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Anal. Chem. 1998, 70, 321R-339R
Gas Chromatography
Gary A. Eiceman*
Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003 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
themes highlighted in this review, and this was particularly true with detectors and field instruments.
REVIEWS, BOOKS, AND GENERAL INTEREST
Review articles appeared on subjects closely paralleling the previous format here, including highlights with multidimensional chromatography and extraction of information from chromato- graphic data among others. A noteworthy effort was a review of attempts toward creating unified chromatography, i.e., a single instrument supporting GC, LC, and SFC methods (A1). The long- standing interest in chromatography through specific retention principles was represented by a review of complexation gas chromatography and molecular recognition in separations (A2).
Significant amounts of research from the former eastern bloc nations have become conveniently available as translations, typified Pattern Recognition and Artificial Intelligence by a review on physiochemical fundamentals of capillary gas chromatography (A3), where emphasis was given to retention High-Speed and Portable Gas Chromatography mechanisms. A third area of advance, that of data extraction from chromatographic information, was reviewed for artificial intel- ligence tools (A4) and multivariate mathematical models forevaluation of retention data matrixes (A5).
This review of the fundamental developments in gas chroma- Multidimensional gas chromatography received substantial tography (GC) includes articles published from 1996 and 1997 treatment, with reviews reflecting the growing interest in GC- and an occasional citation prior to 1996. The literature was GC or liquid chromatography (LC)-GC and the creation of a reviewed principally using CA Selects for Gas Chromatography from substantive body of experience and research. A broad argument Chemical Abstracts Service, and some significant articles from for multidimensional separations focused on a comprehensive late 1997 may be missing from the review. In addition, the on- approach with theory (A6), and others presented reviews with line SciSearch Database (Institute for Scientific Information) specific applications to aromas (A7) and environmental samples capability was used to abstract review articles or books. As with (A8). A multiple detector alternative to multiple columns was the prior recent reviews, emphasis has been given to the identification and discussion of selected developments, rather than Reviews on chromatography with emphasis on uses directed a presentation of a comprehensive literature search, now available toward specific materials were published and are included here widely through computer-based resources.
since the presentations may provide insights into limitations and During the last two years, several themes emerged from a basic challenges in GC. The articles were directed toward review of the literature. Multidimensional gas chromatography pheromones (A10), odorants in foods (A11), pesticides (A12), and has undergone a transformation encompassing a broad range of activity, including attempts to establish methods using chromato- Two books published during this review cycle included a graphic principles rather than a totally empirical approach.
discussion of the theory and practice of headspace sampling with Another trend noted was a comparatively large effort in chro- GC (A14) and a GC/MS handbook with little fundamental and matographic theory through modeling efforts; these presumably wholly pragmatic importance (A15). The presentation of data in became resurgent with inexpensive and powerful computing tools.
this handbook reflects the need for continued work in artificial Finally, an impressive level of activity was noted through the intelligence in data treatment per section below.
Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 SOLID ADSORBENTS AND SUPPORTS
amorphous silicas, because of the possibility of a partial adsorption Several types of liquid phases have been reported during this of alkane chains between the layers of this silica.
review cycle and include synthetic organic phases, chiral and Reports on other natural adsorbents and supports, including natural phases, inorganic salts, and metal-based phases. As in alumina, quartz, various types of clays, and cellulose, were given prior reviews, discussions in this section have been restricted to during this review period. The diffusion of ethyl methyl ketone, reports in which solid adsorbents are discussed or characterized methyl alcohol, and acetaldehyde vapors on alumina was studied with emphasis on new or modified materials. As in past reviews, (B21). Carbon-modified aluminum oxide columns were used to inverse gas chromatography (IGC) was an important method for separate saturated hydrocarbons (B22). In this report, the investigating surface structure and interactions between solid investigators found that the adsorption capacity of adsorbents with respect to saturated hydrocarbons increases significantly with Natural Adsorbents.
carbon content. A study on the adsorption of water vapor on adsorption and chromatographic properties of carbon adsorbents activated alumina also appeared (B23). This report indicated that was evaluated (B1). In this study, the arrangement of hexagonal a molecule of water is bound to two adjacent hydroxyl groups.
layers in the nanostructure of carbonaceous materials was Others evaluated aluminum oxide-coated porous layer open determined to help their adsorption and GC properties. The tubular columns for the analyses of propylene and ethylene (B24).
intramolecular interactions in GC on carbon black coated with Others studied the adsorption of gaseous chlorides and oxychlo- monolayers of hydrocarbons with different electronic structures rides on quartz chromatographic columns (B25).
was also explored (B2), and a series of novel GC graphite-coated The effect of thermal and chemical treatments on the variation capillary columns were evaluated by using polarity parameters of specific surface area of porous bentonite as a support in gas (B3). A hexafluoropropylene epoxide-coated graphitized carbon chromatography was studied (B26). These processes changed black adsorbent was used to determine the retention character- the surface structure of bare bentonite and contributed to a good istics of 13 heavier than ethane-based and eight ethene-based separation of hydrocarbon mixtures. Others modified diatoma- halocarbon fluids (B4). The relative retention data were fitted to ceous supports with phenol-formaldehyde, polyoxyalkylene- linear models for the purpose of predicting retention behavior of polyurethane, polymethacrylate, and epoxy resins to investigate these compounds to facilitate chromatographic analysis. Also, a the possible use of the obtained sorbents in gas chromatography similar hexafluoropropylene epoxide-modified graphitized carbon (B27). A method for the deactivation of diatomaceous solid material was used to determine the Kovats retention indexes of supports based on the adsorption of polyethyleneimine and cross- halocarbons (B5). Gas-solid chromatography was used to linking by a bidentate reagent was also reported (B28). The determine the Henry’s law gas-solid second virial coefficients retention ability of different types of solid supports with respect for hydrocarbons, chlorofluorocarbons, ethers, and sulfur hexaflu- to active agents was studied using NaX-type zeolites (B29). The oride with a microporous carbon adsorbent (B6). The thermo- relative contribution of the zeolite to the overall adsorption of dynamics of gas adsorption on coal was also studied (B7). Others certain hydrocarbons was determined. The kinetic parameters attempted to alter or improve carbon black adsorptivity through for the ring opening of cyclohexane over modified ZSM-5 zeolites surface modifications with a high-frequency plasma (B8).
were also studied (B30). The separation of aliphatic alcohols was Modified GC equations were studied with nonporous silica successfully performed on a packed column with a support coated particles packed into fused silica capillary columns (B9), and the with cellulose tribenzoate, and GC temperature programming adsorption properties of silica gels with chemically bonded improved the separation (B31). A report showed the selectivity aminopropyl and guanidinoethanethiol groups were also investi- of a saltwater stationary phase for the separation of mono- and gated (B10). Column packing containing N-benzoylthioureacop- polyhydroxy isomers on a chromatographic column containing per(II) complexes chemically bonded to silica supports were used H2O (B32). In summary, the studies to study the specific interactions of this modified silica with suggest a high level of sustained development and discovery of electron-donor adsorbates such as ketones, ethers, and nitroal- natural materials to be used for separations in GC methods.
kanes (B11). More recently, the same investigators studied the Synthetic Adsorbents.
adsorptive properties of silica chemically modified by Cu(II) utilized to investigate properties of several materials. Reports that complexes via amino groups (B12). The surface of silica was also used IGC included the following: the structural characterization modified using octadecyl (B13, B14), amino, and guanidino groups of the deactivation of silica surfaces with a silanol-terminated (B15). A review on molecular statistical modeling and gas polysiloxane (B33); the examination of acid-base and some other chromatographic studies of hydrocarbons on modified layered properties of solid materials (B34, B35); the estimation of surface silicates and silica in the Henry’s law region was reported (B16).
