Anal. Chem. 1996, 68, 291R-308R
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 Behnam Davani
Sigma Chemical Company, St. Louis, Missouri 63178 Jorge Gardea-Torresday
Department of Chemistry, University of Texas, El Paso, El Paso, Texas 79968 Review Contents Books, Reviews, and General Interest Solid Adsorbents and Supports Natural Adsorbents Synthetic Adsorbents Liquid Phases Synthetic Organic Phases Chiral Phases and Natural Phases Inorganic Salts and Metal-Based Phases Chromatographic Theory Structure/Retention Studies Sorption and Solvation Processes Columns and Column Technology Multidimensional Gas Chromatography GC/GC SFC/GC and LC/GC Data Processing and Quantitative Aspects Pattern Recognition and Data Processing Artificial Intelligence, Data Bases, and Optimization Peak Analysis and Processing Quantitative Aspects of GC High-Speed and Portable Gas Chromatography Gas Chromatographic Detectors Mass Spectrometric Detection Ambient Pressure Ionization Detection Photometric Detection Other Modes of Detection Literature Cited
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This review of the fundamental developments in gas chromatography (GC) includes articles published from 1994 and 1995 and some works previous to 1994. The literature was reviewed principally using CA Selects for Gas Chromatography from the Chemical Abstracts Service and some significant articles from late 1995 may be missing from the review. In addition, the on-line SciSearch database (Institute for Scientific Information) capability was used to abstract review articles, and newly published books on GC were sought through the MELVYLCAT capability at the University of California. The restructuring of Gas Chromatography, initiated four years ago, has been completed in the present version to reflect the emerging trends in GC while retaining the core themes of past reviews. These changes include the following: S0003-2700(96)00013-3 CCC: $25.00
© 1996 American Chemical Society
the consolidation of all column technology into one section; a redefinition of the sections on adsorbents and liquid phases to include only the creation and study of new materials; the combination of chromatographic principles into a single section, i.e., chromatographic theory; the addition of wholly new sections for multidimensional GC and for high-speed or field GC; and the addition of an opening section on general interest. As with the prior recent reviews, the emphasis has been on the identification and discussion of selected developments rather than the presentation of a comprehensive literature search. BOOKS, REVIEWS, AND GENERAL INTEREST Articles appeared on subjects of general interest in GC including a presentation of the new unified chromatographic nomenclature from the International Union of Pure & Applied Chemistry (A1). Comparisons were made to existing terms in this guide which should be helpful for anyone with interests in GC and a concern for a systematic vocabulary in chromatography. Another item of general and long-standing interest in GC, the standardization of retention indexes, was addressed through an exploration of the experimental boundaries where a standard retention index library would be valid (A2). A region of agreement between experimental results and library listings was bracketed by relationships between phase ratio, to, and rate of temperature programming. A review of retention indexes was given with an expansive view of the development of data bases and should provide a useful summary or an indicator of future directions for refinements in retention indexes (A3). An objection was raised over the present, stagnant condition in the development of capillary GC (A4); it was proposed that the full possibilities for efficiency with capillary columns have not be attained. A review was presented (A5) on the principles of a form of GC where the stationary phase is a gas trapped in the pores of a solid support and the mobile phase is a liquid. This permutation of GC, known as liquid/gas chromatography (LGC), is in the early stages of development and results suggest a genuine phenomenon in separations. However, limitations in the range of volatility for analytes may restrict the general utility of LGC. General education in GC was afforded by a new edition of an existing book (A6) and through an offering of graphics on GC principles on the World Analytical Chemistry, Vol. 68, No. 12, June 15, 1996 291R
Wide Web at http://odin.chemistry.uakron.edu/chemsep/index.html (A7). A search for reviews showed more than 206 citations, although only 10% involved principles of GC rather than specific applications. Noteworthy, was a review of liquid phases (A8), where the role of intermolecular forces in separations was described within the context of definitions of selectivity. In particular, the various classification schemes based on intermolecular forces were discussed with respect to structure/separation relationships. In a related article (A9), a contemporary review of GC phases was provided when the uses of specific liquid phases in GC were linked to foundational principles. Other reviews provided incremental contributions. SOLID ADSORBENTS AND SUPPORTS Discussions in this section have been restricted to reports where solid adsorbents are described or characterized with emphasis on new or modified materials. Studies on the structure of solid surfaces have been included, though investigations of adsorption mechanisms have been placed, mostly in the section on Chromatographic Theory. As in prior reviews, inverse gas chromatography (IGC) was a prominent approach to exploring surface structure and interactions between solid surfaces and probe solutes. Natural Adsorbents. Graphite layers in open tubular columns were evaluated for several uses including the analysis of priority pollutants (B1), the determination of components in petroleum (B2), and the separation of oils from wild plants (B3). Thermal conditioning of carbon to improve chromatographic properties was also explored (B4-B6), and the importance of physicochemical parameters in governing retention properties of different carbon materials was described (B7-B14). The micropore structure of carbon was identified as a primary factor in adsorption properties (B15). Others attempted to alter or customize carbon adsorptivity through modifications of the surface with a microwave plasma (B16), with silane [bis(triethoxysilylpropyl) tetrasulfide] derivatization (B17), and with oxidation processes (B18). The interaction between vinyltriethoxysilane and aerosil and active charcoal surfaces was also studied (B19). Column packing containing β-diketonate chelates of Cu(II) and Ni(II) chemically bonded to phosphinated silica surfaces were used to separate nucleophilic species by metal complex formation (B20). These sorbents were capable of selectively retaining unsaturated linear and cyclic hydrocarbons. The same investigators studied the specific interactions for some ketones, ethers, and nitroalkanes with Cu(II) and Ni(II) acetyloacetonates chemically bonded to silica surfaces (B21). The surface of silica was also modified using mono- and difunctional perfluorosilanes (B22). Differences were observed in retention behavior between alkyland perfluoro-chain-grafted silicas. Silica gel, modified with a polystyrene layer, was evaluated for adsorptive properties related to the porous structure of the gel (B23). The chromatographic packing was evaluated with mixtures of aliphatic hydrocarbons. Silica packing were modified with 3-[3-(trimethoxysilyl)propyl]2,4-pentenedione and then treated with Pd(II) and Cu(II) by bonding through the β-diketonate groups (B24, B25). The separation of ketone mixtures confirmed the usefulness of these chromatographic packings. The high stability and simple synthesis from inexpensive starting materials made it possible to use the N-benzoylthiourea groups as modifiers of silica for complex292R
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ation gas chromatography (B26). The chelating properties of N-benzoylthiourea were utilized to bond Cu(II) ions with silica to provide π-type interactions with electron-donor adsorbates. A saltmodified silica gel adsorbent, coated with disodium hydrogen phosphide was used in a partial resolution of 2-hexene and 2-heptene (B27). Another silica derivative, octadecylsilyl silica gel, was evaluated for rates of adsorption for the gas-phase adsorption of n-alkanes and benzene derivatives (B28). Intraparticle diffusion in mass-transport resistance was a dominant feature with such packings, and surface diffusion was a controlling factor in intraparticle diffusion. Reports on four other natural adsorbents and supports, including alumina, glass beads, clays, and cellulose, were given during this review period. The chromatographic efficiency of alumina, modified thermally, was measured using gaseous hydrocarbons (B29). The structure and properties of alumina were apparently changed after high-temperature calcination and deactivation with alkali metal chlorides. Aluminum oxide was used in capillary columns to separate chloro-fluoro-containing hydrocarbons (B30) and for the separation of gases and volatile organic compounds (B31). A report also appeared on the study of the surface of aluminum oxides pretreated with phosphonic acid derivatives (B32), where both benzene and perfluorobenzene were used as probes. The results obtained were compared to spectroscopic studies to provide an understanding of the surface structure. A new approach to modifying glass beads as support was described (B33) through etching the surface of glass beads; significant advantages were found in the separation of fatty acid methyl esters over unetched glass beads. Columns were also created using nonporous glass beads to support particular materials. For example, 12-ammonium tungstophosphate was used in the separation of C1-C4 hydrocarbons (B34). Zeolites with controlled