Anal. Chem. 1996, 68, 569R-586R
Capillary Electrophoresis Robert L. St. Claire, III
Division of Analytical Sciences, GlaxoWellcome Inc., Research Triangle Park, North Carolina 27709 Review Contents Books Reviews Basic Principles Separation Systems Detection Strategies Basic Principles Electrophoretic Mobility and Field Strength Electroosmotic Flow (EOF) Thermal Effects Solute-Wall Interactions Electrodispersion Fundamental Studies on Capillary Shape, Size, and Surface Characteristics Comprehensive Evaluation and Optimization of Performance Sample Introduction On-Line Coupling Electrophoretic Stacking Ultra-Low-Volume Sampling Injection Artifacts Separation Systems Coated Capillaries Novel Open-Tubular Capillary Geometries and Materials Packed Capillaries Planar Glass Microstructures Coupled-Column Systems Buffer Composition for CZE and MEKC Gels, Polymer Networks, and Modifiers for CGE Modulation of Physical Parameters Fraction Collection Detection Strategies UV-Visible Absorbance Refractive Index and Thermooptical Absorbance Detection Fluorescence Detection Chemiluminescence Mass Spectrometry Electrochemical Detection (EC) Other Detection Strategies Literature Cited
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This reports represents the fourth in a series of fundamental reviews on capillary electrophoresis (CE). The modes of CE examined in this review include the following: capillary zone electrophoresis (CZE), micellar electrokinetic chromatography (MEKC), capillary gel electrophoresis (CGE), capillary isoelectric focusing (CIEF), capillary isotachophoresis (CITP), and capillary electrochromatography (CEC). This review focuses on material published between October 1993 and October 1995. Currently, nearly 1500 documents on the field of CE are published annually. This review, as a fundamental review, examines primarily those documents in which the emphasis is on basic principles, sample introduction, separation systems, and detection strategies. Those documents in which there is greater emphasis on specific CE S0003-2700(96)00018-2 CCC: $25.00
© 1996 American Chemical Society
applications, while not included in this review, can be accessed in the numerous application reviews that also exist. BOOKS Over this two-year review period, five books were published in the field of CE (1-5). The lengthy CRC handbook, edited by Landers (5), contains five chapters devoted to introductory-level discussions on the various modes of CE, as well as optical and mass spectrometric detection techniques. The CRC handbook also contains five chapters devoted to practical and theoretical considerations, such as sample matrix effects and control of electroosmotic flow (EOF). The introductory-level text by Baker (1) is similar, both in size and content, to other recent texts published in the field, with one notable exception. Baker did a particularly good job of presenting a practical guide to basic principles and instrumental techniques. This text would be particularly useful for those just beginning to work with CE methods development. REVIEWS Basic Principles. Monnig and Kennedy authored the third review in this fundamental review series. They primarily focused on fundamentals, such as modeling of separation performance and instrumentation (6). More recently, Issaq and co-workers prepared an extensive review of various experimental approaches for the optimization of CZE experimental parameters based on theoretical and practical concepts (7). Early in this review period, Khaledi and co-workers prepared a summary of literature focused on controlling migration behavior in CE (8). The optimization of selectivity in MEKC was the subject of a recent review by Corstjens and co-workers (9). Several models describing migration behavior were discussed. There has been increasing interest in CEC over this review period, and Dittmann and co-workers (10) have reported the results of several theoretical and experimental studies comparing CEC to HPLC. The utility of CE in biopolymer and chiral separations continues be evaluated. Baba (11) summarized fundamental studies which focused on the prediction of migration times of oligonucleotides. Terebe and co-workers prepared an extensive review of the separation theory associated with several CE techniques from the standpoint of the optimization of selectivity and resolution in enantiomeric separations (12). An extensive review of chiral separations that focused on fundamental studies on enantiomerchiral selector equilibrium was prepared by Vespalec and Bocek (13). Separation Systems. Surface Modification of Capillaries and Planar Chips. El Rassi and Nashabeh conducted an extensive literature survey on the modification of fused-silica capillaries with hydrophilic coatings (14). Schomburg and co-workers reviewed the impact of ionic and nonionic polymeric wall modifiers on efficiency, selectivity, and resolution (15). Open-tubular CEC using capillaries coated with a chiral stationary phase was Analytical Chemistry, Vol. 68, No. 12, June 15, 1996 569R
examined by Vindevogel and Sandra (16). Capillary electrophoresis with immobilized polysiloxane-bonded cyclodextrins was reviewed by Jung and co-workers (17). Wehr’s general review of CE columns (18), and Wirth’s and Fatunmbi’s discussion on self-assembled monolayers (19), provide additional information on coated CE capillaries. Planar chip technology for CE was reviewed by Manz and co-workers (20-22). Buffer Composition. This section covers all aspects of buffer composition, except linear hydrophilic polymers, which are reviewed in a separate section. Janini and Issaq prepared a general review on the role of the buffer in CZE (23). Optimization of biopolymer separations by various experimental means, including the adjustment of buffer pH and the use of buffer additives, was recently summarized by Bruin and Paulus (24). A review of the effects of organic solvents in CE was prepared by Kenndler (25). Various CZE and MEKC methods utilizing unique buffers that facilitate migration of neutral carbohydrates were reviewed by Oefner and co-workers (26). The use of chiral selectors in CE to affect the separation of enantiomers continues to be a subject of great interest. Several general reviews that contain useful information on this subject were prepared (27-29). Fanali and Kilar prepared a review on the utility of cyclodextrins as chiral selectors in CE (30). Gels and Sieving Polymers. Several reviews discussed the utility of both cross-linked systems and entangled polymer solutions in biopolymer separations (24, 31-33). The use of entangled polymer solutions in CZE has gained significant popularity during this review period and was the subject of the comprehensive article by Kenndler and Poppe (34). Detection Strategies. Optical Detection. Yeung has compiled two extensive reviews which discuss the topics of absorbance, refractive index, fluorescence, chemiluminescence, indirect detection modes, and derivatization methods (35, 36). A comprehensive summary of direct and indirect UV methods for underivatized carbohydrates was prepared by Rassi and Smith (37). Refractive index detection of carbohydrates was recently reviewed by Bruno and Krattiger (38). Saz and Diez-Masa reviewed and discussed the utility of thermooptical spectroscopy as a detection mode in CE (39). The growing utility of laser-induced fluorescence detection schemes in CE was the subject of several reviews over this period (40-42). The application of diode lasers, as well as lasers operating in the deep UV, were included in these discussions. Baeyens and co-workers produced a review of chemiluminescence detection in CE (43). Krull and co-workers (44) and Szulc and Krull (45) prepared extensive summaries of various pre- and postcapillary derivatization methods utilized in optical detection procedures. Cui and co-workers (46) have recently reviewed derivatization techniques specific for UV and fluorescence detection of certain amino acid residues. Mass Spectrometry (MS) and Nuclear Magnetic Resonance (NMR). The review by Cai and Henion (47) discussed the most recent advances in coupling MS to CE. This review focused on the latest developments in both on-line and off-line ionization techniques, as well as various means that can improve the technique’s relatively poor concentration sensitivity. Several additional reviews on this topic were published (48-51). In a recent review of the on-line use of NMR in separation chemistry, 570R