energy of modified TiO2 pigments (B36); the surface characteriza- The results of this study can be used to develop new, efficient tion of cellulose fibers (B37); the determination of the properties adsorbents and supports based on layered silicates and silica. A of the films formed by organic substances on a silica gel surface salt-modified silica gel adsorbent, coated with disodium hydrogen (B38); the measurement of the surface energies of spherical phosphide, was also studied (B17). The dispersive and specific cellulose beads (B39); the determination of the chemical and adsorption energies of alkanes (B18) and benzene on silica gel morphological characteristics of inorganic sorbents with respect were also studied (B19). Other types of silicas, amorphous silicas to gas adsorption (B40); the characterization of the cork surface and crystalline silicic acid, were evaluated for their alkane (B41); the determination of intermolecular interactions for hy- adsorption energies (B20). The value of the surface energy of drocarbons on Wyodak coal (B42); the assessment of the surface the crystalline silicic acid was found to be higher than that of the energies of theophylline and caffeine (B43); the estimation of the Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 thermodynamics of adsorption of n-alkanes on maleated wood nematic, smectic, cholesteric, low-molecular, high-molecular, fibers (B44); the investigation of surface properties of protective crown ether, and macromolecular crown ether liquid crystals).
coatings of optical fibers (B45); the study of the physicochemical Several GC stationary phases containing crown ethers were properties of polypyrrole-silica nanocomposites (B46); and the reported (C10-C12). Two new chiral polysiloxanes containing determination of the surface properties of illites and kaolinites crown ethers were prepared for capillary GC (C10). Others (B47). In addition, surface adsorption isotherms, solubility parameters, thermodynamic interactions, and glass transitions C6 carboxylic acids when using benzo-15-crown-5 as were all characterized using IGC for several new polymeric stationary phase (C11). A new calixcrown polysiloxane GC materials (B48-B62). Other studies showed the adsorption and stationary phase was synthesized (C12). This phase, in which gas chromatographic properties of microspherical hyper-cross- the calixcrown monomers lie in the main chain of polysiloxane, linked polystyrene sorbents (B63). The highest retentions for showed good separation properties for nitro-, chloro-, and methyl- various types of organic compounds were observed on polymer substituted benzene or phenol isomers. Other new developments microspheres with the highest degrees of cross-linking. In with liquid phases were made with more traditional polymers, such another report (B64), the adsorption properties of the porous as dicyanobiphenyl polysiloxane stationary phases (C13). Others polymers Porapak R and Porapak T were studied. Others studied evaluated chemically bonded squalene phases for the separation the adsorption of various organic compounds, including alkanes, of hydrocarbons (C14, C15). Also, the effect of the dioxa[11]- aromatic hydrocarbons, aldehydes, ketones, and esters, on Porolas paracyclophane group in polysiloxane stationary phases was polymers (B65). The vinyl chloride adsorption properties of the examined (C16). The incorporation of n-alkyl groups on cyclic polymer poly(vinyl chloride) were also studied (B66). Others siloxane bonded phases was also studied (C17). The incorpora- reported the structure and gas chromatographic properties of new tion of octyl and octadecyl groups on the siloxane skeleton greatly sorbents prepared by the radiation-stimulated polymerization of improved the retention capacity for light hydrocarbons. This 2-butyne-1,4-diol, 2,4-hexadienoic acid, and 1,2,3-propenetricar- behavior was due to the high surface coverage of the packing boxylic acid on the surface of polysorb-1, a styrene-divinylben- material and, thus, to a better solute-stationary phase interaction.
zene copolymer (B67). The retention properties of 15 hydrocar- In addition, the effect of alkylbenzene groups (C18), N-alkylimi- bons on a new GC stationary phase, poly(perfluoroalkyl ether), dazole groups (C19), and phenyl groups (C20) in polysiloxane were also reported (B68). The hydrocarbon-perfluoro compound stationary phases was investigated. Others examined the pos- interactions showed pronounced positive deviations from the ideal sibility of using three polar-type liquids containing (methyl)oxy- behavior and can be attributed to repulsions between the alkane, cyanoalkane, and alkanethiol groups as GC stationary hydrocarbon and the perfluorocompound segment. In another phases (C21). Chemically bonded cyclic organosiloxanes-silica work (B69), quantitative structure-retention relations in GSC gels were evaluated (C22) for possible use in microcolumn GC were employed as a method to study the inclusion properties of of light hydrocarbons. Two reports appeared on the use of the p-tert-butylcalix[4]arene phase in a micropacked column.
dicumyl peroxide for the cross-linking of GC stationary phases Synthetic inorganic materials were the subject of one study that (C23, C24). Two related studies (C25, C26) showed the variation reported the use of thorium bis(monodecyl phosphate) as a GC of selectivity among many polysiloxane stationary phases for GC.
The selectivity differences were explained in terms of differencesin the cohesive energy of the solvents and their capacities for LIQUID PHASES
dispersion, dipole-type hydrogen bonding, and electron pair Synthetic Organic Phases.
complexation interactions. These reports concluded by speculat- liquid GC stationary phases ranging from squalane (retention ing on the needs for new phases to explore the full selectivity polarity ) 0) to bis(cyanoethoxy)formamide (retention polarity potential in GC. Others evaluated resorcarene derivatives for the ) 144.6) was reported (C1). Several liquid crystals were studied separation of substituted benzenes (C27), and 1(R)-trans-N,N′- as possible stationary phases in GC. The separation properties 1,2-cyclohexylenebisbenzamide for the separation of L-2-hydroxy- of hydrocarbons were examined with AVIK-85 (a tetrahydroxy- glutaric acid in urine (C28).
quinone derivative) on Chromosob-W and Silochrom C-80 sup- Chiral Phases and Natural Phases.
ports (C2). The same AVIK-85 liquid crystal was evaluated review, the most prevalent phases in this section were based on through various cycles of heating and cooling of a chromato- cyclodextrin and cyclodextrin derivatives (C29-C64). Cyclodex- graphic column (C3). Other liquid crystal phases were evaluated trin and modified cyclodextrin stationary phases were used for for the separation of 2,3,7,8-substituted chlorinated dioxin isomers the separation of stereoisomers of 3,4-diphenylcyclopentene (C29), in capillary columns (C4), the separation of isomeric phthalic acids methyl 2-chloropropionate (C30), volatile compounds in oils (C5), and selectivity for polycyclic and aromatic compounds (C6).
(C31-C33), PCBs (C34), organophosphorus chemical warfare Others compared two azobenzene liquid crystal stationary phases agents (C35), xylenes (C36), fatty acid methyl esters (C37), in open tubular column GC for the isomeric separation of various furanoids (C38), aromatic alcohols (C39), di- and trisubstituted types of organic compounds (C7). The separation of positional benzene (C40, C41), methyl and phenols (C42), R-campholene isomers of aliphatic, aromatic, and polyaromatic hydrocarbons of and fencholene derivatives (C43), amino alcohols (C44), and DDT three laterally substituted liquid crystal stationary phases was also (C45). The mechanisms of separation of cyclodextrin and reported (C8). A review with 101 references on the development modified cyclodextrin stationary phases are based on van der of liquid crystals as stationary phases in GC was given (C9). This Waals forces (C46), hydrogen bonding (C47, C48), sizes of the review includes the classification of liquid crystals for GC (e.g., inclusion cavity (C49-C51), polar interactions (C52-C54), and Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 steric interactions (C55). Others found a synergistic effect when Table 1. Examples of Physicochemical
cyclodextrin stationary phases were mixed with resorcarene Parameters for Solution Studies Using Gas
(C56), and the necessity of an aromatic system for enantioselective Chromatography
phases was determined (C57). In addition, cyclodextrin and adsorption coefficients with improved equation for modified cyclodextrin stationary phases were examined to deter- partition coefficients of benzenes and essental oils mine the contribution of thermodynamic parameters to chiral activity coefficients with interfacial adsorption C58-C60), the influence of diluting the phases on Gibbs energies of solution and molecular structures enantioselectivity (C61-C63), and the influence of structural donor-acceptor associations with enthalpy measurements characteristics on chiral selectivity (C64).
thermodynamic parameters for solubility with The adsorption properties of a GC glucose-modified silica surface were reported (C65). The additional glucose modification London dispersive component of the surface free energy activity coefficients and Flory-Huggins interaction parameter increased the adsorption potential for molecules with polar partial molar enthalpies of mixing at infinite dilution functional groups. The resolution of enantiomers of various compounds was evaluated using L-valine-tert-butylamide (C66, C67), chirasil-Val (C68, C69), L-tert-leucine-tert-butylamide (C70),and tripeptide derivatives (C71).