levels of Ca(II) were evaluated with the adsorption of nitrogen, oxygen, and argon (B35) and exhibited a pronounced dependence on Ca(II). Several clay materials were studied for the separation of carbon dioxide from N2-CO2 mixtures (B36). Zeolite and sepiolite were chromatographically active to 673 K though montmorillonite, saponite, and silica gel showed no separating properties above 573 K. Others made treated clays, organosmectites, as packing materials for GC columns to separate mixtures of C1-C4 hydrocarbons (B37). Surface deactivation of diatomaceous solid supports in GC was described through the adsorption of poly(ethylenimine) and cross-linking by a bidentate reagent (B38). The surface adsorption properties of illites and kaolinites of various origins and formation conditions were measured by GC by using branched alkane probes (B39). Modified cellulose was used in GC separations and included treatments with tribenzoate (B40) and with trifluoroethoxyacetic acid (B41). Also, the acid/base surface energy of microcrystalline cellulose was determined (B42). Three cellulose surfaces all exhibited acidic and basic character, but were predominantly acidic in nature. The adsorption properties of several alkanes over a wide range of temperatures on the surface of amorphous cellulose were reported (B43, B44). The results obtained were correlated to surface free energy and to several thermodynamic functions. The reports in total suggest a remarkable level of sustained development and exploration of natural materials by GC methods and for separations. Synthetic Adsorbents. Inverse gas chromatography (IGC) was used to explore properties of several materials, and studies
have included the following: the compatibility of cross-linked polyurethane (as the stationary phase) with liquids at different temperatures (B45); the nature of polymer/probe interactions in a column containing acrylonitrile (as copolymer), methyl acrylate, and sodium 2-acrylamido-2-methylpropane-1-sulfonate (B46); and the chromatographic properties of polycarbonate and poly(ethylene terephthalate) (B47). In addition, surface energies, thermodynamic interactions, solubility parameters, and glass transitions were all characterized using IGC for several new polymeric materials (B48-B56). Other studies indicated that the polymer pore size controls the separation of water and NH3 on porous-layer open tubular (PLOT) columns when divinylbenzene copolymers and dimethacrylate homopolymers were used as stationary phases (B57). In another work (B58), the behavior of GC columns packed with the copolymer of methacrylic ester of (p,p′-dihydroxydiphenyl)propane diglycidyl ether, and divinylbenzene was evaluated using selectivity studies where the molar ratios of monomers in three polymer were varied. Others studied plasma modification of polymer sorbents such as Polysorb-1 (B59, B60). This method of modification was recommended for improving the efficiency of polymer sorbents for the separation of polar compounds. Another new stationary phase containing porous bis(methacryloxymethyl)-m-xylene-divinylbenzene copolymers was synthesized and characterized (B61). The general selectivity of the copolymers was determined by the selectivity triangle procedure. The surface properties of organic adsorbents based on symmetrical heptazine series were reported (B62) to exhibit high surface uniformity with a low dispersion potential and surface centers capable of forming hydrogen bonds with functional groups of adsorbates. Synthetic inorganic materials were the subject of some interest, for example, with synthetic titanium(IV) phosphonate inorganic ion exchangers (B63). Substantial modification of a natural material, iron oxide, with stearic acid and octadecylamine was found to decrease adsorption activity with respect to more polar molecules with high electron densities(B64). For the first time, fullerene C60 has been reported as a chromatographic material (B65, B66). The authors determined that the dispersion interactions of fullerene influenced adsorption. Fullerene is weakly polarizable and has some hydrogen bond basicity, commensurate with its behavior as a giant close-cage alkene rather than an aromatic molecule. For the purpose of this review, fullerene was classified as a synthetic adsorbent, even though it has been found in natural sources. Retention parameters of benzene and its alkyl and halo derivatives were determined on 4-methoxy-4′-(ethoxyazoxy)benzene, aminosilochrom, and fluorinated graphite, along with other natural adsorbents (B67). The contributions of various types of interactions to the retention of these compounds were linked to structure and physicochemical properties. LIQUID PHASES The emphasis in this section is now toward new or modified materials, and studies on solvation are included in the section on Chromatographic Theory. Several types of liquid phases have been described during this review cycle and include synthetic organic phases, chiral and natural phases, and also inorganic salts and metal-based phases. Synthetic Organic Phases. Several types of liquid crystals were investigated as possible stationary phases in GC. The separation behavior of two nematic liquid crystals was evaluated
using hydrocarbon positional isomers, aromatic hydrocarbons, geometric isomers, and volatile aromatic compounds (C1). A comparison was made between the influence of internal and external, NO2-liquid crystals. Other crystalline stationary phases were assessed for thermodynamics of solution using alkanes (C2), shape selectivity (C3, C4), phase transition of monophases (C5), and resolution of polyacrylic aromatic hydrocarbons and polycyclic aromatic sulfur heterocyclic compounds (C6). Various stationary phases using crown ethers were reported (C7-C14). Two new open crown ether polysiloxane stationary phases were prepared for capillary GC (C7). These phases showed unique selectivity for alcohols, aromatic hydrocarbons, and N-containing compounds. Other two crown ether-substituted polysiloxanes were reported for open tubular column GC (C8). A good selectivity of these phases was found for the separation of six dimethylphenol isomers. Others reported excellent selectivity for the separation of polar positional isomers such as phenols and dinitrotoluenes when using phases containing dibenzo crown ethers (C9, C10). Crown ether side-chain polysiloxanes (C11) with and without liquid crystalline character were characterized and a side-chain liquid crystal polysiloxane containing a crown ether with a long spacer was studied (C12). This phase exhibited the retention properties of both liquid crystal and crown ether stationary phases and possessed higher efficiency and selectivity than phases with short spacers. Another phase containing polycrown ethers was used for capillary column GC (C13) and exhibited a weak polarity and higher selectivity for some positional isomers such as cresol and dimethylphenol isomers. A review with 42 references on the application of crown ether compounds as GC stationary phases was given (C14). In this review, the unique separating properties of crown ethers were attributed to cavity size and the strong electronegative effect of heteroatoms. Other developments with liquid phases were made by using more conventional polymers such as cyanobiphenyl-substituted polysiloxanes (C15-C18). Some of the cyanobiphenyl stationary phases showed polarities higher than those for Carbowax 20M or 50% cyanopropyl. Others evaluated N-acylamides for the separation of volatile oil constituents (C19) and for the resolution of enantiomers of amino acid derivatives (C20). The effect of the trifluoropropyl group in polysiloxane stationary phases was explored (C21). Substances with electron-donor groups show maximum retention for a trifluoropropyl group content of ∼30% while the retention of hydrocarbons, halogenated compounds, and alcohols decreased with an increase in the degree of trifluoropropyl group substitution. New polysiloxane stationary phases with various polarities were developed (C22-C25). Polar phases containing 4-(methylsulfonyl)phenyl (C22) and polyhydroxy substitution (C23) were reported. In addition, medium-polarity phases containing methoxy-terminated (C24) and silanol-terminated silarylene/siloxane groups (C25) were studied. A new nonpolar methyloctadecyl polysiloxane phase was shown (C26) to have a distinct selectivity in the separation of polychlorinated biphenyls (PCBs) and a significantly improved range of operating temperatures. Chiral Phases and Natural Phases. As in the past review, the most common phases in this category were based on cyclodextrin and cyclodextrin derivatives (C27-C68). Three comprehensive reviews appeared on cyclodextrin-based stationary phases for the separation of enantiomers (C27-C29). The mechanisms of separation of cyclodextrins and modified cycloAnalytical Chemistry, Vol. 68, No. 12, June 15, 1996