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Albert included articles on the latest developments in coupling NMR to CE (52). Electrochemical Methods. The coupling of CE to various electrochemical methods was discussed in two reviews produced over this period (53, 54). BASIC PRINCIPLES Electrophoretic Mobility and Field Strength. The effect of pH and ionic strength of the buffer on the migration behavior of peptides was studied by Cifuentes and Poppe (55). They utilized a computer program, developed in their laboratory, known as SPPMCE (system for predictions of peptide migration in CE). This program, capable of predicting pKa values and electrophoretic mobility (µep) based on structure, was tested successfully with 25 different buffer systems covering a pH range of 2-11 and a concentration range of 5-100 mM. Several additional studies on the evaluation and simulation of peptide migration, as a function of structure and buffer composition, were reported (56-58). Cordier and co-workers reported on the use of computer-aided prediction of migration times of oligodeoxyribonucleotides in CGE using base-specific migration coefficients (59). They noted some difficulties in making accurate correlations due to the inherent stability problems with gels, the effects of the secondary structure of gels on migration, and negligible differences in migration times for oligonucleotides with greater than 50 bases. Friedl and coworkers reported on the empirical derivation of an effective µep (electroosmotic flow suppressed) relationship for multivalent organic anions (60). They reported this expression was valid over an ionic strength range of 0.001-0.1 M and a charge number of 1-6. Timerbaev discussed recently basic principles and several models relevant to the migration of metal ions in CE via metalcomplex formation (61). The development of a valid migration model for lanthanide ions complexed to EDTA and similar analogs was reported by Timerbaev and Semenova (62). This model was developed to relate µep to the chemical nature of these complexes and system variables such as buffer pH. Yang and co-workers reported on a migration model for inorganic cations in capillary ion electrophoresis (63). With this model, effective µep was described in terms of absolute µep, complex formation constants, and concentrations of complexing reagents and organic solvents in the buffer. A series of monosubstituted alkylpyridines were examined for molecular size/shape characteristics in CE in order to facilitate discrimination between derivatives with differing chain length and positional isomers (64). In CZE, the overall velocity (electrophoretic and electroosmotic) of an ion decreases as the concentration of the background electrolyte increases. This occurs, in part, because of a reduction in overall mobility caused by changes in various factors (effective charge of ion, solvated size of ion, buffer viscosity, zeta potential (ζ)). The other component of overall velocity is the effective electric field strength (Eeff). Jumppanen and Riekkola developed a marker technique in which two to four markers of known µep were used to accurately determine Eeff and electroosmotic flow velocity (veo) in a system (65). In another study, they evaluated the degree to which an increase in the concentration of the buffer electrolyte and the hydrated size of a cationic counterion can reduce Eeff (66). Williams and Vigh studied the consequences of long potential ramps (as long as 20 s on several commercial CE instruments) on the determination of accurate µep values (67).
They proposed an expression that can be used to correct for this effect. Electroosmotic Flow (EOF). Several methods for the online measurement of veo in fused-silica capillaries (68, 69) and an off-line measurement in plastic capillaries (70) were reported. Lee and co-workers employed a method of introducing a reference flow of a fluorescent marker into the running buffer downstream from the detection zone (68). This technique detected flow rate changes as small as 1% and was recommended as a means to correct for variation in migration times of analytes that resulted from changes in the surface of capillaries used in bioanalytical applications. Kuzdzal and co-workers presented a method for the simultaneous measurement of EOF and the migration time of the pseudostationary phase in MEKC (71). Taylor and Yeung reported on an imaging system composed of microscope optics and a charged-coupled device (CCD) camera suitable for analysis of the movement of the liquid core within silica capillaries under typical CE conditions (72). Results were discussed in relation to the study of zone dispersion in CE. Kasicka and co-workers reported on a CE system with double UV detection for measurement of EOF (73). Electroosmotic flow and zone dispersion were the subject of several studies over this review period. Gas and co-workers derived an expression for the plate height contribution by EOF in terms of the diffusion coefficient of the analyte, veo, and the thickness of the electrical double layer in cylindrical capillaries with longitudinally uniform ζ (74). They also compared the contribution to zone dispersion from EOF to that from longitudinal diffusion and extracolumn effects. The subject of EOF in CZE with a longitudinally nonuniform ζ was explored in great depth by several experts in the field. Bello and co-workers demonstrated a dependence of electroosmotic mobility (µeo) on the applied electric field that was not related to temperature effects (75). In addition, they observed different µeo values using different electrophoresis units with the same applied voltage. They concluded that these effects resulted from radial electrical fields existing, to some degree, in any electrophoresis unit. Keely and co-workers reported modeling flow profiles and dispersion in a system where a known radial voltage was applied across the capillary wall via a conductive shield (76, 77). This approach allowed for a linear variation of the radial voltage and ζ along the length of the capillary and for the development of theory describing the velocity profile and dispersion such a situation would create. Wu and co-workers utilized an applied radial electric field to study dispersion that was induced by a leakage current phenomenon at the capillary-solution interface (78). Recently, the velocity field of EOF in capillaries with nonuniform ζ was modeled with Navier-Stokes equations (79). The relationship between the nature of the buffer cation and EOF was studied experimentally and theoretically. Dickens and co-workers (80) and Jumppanen and co-workers (81) evaluated the effects of various alkali metals and metal-ammonia complexes on EOF. Jumppanen observed that veo decreased as the hydrated radius of alkali metal decreased. Huang and co-workers conducted mechanistic studies of the capillary-solution interface using a radial electrical potential gradient and several inorganic and organic cations (82). Tavares and McGuffin recently proposed that the capillary surface behaves like an ion-selective electrode because of an observed correlation between ζ and cation activity (83).