Other retention models were described and were useful in Inorganic Salts and Metal-Based Phases. The chromato-
restricted models for prediction, such as the use of number of graphic properties of stearyl-1-R-naphthyl acetate were examined chlorines in predicting PCB retention (D13). Others have used as a GC stationary phase (C72). This phase falls in the medium carbon numbers in alkyl chains (D14), boiling points (D15), polar category and can be a versatile, easily available phase for enthalpy (D16), and combinations of these (D17). On the whole, GC. A GC stationary phase containing chiral chelates of europium these attempts are directed at fairly narrow variations on structure was found to exhibit high selectivity for nucleophilic solutes (C73).
and will be useful for targeted understandings. In contrast, the Successful enantiomeric separation of selected alcohols and works referenced in the previous paragraph have universal ketones was obtained. However, no separation of chiral com- pounds containing double bonds and chloro aliphatic compounds Predictions on the role of temperature on retention dates from was observed. Others evaluated copper(II) chelates of tetraden- before the inception of temperature programming, though oven tate -ketomaines as GC stationary phases (C74). These phases programming has elevated the importance on linking retentions showed a high potential for the separation of alcohols, ketones, from isothermal and programmed temperature experiments. This and heteroaromatic compounds. The complexing GC stationary has reached an advanced stage of refinement, and some have phases containing tris[3-((trifluoromethyl)hydroxymethylene)cam- reported differences of below 1% between predicted and measured phorato]-derivatives of lanthanides were also characterized (C75).
retention times (D18-D20). A flexible model, in which temper- In another report (C76), IGC was used to determine the solubility ature plateaus are allowed, showed errors of 4% on retention and and polarity parameters for pyridinecarboxamides and their 10% on peak widths (D21). An attempt to minimize the number of experiments for such accomplishments was described (D22),and one model included a detailed consideration of column CHROMATOGRAPHIC THEORY
The major themes for this section are based on studies where General retention within gas chromatography underwent chromatographic behavior is associated with the molecular significant advances during the last two years, with noteworthy structure of solute or stationary phase and the connections advances in a unified retention concept (D24), in a universal between thermodynamic parameters on efficiency, resolution, or database (D25), and in classification of stationary phases using Kovats coefficients (D26, D27). These works have a common Structure-Retention Studies.
aim, which is to free investigators from the inconvenience of noted in this subject, with over 73 articles on a main premise: empirical determinations of retention when various stationary Can retention times be predicted from molecular structure? Three phases are employed. While developments toward this goal descriptors or guides for linking structure to retention have seemed promising (D26), interfacial adsorption for polar phases emerged: topologic, geometric, and electronic terms (D1-D4).
was recognized as a variable which cannot be ignored (D25) and In general, these tools are still in a semiempirical state of may defeat wishes for a highly refined and broadly applicable set development, where the models are being developed through correlations between retention indexes and molecular details. In Thermodynamic Parameters.
other models, molar volume and solubility parameters have solvent interactions has been a mainstay in gas chromatography formed the basis of correlations (D5). Still others have used for decades, and this area saw active development, even in 1996- selected descriptors, including topologic (D6), electronic interac- 1997. Since the studies are often very specific and only together tions (D7), conformations (D8), and van der Waals volumes (D9).
comprise significant importance for GC, the results are sum- In these, molar volumes appear to be an influential descriptor marized in Table 1. The listing demonstrates that GC has been (D10, D11), except where gas solid chromatography was involved maintained as a research tool for gleaning constants and physical terms for molecular events or properties.
Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 to correlate GC equations with thermodynamic parameters (E16) Table 2. Studies for Various GC Parameters
and to determine the adsorption mechanism of n-alkane retention with various liquid stationary phases (E17). Also, a super long mathematical model for asymmetry in peak shape modeling molecular basis for separation of enantiomers equilibrium-dispersive model for elution band profiles for the separation of gasoline (E18). The column had 1.3 million model for separations of multiple phases such as GC-GC effective plates and was built up by connecting nine 50-m columns assessment of corrected retention volumes in GC dependence of retention on carrier gas pressure in GSC in series. This new column resolved up to 970 components in a specific retention volumes for capillary GC with ∼1% error specific retention volumes calculated from retention indexes dimensional packed-capillary column was reported (E19). In the new polarity scales for phases using Kovats coefficients two-dimensional system, the column efficiency was determined discovery of failure with variance additivity in by the solute’s retention time and the ratio of peak spreading in void volume and mobile phase volume measured the precolumn and the main column. It was concluded that, in model for flow control in temperature programmed GC order to improve column efficiency, matching of the precolumn influence of coating thickness and temperature on dead time and main column conditions was necessary. Others reported (for comparison of dead time methods: Grobler-Balizs the first time) the chemical modification of open tubular columns moving focusing in chromatographic resolution with a functionally selective immobilized stationary phase (E20).
The column modification was performed by the oxidation of the A general category of miscellaneous facets with the theory of polyol polymer with nitric acid. The optimization of temperature- gas chromatography showed a remarkable degree of diversity and programmed GC separations using off-line simplex optimization level of activity. In some works, resolution and peak shape were and column selection was studied (E21). This study indicated the main considerations while in others, influences on retention that fully off-line simulation and optimization of single ramp and were explored; still others addressed what are often neglected multiramp temperature-programmed GC separations as well as issues in GC, such as dead time and void volume. These are listed column selection was possible. A subambient GC temperature in Table 2 and suggest a broad range of study in this domain of programming method using two isothermal gas chromatographs was also reported (E22). Another report in this area was thedevelopment of a thermal desorption modulator for GC (E23). In COLUMNS AND COLUMN TECHNOLOGY
addition, automatic thermal desorption in GC was used for the The major themes of this section are new developments and analysis of volatile food components (E24, E25). A new GasPro improvements in column designs as well as new attempts at GC PLOT column for the separation of gases and highly volatile creating columns for evaluation of chromatographic principles or for enhanced selectivity. A review with 11 references appeared on the use of metal capillary GC columns as an alternative to fused between the GasPro column and a conventional doped aluminum silica columns (E1). A new wall-coated open tubular metal column oxide capillary column. The GasPro column was not adversely was developed for the separation of petroleum-derived waxes and affected by water, carbon dioxide, and sulfur gases, and it appeared high-molecular-mass linear alcohols and acids (E2). Others made to be more inert than the aluminum oxide column, since it did new inert stainless steel capillary columns by a multigradient layer not cause decomposition of most reactive analytes. Also, a new technology (E3). Also, a new deactivated metal capillary GC type of quartz-lined aluminum capillary GC column coated with column was used for the determination of trace amounts of graphitized carbon black was prepared (E27). This new column triazolam in serum (E4). This metal column exhibited excellent was resistant to drastic acidic or alkaline treatments, and it was thermostability at high temperatures. Several new GC porous evaluated for the analysis of amines, VOCs, and oil products.
layer open tubular (PLOT) columns were also reported (E5-E10).
