, its ionization mechanism is still not well understood and fundamental investigations into its chemistry is 142R

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still an active area of research. Confirming earlier studies that oxygen enhances sensitivity of an FID, an oxygen-enriched flame gas improved the sensitivity of the FID by 28-100% for compounds such as aromatic hydrocarbons (I2). One approach to predict the FID response resorted to an artificial neural network (I3) with good predictive powers for the response factors of several organic compounds. A complete review of the mechanism of the FID was presented with reiteration that the all-important ion for the FID response was the formylium ion, CHO+ (I4). Using a capillary probe inside the flame, it was shown that all hydrocarbons degrade to methane at low temperatures by the reaction of hydrogen atoms in the precombustion zone of the flame. The electron capture detector, another of the early GC detectors, continued to undergo development during the past two years. By modifying the design, improvements in sensitivity, linearity, dynamic range, and ruggedness were made for a micro-ECD (I5). Several other novel detector designs were also reported during the previous two years. Gas-phase electron diffraction was used as a method for detecting and identifying isomers (I6). Penning ionization with competitive absorption served to develop a detection method for argon (I7). Because a routine oxygen-selective detector for gas chromatography still does not exist, research in this area is still active. Progress toward this goal was achieved by the development of a negative, surface ionization detector designed for the detection of oxygen-containing compounds (I8). In addition, a detector for alcohols was reported based on gasphase molecular absorption spectrometry. The use of secondary electrospray ionization for the ionization and detection of volatile compounds was corroborated with a multiple channel, electrospray-ionization mass spectrometer after gas chromatography (I9). Although the realization of the full potential of analytical chemistry on a chip is still in the future, development of a molecular emission detector on a chip offered renewed promise for this challenging technology (I10). Using a plasma as an excitation source, methane could be detected with a limit of 3 pg/s. Gary A. Eiceman is a Professor in the Department of Chemistry and Biochemistry at New Mexico State University in Las Cruces, NM. He received his Ph.D. degree in 1978 at University of Colorado, was a postdoctoral fellow at the University of Waterloo (Ontario, Canada) from 1978 to 1980, and joined the faculty at NMSU in 1980. In 1987-19888, he was a Senior Research Fellow at the U.S. Army Chemical Research Development and Engineering Center (Aberdeen Proving Grounds, MD) and was a Senior Associate of the National Research Council in 1992. His research interests include the development of gas chromatography for environmental analyses, the advancement of GC-ion mobility spectrometry for chemical separations, and the creation of chromatographic phases from natural materials such as clays. He is author or coauthor of over 120 journal articles or reviews and coauthor of the book, Ion Mobility Spectrometry. He teaches instrumentation and electronics, quantitative analysis, and freshman chemistry at NMSU.

Jorge L. Gardea-Torresdey is a Professor of Chemistry at The University of Texas at El Paso in El Paso, TX. He received his Ph.D. in 1988 at New Mexico State University in Las Cruces, NM. His research interests presently include environmental chemistry of hazardous heavy metals and organic compounds, gas chromatography, gas chromatography/mass spectrometry, atomic absorption and emission spectroscopy, inductively coupled plasma/mass spectrometry, X-ray absorption spectroscopy, and investigation of metal binding to biological systems for remediation of contaminated waters and soils (through phytoremediation). He has authored or coauthored over 90 research articles and book chapters and holds four U.S. patents for environmental remediation. He has taught analytical chemistry and instrumental analysis at the undergraduate level and advanced analytical chemistry and environmental chemistry at the graduate level.

Herbert H. Hill, Jr. is a Professor of Chemistry at Washington State University. His research interests include gas chromatography, supercritical fluid chromatography, ion mobility spectrometry, ambient pressure ionization sources, and mass spectrometry. He received his B.S. degree in 1970 from Rhodes College in Memphis, TN, his M.S. degree in 1973 from the University of Missouri, Columbia, MO, and his Ph.D. degree in 1975 from Dalhousie University, Halifax, Nova Scotia, Canada. In 1975, he was a postdoctoral fellow at the University of Waterloo, Ontario, Canada. In 1983-1984, he was a visiting professor at Kyoto University, Kyoto, Japan, and in 1993-1994, he was a visiting professor at Bayreuth University, Bayreuth, Germany. In 1989, he received the Keene P. Dimick award in Chromatography for his work in chromatographic detection methods. He has been on the faculty at Washington State University since 1976.