Kitagawa and Tsuda recently developed an equation for the relationship between the buffer pH and EOF in CEC (84). They utilized this information to estimate the dissociation constant for the unreacted silanol groups on the surface of the octadecylsilane (C18)-silica. Thermal Effects. The limit to which higher field strengths can improve efficiency and resolution is determined by the Joule heat generated by the passage of electrical current in CE. The consequence of this heat generation on buffer viscosity and, ultimately, system performance was an on-going topic of study over this review period. A two-part discussion on thermal effects, from both experimental and theoretical perspectives, was presented by Knox and McCormack (85, 86). Knox’s work was further reviewed by Reijenga (87). Several groups reported on the development of numerical simulation programs for the prediction of buffer temperatures under various experimental conditions and for the estimation of the time scale of thermal gradient formation (88, 89). Liu and co-workers reported on the measurement of steady-state and transient intracapillary temperature gradients with Raman microthermometry (90). Several studies on thermal effects were relevant to CE method development. Bello and co-workers examined experimentally and theoretically how thermal fluctuations at the outer surface of the capillary induced fluctuations in the current and the baseline in CE (91). Knox and McCormack reported that applying the electric field after sample introduction can cause sample loss if the rate of thermal expansion of the liquid in the capillary, due to Joule heating, is more rapid than the electromigration rate of the slowest moving analyte (92). The relationship between column temperature and DNA fragment mobility in CGE was evaluated experimentally by Lu and co-workers (93). They concluded that the biased reptation model did not predict the observed temperature dependence of mobility and that the fragment length required for onset of biased reptation actually decreased with increasing temperature. The manner of temperature regulation of the capillary determines the extent of thermal effects. Cifuentes and co-workers recently compared the thermostating characteristics of air versus helium in both forced and natural convective cooling modes (94). Improved efficiency was demonstrated with helium. Solute-Wall Interactions. With few exceptions, the adsorption of analytes to the capillary wall is an undesirable event. The consequences of such interactions can range from asymmetric zone dispersion to a permanent adsorption of the analyte to the capillary wall resulting in sample loss and column fouling. Experimental and computer simulation studies of wall adsorption were reported by several groups (95-98). In particular, the recent work of Ermakov (95) examined, using experimentation and modeling, the adsorption of small monovalent cations versus polycations (poly(L-histidine)) as a function of pH, buffer additives, and column washing with NaOH. They reported that, at any pH above 3.0, significant adsorption of polycations occurred, ultimately to the point of reversing the direction of EOF. Strong adsorption of monovalent cations was not observed. The wall adsorption of small anions in CE, induced by ppblevel concentrations of cationic species in the buffer, was reported by Gassner and co-workers (99). They observed severe peak tailing of di- and tribasic benzoates and determined this effect resulted from the interaction of these acids with Fe(III) ions, which Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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were themselves adsorbed, to the surface of uncoated and coated capillaries. Electrodispersion. If conductivity differences exist between the sample zone and the surrounding buffer, field strength will not be uniform along the capillary. When this condition is coupled with the naturally occurring longitudinal diffusion, significant peak dispersion can result. Ermakov and co-workers (100, 101) contributed significantly to the understanding of the fundamental nature and practical consequences of electrodispersion. Ermakov (101) reported the development of a new computer simulation program that utilized typical experimental parameters to correctly predict and access peak fronting and tailing phenomena resulting from conductivity differences between the sample and surrounding buffer. In a two-part report, Beckers recently discussed a model for electrodispersion and a computer program that generates simulated electropherograms which depict the impact of various experimental parameters on electrodispersion (102, 103). Bello and co-workers recently developed a theoretical description for zone evolution within the capillary as a function of both electrodispersion and solute-wall interactions (104). Experimental approaches designed to minimize electrodispersion through manipulation of the buffer co-ion were reported (105-107). Rawjee and co-workers developed a model that could be used to determine the appropriate adjustments in experimental parameters to adjust the mobility of the buffer co-ion to more closely match the mobility of the analyte (106). Fundamental Studies on Capillary Shape, Size, and Surface Characteristics. Cifuentes and Poppe conducted a theoretical study on the use of rectangular columns in CE (108). They focused on issues such as thermal effects, efficiency, and sample capacity. Because of the inverse relationship between electrical resistance and capillary cross-sectional area, the effective electrical field varies along the capillary if the inside diameter is not constant. Slater and Mayer (109) examined resolution as a function of variability in the capillary inside diameter and developed a theoretical model to evaluate the consequences of this relationship for capillary surfaces and thin films. Barberi and coworkers reported on the use of an atomic force microscopy to examine the inner surface of a fused-silica capillary wall (110). They reported that the inner surface was quite smooth and that surface roughness did not exist above the thickness of the diffuse double layer (>10 nm). Comprehensive Evaluation and Optimization of Performance. Capillary Zone Electrophoresis. Several computational optimization schemes were reported during this review period (111-115). Specifically, the studies by Reijenga and Kenndler (111, 112) described the development of an instrumental simulator that produced electropherograms where effective mobilities were corrected for temperature and concentration effects. The evaluation of the effects of diffusion, EOF, electrodispersion, thermal effects, and extracolumn issues such as injection and detection was also possible with this simulator. Virtanen discussed the conditions under which it was valid to determine total peak variance from the sum of the individual variances (116). The development of resolution models for isomer separations with CZE, with emphasis on weak acids, was reported by several groups (114, 117-122). Of particular significance was the work of Rawjee and Vigh (117). They developed a model capable of predicting the optimum buffer pH, EOF, and applied potential and concentration of the chiral selector based on whether the non572R