Others reported a new type of high-performance submicroparticle Some characteristics of these new PLOT columns included the packed column (E28). This column was found to have properties in situ polymerization of the monomer (E5, E6), a new method similar to those of capillary columns for the analysis of high- of silica modification (E7), a less reactive column which is boiling-point compounds at relatively low column temperatures.
sufficiently inert to separate volatile chlorofluorocarbons (E8), a A packed glass capillary column with multicores was also prepared charcoal-based column capable of resolving light hydrocarbons and was shown to have a better permeability than conventional (E9), and an ultrafine zeolite-based column with high speed and packed capillary columns (E29). In addition, a GC capillary selectivity (E10). Others also reported the preparation of a zeolite column which can be directly heated by an internal chromium/ membrane PLOT column by in situ synthesis (E11). A new type nickel wire was developed (E30). The separation of C - of SCOT column with ultrafine zeolite was also described (E12).
hydrocarbons was demonstrated with this column, and it was The ultrafine zeolite had a fine and uniform particle size, large compared to conventional column oven heating. High-tempera- specific surface area, regular crystal structure, and similar chemi- ture GC capillary columns, which can be used up to 360 °C, were cal constitution to glass. Two new methods for the preparation prepared and characterized (E31). Others developed a new type of microcolumns were also evaluated (E13, E14). These columns of strongly polar organic polymer PLOT column by in situ contained cross-linked, bonded polysiloxane stationary phases of copolymerization of acrylonitrile and divinylbenzene (E32). Also, well-defined thickness (E13) and (aminopropylsilyl)dithiooxamide a way of controlling the maximum allowable oven temperature bonded phases (E14). The use of a short microcapillary GC during on-column injection by the column pressure drop was column (i.d. ) 50 µm) for rapid triglyceride analysis in fats and suggested (E33). In this report, an arrangement using a restrictor oils was reported (E15). Open tubular GC columns were used at the column outlet for adjustment of the column inlet pressure Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 was evaluated. The polarity and adsorptivity of nondeactivated carbons in edible oils (F18). In another article, the conditions of GC capillary surfaces were tested (E34). The effect of various sampling the first column were explored relevant to recovery of leaching and etching procedures used for column preparation, reference standards in determination of polychlorinated biphenyls as well as the stability of uncoated fused silica precolumns toward (F19). Finally, the linearity and precision for LC-GC were water and some organic solvents, was studied in this report.
documented using phyrethorid insecticides in fruit extracts (F20).
Recoveries were good, and reproducibility was high (0.6-6.6%).
MULTIDIMENSIONAL GAS CHROMATOGRAPHY
The growing trend in multidimensional gas chromatography DATA PROCESSING AND QUANTITATIVE
was distinguished during this review cycle by advances in the subject of modeling and optimizing two-dimensional gas chroma- Previously, data processing was regarded as a means to provide tography and by a dramatic increase in liquid chromatography insights into chromatographic events, and the efforts were limited (LC)-GC techniques. Applications continued to lead in all in scope. During this last review cycle, a remarkable surge reports, as true throughout gas chromatography literature every- occurred in reports on data processing. This occurred in both where, though the foundations of a systematic methodology are the scope and the number of studies. The causes for this vitalityand diversity are unknown but might be attributed to the being explored, fortunately. Automation and hardware were minor availability of powerful and affordable computers.
themes, in contrast to reports from the past decade.
Optimization and Simulations. Optimizations and simula-
Models and optimization constitute an encouraging develop- tions represented a significant portion of work in this section and ment in multidimensional gas chromatography, since, without included an examination of signal processing, such as the foundational tools for designing methods or interpreting results, performance of digital processing for selected ion monitoring (G1).
the subject will remain in a highly empirical condition. The article Computer modeling was used for optimizing column parameters that best represents this involved modeling retention and separa- via commercial software (G2) and through neural networks (G3).
tions in three capillary columns (F1). In three other articles, a Selectivity in GC was optimized and verified experimentally main theme was optimization of selectivity in two-dimensional GC through simplex methods, enhanced through the use of a (F2-F4). These articles are not directly available in English and polynomial gleaned from preliminary measurements (G4).
are noted for their importance in the development of multidimen- Advances in comparison of retentions were made using sional gas chromatography. Elsewhere, attempts to maximize 2-D thermodynamic retention indexes (G5) and used with simulations GC were made by exploiting thermal modulation for complete to explore separations by GC (G6). Predictions of numbers of characterization of every peak eluting from the first column (F5, components were accomplished via a statistical model of overlap (G7). The high level of current capabilities in the overlap between A remarkable activity in the coupling of LC with GC has computation and separations is illustrated by simulations of occurred, despite a first impression of apparent incompatibility retention for highly complex mixtures (G8) and for simultaneous in strengths of LC for large nonvolatile molecules. However, in programming of temperature and column head pressure (G9).
nearly all reports for LC-GC, the liquid chromatograph was used Pattern Recognition and Artificial Intelligence. As in past
principally for prefractionation of a complex mixture to isolate a activity within this section, multivariate data analysis has been targeted GC-compatible substance. Examples include polycylic used powerfully with complex mixtures, suggesting that tools aromatic hydrocarbons (F7, F8), alcohols and sterols (F9), exists for coping with the large information density presented by morphine and analogues (F10), PCBs and pesticides (F11), -blocker (F12), and thiols and other sulfur-containing compounds become an element of foundational significance in handling (F13). Though these are application-driven reports, they are noted chromatographic data, extending and expanding the meaning of here to document the activity in one future direction for multidi- chromatographic resolution. Also, as in the past, the best mensional gas chromatography and the relevant basic research examples come from complex natural or environmental samples.
These have included blueberries (G10), orange juice (G11), A persistent subject in multidimensional gas chromatography microbial defects in milk (G12, G13) and fresh cabbage (G14) has been the hardware to combine multiple columns without for foods. Neural networks were found to correlate sensory degradation in peak shape and automation of instrumentation. This evaluation with chromatographic headspace analysis with good was a weakly active area in the last review cycle and is best typified success (G15). Complex processes of weathering for a complex by interest in valve connections or traps between columns in GC- sample, jet fuel, were successfully modeled and linked to dis- GC (F14, F15) and LC-GC (F16). A noteworthy variation from criminant functions, demonstrating the advanced role of artificial linearly coupled columns is that of parallel columns, after a intelligence methods with chromatography (G16). Similar success precolumn, with a single detector (F17). In this work, retention was obtained with plastic-bonded explosives (G17). These routes behavior was modeled and was 0.3-0.6% of predictions.
to data processing are sufficiently mature to become common tools Enhancements in confidence or specificity of detection via resolution with two stationary phases was and remains a motiva- An ancillary challenge to handling complex data sets for refined tion for multidimensional gas chromatography. Consequently, the understanding of total composition or sources assignment is the literature in this subject is weighted toward applications, as is true extraction of specific information from simple or complex chro- in all of gas chromatography. A few articles illustrate a mixture matographic data. Information theory and calibration standards of application and critical evaluation, such as the comparison of were used to establish chromatographic performance (G18), and LC-GC versus the official analytical method for steroidal hydro- throughput increases were controlled with an expert system Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 (G19). Chromatographic separation was also established using of analysis were obtained for a series of pesticides (H16).
simplified Fourier analyses, with additional benefits of disclosing Multidimensional HSGC showed low-capacity factor compound overloading effects (G20). Extraction of information from GC/ separations (H17) with many possible applications (H18). Two MS screenings was accomplished automatically with low S/N columns with opposite stationary phases (polar and nonpolar) were levels (G21) and through differential results (G22). The automa- optimized by adjusting the midpoint pressure to attain maximum tion of retention index comparison attracted some effort, including resolution for a critical pair (H19).
a merging of isothermal and programmed data sets (G23) and Injection techniques for HSGC must provide narrow band- the use of diverse literature sources (G24). Such advances will widths due to fast analysis time requirements without compro- be necessary for full exploitation of chromatographic data in mises in resolution (H20). Cryogenic inlets were the most popular chemical identifications, as typified by a new software program method of introduction. Two cryofocusing inlets provided band- widths between 5 and 10 ms (H21). A cryogenically controlled Quantitative Aspects. The subject of quantitative determi-
microloop gave bandwidths of <10 ms (H22) and was compared nations by GC received small but refined examinations during with a capacitance-heated metal cryotrap in a recent study (H23).
this review cycle. For example, the influence of matrixes on A cryotrap/thermal desorption inlet with 10 different deactivated pesticide determinations was addressed (G26). Matrixes can tubes was evaluated and found to provide minimal thermal reduce losses of pesticides on surfaces, and matrix standard decomposition in 6 of the 10 tubes (H24).
calibration solutions were described. In another work, the signal As with injectors, compatible detectors for HSGC must be fast associated with a chromatographic blank was obtained through enough to resolve narrow peaks, which requires minimal dead use of background noise, suited to further statistical evaluation volume and fast electronics (H25). When a mass spectrometer (G27). The determination of morphine in opium using complex is coupled to HSGC (H26), fast scanning rates are necessary, and pyrolysate chromatograms was advanced using principle compo- scanning rate limits of >25 000 amu/s have been shown when a nent analysis and provided quantitative results (G28). Quantitative quadrupole mass filter was implemented (H27). Utilizing a fast determinations with gas chromatography-mass spectrometry double-focusing mass spectrometer showed improvements in both were also explored and characterized for variance from various detection limits and interferences (H28).