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dextrins are based on polar interactions (C30, C31), changes in conformation (C32), sizes of the inclusion cavity (C33-C39), and steric interactions (C40-C44). Also, modified cyclodextrins were evaluated for the influence of diluting the phases on enantioselectivity (C45-C53) and for the contribution of thermodynamic parameters to chiral resolution (C54). Gas chromatography on polysiloxane-anchored cyclodextrin derivatives has been used to separate the enantiomers of racemates of different classes of compounds ranging from alkanes to highly polar compounds, such as underivatized diols and free acids (C55-C57). In addition, modified cyclodextrin stationary phases were used for the enantiomeric separation of R-hexachlorocyclohexane (R-HCH) and pentachlorocyclohexene (PCCH) (C58), PCBs (C59, C60), amino acids (C61, C62), amphetamine and methamphetamine (C63), essential oils (C64-C66), and flavor compounds in various biological materials (C67). Chemically bonded natural free fatty acid stationary phases were used for the quantitative determination of vanillin (C68) and for the resolution of underivatized fatty acids (C69). In another study (C70), derivatized corn syrup polysaccharides were used as stationary phases for the enantiomeric separation of aromatic amine and amino alcohol compounds. Hydrogen-bonding and dipole/dipole interactions augmented by steric hindrances were probably the major factors that lead to chiral separations by these new stationary phases. The resolution of enantiomers of basic imidazol-2-yl-substituted alcohols with L-Chirasil-Val as the stationary phase was evaluated (C71), and the mechanism for the separation was associated with the hydroxyl group. Resolution of D,L-amino acids was accomplished using L-phenylalanine tetraamides, and separations were controlled by the length and polarity of the oxaalkanoyl bridge spacing the diamide chains and the effects of steric hindrances (C72). Inorganic Salts and Metal-Based Phases. The chromatographic properties of tetra-n-butylammonium n-alkanesulfonate salts were evaluated as a stationary phase using the Rohrschneider-McReynolds system (C73). The specific retention volumes for the polar selectivity probes were generally not affected by an increase in the carbon number. However, the specific retention volumes for the n-alkane retention index markers increased dramatically as the anion carbon number was increased. Liquid organic nitrates where shown to exhibit good selectivity for strong dipole or hydrogen-bonding molecules, such as alcohol (C74). These phases provided good efficiency for ketones and monosubstituted benzene derivatives, but not for hydrocarbons. Other studies reported the separation of unsaturated compounds, such as alkenes with stationary phases based on Rh(I) carbonyl complexes (C75) and the enantiomer separation with a polysiloxane containing chemically bonded chiral metal complexes derived from nickel(II)-Chirasil (C76). The enantioseparation of volatile chiral nucleophiles on a stationary phase containing a chiral substituted tris-β-diketonate of europium was studied by complexation GC (C77). The contribution to solute retention was attributed to electron donation toward a metal ion and steric interactions between solute molecules and chiral ligands of the europium compound. Two developments in liquid phases could not possibly be inserted in any of the categories mentioned above and will be discussed individually. A chemometric classification of the selectivity of commonly used GC stationary phases was reported (C78). In this study, principal component analysis and hierarchi294R
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cal techniques were used to classify the phases by their similarity for specific intermolecular interactions. In another study (C79), the principal component analysis was applied to two sets of data consisting of the gas/liquid partition coefficients for 30 solutes on 22 stationary phases or 67 solutes on 10 stationary phases. Three or four factors were required to characterize the results and were identified as contributions from cavity formation and dispersion interactions, solvent H-bond base interactions, and orientation interactions typical of aromatic and aliphatic compounds. CHROMATOGRAPHIC THEORY The major themes for this section are based on studies where chromatographic behavior is associated with the molecular structure of solute or stationary phase and the relationships between thermodynamic parameters on efficiency, resolution, or retention. Included here are the minor but long-lived explorations of column parameters within GC theory. Structure/Retention Studies. Predictive models using structure/retention relationships were investigated for several chemical classes including alkenes (D1), polybrominated dibenzop-dioxins (D2), polychlorinated diphenyl ethers (D3), alkylbenzene (D4), polychlorinated naphthalenes (D5), and some benzene derivatives (D6). A computer program was developed to estimate the retention time of a wide range of organic compounds on squalane liquid phase (D7). In this model, the Henry’s constant was calculated using vapor pressure and activity coefficient and this was related to the Kovats retention index. Qualitative structure/property relationships (QSPR) were employed to predict the retention times and response factors for a range of organic compounds (D8). Using statistical treatment, the most important descriptors were identified and retention and response factors were predicted for unavailable compounds. Quantitative structure/ retention relationships (QSRR) were also applied to alkylbenzenes and naphthalenes (D9). In this study, suitable descriptors were selected based on the extent of correlation with the retention using linear modeling. Additionally, it was concluded that indicator descriptors were more suitable than the more complicated quantum chemistry indexes for practical chromatographic applications. The QSRR models were successfully applied for congener-specific identification of PCBs in environmental samples (D10) and for prediction of naphthalene retention on five stationary phases (D11). In other studies, correlations were investigated between retention indexes of linear alkylbenzene isomers and molecular connectivity indexes (D12) and between retention data of polycyclic aromatic hydrocarbons and several molecular descriptors (D13). Equations were developed to predict the retention times, column efficiency, and resolution for eight p-hydroxybenzoic esters as a function of temperature and flow rate (D14, D15). The dependence of retention of hydrocarbon gases on nature and pressure of carrier gas in capillary adsorption chromatography was evaluated (D16). The retention mechanism of hydrocarbons was examined using modified alumina coated with diphenyl phthalate (D17). In this work, some adsorption effects on retention behavior of hydrocarbons were observed at column temperatures below the melting point of the stationary phase. Theory based on extended statistical model of overlap was proposed to predict and characterize the number of components in small regions of separations (D18). In a review with 31
references, the role of solute/stationary phase interactions and the possibility of using gas chromatography for the determination of molecular hydrophobicity were discussed (D19). Studies and calculation methods on programmed temperature retention indexes under different chromatographic conditions were reviewed (D20). Systems for calculating such retention indexes in programmed temperature GC were evaluated for polycyclic aromatic hydrocarbons (D21, D22) and n-alkanes (D23, D24). Methods were proposed to forecast retention times under nonisothermal condition using a few sets of isothermal experiments (D25, D26). Models were used to predict retention and column efficiency (HETP) in programmed temperature GC and to subsequently simulate peak widths from the HETP measurement (D27, D28). Sorption and Solvation Processes. Solvation parameters were gleaned from GC measurements for functionally substituted aromatic compounds and heterocyclic compounds (D29). Applications of solvation equations of Abraham and Poole to gas/ liquid partition coefficients for a large number of varied solutes on an acidic (H-bond acidity) stationary phase produced compatible qualitative and quantitative results (D30). A comparison of the Abraham model with an alternative cavity model proposed for the sum of the cavity and dispersions contributions to solvation showed similar trends as a function of temperature, but differences existed in the contribution of the cavity dispersion term to the overall free energy change for the solvation process (D31). The influence of temperature on the retention mechanism and solvation interactions of various solutes in several stationary phases of different polarity were discussed (D32), and the criteria for relative importance of partition versus adsorption were outlined. Solute retention in capillary GC column was correlated with a linear solvation energy relationship (LSER) and the stationary phase was quantitatively assessed (D33). Furthermore, the effect of column temperature and the main contributions to retention were determined from an LSER equation. Gas/liquid partition coefficients in n-haxadecane were determined by packed and capillary column and were shown to include contributions from interfacial adsorption, long a concern in gas/liquid chromatography (D34). In this study, headspace GC, which is free from the effects of interfacial adsorption, was used to determine partition coefficients and infinite dilution activity coefficients on a large number of nonpolar and polar solutes in n-hexadecane. Additionally, the corrected activity coefficients were used to interpret the partition process and to reevaluate gas/liquid solvation in n-hexadecane. The phenomenon of enthalpy/entropy compensation in capillary GC was examined using statistical procedures (D35). Enthalpy/entropy compensation was observed for probe solutes within a homologous series as predicted by the LSER method. This observation agreed with the principle that the differences in retention between the selected solutes were governed by a single type of parameter-controlled intermolecular interaction. However, this phenomenon was not observed for probe solutes of different size, polarity, hydrogen bond donor, and acceptor strength. Inverse gas chromatography (IGC) was used to characterize and study solubility parameters for Superox 20M deposited on glass capillary columns (D36), oligo(oxyethylene) derivatives (D37), and a group of R,ω-diamino oligoethers (D38). Other studies were reported for investigation of polymer/solvent interactions using IGC (D39-D42). The specific retention volume values for several probes in poly(propylene oxide) were successfully predicted using the solvation equation proposed by Morales and Acosta (D42).