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dissociated form (type I), the dissociated form (type II), or both forms of the acid selectively complexed with the chiral selector. Models that examined performance in the separation of anions without buffer additives (123) and with a cationic surfactant below the critical micelle concentration (cmc) (124) were also reported. Micellar Electrokinetic Chromatography. The development of modeling programs for MEKC of neutral analytes was an active area of research over this review period (125-129). Muijselaar and co-workers theoretically and experimentally examined numerous experimental factors that affect the elution window and capacity factors in MEKC (125). Reijenga and Hutta produced a MEKC training simulation program that demonstrated the effects of a variety of instrumental parameters and buffer compositions on separation (126). Evaluation of the influence of temperature and ionic strength on the cmc and various dispersive forces was also possible. Jimidar and co-workers developed both empirical and theoretical models to characterize the migration of anions in the presence of a cationic surfactant pseudostationary phase (130). Once a means for predicting mobility was achieved, a selectivity optimization scheme was developed. Pyell and Buetehorn developed a computer-assisted MEKC optimization scheme for surfactant and modifier concentrations (131) and for capillary temperature (132). Capillary Gel Electrophoresis. The advantage of using CGE, over slab gel methods, for DNA sequencing is the speed and efficiency (reduced longitudinal diffusion) afforded by the high field strengths possible with capillaries. Luckey and Smith examined the relationship between optimum field strength and DNA fragment length (133). Sun and Hartwick recently reported on a multipoint detection method that evaluated peak dispersion of oligonucleotides as a function of field strength (134). The utility of this detection method for the evaluation of dispersion that originates from sample introduction was also discussed. Slater and co-workers recently calculated the longitudinal and transverse diffusion coefficients of a DNA molecule as it reptated through a dense polymer matrix (135). They demonstrated that both diffusion coefficients increased with increasing the electric field, but that the transverse coefficient dominated for field strengths normally encountered in CGE. They also reported that the fielddependent diffusion coefficients may dictate an optimum field strength, independent of Joule heating effects. The important implications of these findings to optimization strategies in CGE were also discussed. Barron and co-workers developed a transient entanglement coupling model for DNA separations in ultradilute polymer solutions (136). They concluded that the Ogston and reptation models utilized to describe CGE were not appropriate for characterization of DNA separations in dilute solutions of polymer networks. Optimization of performance in CGE by manipulation of several experimental parameters other than just field strength was also reported (137-139). Capillary Electrochromatography. CEC is a separation technique that utilizes an electric field and electroosmotic flow to drive mobile phase through capillary liquid chromatography columns. While information about this technique has existed for over a decade, interest in CEC has significantly increased over the last several years. Rebscher and Pyell developed a method that differentiates between the several sources of zone dispersion in
CEC with a C18-silica stationary phase (140). Unique opportunities for retention in CEC with capillaries packed with a sizeexclusion stationary phase were reported (141, 142). Basak and Ladisch have conducted additional mechanistic studies on the separation of proteins in this version of CEC (142). The unique retention component in this system was reported to be an electrically induced adsorption of the analyte to the stationaryphase surface. SAMPLE INTRODUCTION On-Line Coupling. Sample Enrichment with Capillary Isotachophoresis. The utility of hydrodynamic counterflow in CITP/ CZE for enhanced analyte focusing was explored by several groups (143, 144). Specifically, Reinhoud and co-workers examined, in a single capillary system, the variation in sample zone velocity and current during the focusing procedure, and they were able to use this information to determine the velocity of hydrodynamic flow needed to counterbalance the sample zone velocity (144). In addition, the correlation between isotachophoretic current and sample zone position in the capillary was used to determine the appropriate moment to switch the system to the CZE mode. The combination of liquid-liquid electroextraction from an organic phase with CITP coupled on-line with CZE was reported (145). Sample Enrichment with Chromatographic Methods. The topic of on-line preconcentration with chromatographic methods received considerable attention over this review period. In studies with C18 concentrators, Beattie and co-workers (146) reported a sensitivity enhancement that was greater than 700-fold, whereas Tomlinson and co-workers (147) reported anomalous behavior in the EOF in the capillary as a function of the bed volume of the C18-silica packing. Tomlinson and co-workers also reported on the use of on-line impregnated membranes as preconcentrators (148, 149). Cole and Kennedy recently described selective online immunoaffinity preconcentration on a protein G chromatographic support (150). In this study, the capillary affinity column was coupled to the electrophoresis system with a flow-gated interface, and a preconcentration enhancement of 1000-fold was reported. Sample Introduction via Microdialysis. The utility of microdialysis for direct in vivo monitoring of biologically relevant analytes continues to be explored. The on-line coupling of microdialysis to MEKC (151) and CZE (152) was presented. Specifically, the microdialysis-MEKC interface described by Hogan and coworkers provided a precision of 2.6% RSD, and the entire system facilitated in vivo analysis with a 90-s temporal resolution (151). On-Line Precolumn Reactions. Several investigations of the online coupling of chemical reaction to CE were reported. Jacobson and co-workers reported on the construction of a microchip that can perform chemical reactions and CE sequentially (153). Licklider and Kuhr, constructed an enzyme-catalyzed microreactor coupled to CZE for peptide mapping (154). Zhou and co-workers developed an on-line reactor to create fluorescent derivatives of primary amines (155). Electrophoretic Stacking. Differences in sample zone and buffer zone conductivity can contribute to electrodispersion and peak asymmetry. However, when the analyte and its buffer coion have similar µep and the sample zone has a lower overall conductivity relative to the conductivity of the buffer zone, symmetrical analyte focusing or stacking will occur. Several
studies focused on the aspects of stacking anions (156, 157) and inorganic cations and anions (158) were reported. In particular, Burgi reported on the use of diethylenetriamine to suppress EOF during field-amplified injection (electrokinetic stacking) (157). Dasgupta and Liu attached a second capillary to the terminal (postdetector) end of a CZE system (159). By controlling the direction and strength of the electric field applied to the second capillary, they were able to control EOF during a stacking procedure in the first capillary and improve overall separation efficiency. Hjerten and co-workers (160, 161) and Liao and co-workers (162) devoted significant effort to the development of wholecapillary injection techniques for ampholytes such proteins and peptides. A concentration enhancement of 400-1000-fold and procedures to desalt biological samples containing proteins prior to stacking were also reported. Zhang and co-workers developed a model to evaluate the influence of the sample injection time, in stacking and nonstacking modes, on migration time (163). It was reported that the model correctly accounted for experimentally observed increases in migration time, in the stacking mode, with increased injection