steps of an entire method for cholesterol. Not surprisingly, sample One tradeoff for HSGC is the loss of capacity due to the smaller collection and preparation contributed the bulk of variance, and diameter and shorter columns. Application of packed capillary the instrumental determination was less than 30% of the total columns in HSGC has been shown to improve capacity and (G29). Methods for testing scanning mass spectrometry for selectivity (H29) while obtaining high-speed separation for light linearity of response were presented (G30), and minimum detec- hydrocarbons (H30). By implementing supercritical fluid/gas tion limits were approached using principal component analysis chromatography conditions along the column, less volatile com- pounds can also be separated by using packed capillary columns(H31). A multicapillary column improved capacity while maintain- HIGH-SPEED AND PORTABLE GAS
ing the efficiency obtained with small internal diameter columns CHROMATOGRAPHY
In recent years, the need for rapid and portable analytical measurements has led to the development of gas chromatographs Many portable HSGC instruments are being used for on-site with fast separation times and small instrumentation (H1). Several analysis (H33). A micro-GC coupled with a thermal conductivity recent reviews examined the techniques for increasing analysis detector (TCD) was shown to separate CO2 and C1 C6 alkanes times in GC, such as using short narrow-bore columns (H2, H3) within 30 s. Petroleum industry application included detection and optimizing the mobile phase parameters (H4). Theoretical of H2S and COS impurities (H34) and rapid screening for gasoline expressions for typical high-speed gas chromatography (HSGC) to diesel range organic compounds (H35). Environmental prob- conditions have been derived and compared with the Van Deemter lems are a major application for portable GC systems due to the equation (H5). Differences were found due to the higher pressure complexity of the samples (H36). Recent separations in this area drop across the column for HSGC. Conventional GC instruments include separation of multiple pesticide residues in agricultural have been reviewed for compatibility with HSGC conditions (H6), products (H36), air analysis (H37), and the detection of polychlo- and one major problem discovered was due to slow data acquisi- rinated biphenyls (PCBs) (H38). Other applications which have tion rates and large amplifier constants (H7).
recently been tested with HSGC involve leak verification (H39), New commercial HSGC systems have been presented which identification of pesticides in plasma samples (H40), and separa- utilize smaller thermal ovens (H8) and innovative column heating tion of thermally labile steroids, carbamates, and drugs (H41).
arrangements (H9). Column heaters based on resistive heating Because gas chromatographic instruments can be made light were shown to be compact, utilize minimal power, and decrease and energy efficiency, considerable emphasis over the past two sample analysis times (H10). Resistive-heating combined with years has been placed on developing portable gas chromatographs temperature gradients along one column (H11) resolved 13 for the separation and detection of target compounds in the field compounds with baseline separation in 3.5 s (H12) and was shown (H42). Gas chromatographs have been developed which are truly to be compatible with the addition of a second column (H13).
portable, with approximately 100 W of power at peak-to-peak power Dual-column HSGC has been investigated for both the theoretical consumption (H43). More importantly, small GCs are not limited possibilities (H14) and its applications (H15). Two-column to operation in the field. In the laboratory, they have the separations with high-resolution mass spectrometry (HRMS), advantages of operation with minimal consumption of utilities such improved sensitivity, short analysis times, and decreased costs as compressed gases, electricity, space, and so forth.
Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 The primary focus of research and development in field gas instrumental control methods have been developed for more chromatography (FGC) during the past two years has been precise control of the operating temperatures and gas flow rates instrument miniaturization. The ultimate miniature FGC system (H61, H62). One approach for improved injection technologies was designed and developed using Si micromachining and is called thermal modulation (H63). Thermal modulation was integrated circuit processing techniques (H44). The chromato- produced from the rotation of a heater element over a capillary graph consisted of a 10-µm-long sampling loop, a 0.9-m rectangular- column. This rotation accumulated the analyte and then focused shaped column, and an injection loop and column each with a it into a sharp concentrated pulse, injecting the pulse onto the width of 300 µm and height of 10 µm. The column was coated GC column. Because the modulation was produced from a with a 0.2-µm thickness of Cu phthalocyanine as the stationary controlled rotation of the heater module, repetitive and reproduc- phase. Detection was based on a dual detection scheme using a ible sample injections were possible.
coated chemiresistor and thermal conductivity detection. The One major problem of FGC when compared to laboratory- complete FGC system was packaged in less than 23 cm2 and was based instrumentation is the reduced resolution that most field 2.5 mm high. Although limited in scope to the detection of instruments exhibit. This reduced resolution is compensated for, ammonia and nitrogen dioxide, this miniature chromatograph in part, by the use of selective detectors. Detectors such as the offers exciting possibilities for future field instruments.
electron capture detector (H48), the photoionization detector One of the more novel approaches reported for FGC during (H47, H64), and a miniature dual flame photometric detector for the review period was the miniaturization of a tandem GC phosphorus and sulfur compounds (H57) have all been interfaced instrument (H45). This potential field-portable GC-GC method of separation involved dynamic coupling between two short The most versatile selective detector for GC is the ion mobility capillary columns. Each column could be independently optimized spectrometer or detector; recently, it, too, has been coupled to with regard to temperature and flow rate. Dynamic coupling was gas chromatographs for field analysis (H65, H66). A new hand- accomplished by automated vapor sampling (AVS) techniques.
portable GC/ion mobility spectrometer has been constructed and Thus, a slowly eluting GC peak from the first column can be named the EVM II (environmental vapor monitor). The unique rapidly sampled into the second column, providing a two- feature of this instrument was the GC column-to-source interface, dimensional separation of complex samples. The feasibility of this in which a transfer line was used as the chromatographic column.
tandem GC approach was demonstrated by two-dimensional The concept has led to the term transfer line gas chromatography C6 ketones. Other miniature FGC systems have (TLGC). The utility of this hand-held GC ion mobility detector also been reported (H46, H47). A detachable column cartridge instrument was demonstrated by the separation and detection of was developed which permitted the substitution of columns in chemical warfare agents such as DMMP within 20 s (H67). One interesting application of GC/ion mobility spectrometry was for The most common uses for field gas chromatography (FGC) the determination of fish freshness (H68, H69). The presence have been in the determination of volatile and semivolatile organic of 1,5-diaminopentane (cadaverine) was detected after 4 days, compounds in the atmosphere. Target compounds for on-site indicating the on-set of fish decay. Coupling an automated vapor screening by FGC include benzene in complex environments at sampler with a transfer line gas chromatograph (AVS-TLGC) and the ppm level (H49), dimethyl sulfide (DMS) and carbon disulfide an extremely small ion mobility spectrometry detection device (15 (CS2) (H50), and polychlorinated biphenyl (H51). Indoor air in.3), an attempt was made to construct a personal chemical hazard pollutants such toluene, R-pinene, and 1,4-dichlorobezene were determined, with detection limits in the low microgram per cubicmeter level ( Mass spectrometry is a mature technology and, perhaps, the H52), and long-lived species were identified in the upper troposphere and lower stratosphere (H53). Methylphos- ultimate GC detector, but due to size, vacuum, and energy phonic acid esters were detected at the 5 ppb level in air (H54).
requirements, it has been limited as a field method. Nonetheless, Other volatile organics compounds (VOCs) in the atmosphere considerable attention has been paid to the development of this (H55) were targeted, as well as soil gases containing BTEX and technique due to the tremendous benefits that field portable gas chlorinated solvents (H56). Chemicals for chemical weapons chromatography/mass spectrometry would provide. One portable convention treaty verification (H57) and explosives in air from a GC/mass spectrometer was contained in a standard size suitcase, walk-through sampling module which provides automatic screen- weighed about 70 lbs, and required about 600 W of energy for ing of humans at rates of 10/min (H58) have been screened by operation under peak load conditions (H71). Such a portable GC/ FGC. These chemicals encompass a wide range of vapor pressure MS system can be transported to a clandestine laboratory for the on-site identification of illicit drugs and other related compounds Problems associated with FGC have included the fluctuation (H72). A “roving” GC/MS, mounted on a golf cart, was capable of retention times and the nonreproducibility of injections.