However, the results for polymer/solvent interaction parameters did not correlate well using this model. The chromatographic properties of poly(ethyleneglycol) 20 M (PEG 20 M) coated on the surface of a diatomaceous earth-type support was investigated (D43), and the solid form of PEG 20 M exhibited partition mechanisms even below the melting range. The adsorption isotherm of C6H6 on silica gel and the retention data of probe solutes as a function of adsorbed C6H6 were used to test the current models for adsorption of binary mixtures (D44). In another study, the adsorption energy distributions of several molecular probes on unmodified and modified silica were determined from the chromatographic retention data (D45). The chemical structure of the probe and the adsorbent surface were studied from the differences observed for the specific capacities of adsorption and for the energy distributions of the unmodified and modified silica samples. COLUMNS AND COLUMN TECHNOLOGY In this section, improvements in column designs and materials as well as new approaches to creating columns for enhanced selectivity or examination of chromatographic principles are noted particularly when the improvements are described or discussed in terms of GC principles. A silicon micromachined GC system with a thin film copper phthalocyanine (CuPc) stationary phase was designed and fabricated (E1, E2) and involved a novel integrated circuit processing technique developed to sublime the CuPc coating on the column walls. This miniature GC system was successfully evaluated for the separation of ppm concentration levels of ammonia and nitrogen dioxide. A deactivated metal (stainless steel) capillary was prepared by oxidizing the metal surface prior to the monosilane treatment and was shown to exhibit thermal stability at >400 °C (E3). A high-pressure microbore packed column GC using common microbore packed columns was studied (E4). The effects of stationary and mobile phases, the mobile linear velocity, and the column temperature were investigated using this column. An inorganic porous chromatographic column containing three-dimensional throughholes with 500-nm diameter and fine holes with 5-100-nm diameter was reported (E5). A method based on thermal treatment was discussed for silicon gum of OV-225 type for open tubular GC (E6). This stationary phase showed good thermal stability as well good selectivity for azarrenes. Capillary columns packed with crystals of NaX zeolite were shown to have superior heat-transfer and external mass-transfer characteristics (E7, E8). This type of column was successfully evaluated by measuring the diffusion for a series of hydrocarbons. Improved sample introduction to columns for enhanced sensitivity was the subject of a few investigations. These included use of coated precolumn solvent focusing (E9) and independently temperature-programmed uncoated precolumn (E10). Another development in this area was the evaluation and optimization of programmable temperature vaporizing injection techniques for specific applications (E11E14). MULTIDIMENSIONAL GAS CHROMATOGRAPHY During the late 1980s, a small number of articles were published annually on multidimensional gas chromatography (MDGC) with separations made on-line using multiple GC columns. A rise in interest in MDGC in the early 1990s can be Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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Table 1 ref Instrumentation description of pyrolysis MDGC system with emphasis on minimizing cold spots and dead volume connections without dead volume exploration of recycling sample for GCn experiments
F7 F8
Separations pine needle oil by GC/GC/MS pyrolysis distillate of C9-C10 aromatic hydrocarbons volatiles in green coffee beans, tea leaves, and peanuts chiral separations of optically active compounds in essential oils fatty acids in fish oil pesticides in blood
F9 F10 F11 F12 F13 F14
F6
seen in an increased number of published journal articles, and this trend has continued during the last review period. For example, over 64 articles were found for MDGC for 1994 and 1995. This trend in interest was noted in the last review though reports previously had been devoted to hardware (i.e., switching valves and connections) and applications. The limited attention given to principles was noted in the last review as a weakness and a potential difficulty for the future of MDGC. Fortunately, reports have appeared on the creation of a framework of principles and fundamentals for MDGC. There has also been a sustained interest in applications of MDGC (paralleling the development of traditional GC), and this suggests that the increased cost and complexity of MDGC can be compensated with sufficient improvements in separation efficiency. Both reviews of MDGC and reports on exploration of fundamentals will be noted below. GC/GC. Broad treatments of the entire concept and practice of MDGC were given in two reviews (F1, F2) which provide primers on MDGC and descriptions of the current technical status of MDGC. The intrinsic selectivity of MDGC was shown to be associated with the strategies for locating and sampling peaks (F3). The precision of sampling a peak using heart-cut methods was the premiere factor in controlling selectivity. This finding was supported indirectly by the extrapolations of retention indexes for PCBs with heart cuts (F4). One of the most encouraging reports in MDGC was the treatment or development of a predictive model based upon a statistical theory of overlap for multidimensional separations including MDGC (F5). While results were encouraging, limitations in quantitative performance of this approach were associated with limits on knowledge of parameters. Nonetheless, this article and others mark the first indications that the principles of MDGC are being addressed and foundations are being established for MDGC as an analytical technique in GC. Over 28 articles appeared on particular separations using MDGC with samples as diverse as pine needle oil, enantioselective flavors, and pesticides. A few of these are noted also in Table 1 and the listing illustrates the variety of samples where separations by MDGC are regarded as advantageous over traditional singlecolumn separations. The complexity of samples also illustrates the need for advanced tool of management and optimization of MDGC; the development of such tools using principles is still in nascent stages. SFC/GC and LC/GC. Variations of MDGC include supercritical fluid chromatography/gas chromatography (SFC/GC) and liquid chromatography/gas chromatography (LC/GC). The reports for SFC/GC were based in analytical measurements including organic compounds in atmospheric aerosols (F15), citrus essential oils (F16), pesticides in food (F17) and gasoline in environmental samples (F18). These developments are reminis296R
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cent of the last decade of reports in MDGC where experimental accomplishments and applications preceded theoretical developments. A similar condition exists for LC/GC, where major concerns in interfacing are being solved with programmed temperature injectors (F19), solvent vapor exits (F20), and by removing ion-pair reagents from the LC effluent (F21). This central concern with LC/GC interfaces where sample and solvent must be volatilized and transferred to a capillary GC column was reviewed (F22). An exception to the predominant concern with interfaces was a review in which principles of LC/GC were discussed and techniques and instrumentation were described (F23). As with MDGC of a few years ago, few tools are available to assess the possibilities of LC/GC from principles and experimental challenges loom. DATA PROCESSING AND QUANTITATIVE ASPECTS The processing of chromatographic data can supplement advances in the principles of GC and can provide insights into chromatographic events. Gas chromatographic theory may not be addressed directly in some of these studies but recent developments in data processing represent new approaches to extracting information from chromatograms. These developments have been facilitated through advanced, commercially available, software for neural networks, multivariate analysis methods, and optimization routines. Consequently, data processing should be regarded as a potential tool for chromatographic research whether the techniques are applied directly for separation investigations or indirectly for applications of GC. Pattern Recognition and Data Processing. An illustration of how advanced methods can be used to handle large amounts of chromatographic information was demonstrated with an expert system where several data processing techniques were integrated to guide chemical determinations (G1). Accuracy and reproducibility of determinations of commercial PCB samplers were enhanced in this approach. Another example of improvements in total systems was the correction with overlapping peaks for isotope ratio determinations in GC/MS (G2). An improvement for a subtle bias was made using automated curve fitting. This problem was also approached through chemometric methods to guide experimental conditions for the determination of isotopic ratios for chlorine, bromine, and sulfur with GC/MS measurements (G3). The broad subject of recognizing patterns within complex chromatographic data was addressed using multiple peak identifications and a battery of statistical tests (G4). The subject of peak area versus retention time to correct for minor variations in retention times was solved with a least-squares procedure added with linear combination techniques (G5). One helpful primer on signal processing and correlation techniques was provided as a review (G6). The possibilities of combining information from high-resolution GC of complex samples with data processing tools are remarkable and can provide additional insights into the importance of experimental results. An example of this genre is the use of multivariate statistical methods to extract information on the chemistry of cocoa roasting and the origins of cocoa from chromatograms of associated vapors (G7). Similar studies were reported for Italian peppermint (G8), morphine alkaloids in opium (G9), volatiles in coffee cultivars (G10), and pollutants in airborne particulate matter (G11). While these do not directly refer to