times. Ultra-Low-Volume Sampling. As the technology develops to inject ever smaller volumes of analytes into a CE system, opportunities to conduct ever faster separations become more possible. Several groups have contributed to this area over the last two years. Fishman and co-workers evaluated sample injection by spontaneous fluid displacement (164, 165). They reported that with a small droplet of sample solution hanging from the capillary tip, there was sufficient interfacial pressure across the curved surface of the droplet to drive some of the liquid into the capillary, and this effect could be varied by capillary outside diameter. With this method, it was reported that 3.5 nL was injected with a 5.8 ( 0.7% RSD. The utility of this method for injection of volumes significantly less than 1 nL was discussed. Sziele and co-workers described a microdrop injector capable of injecting 113 pL with a RSD less than 3% (166). Moore and Jorgenson continued to evaluate their optical-gating system for high-speed analysis with studies that examined the system’s contribution to total peak variance (167) and its utility in highspeed analysis of cis-trans isomerization (168). Injection Artifacts. Kleparnik and co-workers experimentally and theoretically evaluated injection bias of DNA fragments in CGE with a liquefied agarose sieving medium (169). Using both electrokinetic and hydrodynamic injection, the observed size bias was attributed to different mobilities in the sieving medium and electroosmosis. Crabtree and co-workers reported on the effect of acetonitrile in the sample solution on peak shape in MEKC (170). Disturbances in the baseline UV signal of an electropherogram that were related to the ionic composition of the sample solution were examined in detail by Beckers (171, 172). Ermakov probed, with experimental and theoretical studies, the relationship between interactions between the sample and buffer electrolytes and artifactual peak splitting observed in CE (173, 174). Ermakov demonstrated that when weak acids or bases were analyzed at high sample loads (as compared to the concentration of the buffering co-ion), the analyte peak was split into two visually distinct zones. The leading zone represented the charged form of the analyte, and the trailing zone represented the uncharged form. In the 1995 study, Ermakov demonstrated Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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the same effect with amphoteric compounds in which a single analyte was split into three zones representing the cationic, anionic, and zwitterionic forms. SEPARATION SYSTEMS Coated Capillaries. This section reviews all materials and methods relevant to the creation of bonded or adhered phases along the capillary wall, except those dynamic procedures involving the use of buffer additives which will be examined later in this review. Non-Ionic Phases. In general, the utilization of non-ionic phases eliminates EOF by decreasing effective charge on the capillary wall and by increasing viscosity along the surface of the wall. This practice is frequently employed to improve efficiency and recovery of cationic biopolymers (proteins and peptides) by preventing solute-wall interactions. New procedures for coating capillaries with cellulose derivatives (175, 176), cross-linked dextrans (177), poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers (178), and titanium oxide or aluminum oxide (179) were reported. In particular, Huang and co-workers (175, 180) reported on a process in which an alkylsilane monolayer was first bonded to the capillary followed by the attachment of the non-ionic phase. A hydrolytic stability of several weeks at pH 2-10 was reported. The subject of chemical stability of non-ionic coated columns was also addressed by Cole and co-workers in a CE/MS environment (181) and by Nakatani and co-workers in their comprehensive studies on polyacrylamide coatings (182). Ionic Phases. The application of ionic phases affords the opportunity to prevent solute-wall interactions via ionic repulsion and to reverse the direction of EOF, which can enhance resolution. Cationic phases have found great utility in the analysis of basic proteins and peptides and metal ions. Several new methods for the attachment of cationic phases were reported (183-188). Specifically, Sun and co-workers created a chitosan-coated capillary (187). They reported this capillary had a cationic surface below pH 6.5 and an RSD of EOF of 0.77% and showed a 2.4% change in EOF after 10 h of analysis of basic analytes. The synthesis and evaluation of an anionic polymer-coated capillary with pH-independent EOF (189) and its utility in MEKC, under acidic conditions, (190) were reported. This process, developed by Sun and co-workers (189), created capillaries in which the EOF could be predetermined, at the time of synthesis by varying the ratio of sodium [2-(acrylamido)-2-methyl]propanesulfonate to the neutral acrylamide in the polymer matrix. A variation in EOF, column to column, of 1.7% RSD was reported. The pH-independent EOF changed less than 1.5% over 20 days of routine use. Specialty Phases. Several methods offering unique selectivity were reported. The attachment of chelating functional groups (191), a chiral stationary phase (192, 193), and an immobilized enzyme (194) were reported. Guo and Colon recently fabricated an organic-inorganic hybrid material that could be layered as a thin film on the inner wall of fused-silica capillaries (195). They reported efficiencies of 500 000 plates/m for open-tubular CEC analysis of polycyclic aromatic hydrocarbons. Novel Open-Tubular Capillary Geometries and Materials. Geometries. The use of fused-silica capillaries with microconcentric shapes (196) and tapered shapes (197) was reported. The construction of the microconcentric columns by Fujimoto and co574R
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workers involved winding a plastic line, helically, around a smalldiameter fused-silica capillary and then inserting this unit into a larger diameter capillary (196). It was reported that this configuration afforded higher sample loading than possible with conventional CE, and its utility in a micropreparative application was explored. Materials. The preparation of capillaries from hollow polypropylene fibers was reported (198, 199). Specifically, Liu and co-workers developed an 85-µm-i.d., polyacrylamide surface-coated, polypropylene, hollow fiber column that produced 350 000 theoretical plates for a model protein (199). Packed Capillaries. New procedures to prepare packed columns for CEC continued to be explored (200-204). This is due, in large part, to the efficiency advantage of EOF in CEC over the laminar flow that is characteristic of conventional chromatographic separations and the increased sample capacity of CEC relative to CE. Specifically, Boughtflower and co-workers described a novel pressurized ultrasound device for efficient production of packed capillaries (201), and Li and Lloyd conducted a comprehensive evaluation of the experimental factors in the preparation and utilization of capillaries packed with R 1-acid glycoprotein (202) and β-cyclodextrin (204) chiral stationary phases. Planar Glass Microstructures. The increasing interest in planar glass microstructures (microchips) is a consequence of their ability to inject small volumes (60 pL) (205) under fast, highfield conditions (2500 V/cm) (206) with efficiency (0.3-µm plate height) (207). Coated Separation Channel. Channels containing bonded polyamide (208), polyacrylamide (209), and C18 (210) were reported. The work of Jacobson and co-workers (210) reported a plate height as low as 5.0 µm for a retained compound with open-channel