of analyzing up to 1000 samples/h while traveling at a speed of Humidity may also affect the results of analyses (H59). In general, 20 mi/h (H73). Portable GC/MS instruments were also used for however, understanding the difference between field screening industrial hygiene applications (H74). The primary advantage of and field analysis can reduce the problems associated with field portable GC/MS instruments is the large number of applications measurement (H60). By adjusting the data quality requirements for with this method is useful. As the need for field measurements with the time requirements, optimal parametric conditions can increases and mass spectrometry becomes miniaturized, the be selected in which reliable on-site data can be obtained. In number of field portable GC/MS systems can be expected to efforts to minimize problems associated with field GCs, several Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 GAS CHROMATOGRAPHIC DETECTORS
in water (I15). A new type of HID based on dc plasma ionization In addition to high resolution and speed, gas chromatography was reported for use with GC (I16, I17) and applied to the offers the considerable advantage of interfacing with a wide variety measurements of flammable gases in coal gas (I18).
of detection methods. The myriad of detectors available for gas (c) Nitrogen-Phosphorus Detector (NPD). The most
chromatography can be separated into two primary categories: common GC detector based on the ionization of the analyte in ionization detectors and optical detectors.
the presence of a heated alkali source is the nitrogen-phosphorus Ambient Pressure Ionization Detectors. Ionization detec-
detector. Developed to maturity, this detector was used in a tors remain the most common devices used to measure analytes number of novel applications. Quantification of the metabolite separated by gas chromatography. Although these methods are dichloroethylcyclophosphamide was preformed after direct capil- reaching a mature state of development, considerable research lary gas chromatography without prior derivatization, with a activity still exists. Research and development on the following detection limit of 1 ng/mL (I19). Other applications to biological ionization detectors have been reported in the literature during matrixes included determination of yohimbine in commercial the past two years (listed in order of presentation): the flame yohimbe products, study of a dietary supplement alternative to ionization detector (FID), the helium ionization detector (HID), anabolic steroids (I20), nitrate analysis in biological fluids, in the nitrogen-phosphorus detector (NPD), the electron capture which nitrobenzene was produced from the dissolved nitrate (I21), detector (ECD), the surface ionization detector (SID), the pho- cyclophosphamide metabolites (I22), simultaneous quantification toionization detector (PID), the ion mobility detector (IMD), and of two antidepressant drugs, fluoxetine and desipramine (I23), and quantification of clozapine (I24) and zolpidem and zopiclone (a) Flame Ionization Detector (FID). The detector work-
(I25) in human plasma or serum. The NPD was also applied to horse for gas chromatography remains the carbon-selective flame food analysis for the determination of fungicide residues in ionization detector. Several new applications for this detector were cucumbers (I26), herbicides in drinking water (I27), and imazalil reported. They included the determination of castor oil fatty acid residues in lemons (I28). An important environmental application composition (I1), an Empore disk elution method coupled with of the NPD was the determination of underivatized nitrophenols injection port derivatization for the quantitative determination of in groundwater (I29). Simultaneous determination of 15 organo- linear alkyl benzenesulfonates (I2), the determination of parac- nitrogen pesticides was accomplished with a flame thermionic etamol and dicyclomine hydrochloride (I3), and dual-column detector (I30). Also, online determination of organophosphorus hydrocarbon analysis (I4). Related to these routine applications pesticides in water by solid-phase microextraction techniques was was a simple and efficient method for the determination of reported (I31). One approach was investigated for the selective retention parameters using a methane marker device (I5).
detection of oxygenated volatile organic compounds (I32).
Developmental advances in FID included several modifications In one, more fundamental study, severe tailing of phosphorus in design. A compact and low-fuel FID was developed for portable pesticides was found to be associated with the thermionic GCs (I6). Fuel flow rates were as low as 12-15 mL/min, and ionization source and not the column (I33). A novel type of alkali oxidant flow rates were in the range of 120-150 mL/min. Another source was reported in which the alkali salt was continuously improvement to FIDs was reportedly made by the incorporation introduced into the detector as a solution in water by means of a of a precombustion chamber to mix the fuel and sample gas (I7).
liquid chromatographic pump (I34). This design had the advan- Similarly, a premixed FID was developed by adding hydrogen and tage of continuous refreshment of the source. Also, an alkali flame air at the same flow rate to the outlet of the capillary column (I8, ionization detector (AFID) was reported in which an alkali chip I9). Both hydrogen and oxygen were produced by an electrolyzer was mounted on top of the collecting electrode (I35) or the that was incorporated into an FID design (I10). This electrolyzer temperature of the alkali source could be varied (I36). Flame- flame ionization detector (EFID) was similar to a standard FID, free ionization detectors were reported which used a heated except that the flame tip had a narrower bore to prevent flame ionization tube of high-purity alumina (I37, I38).
flashback and the entire detector structure needed to be main- (d) Electron Capture Detector (ECD). Sensitive and selec-
tained at a temperature greater than 100 °C in order to prevent tive for halogenated and other electronegative compounds, the water condensation. Sensitivity of the EFID was similar to that electron capture detector (ECD) remains one of the most widely of the FID, but detectivity was improved by a factor of 2.
used GC detectors. Novel applications reported during the past Other developments included a dual-channel detector in which two years include the following: the shipboard analysis of the effluent from a reactive flow luminescence detector (RFD) halocarbons in seawater and air for oceanographic tracer studies was burned to form a tandem, stable, air-rich flame ionization (I39), the determination of chlorobutanol in mouse serum, urine, detector, providing dual response of compounds from a single and embryos (I40), the measurement of organochlorine com- separation (I11). Fundamental research on the FID has included pounds in milk products (I41), the identification of pesticides and a mechanistic study of isotope and heteroatoms affects on the other organochlorides in water (I42), organochorine pesticides relative molar response (RMR). RMR did not change when in edible oils and fats (I43), metabolites from permethrin, and deuterium was substituted for hydrogen in most hydrocarbons cypermethrin in foods (I44), and the determination of phenols (I12). The fast response time of a flame ionization detector was from aqueous solutions as bromo derivatives (I45). A brominated internal standard was found to be useful for the determination of (b) Helium Ionization Detector (HID). The helium ioniza-
organochlorine pesticides (I46).
tion detector (HID) was most often used for the detection of inert In more fundamental studies, a nonradioactive ionization gases (I14). In one example, traces of Ar and N2 were determined source was compared with a radioactive source. Initially, the Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 nonradioactive source was found to be 20 times more sensitive personal chemical warfare detector (I66). In addition, IMS has than the radioactive source, but after 9 months of use the been used as a detection method after pyrolysis GC. With this sensitivity of the nonradioactive source decayed to levels equiva- system, bacillus spore detection was achieved through the lent to the radioactive source (I47). In a similar study, it was characteristic pyrolysis decomposition products of the spores, found that a pulsed discharge electron capture detector, a which were identified as dipicolinic and picolinic acid. The nonradioactive source, was more sensitive than the radioactive detection limit was found to be on the order of 100 ng of bacillus source for most compounds, covered a wide dynamic response Other IMS investigations have included the range similar to the radioactive source, and demonstrated a development of an IMS with an internal gas chromatographic temperature dependence similar to that of the radioactive source.
column (I68) and IMS detectors using tritium ionization and A kinetic description of the detector response was provided (I48).
photoionization sources (I69).
Amine response in a radioactive source demonstrated that the (h) Glow Discharge Detector (GDD). A novel detection
relative molar sensitivity factor, TM, was correlated with the pKBH method investigated for GC was glow discharge ionization (I70).
of the individual amine (I49). Finally, it was noted that the The effect of discharge gas on the GC response was reported addition of ammonia to the nitrogen makeup gas in amounts as (I71), along with analytical characteristics for organic compounds large as 20% increased the response for various chlorinated (I72). Glow discharge detectors have also been used as ionization sources for gas chromatography/mass spectrometry (I73).