chromatographic processes, the approach to handling complex chromatographic information may represent a model for handling chromatographic research results. Artificial Intelligence, Data Bases, and Optimization. Developments in artificial intelligence were represented with neural networks which were explored to predict GC retention index data and were based upon electrotopographic indexes (G12). In a direct application, the composition of fuel was associated to octane number using gas chromatographic analyses as inputs for neural networks (G13). Particularly noteworthy was the attempt at simultaneous optimization of chromatographic systems using simplex methods (G14). Simplex optimization was also used for cross-correlation of peaks, as described in the next section (G15). A graphical molecular data base was created to manage the burgeoning literature reports on enantiomer separations (G16); gleaning literature reports failed to fully satisfy the information necessary for a complete data base. The presence of matrix effects from the column effluent in GC/MS on mass spectra, when compared to standard reference libraries, was improved through a macroprogram with reconstructed ion chromatograms (G17). The importance of including reference chemicals with links to chromatographic retention, long a missing part of software for GC/MS data systems, was demonstrated in a limited manner for arson accelerants (G18). A construction of target compound chromatograms assisted in interpreting the complex chromatograms with background interferents. Peak Analysis and Processing. There remains a remarkable level of interest in the treatment of peaks in chromatograms for quantitative purposes and in the extraction of information, particularly when peaks overlap. Moreover, some predictive skills have also been sought. Methods to predict peak widths in temperature-programmed regimes were explored (G19) and then evaluated with respect to peak asymmetries and resolution (G20). Similar studies were made elsewhere (G21). Two attempts to extract information from overlapped peaks were attempted through an extended statistical model (G22) and through deconvolution theory (G23). Quantitative Aspects of GC. Comparison of results between laboratories, long a concern in GC, was approached through two interlaboratory or collaborative studies. For ethylcarbamate in alcoholic beverages and soy sauce (G24), the precision of determination was better than 10% for 17 laboratories. For extracts from a bacterium, Vibrio vulnificus (G25), the percentage of correctly identified isolates using GC profiles for fatty acids was 93.7% from 13 laboratories. The importance of chromatographic performance or efficiency was not addressed in either study. The limits of detection in GC were explored using two methods including a statistical approach and should be a useful reference and guide for such determinations (G26). Limits of detection were improved through a thermal modulation event between the column and detector with a factor of 15 improvement in detector response and an improvement of a factor of 5 in signal-to-noise ratio with a flame ionization detector (G27). Relative response with a flame ionization detector to fatty acids was modeled and linked with regression analysis to the number of carbon atoms (G28).
HIGH-SPEED AND PORTABLE GAS CHROMATOGRAPHY A major trend in gas chromatography in recent years has been fast separations using small instruments (H1). In high-speed gas chromatography (HSGC), separations can be measured in seconds rather than minutes (H2) and this speed presents a challenge for sample introduction (H3), temperature programming (H4), and detection (H5). As with all chromatographic methods, speed and resolution in HSGC were inversely related (H6). One approach for increasing selectivity of HSGC was to use a second column of differing polarity to obtain a multidimensional separation (H7, H8) as discussed in a section above. Further refinement of the multidimensional separation was investigated by modifying the pressure between the two columns (H9). This pressure-tunable approach for separation selectivity changed the contribution of each column to the overall separation, by modifying the relative residence times of components in the columns. One of the primary objectives for developing HSGC was to provide near-real-time monitoring for field applications (H10). For applications in the field, portability is important. Thus, a highspeed, field-portable micro-GC for analysis of gases and volatile solvents was micromachined (H11, H12). Evaluation of underground fuel spills was reported by the use of pattern recognition analysis of high-speed gas chromatograms (H13). For the most part, field analyses were conducted with portable (H14, H15) or transportable (H16) gas chromatographs. While not all portable gas chromatographs were capable of providing high-speed separations, separations were usually rapid when compared with common laboratory-based techniques. Construction was described for a portable GC that was modular in nature, and this design enabled quick and effective on-site repairs (H17, H18). Field substitution of chromatographic columns was facilitated by the development of a capillary tube connector (H19). Cryotrapping (H20) was one of the more popular sample introduction methods investigated. A capillary metal cold trap was described in which no mechanical components were required (H21). Solid-phase extraction (SPE) was also investigated as an introduction method. In one study, a drying agent was placed between the SPE and the column to ensure water-free solvent introduction onto the GC (H22). Microextraction was shown to be an efficient method for introduction into a high-speed GC (H23). With this method, the separation of 28 compounds was demonstrated in 150 s. For the field analysis of non-volatile compounds, pyrolysis prior to injection was reported to provide microbiological information on whole microorganisms (H24). Detection methods for samples separated in the field have been evaluated with emphasis on compatibility with on-site analyses. Rapid GC determination of hydrocarbons by selective absorption and flame ionization detection provided a fast one-step technique for the class resolution of hydrocarbons into aromatics, olefins, and saturated compounds (H25). Thermal desorption modulation of the sample between the column and detector was shown to enhance sensitivity signal-to-noise ratio and detection limit (H26). A special sorption pump developed for mass spectrometry aided in the development of a portable GC/mass spectrometer (H27). Gas chromatographic application to field analyses were reported for a wide variety of analytes including organochlorine pesticides (H28), semivolatile organic compounds (H29), carbon dioxide (H30), contraband drugs (H31), and natural gas (H32). One emphasis on a field GC has been the continuous monitoring Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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of selected components. For example, continuous GC measurement methods were developed for methane (H33), non-methane C2-C10 hydrocarbons (H34), aromatic hydrocarbons (H35), and atmospheric ammonia (H36). For effective continuous monitoring operation, several automated GCs were developed (H37-H41). GAS CHROMATOGRAPHIC DETECTORS Of the 1300 papers reviewed for this section, ∼58% used mass spectrometry as the detection method after gas chromatography while only 23% used ambient pressure ionization detectors, 15% used photometric detectors, and 4% used detection methods that cannot be placed in either of these three categories. Although detectors such as the flame ionization detector have become so routine that the bulk of analysis for which they are used today may not be reported in the literature, the surge in mass spectrometry as a detection method for gas chromatography over the past 10 years is clearly reflected in the percentage of research published over the past two years. Decreases in cost and complexity along with increases in stability and durability have caused this boom in mass spectrometry to the point where mass spectrometers have become the most common of GC detectors. This review will discuss GC detectors in the order of their frequency of use as reported in the literature during the past two years: mass spectrometric detection > ambient pressure ionization detection > photometric detection > other detection methods. Mass Spectrometric Detection. As was evidenced by the growing number of publications investigating mass spectrometry as a detection method for gas chromatography, the ease and reliability with which mass spectrometers can be interfaced to gas chromatographs have led to their general acceptance as GC detectors. It is interesting to note that what is not being discussed in the literature can be as informative as what is being discussed. Methods used to interface gas chromatographs to mass spectrometers were not a focus of investigation. The dominant use of capillary gas chromatographic columns has eliminated the need for intensive research into interface methodologies. Topics which did receive much research attention during the past two years include ionization sources, identification methods, quantification methods, and sample preparation methods. Ionization Sources. Electron impact (EI) was the most common ionization method used for GC/MS during the past two years. Whether ions were separated by quadrupole mass spectrometry (I1-I3), ion trap mass spectrometry (I4-I8), or high-resolution magnetic sector mass spectrometry (I9-I11), characteristic mass spectral EI scans were used to identify and confirm a wide variety of volatile and semivolatile organic compounds. Although timeof-flight mass separators were originally interfaced to gas chromatographs (I12), their use as a GC detection method has waned. Example applications include the GC/EIMS determination of volatile organic compounds (I13-I20), dioxins (I21-I24), drugs (I25-I35), and pesticides (I36-I42). In addition, GC/EIMS was used for the analysis of a variety of compounds in complex matrixes including urine (I43-I50), blood (I51-I56), and food (I57-I60). Chemical ionization (CI) was the next most often used source for mass spectrometry. As with electron impact ionization, chemical ionization is well suited for interfacing with capillary gas chromatographic sample introduction. While this soft ionization method provided less structural information, the lack of fragmentation and the high efficiency of ionization led to simple mass 298R