CEC on a C18-coated microchip. Multichannel Arrays. Woolley and Mathies have reported on the feasibility of high-speed DNA sequencing by conducting capillary array electrophoresis on microchips (211, 212). The effects of the electric field, injection procedure, and size and shape of the separation and injection channels on performance were reported (211). Column Switching. Burggraf and co-workers developed a novel CE-based column switching system on a planar glass structure that they referred to as synchronized cyclic capillary electrophoresis (SCCE) (213-215). This structure was composed of four capillaries of 20 mm in length arranged in a square. They reported on moving one component around the system (one cycle) in 1 min with 2.5 kV, and they evaluated peak dispersion during this process (215). Coupled-Column Systems. Mesarous and Ewing evaluated the lateral dispersion, longitudinal dispersion, and signal intensity associated with continuous electrophoretic separations in which CZE is coupled to open-channel structures with rectangular cross sections (216). Krasensky and co-workers coupled, via a bifurcation block, one capillary containing a buffer optimized for a chiral separation with another capillary containing a buffer optimized for analyte detection (217). The implications of this dual-buffer approach to separation and detection of any analyte were discussed. The development of multidimensional separation systems in which chromatographic processes are coupled to CE, which substantially increases the overall system peak capacity, continued
to be explored over this review period (218-221). In particular, Moore and Jorgenson developed a three-dimensional separation scheme in which peptides were first sampled repetitively on the basis of molecular weight over a period of several hours from a size exclusion column. These fractions were then directed into a reversed-phase liquid chromatography/CZE two-dimensional system with an analysis time of 7 min (218). Buffer Composition for CZE and MEKC. Surfactants. At levels below the cmc, charged surfactants can add selectivity via ion-pairing interactions with charged analytes. Surfactants at concentrations above their cmc function as a pseudostationary phase. Surfactants can also be used to modify the capillary wall and, thus, modify EOF. As with all surfactants, they can also assist in the solubilization of hydrophobic solutes. Surfactants: Anionic. Several developments in anionic surfactants were reported (222-227). In the work reported by Shi and Fritz, a novel surfactant, sodium dioctyl sulfosuccinate (DOSS), was added to an aqueous solution containing 40% (v/v) acetonitrile (223). They reported that different electrophoretic mobilities for nonionic organic compounds were created as a function of the strength of the analyte/DOSS association in a nonmicellar environment. The work of Smith and El Rassi (224226) focused on MEKC with in situ charged micelles. In their most recent approach, alkylglucoside surfactants were complexed with butylboronate. It was reported that, with the manipulation of the exact composition of this complex and buffer pH, micelles with a range of hydrophobic character and charge density were created. Surfactants: Cationic. The utilization of poly(dimethyldiallylammonium) chloride (228) to induce charge reversal of the capillary wall and inhibit protein adsorption was reported. Shi and Fritz utilized a tetraheptylammonium salt in an aqueous acetonitrile buffer to separate nonionic organic compounds (229). Several examples of the use of cationic surfactants (at levels above and below their cmc) for the separation of anions were reported (130, 230-232). Specifically, Jimidar and co-workers fitted experimental data into several models designed to predict migration behavior of anions as a function of both the ionic dissociation and micellar partitioning events occurring in this complex process (130). Oda and co-workers reported on the superiority (speed and resolving power) of decamethonium bromide as an additive over the amine modifier 1,4-diaminobutane in the separation of ovalbumin glycoforms (233). Surfactants: Neutral and Zwitterionic. Matsubara and Terabe reported the separation of dansylamino acids with micelles of the nonionic surfactant Tween 20 (234). Greve and co-workers utilized the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3ammonio-1-propanesulfonate near its cmc for a peptide separation (235). Mazzeo and co-workers prepared the optically active zwitterionic surfactant (S)-N-(dodecoxycarbonyl)valine for chiral MEKC applications (236). The use of a zwitterionic surfactant in CE/MS of peptides was also reported (237). Surfactants: Mixed Systems. Ahuja and co-workers examined ways to increase the elution range in MEKC by using a nonionicanionic mixed micellar system (238). Emmer and Roeraade evaluated the utility of mixtures of zwitterionic and cationic fluorosurfactants for the CE of proteins (239). They demonstrated alteration of EOF direction with changes in buffer pH. They also observed changes in selectivity with changes in the proportion of the zwitterionic to the cationic surfactant.
Amine Modifiers. Small alkylamines have been employed to reverse EOF for fused-silica capillaries and afford selectivity to the system by participation in ion-pairing interactions with analytes. The utility of alkyldiamines for this function was examined (240-243). Corradini and co-workers recently examined the adsorption of their diamine in relation to its interaction with the electrical double layer on the capillary surface (240, 241). Iki and co-workers reported on an ion association reaction between the tetrabutylammonium ion and highly charged metal chelates that was critical for resolution (244). Cationic Polyelectrolytes. The use of cationic polyelectrolytes for reversal of EOF and resolution of anions was reported by several groups (245-247). Hinze and co-workers described the preparation and properties of a new cationic polyelectrolyte with dimethylammonium charged centers interconnected by methylene groups (248). They described this material as a micelle-mimetic agent because of its ability to form intramolecular aggregates which can function like surfactant micelles. Solutions of these materials were reported to cause far less foaming than was typical for traditional micelles. Organic Modifiers and Other Solvent Systems. Several groups investigated the utility of organic solvent modifiers in CZE (249251). Specifically, Zhang and co-workers examined buffer systems for the analysis of hydrophobic cations that had a pH lower than the analyte’s pKa and contained 40-80% (v/v) of an organic solvent. They found this system superior to buffer systems employing micelles or ion pair-forming buffer additives (251). The utility of poly(ethylene glycol) (PEG) as a buffer additive in CITP (252) and CZE (253, 254) was reported. Utilizing CZE, Esaka and co-workers reported that PEG could alter the migration of analytes with hydroxyl, amide, and amine groups via hydrogen bond formation. Greenaway and co-workers reported significant improvements in the resolution of hydrophobic compounds in MEKC when mixtures of deuterium oxide and deuterated methanol were used instead of water and methanol (255). Kitagawa and Tsuda reported that the EOF velocity in a CEC study was almost constant with 30-90% (v/v) methanol in the elution solvent (256). Several groups investigated the use of a nonaqueous separation medium (257-261). In a recent study by Jansson and Roeraade, the separation medium was N-methylformamide (NMF) (258). They explored ways to use the advantageous properties of NMF, such as its amphiprotic character, high dielectric constant, high solubilizing power, and low