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(G2) Demir, C.; Hindmarch, P.; Brereton, R. G. Analyst 2000, 125 (2), 287-292 (G3) Fujikura, T.; Sakamoto, K.; Shimozawa, J. T. Anal. Chim. Acta 1997, 351 (1-3), 387-396. (G4) Echarri, I.; Nerin, C.; Domeno, C.; Cacho, J.; Wells, D. E. Analyst (Cambridge, U. K.) 1998, 123 (3), 421-427. (G5) Kursawe, P,; Zinn, P. Dtsch. Lebensm.-Rundsch. 1999, 95 (11), 453-457. (G6) Lavine, B. K.; Moores. A. J.; Mayfield, H. T.; Faruque, A. Anal. Lett. 1998, 31 (15). 2805-2822. (G7) Heberger, K.; Gorgenyi, M. J. Chromtatogr., A 1999, 845 (12), 21-31. (G8) Schirm, B.; Watzig, H. Chromatographia 1998, 48 (5-6), 331346. (G9) Kinghorn, R. M.; Marriott, P. J. HRC, J. High Resolut. Chromatogr. 1998, 21 (1), 32-38. (G10) Baumard, P.; Budzinski, H. Analusis 1997, 25 (7), 246-252. (G11) Beens, J.; Boelens, H.; Tijssen, R.; Blomberg, J. HRC, J. High Resolut. Chromatogr. 1998, 21 (1), 47-54. HIGH-SPEED GAS CHROMATOGRAPHY (H1) Sacks, R.; Smith, H.; Nowark, M.; Anal. Chem. 1998, 70 (1), 29A-37A. (H2) Cremers, C. A.; Leclercq, P. A. J. Chromatogr., A 1999, 842 (1+2), 3-13. (H3) Leonard, C.; Grall, A.; Sacks, R. Anal. Chem. 1999, 71 (11), 2123-2129. (H4) Smith, H.; Sacks, R. Anal. Chem. 1998, 70 (23), 4960-4966. (H5) Prazen, B. J.; Bruckner, C. A.; Synovec, R. E.; Kowalski, B. R. J. Microcolumn Sep. 1999, 11 (2), 97-107. (H6) Leclercq, P. A.; Cramers, C. A. Mass Spectrom. Rev. 1998, 17 (1), 37-49. (H7) Leonard, C.; Sacks, R. Anal. Chem. 1999, 71 (22), 5177-5184. (H8) Smith, H.; Zellers, E. T.; Sacks, Richard Anal. Chem. 1999, 71 (8), 1610-1616. (H9) Shahar, T.; Dagan, S.; Amirav, A. J. Am. Soc. Mass Spectrom. 1998, 9 (6), 628-637. DETECTORS (I1) Lohnes, M. T.; Guy, R. D.; Wentzell, P. D. Anal. Chim. Acta 1999, 389 (1-3), 95-113. (I2) Hilscher, W. German Patent DE 19,653,346 (Cl. G01N30/68), 2 July 1998, Appl. 19,653,346, 20 December 1996. (I3) Jalali-Heravi, M.; Fatemi, M. H. J. Chromatogr., A 1998, 825 (2), 161-169. (I4) Holm, T. J. Chromatogr., A 1999, 842 (1+2), 221-227. (I5) Klee, M. S.; William, M. D.; Chang, I.; Murphy, J. J. High Resolut. Chromatogr. 1999, 22 (1), 24-28. (I6) Huang, J.; Zhu, X.; Lennon, P. J.; Bartell, L. S. J. Chromatogr., A 1998, 793 (1), 209-213. (I7) Skaggs, R. L.; Aguin, M. L.; Lewis, W. M.; Birks, J. W. Anal. Chem. 1998, 70 (16), 3493-3497. (I8) Kishi, H.; Arimoto, H.; Fujii, T. Anal. Chem. 1998, 70 (16), 3488-3492. (I9) Lee, C.-Y.; Shiea, J. Anal. Chem. 1998, 70 (13), 2757-2761. (I10) Eijkel, J. C. T.; Stoeri, H.; Manz, A. Anal. Chem. 1999, 71 (14), 2600-2606.

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