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spectra with high sensitivity (I61, I62). Both positive ion chemical ionization and negative chemical ionization were investigated (I63, I64). Ammonia, due to its selective proton-transfer properties, was investigated as a reagent gas for positive-mode chemical ionization (I65). Negative ion monitoring was chosen for a number of fluorinated derivatives when ammonia was the reagent gas (I66, I67), but the primary methodology used for negative ion monitoring was that of electron capture chemical ionization (I68-I72). Other reagent gases used in CI included acetonitrile vapor (I73) and methane (I74-I75). Reproducibility and reliability of chemical ionization methods were evaluated by several laboratories. In one international validation study, all laboratories successfully confirmed the presence of chloramphenicol in bovine muscle at the 0.6 ppb with no false positives in blank tissues (I76). There were a number of investigations which demonstrated the high sensitivity of negative ion chemical ionization methodology (I77-I84). Femtomole detection was reported for zacopride in human plasma (I85), corisol in human plasma (I86), amino acids and dipeptides in urine (I87), and jasmonic acid (I88). The third type of ionization source used for mass spectrometry after gas chromatography was the inductively coupled plasma (ICP). The primary application of GC/ICPMS was for the determination of organometallics such as volatile chelate complexes (I89) and metalloporphyrins (I90) with detection limits in the microgram per gram range. The response of halogenated compounds was investigated using a helium low-pressure ICP source and found to have absolute detection limits at the low picrogram level with a linear range of 2-3 orders of magnitude (I91). Identification Methods. The most common method for identification in GC/MS was the use of a full-scan EI source mass spectrum which was matched by characteristic algorithms to standard reference data. For compounds in complex matrices or compounds that give similar spectra, MS/MS methodologies aided identification. For example, phosphate esters in the presence of interfering hydrocarbons were identified by monitoring daughter, parent, and constant neutral loss spectra of the phosphate esters during the capillary run (I92). Other applications of GC/MS/ MS reported were the detection of nucleotide bases in ancient seeds (I93), a rapid screening technique for dioxins in complex environmental matrixes (I94), the analysis of chemical warfare samples (I95), the determination of antipyrine metabolites in biological material (I96), and the confirmation of anabolic steroids in urine (I97). Novel developments in GC/MS/MS technologies included the use of a bench-top ion trap system (I98, I99) and automation (I100, I101). When EI sources were used as the primary ionization source for MS/MS, sensitivity resembled that of an EIMS system (I102-I104). Sensitivity was significantly improved when CI sources were employed as the ionization source (I105). For compounds that do not have clear and unique mass spectral patterns, FT-IR was used to provide additional qualitative information. Two approaches were employed: on-line and matrix isolation. With on-line FT-IR/MS, the GC effluent was passed through the non-destructive FT-IR cell prior to entering the ionization source of the mass spectrometer. Using this on-line GC/FT-IR/MS approach, identification of cis/trans isomers of polyunsaturated fatty acids was demonstrated but, because IR is much less sensitive than MS, 25 ng/µL was required to observe
the weak cis and trans bands (I106). Other problems solved with FT-IR/MS included the identification of over 60 compounds in Douglas fir needles and twigs (I107), components of a commercial liquid smoke flavoring (I108) and the isomers from the monoand dinitration of phenyl- and diphenylacetic acids (I109). Trace analysis using FT-IR/MS required that the sample be deposited off-line for matrix-isolation FT-IR spectrometry. Selective thermolysis of the enol forms of acetoacetates during gas chromatography was identified using the off-line technique (I110) as well as the identification of allelochemicals and pheromones (I111). Isotope ratio mass spectrometry (IRMS) after GC was also investigated for sample identification. The typical IRMS experiment consisted of separation of the organic compounds by gas chromatography, combustion on-line, and the isotopic ratio of the combustion products measured by an isotope ratio mass spectrometer (I112). For GC/IRMS, most of the isotope ratios measured were those of 13C/12C and were used to study kerogens and kerogen precursors (I113), fatty acids in marine sediment (I114), and organic residues of archaeological origin (I115). In addition, GC/IRMS was used to determine the difference between natural and synthetic perfumes and flavors (I116-I119), measure metabolic rates (I120-I123), investigate atmospheric samples (I124, I125), and evaluate hydroxyl radicals in aqueous solutions (I126). Optimization of IRMS for gas chromatography compared various operating conditions of the combustion tube using CuO and NiO. In general NiO-based systems were recommended with the caution that Ni-bound carbon may form and lead to inaccurate isotopic results (I127). At low signal levels, it was recommended that curve fitting may aid in the production of high-precision isotope ratios (I128). Splitting the GC effluent between an IR/ MS and an ion trap mass spectrometer provided simultaneous mass spectra with isotope ratios of a component eluting from the gas chromatograph (I129). Quantification Methods. The most prevalent method of operating a mass spectrometer for detection of compounds eluted from a gas chromatograph was selected ion monitoring (SIM). Also called single-ion monitoring, SIM provided enhanced sensitivity over full-scan methods by continuously monitoring a selected ion of interest. Multiple ion monitoring (MIM) was also reported, which permitted the simultaneous detection of several target compounds (I130). By programming the SIM to search for target ions, SIM could be used for multicomponent detection (I131). Trace level components of mixtures that could not be detected by full-scan mass spectrometry were successfully confirmed by accurate mass measurements using high-resolution SIM (I132I135). With either electron impact mass spectrometry (I136, I137) or chemical ionization mass spectrometry (I138), operation in the SIM mode improved both sensitivity and selectivity. A primary advantage of SIM was the elimination of interferences commonly found in complex matrixes (I139). While detection limits varied, SIM detection limits were in the range of 0.1-100 ng/mL (I140-I143). For quantitative analyses, the most powerful methodology was isotope dilution mass spectrometry. With this approach, isotopes of the target compound were directly incorporated into the sample to serve as an internal standard. Losses of the target compound due to its chemical behavior during the analysis were corrected relative to the isotopic internal standard (I144). The quantification of lignans and isoflavonoids in feces (I145, I146), pesticides and pollutants in aqueous samples (I147-I50), and PAHs in drug
formulations (I151) were reported by isotope dilution mass spectrometry. The most common application of isotope dilution mass spectrometry was for the analysis of biological samples. Methods for the quantification of thyroxine in serum (I152), chromium in whole blood (I153) and urine (I154), toluene in blood (I155), tellurium (I156) and lead (I157) in urine, oxaloacetate and R-ketoglutarate in blood and tissue (I158), 7a-hydroxy4-cholesten-3-one in plasma (I159), and red blood cell folates (I160) were reported. Sample Pretreatment Methods. Many of the investigations over the past two years have focused on the pretreatment of the sample in order to improve separation, sensitivity, or both. Methods that received the most research activity were purge-and-trap systems for volatiles, derivatization for semi- and nonvolatile polar compounds, and pyrolysis for polymeric materials. For the detection of volatile compounds dissolved in aqueous samples, purge and trap offered efficient methods for concentrating the sample before GC/MS. Evaluations of the method were reported for the determination of smoke contamination of foods and packaging materials (I161), volatile organic compounds from hydrothermal sites (I162), benzene in denture adhesives (I163), and trihalomethanes in processed foods (I164). Purge-and-trap methods were found to be sensitive to operating parameters and could be greatly improved through careful optimization (I165). Sample derivatization was used to increase the volatility of the compounds for gas chromatography or to tag the compound with a particularly sensitive moiety for mass spectral detection. Silylation was the most common derivatization procedure, and compounds found to be well suited for silylation followed by GC/ MS analysis were the following: benzodiazepines (I166, I167), carbohydrates (I168), amino acids (I169), amino alcohols, carboxylic acids (I170-I172), hydroxyquinones (I173), sugars (I174), sterols (I175, I176), phenols (I177), diols (I178), flurozepam (I179), and fatty acids (I180). In the fatty acid study, it was