conductivity. They reported a high µeo and excellent reproducibility in migration rates, even while operating at a field strength of 1000 V/cm. The utility of this separation medium for the fast analysis of compounds with poor water solubility was noted. Chiral Selectors: Cyclodextrins. A cyclodextrin (Cd) is a cyclic oligosaccharide containing six to eight glucopyranose units arranged in such a way as to create a cavity. This cavity can separate molecules on the basis of their size relative to the Cd cavity and on the stereoselectivity due to the optically active nature of the carbohydrate units. This specific topic has received more attention over this review period than any other area within the field of CE. The utilization of modified, charged, and neutral Cds in CZE has received the greatest attention (117, 262-276). Among these studies, particular attention was paid to the use of new polyanions of β-Cds (263, 264, 268, 276). Specifically, Tait and co-workers reported that sulfobutyl ether β-Cd was superior Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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to neutral selectors, such as β-Cd and heptakis(2,6-dimethyl)-βCd, for enantomeric separation of several amines (276). Chankvetadze and co-workers using sulfoethyl ether β-Cd, in comparison with sulfobutyl ether β-Cd, reported that possible differences in resolution and efficiency observed with these two selectors were determined mainly by their differences in countercurrent mobility and not by spacer length (264). Rawjee and co-workers reported on peak resolution models for enantiomeric separations of weak acids (117) and weak bases (272) with hydroxypropyl β-Cd. Resolution as a function of buffer pH and concentration of the chiral selector was examined. Yoshinaga and Tanaka recently reported that the enhancement in enantioselectivity of a specific class of Cds, in the presence of several urea derivatives, was strongly dependent on the class of Cds examined (277). Chiral Selectors: Noncyclic Oligosaccharides and Polysaccharides. As is the case with Cds, the presence of optically active carbohydrates within this group of modifiers affords them their chiral selectivity. Several groups examined oligosaccharides with glucopyranose units joined via R-1,4 linkages (278-281). Stalcup and Agyei reported on the use of heparin, an acid mucopolysaccharide (282). They examined the chiral recognition mechanism based on electrostatic interactions with nitrogens on the analyte and analyte size. Chiral Selectors: Proteins. The use of various serum albumins (283-285) and human serum transferrin (286) as chiral selectors in CZE was reported. Specifically, the study of Yang and Hage examined the theory and fundamental mechanisms involved in chiral separations performed with proteins as chiral selectors (285). Factors such as the role of the protein adsorbed on the capillary wall and the protein in the buffer on stereoselectivity were examined. Chiral Selectors: Crown Ethers. The utility of 18-crown-6 tetracarboxylic acid (18C6H4) as a chiral selector was explored by several groups (287-292). In the study reported by Kuhn and co-workers (288), a synergistic effect was observed when a mixture of 18C6H4 and a Cd was used for the enantiomeric separation of amines which could not be resolved by either selector alone. In the most recent work of Schmid and Guebitz (290), the resolution of a dipeptide with two stereogenic centers into its four isomers was achieved with 18C6H4. Chiral Selectors: Macrocyclic Antibiotics. The use of the antibiotics ristocetin A (293), rifamycin B (294), and vancomycin (295, 296) as chiral selectors in CZE was reported. Rundlett and Armstrong evaluated the effect of micelles and mixed micelles on the enantiomeric selectivity of vancomycin. They found that adding sodium dodecyl sulfate (SDS) increased efficiency, reversed elution order, and reduced overall analysis time (297). They derived an equation that described analyte interaction with vancomycin in the micelle and vancomycin free in the buffer, as well as any wall interactions. Complexing Agents for Metals and Anions. The utility of various organic acids as complexing agents that assist in the electrophoretic separation of metal ions was explored (298-300). Timerbaev and co-workers examined metal ion separation via complexation with 2,6-diacetylpyridine bis(N-methylenepyridiniohydrazone) (301). They utilized this complexing agent in a MEKC mode with tetradecyltrimethylammonium bromide as the micellar agent and sodium octanesulfonate as an ion-pairing counterion to separate 14 metal ions in 12 min at 15 kV. 576R
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Wiley recently conducted a CZE separation of poly(carboxylic acids) via Cu(II) complexation (302). Complexation with Borate. Stefansson and Novotny reported on the use of borate complexation in the enantiomeric separation of monosaccharides (303) and the separation of oligosaccharides (304). Tadey and Purdy explored how borate complexation with monophosphorylated nucleotide positional isomers affected resolution in the presence of β-Cd (305). Other Additives for Reduction of Wall Effects. The use of zwitterionic buffer additives (306) and chitosan, a natural cationic polymer (307), to reduce protein adsorption to the fused-silica capillary wall was reported. Okafo and co-workers reduced this protein adsorption by using an additive, the sodium salt of phytic acid, which would ion pair with the proteins and prevent the interaction (308). Wu and co-workers utilized a buffer system containing a mixture of an anionic surfactant and a serum albumin for the separation of porphyrin isomers (309). This combination was reported to offer enhanced selectivity, analyte solubilization, and reduced wall interactions. Other Pseudostationary-Phase Additives for Nonenantiomeric Separations. The utility of the linear polymer poly(vinylpyrrolidone) for the separation of diastereomers (310) and the use of a two-component polymer mixture of poly(ethylene oxide)-polydextran for the separation of small molecular weight compounds (311) was reported. Other additives used were liposomes (312), suspended chromatographic particles (313), starburst dendrimers (314), macrocyclic oligomers, calixarenes (315), and crown ethers (316, 317). Additional uses of cyclodextrins, other than for enantiomeric separations, were reported. Cooper and Sepaniak examined possible mechanisms for the separation of benzopyrene substitutional isomers in an MEKC environment with a buffer containing SDS and γ-Cd (318). Sepaniak and co-workers recently developed an approach in which a buffer containing a mixture of neutral and charged Cds was utilized successfully to separate polyaromatic hydrocarbons (PAHs) (319). The advantages of this approach over an MEKC approach were examined in this comprehensive study. Additional in-depth studies on the utility of Cds in MEKC analysis of PAHs (320) and Cds in the CZE analysis of PAHs (321) were reported. Other Issues: Supporting Electrolyte Composition. Hjerten and co-workers presented a comprehensive discussion on buffer compositions suitable for high-speed analysis with field strengths in the vicinity of 2000 V/cm (322). Zhu and co-workers examined the variation in the pH of the supporting electrolyte as a result of electrolysis (323). Gels, Polymer Networks, and Modifiers for CGE. Gels. Chen and co-workers reported on a new method for the preparation of void free 25-µm-diameter polyacrylamide capillaries (up to 30% T + 5% C) in 5 h (324). Several studies that focused on agarose gels were also reported (325-327). In particular, Hjerten and co-workers examined a UV-transparent, methoxylated agarose with a gelling temperature of 25.6 °C (325). It was reported that at 35-40 °C, this agarose gel was sufficiently liquefied to be expelled out of the capillary, allowing replenishment of the same capillary with fresh agarose. They demonstrated that this replaceable gel compared favorably with cross-linked polyacrylamide for the separation of proteins and DNA fragments. Polymer Networks: Linear Polyacrylamide. The utility of these sieving polymer networks for separation of oligonucleotides and