(e) Surface Ionization Detector (SID). Over the past few
Mass Spectrometric Detectors. Is gas chromatography an
years, surface ionization detectors have been given considerable inlet for mass spectrometry, or is mass spectrometry a detector attention for the determination of organic compounds with low for gas chromatography? In past gas chromatography reviews, ionization potentials. Recently, however, a novel design for surface the former concept prevailed, and the review of GC/MS methods ionization detection was reported based on hyperthermal positive was covered only in the mass spectrometry section. Today, surface ionization (I51-I54). The primary requirement for the however, given the growth in number of applications for which operation of this detector was the use of a supersonic free jet GC/MS instruments are well suited, the decrease in size and nozzle to introduce the sample to a high-work-function surface of required operator expertise, and the increase in reliability and rhenium oxide. The primary advantage of this new SID is that it ruggedness, mass spectrometry has become a simple but powerful produced a higher sensitivity for all organic compounds, providing detector for gas chromatography. Thus, for the first time, mass a universal GC response. Detection limits of 10-13 g/s for pyrene spectrometers are reviewed in this section along with other and 10-12 g/s for toluene were reported, with a linear dynamic standard, routine gas chromatographic detectors. While all mass range of 106. A second surface ionization detector provided a spectrometric detectors fall into the category of ionization detec- unique sensitivity for tertiary amines, with detection limits down tors, they have been split into a separate category in this review to 10-14 g/s (I55, I56). Finally, a GC/SID system was reported due to the breadth of ionization mechanisms and GC applications for the detection of underivatized codeine and dihydrocodeine reported in the literature during the past two years. These mass (I57), strychnine (I58), and various benzamides (I59) in body spectrometric detectors (MSDs) have been separated according to their ionization mechanisms for this review.
(f) Photoionization Detector (PID). Although some method
(a) Electron Impact Ionization. Electron impact ionization
development using PID was reported, such as the monitoring of is, of course, the oldest of the ionization methods currently used.
volatile contaminants in wastewaters (I60), much of the research Its mechanism is well understood, and the fragmentation patterns revolved around the development of novel photoionization detec- produce valuable information for the identification of organic tors. One study incorporated the use of selective photoionization compounds. GC/MS applications using this ionization source detectors with nonselective detectors to provide both quantitative have been developed for environmental (I74-I82), agricultural and qualitative information about the sample (I61). Another used (I83-I85), food science (I86-I88), biological (I89-I100), forensic a variety of rare gases to identify and quantify unknown com- (I101), petroleum (I102, I103) and industrial (I104, I105) samples.
pounds (I62), while a pulsed discharge in He was demonstrated On a more fundamental level, a novel ionizer was reported (I106), as a He photoionization detector with a selective photoionization and region II of the a/q stability diagram was used for fast mode of operation (I63). The measuring chamber arrangement scanning of a linear quadrupole mass spectrometer. The purpose of a photoionization detector was modified by dividing the main of this experiment was to evaluate this method as a means of chamber into several smaller volumes, permitting these smaller detection for high-speed gas chromatography. Scan rates of 1000 volumes to share a common anode and cathode (I64). Benzene scans/s were obtained, with mass spectral peaks resolved over relative response factors for the PID were reported in which energy models were developed using HyperChem and compared Using ion trap mass spectrometry (ITMS) with GC/MS, airborne carbonyl compounds were separated and detected as (g) Ion Mobility Detector (IMD). The number of applica-
their pfbha oximes (I108), residues of chlorinated pesticides in tions of ion mobility spectrometry as a detector for gas chroma- eggs of the gray heron were identified (I109), and pesticides in tography continues to grow. Over the past two years, much of the marine environment at the low nanograms per liter level were the emphasis on ion mobility detection after gas chromatography determined (I110). Novel developments in ion trap mass spec- as been in the area of portable analytical instruments. Many of trometers included a three-dimensional quadrupole ion trap the IMS references are discussed in that section of this review.
coupled to a capillary GC (I111) and the addition of a sample One major thrust in the development of GC/IMS has been as a inlet with a pressure working range from 1 to 1000 mbar (I112).
Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 Isotope ratio mass spectrometry was used with GC for the high- time-of-flight mass spectrometry was reported with isomer- precision D/H measurement from organic mixtures (I113) and selective multiphoton ionization (I135, I136).
the determination of 15N of N2 and N2O in soil atmosphere (I114).
Optical Detectors. As with ionization detectors, optical
In general, gas chromatography was coupled to isotope ratio mass detectors fall into a wide range of categories. Atomic and spectrometry via a combustion furnace (I115, I116).
molecular absorption and emission provide multiple approaches (b) Chemical Ionization. Chemical ionization, a less ener-
for both qualitative and quantitative information from samples getic ionization method than EI, is often used with EI to aid in identifying the molecular ion and to increase the sensitivity of a (a) Atomic Absorption Detectors. Coupling gas chroma-
method. As with the EI source, fundamental investigations of the tography to atomic absorption spectrometry (AAS) provided ionization mechanism have been extensive, but novel GC/MS selective detection for many compounds. Tetramethyllead and applications are still being developed. 4-(Carbethoxy)hexafluo- tetraethyllead were used as test analytes in the evaluation of a robutyryl chloride was used for the derivatization of ethylene quartz tube atomizer (I137). A comparison of GC/AAS with glycol from human serum. This derivatization product produced differential anodic stripping voltametry for organolead was re- a distinct protonated molecular ion peak at m/z of 563, providing ported (I138). A simple and reliable method for the detection of an unambiguous confirmation of ethylene glycol (I117). When organolead compounds in a water sample was developed (I139).
ammonia was used as a reagent gas, direct and rapid characteriza- GC/AAS proved to be especially useful for the speciation of tion of paraformaldehydes after pyrolysis gas chromatography was organomercury compounds. Mercury speciation in natural water possible (I118). Solid-phase extraction and positive chemical samples (I140) and in human body fluids (I141) was reported.
ionization mass spectrometry were used for the analysis of cocaine GC/AAS was also applied to the analysis of organotin species and its metabolites (I119). Other positive chemical ionization (I142). A novel atomic absorption technique after gas chroma- methods included identification of higher alcohols in cosmetics tography was reported using a microwave-induced plasma for the (I120) and the characterization of low-molecular-weight polyeth- determination of chlorinated hydrocarbons (I143). In addition, yleneimines (I121). Negative ion chemical ionization was used element-selective diode laser plasma detection for chlorinated to analyze R- and -endosulfan in biological samples by selectively compounds was reported (I144).
monitoring the product ions at m/z 35 and 37. The selectivity (b) Atomic Emission Detector (AED). The most utilized
and sensitivity of negative ion chemical ionization were demon- GC detector based on optical mechanisms was the atomic strated by the direct measurement of endosulfan in mouse brain emission detector (AED) (I145-I148). It was especially useful without purification of samples (I122). Electron capture negative as a complementary analytical technique to GC/MS for environ- ion chemical ionization was used for the analysis of 5-methox- mental screening (I149) and for the separation and detection of organometalic compounds (I150, I151). With respect to organo- For improved selectivity and sensitivity, GC was coupled to metalic compounds, a number of analytical methods were reported MS/MS instruments. The primary advantage is that a sharp, well- for the determination of organomercury (I152-I157) and orga- resolved GC peak is produced for quantitative analysis, and a notins (I158, I159). Other applications included the determination simple, unambiguous spectrum is available for qualitative confir- of chlorophenols in tap water (I160), measurement polychlori- mation (I124). With this method, as low as 10 pg of an isotope- nated and polybrominated biphenyls in the environment (I161, labeled amino acid was detected in 20 ng of the endogenous I162), and herbicide analysis in soils (I163).
compound (I125). In another example, phosphate esters in diesel Fundamental studies of the AED were also reported (I164).
exhaust were detected and confirmed (I126).