found that response factors for an electron impact ionization source increased linearly with total number of carbons. Finally, a comprehensive screening procedure for the detection of stimulants, narcotics, adrenergic drugs, and their metabolites in human urine was developed to prevent abuse of these drugs in sports. After urine samples were enzymatically hydrolyzed and extracted, residues were selectively derivatized with a mixture of N-methylN-(trimethylsilyl)trifluoroacetamide and N-methylbis(trifluoroacetamide). This enabled the formation of trimethylsilyl derivatives of hydroxyl, acidic, and phenolic groups and the formation of trifluoroacetamide derivatives of primary and secondary amines (I181). In general, fluorinated derivatives served to both increase volatilization of a compound and to make the mass spectrometer in the negative chemical ionization mode sensitive to and selective for the compound (I182-I187). Microwave-induced, rapid preparation of fluoro derivatives were investigated (I188, I189). Conventional procedures require heating the reaction mixture for 30 min at 60 °C for the formation of perfluorooctanoyl derivatives while this process took only 1 min by microwave irradiation. Several unique applications of fluorinated derivatives included the determination of nitro polycyclic aromatic hydrocarbons in airborne particulate matter by derivatization with heptafluorobutryic anhydride (I190), the analysis of petroleum for carboxylic acids after esterification with fluoroalcohols (I191), and the quantificaAnalytical Chemistry, Vol. 68, No. 12, June 15, 1996
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tion of nitrite and nitrate in human urine and plasma as pentafluorobenzyl derivatives (I192). The speed of extraction, reproducibility, and accuracy were important parameters for the evaluation of derivatization methods for GC/MS. A comparison of these parameters was conducted for three different GC/MS screening procedures for the determination of diuretics in urine (I193). While there were too many derivatization methods developed for GC/MS to cover comprehensively in this review, several additional procedures have been selected to demonstrate the breadth of this technique. New mechanisms for derivatization were investigated. Esterification during chloroformate derivatization was proposed based on the formation of an intermediate mixed carboxylic-carbonic acid anhydride followed by the exchange with an alcohol (I194). A one-step conversion of fatty acid derivatives directly from total lipids was reported (I195). The advantage of derivatization prior to combustion isotope ratio mass spectrometry was investigated (I196). Simultaneous hydrothermal decomposition and derivatization were found to be useful for the analysis of polymeric material (I197, I198). Analysis of polymeric and high molecular weight non-volatile material that was not amenable to derivatization methods was accomplished by pyrolyzing the sample and characterizing the pyrolysis products (I199). In this manner, airborne particles collected during space shuttle missions were identified as sources of contamination in the shuttle’s atmosphere (I200). Other applications included polyaromatic hydrocarbons as a function of coal rank (I201), identification of paint media (I202), coal characterization (I203, I204), asphaltene characterization (I205), and determination of furfuraldehyde resins (I206). In a comparative investigation, Py-GC/MS was found to behave differently from direct pyrolysis-mass spectrometry (DPMS). Thiophene derivatives were found in the Py-GC/MS run but not in the DPMS. It was speculated that the reason for this discrepancy was that the thiophene compounds were the terminal species arising from further decomposition of the cyclic styrene sulfides (I207). Other more unique investigations of Py-GC/MS included the evaluation of a method to simultaneously detect cocaine, cocaethylene, and their metabolites and “crack” pyrolysis products (I208). Quantitative pyrolysis was investigated for the quantification of sulfonic acid groups in macromolecular dissolved organic matter (I209). Pyrolysis of whole microorganisms appeared promising as a method for rapid detection and characterization of biological warfare agents (I210). One of the most interesting results of PyGC/MS was the discovery of ether-linked phenolic acids in coastal bermuda grass cell walls (I211). Pyrolysis data also indicated that differences in the maturity of petroleum could be determined by this technique (I212). Other sample preparation methods related to pyrolysis included the UV laser ablation of organic polymers (I213) and the thermal degradation of copolymers (I214). Interferences. While mass spectrometry has fulfilled the promise of a versatile, sensitive, and selective detection method for chromatography, there still remain a number of problems associated with the technique. Many of these problems originated from the gas chromatograph. For example, interconversion of chemical species during analysis was a major concern (I215, I216). Also, interference by substances coeluting with targeted compounds is a general problem in GC/MS. One method to identify coeluting compounds was described using a deuterated 300R
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coeluting internal standard (I217). Another approach to reduce or eliminate interferences was to use extensive cleanup methods prior to analysis. For example, liquid chromatography was investigated as a cleanup and preparation method for GC/MS (I218, I219). Still another approach used to limit interference was data analysis. Curve fitting was employed to restore accuracy for overlapping peaks in GC/combustion isotope ratio mass spectrometry (I220), Fourier analysis was used to characterize chromatograms of Aroclors (I221), and a multivariate statistical analysis of GC/MS was reported for the characterization of compounds generated from roasting cocoa (I222). As mass spectral methods decrease in cost and increase in reliability, they are becoming the detection method of choice for gas chromatography. Nevertheless, the ease of use and unique response characteristics of standard gas chromatographic detection methods still reserve a prominent place in the quantification of gas chromatographic effluents. In the final sections of this review, advances in standard GC detection methods will be briefly discussed. Standard GC detection methods have been divided into the following three sections: ambient pressure ionization detectors, photometric type detectors, and other detectors. Ambient Pressure Ionization Detection. The flame ionization detector (FID) is the most common detector for gas chromatography. Almost every laboratory with a gas chromatograph has at least one FID. Although the FID is a mature, wellcharacterized detector for GC, there were still some interesting fundamental papers published during the past two years. Surprisingly, the temperature of the detector block was found to influence response. Moreover, these changes were determined to be a function of the detector design (I223). The response of the FID was reported to increase when ammonia was either used as the carrier gas or mixed with the nitrogen makeup gas at the 5% level (I224). As is well-known, the addition of alkali salts to a flame increases the response for nitrogen- and phosphorus-containing compounds. A new design for this alkali flame ionization detector was reported in which the tip of the capillary exit was inserted into a nozzle where alkali salts were heated in the presence of O and H atoms (I225). First developed for interfacing to liquid chromatography, the aerosol alkali flame ionization detector introduced alkali salts in the form of an aerosol and was investigated for halogen-selective detection. Detection limits were found to be as low as 321 pg of I/s and 6.7 ng of Br/s (I226). When no flame was used to obtain the nitrogen/phosphorusselective response with alkali salts, a tailing effect was often noticed for phosphorus-containing compounds, presumably due to adsorption of the compound on the hot alkali salt surface. This tailing effect could be reduced in aged detectors by coating the source with rubidium and alumina powder (I227). While thermionic detectors were normally used for nitrogen- and phosphoruscontaining compounds, selective detection of oxygenated volatile organic compounds was observed in the presence of other hydrocarbons (I228). The electron capture detector (ECD) was the second most often used detection method in gas chromatography. Because response with an ECD was more sensitive to parametric conditions and detector design than that of the FID, considerably more investigations of a fundamental nature were reported. Detector temperature (I229), detector voltage (I230), detector design (I231), detector contamination (I232), sample introduction meth-
ods (I233), and standard addition procedures (I234) were all investigated for their effects on ECD response. Upon comparing response factors of toxaphene in the ECD with that of electron capture negative ion mass spectrometry, it was found that the two methods responded in a similar manner (I235). The primary problem associated with electron capture detectors has been the requirement of a radioactive source. In some of the more promising research to date, the elimination of the radioactive source was accomplished by using a pulsed-discharge helium photoionization detector to supply electrons for electron capture detection (I236, I237). The pulsed discharge helium ionization detector also served as a universal ionization detector with detection limits in the lowpicogram range (I238). A new type of dc helium plasma was developed as a detector which used an inexpensive plasma source with a stable background. Detection limits for this detector were reported to be from 0.1 to 10 pg/mL (I239). Electrical discharges were also used to generate a photoionization response. Under the standard operating conditions of a helium discharge photoionization detector, it was found that the response to neon could be improved 6-fold by the addition of 3.8 ppm neon to the helium discharge gas (I240). When Ar was used as the carrier gas with a pulsed discharge, the metastable Ar produced during the discharge served as the ionizing radiation (I241). A microwaveinduced argon ionization detector was reported for the detection of benzene and carbon dioxide (I242). Photometric Detection. Photometric detectors can be divided into three classifications: emission, absorption, and scattering. Light emission methods were generally the most sensitive because, as in fluorescence detection, light was detected on a dark background. Absorption was the next most sensitive while light scattering was usually the least sensitive, as was evidenced by the detection of alcohols in a gas chromatographic effluent by laser light scattering (I243). Limits of detection were only 2-8 µg/s. The most common GC detector based on light emission was the atomic emission detector (AED). The microwave-induced AED (MID) was investigated for instrument-induced effects on peak shape (I244), for parametric effects on oxygen response (I245) and for flow rate effects on interfering emission lines (I246). When optimized, the MID was a specific and sensitive detection method for 13C-labeled molecules with a detection limit of 0.1 pg/s (I247). Investigations of plasmas sustained inside the fused-silica capillary columns were reported using a 350-kHz helium plasma. Due to the small internal diameter of the column, only 1.5-3 mL/ min helium flow was required to maintain the plasma (I248) although it was found that nitrogen-, oxygen-, and phosphoruscontaining compounds tail at these low flows (I249). Selectivity of the radio-frequency helium plasma was enhanced by a device that first split the collimated emission and then filtered, scaled, and subtracted the interference lines from the combined signal, giving a net signal for the analyte (I250). Another device used to enhance selectivity was a low-resolution near-IR monochromator with a 0.2-m focal length and a 0.8-3.2-nm effective bandpass. Under optimized plasma conditions, selective atomic emissions for F-, Cl-, Br-, S-, and P-containing compounds with respect to carbon were between 103 and 104 (I251). Interference effects on detection limits, baseline noise, blank values, and even chromatographic resolution were observed for coextractants and reagent impurities (I252). In a three-detector comparison for the analytical
determination of sulfur-containing compounds, the AED demonstrated the most linearity. It had a larger upper limit and lower detection limit than either of the other detectors (I253). An interlaboratory study of the AED demonstrated the GC/AED precision for various test compounds ranged from 1.3 to 22% RSD and the method precision ranged from 11 to 40% RSD (I254). A novel oscillating plasma glow discharge detector (OPGDD) for GC demonstrated a complex relationship between the oscillating frequency, amplitude, and current (I255). The “fingerprint” identification offered by the OPGDD was improved by changing cell pressure, applied voltage, and electrode spacing (I256), and identification of organic compounds was accomplished by employing a multivariate statistical approach (I257). Several flame photometric detectors (FPDs) were investigated. One based on surface emission was described (I258, I259). Another was reported that improved performance by pulsing the flame (I260). Detection limits for this pulsed flame photometric detector were reported to be 180 fg/s for sulfur, 7 fg/s for phosphorus, and 2 pg/s for nitrogen. The nature of the response from this pulsed flame was modeled and analyzed. A third photometric detector based on reactive-flow luminescence was reported for the detection of sulfur and phosphorus (I261). When a stable, air-rich flame was added at the end of the hydrogen-rich reactive flow, an FID-like response was observed from the flame (I262). With dual-channel response, an integrative algorithm provided a novel method for automatically detecting the response ratio of the two channels (I263). Atomic fluorescence detection was investigated for the determination of organomercury compounds (I264-I266). Detection limits in these studies were reported in the subpicogram per second range. Chemiluminescence provided a highly selective and sensitive method for measuring organic nitrates in the atmosphere (I267). Detectors based on FT-IR (I268), Raman spectroscopy (I269), and atomic absorption (I270) were also investigated during this review period. Other Modes of Detection. The challenge for research and development of many GC detectors, especially those which have been around for a long time, was miniaturization. A microthermal conductivity detector was reported for the detection of CO in blood after capillary GC separation (I271). Other new thermal conductivity detectors were constructed for various applications. A serial detector was developed for capillary chromatography (I272), one was constructed with a thin diaphragm which contained the heatgenerating portion of the detector (I273), and another was reported with a simple-to-build design (I274). While the thermal conductivity detector was one of the oldest vapor-phase detectors investigated, the oldest was undoubtedly the nose. Olfactometry was still found to be an important detection method for gas chromatography (I275). Methods for the determination of aliphatic amines (I276), roasted coffee powders (I277), odor defects in fish (I278), sulfur volatiles (I279), food flavorings (I280), and thermally generated flavors (I281) were reported using olfactometry. One olfactometric technique known as Osme was tested with a series of known compounds (I282). There was good agreement in rating each compound’s odor potency and quality. It compared well with traditional olfactometry techniques and was found to be even fairly quantitative. Electrochemical detectors investigated included a SnO2-based gas sensor for the detection of CO in air (I283), a Pt-based Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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electrochemical detector for the determination of oxidizable organic compounds (I284), and a reduction gas detector for the determination of reactive hydrocarbons (I285, I286). A study of the responses of a reduction gas detector revealed that alkenes were more sensitive than alkanes, that it was more sensitive to alkenes than an FID, its sensitivity increases with increasing HgO bed temperature, and its sensitivity was inversely proportional to the carrier gas flow rate (I287). Electroantennographic detection was developed for the detection of oilseed rape floral volatiles after GC (I288) and for the detection of the stereoisomer of the female sex pheromone of the brown-banded cockroach (I289). Finally, one major trend was the attempted use of multi-GC detectors simultaneously in order to gain additional qualitative information from the analyte. Combinations of detectors included ECD/PID (I290), FID/FPD (I291-I293), ECD/NPD/AED (I294), chemiluminescence/NPD (I295), ECD/FID/NPD (I296), NPD/ FPD (I297), and PID/FID (I298). The combinations of detectors with fundamentally different responses provided unique response ratios for additional qualitative information about the analytes. Detector noise was of general concern whether for single or multiple detectors. A general discussion of noise as applied to GC detectors relating noise, filters, and detection limits (I299) was published and the fundamental noise of three chromatographic detectors was defined and measured (I300). In conclusion, the use of mass spectrometry as a detection method after gas chromatography has been of considerable interest and utility. Yet, we continued to see advances in simple detectors for gas chromatography as well. As the cost of mass spectrometers decreases and their reliability increases we can expect to see an even greater number of mass spectrometers used as gas chromatographic detectors in the future. Gary A. Eiceman is a Professor of Chemistry 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 1987-8, he was a Senior Research Fellow at the U.S. Army Chemical Research Development and Engineering Center (Aberdeen Proving Grounds, MD) and is a visiting lecturer at the Universidad Autonoma de Chihuahua (Mexico). 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 teaches electronics, quantitative analysis, and freshman chemistry at NMSU. 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, and in 1983-1984 he was a visiting professor at Kyoto University, Kyoto, Japan. He has been on the faculty at Washington State University since 1976. Behnam Davani is Project Manager in the Bulk Pharmaceutical Section of Analytical Services Department, Sigma Chemical Co., St. Louis, MO. He received his Ph.D. in 1985 at New Mexico State University. Prior to joining Sigma Chemical Co., Dr. Davani held positions as Project Leader at Midwest Research Institute, Kansas City, MO (1987) and Analytical Organic Department Manager at Hall Kimbrell Environmental Services in Lawrence, KS (1988-1990). His current interests include development of analytical methods with emphasis on chromatography (GC, LC) and mass spectrometry (GC/MS, LC/MS) for separation and quantitation of impurities or degradation products in bulk pharmaceutical substances. Other areas of interests include new approaches to sample extraction and detection for trace organic analysis in complex environmental and energy-related matrices. 302R
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Jorge L. Gardea-Torresdey is an Associate 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, supercritical fluid extraction, atomic absorption and emission spectroscopy, inductively coupled plasma/mass spectrometry and investigation of metal binding to biomaterials for remediation of contaminated waters. He has authored or coauthored over 40 research articles and book chapters and holds three 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.
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