Parametric investigations included temperature effects on quan- (c) Inductively Coupled Plasma Ionization. A powerful
tification (I165) and makeup gas flow on emission (I166). A technique for the separation and speciation of volatile organo- round-robin robin study demonstrated that intralaboratory pro- metalic compounds is capillary gas chromatography coupled to cedures ranged from 1.3 to 22% RSD and that interlaboratory inductively coupled plasma mass spectrometry (ICPMS) (I127).
results ranged from 11 to 40% RSD (I158). Elemental C/Cl and The primary advantage of GC-ICPMS is that the total analyte C/Br ratios deviated by less than 20% from calculated values was transferred into the ICPMS without loss due to nebulization (I128). Several methods for coupling a GC to an ICPMS have Development and evaluation of several novel analytical systems been reported. In one case, the interface did not require any were reported. A direct-current AED was described for use with changes in the ICP and could be completed in less than a minute gas chromatography (I168). Pyrolysis/GC/AED was demon- (I129). For alkyltin compounds, coupling was accomplished with strated by the determination of silicones in air down to the 0.1 a heated stainless steel transfer line (I130). A heated transfer ng/L level (I169). Also, an LC/GC/AED system provided a high- line made from quartz was also reported (I131). Applications of efficiency separation for analytes in a complex sample matrix GC/ICPMS have included organometallics from river and harbor sediments (I132) and the speciation of organolead compounds (c) Molecular Absorption Detectors. The use of molecular
from environmental waters (I133). One interesting application absorption detectors in gas chromatography is less common than was the detection of dimethylselenium in human breath in the other optical detection methods. The primary approach is IR detection. Analysis of cis- and trans-fatty acid isomers after (d) Laser Ionization. Normally, laser ionization methods are
separation by capillary GC was reported to be simple and fast, used for large nonvolatile compounds. However, the combination but its lower detection limit of 5% limited it use (I171). In some of gas chromatography, supersonic beam UV spectroscopy, and cases, sensitivity limitations could be overcome by the use of large Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 Table 3. Applications of Multiple Detectors
pesticide residues in fruits and vegetables organophosphorus and organonitrogen pesticides cyclic hydrocarbon pollutants from combustion gas of coal injection volumes (I172). Fourier transform methods can also Its lower detection limit was approximately 200 ppb (I200).
improve detection limits (I173, I174). The most sensitive ap- Miscellaneous and Multiple Detectors. Miscellaneous GC
proach is matrix-isolation IR spectrometry, but the instrument is detectors not conveniently included in categories discussed above more complicated, and chromatographic resolution may be are covered in this section. An acoustic flame detector (AFD) reduced (I175). In a unique approach, diffuse reflectance Fourier for gas chromatography monitored the frequency of the sonic transform infrared spectrometry was coupled with GC (DRIFT- bursts from a flame. Shifts in the frequency were indications of GC) for catalyst characterization (I176).
analyte presence in the flame (I201). Olfactometry (OD) is one Another molecular adsorption technique coupled a packed of the oldest detection methods for GC, and still, novel methods column with a UV-visible molecular absorption spectrophotom- were reported (I202, I203). Electrolytic conductivity detection eter for the detection of benzene, toluene, 1,4-xylene, 1,2-xylene, (ElConD) coupled with solid-phase extraction was used for the and mesitylene (I177, I178). In addition, the determination of trace analysis of vinclozoline (I204). Radiochemical detectors isoprene in human breath was reported by thermal desorption were used for special applications (I205-I207) and, in a few gas chromatography with UV detection (I179).
specific cases, GCs were interfaced to a nuclear magnetic (d) Molecular Emission Detectors. The flame photometric
resonance spectrometer to provide molecular structural informa- detector (FPD) is, by far, the most common molecular emission tion on the analyte (I208, I209). Semiconductor detectors were detector. Routinely used for the determination of tin- (I180- investigated to replace the FID (I210) and for breathalyzers I185), lead- (I186), germanium- (I187-I189), nitrogen- (I190), (I211). Finally, improvements in thermal conductivity detectors sulfur- (I191), and phosphorus- (I192) containing compounds, ithas the option of multichannel response (I193, I194). An (TCD), for years the primary detection method for gas chroma- improved performance of the detector was demonstrated using a tography, were reported for sensitivity (I212) and for interfacing pulsed mode of operation and optimal operating conditions (I195).
with capillary columns (I213, I214).
Miscellaneous chemiluminescene detectors were reported as The ability to combine GC detectors in a single analysis is a well. The determination of trace amounts of organic explosives, powerful approach for the investigation of complex mixtures and in which 0.2 ng of HMX was detected, was accomplished using a the identification of unknown compounds. The combinations of thermal energy analyzer (TEA) (I196). One detector, called the detectors shown in Table 3 were used in novel ways to solve sulfur chemiluminescene detector, was based on the principle of analytical problems during the past two years.
the formation of sulfur dioxide in a reducing flame. The sulfurdioxide is then detected by its chemiluminescence reaction with Gary A. Eiceman is a Professor in the Department of Chemistry and
Biochemistry at New Mexico State University in Las Cruces, NM. He ozone (I197). Mechanisms were also discussed in the literature received his Ph.D. degree in 1978 at the University of Colorado, was a (I198). Application of the detector was reported for volatile sulfur postdoctoral fellow at the University of Waterloo (Ontario, Canada) from1978 to 1980, and joined the faculty at NMSU in 1980. In 1987-1988, compounds in water, with a linear response from 10 to 100 µg/L he was a Senior Research Fellow at the U.S. Army Chemical Research Development and Engineering Center (Aberdeen Proving Grounds, MD) I199). A selective chemiluminescene detector after GC was also and is a visiting lecturer at the Universidad Autonoma de Chihuahua applied to the detection of nitrogen compounds in crude and (Mexico). His research interests include the development of gas chro-matography for environmental analyses, the advancement of GC/ion refined petroleum fractions. This detector was based on initial mobility spectrometry for chemical separations, and the creation of pyrolysis combustion at 1100 °C in the presence of oxygen, chromatographic phases from natural materials such as clays. He teachesinstrumentation and electronics, quantitative analysis, and freshman followed by a luminescence reaction of the product NO with ozone.
Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 Jorge L. Gardea-Torresdey is an Associate Professor of Chemistry
(B15) Roshina, T. M.; Davydov, V. Y.; Khrustaleva, N. M.; Mandrugin, at The University of Texas at El Paso in El Paso, TX. He received his A. A.; Gurevich, K. B. Adsorpt. Sci. Technol. 1997, 15(3), 147-
Ph.D. in 1988 at New Mexico State University in Las Cruces, NM. His research interests presently include environmental chemistry of hazardous (B16) Tarasevich, Y. I.; Aksenenko, E. V.; Bondarenko, S. V. Stud. heavy metals and organic compounds, gas chromatography, gas chroma- Surf. Sci. Catal. 1996, 99, 539-571.
tography/mass spectrometry, atomic absorption and emission spectroscopy, (B17) Naito, K.; Yasukiyo, E.; Sadaaki, M.; Shinsuke, T. Anal. Sci. inductively coupled plasma/mass spectrometry, and investigation of metal 1995, 11(2), 303-305.
binding to biomaterials for remediation of contaminated waters and soils (B18) Nan-Tran, H.; Gander, B.; Nguyen, V. P.; Gentili, S.; Sabra, F.
(through phytoremediation). He has authored or coauthored over 70 J. Phys. Chem. 1995, 99(11), 3806-3809.
research articles and book chapters and holds three U.S. patents for (B19) Zhu, C.; Kwang, S. Y.; Jon, F. P. Anal. Chem. 1995, 67(9),
environmental remediation. He has taught analytical chemistry and instrumental analysis at the undergraduate level and advanced analytical (B20) Hadjar, H.; Balard, H.; Papirer, E. Colloids Surf., A 1995, 99(1),
chemistry and environmental chemistry at the graduate level. (B21) Khan, M.; Afzal, M. Pak. J. Chem. Soc. Pak. 1996, 18(2), 72-
Herbert H. Hill, Jr., is a Professor of Chemistry at Washington State
(B22) Shalaeva, M. E.; Zheivot, V.; Prokudina, N. A.; Chesnokov, V.
University. His research interests include gas chromatography, super- V.; Malakhov, V. V. J. Anal. Chem. (Transl. of Zh. AnaI. Khim.) critical fluid chromatography, ion mobility spectrometry, ambient pressure 1996, 51(6), 560-564.
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Japan. He has been on the faculty at Washington State University since (B27) Ravelova, P.; Chanev, Kh.; Petsev, N. God. Sofii. Univ. “Sv. Kliment Okhridski”, Khim. Fak. 1995, 88, 99-105.
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