DNA fragments received the attention of several groups (328332). Specifically, Gelfi and co-workers examined different processes for the manufacture of linear poly(acrylamides) that would be optimal for the separation of DNA fragments of the size associated with polymerase chain reaction studies of genetic diseases (50-500 base pairs (bp)) (330). Woolley and Mathies demonstrated DNA sequencing via multichannel-array CGE on a microchip using a polyacrylamide sieving medium (212). Polymer Networks: Cellulose Solutions. Barron and co-workers (333) examined the use of hydroxyethyl cellulose as a sieving matrix in the separation of DNA restriction fragments. Mitnik and co-workers (334) conducted a mechanistic study on the separation of duplex DNA in hydroxypropyl cellulose. Shi and co-workers recently reported on the use of video microscopy to visually observe in CGE the shape-changing entanglement between DNA and hydroxyethyl cellulose (335). Multichannel-array CGE on a microchip using hydroxyethyl cellulose as the sieving matrix was also reported (211). Polymer Networks: Dextrans. Dextrans are branched polysaccharides that have low absorbance in the low-UV and, as such, offer detection advantages over poly(acrylamide) for protein separations. Karim and co-workers observed a significant relationship between dextran molecular weight and the migration time and efficiency in protein separations (336). They reported an unexpectedly rapid separation with enhanced resolution when using dextrans having narrow molecular weight distributions around either 1270 or 5220. Takagi and Karim recently reported that this separation mechanism was different from just sieving by a polymer network (337). Polymer Networks: Poly(ethylene oxide). This topic received considerable attention over this review period with several studies exploring its utility in nucleic acid analysis (338) and protein separations (339-343). Specifically, Fung and Yeung utilized a mixture of two sizes of poly(ethylene oxide) (PEO) in bare fusedsilica capillaries as a sieving matrix for DNA fragments (338). In a comparison study with a 6% T polyacrylamide solution, they observed that the PEO mixture offered identical performance for small DNA fragments, superior performance for larger fragments, increased separation speed, and lower viscosity. Guttman, in his most recent work, examined the effect of PEO chain length and concentration on the separation mechanism of SDS-protein complexes (339). Other Gels and Polymer Networks. The use of linear poly[N(acryloylamino)ethoxyethanol] (344), polyacrylamide with negatively charged functional groups (345), and entangled solutions of poly(vinyl alcohol) (346) were reported. Modifiers. The use of modifiers to impart additional selectivity to CGE can be accomplished by incorporating them into the gel, as well as by adding them to the buffer. Singhal and Xian conducted a comprehensive study into the use of ion-pairing agents that interact with the polymer network and modify its physical properties. They also studied the use of ion-pairing and intercalating agents (ethidium bromide) that interact with DNA fragments and modify their mobilities (347). The use of bis and monointercalating dyes for the separation of duplex DNA in cellulose polymer networks was also reported (348). Cheng and Mitchelson observed improved separation of duplex DNA with (hydroxypropyl)methyl cellulose (HPMC) when glycerol was added to a borate buffer system (349). They reported that borate
formed a linking complex between glycerol and HPMC, allowing for entangled solutions with different pore sizes to be created. Baba and co-workers evaluated the effect of urea concentration on the separation of oligo(thymidylic acids) in capillary affinity gel electrophoresis (CAGE) in which poly(9-vinyladenine) was the affinity ligand (350). Sun and co-workers described a process to covalently link bovine serum albumin to a dextran polymer network to facilitate an enantiomeric separation via CAGE (351). They also demonstrated the utility of a cyclodextrin-dextran polymer network for enantiomeric separations of dansylated amino acids (352). Modulation of Physical Parameters. Pulsed Electric Fields. It was demonstrated that coiled DNA will elongate and align itself in a size-dependent manner, with a constant electric field. This results in a biased reptation that reduces resolution as DNA fragment size increases. To extend the range of resolution, pulsed-field techniques (with and without field inversion) are employed which allow the DNA strands to revert back, momentarily, to the coiled structure in which mobility is more precisely related to molecular weight. Several studies were conducted over this review period that helped to refine this technique (353-357). Kim and Morris (355) demonstrated the resolution of DNA fragments as long as 1.6 million base pairs in 12-13 min by applying rapid pulsed electric fields to a capillary that contained a mixture of hydroxyethyl cellulose and poly(ethylene oxide). Sudor and Novotny evaluated the relationship between the mobility of duplex DNA (5-100 kb) in neutral polymer networks and the frequency and pulse shape of the applied voltage (357). Applied Pressure. Culbertson and Jorgenson utilized a pressure-induced counterflow to retard, halt, or reverse an analyte’s migration through a capillary (358). This manipulation allowed the analyte to remain in the separation field longer than normally possible and yielded efficiencies of 17.3 million plates for fluorescently labeled amino acids. They reported that these efficiencies would allow resolution of µep differences of 1 × 10-7 cm2/V‚s. The application of pressure to assist in moving the analyte toward detection in CIEF (359) and CE/MS, in which high levels of organic solvent are present in the buffer (low EOF) (360), was reported. Eimer and co-workers reported on the use of a pressure gradient in CEC (361). Other Physical Parameters. Bevan and Mutton reported on a freeze-thaw procedure for managing flow in capillaries (362). Janini and co-workers observed the resolution of syn and anti conformers in capillaries operated at 5 °C (363). Razee and coworkers examined the effect of a 10-kG magnetic field (applied perpendicular to the electric field) on EOF and µep (364). Fraction Collection. The technical challenges and scientific rewards associated with development of fraction collection devices for micropreparative CE continued to be explored (365-371). In particular, Effenhauser and co-workers developed a method to selectively isolate a single 300-pL zone following fast CE on a microchip by using automated switching of the applied potential on microchannel system (365). Muller and co-workers described a high-precision fraction collector that utilized a detector near the end of the capillary and a sheath liquid to facilitate continuous collection of up to 60 fractions (1 µL or less) automatically (370). DETECTION STRATEGIES UV-Visible Absorbance. Detector Design. New, on-column designs that focused on increasing optical path length by increasAnalytical Chemistry, Vol. 68, No. 12, June 15, 1996
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ing column diameter were described and evaluated by Xue and Yeung (372) and Liu and Dasgupta (373). Moring and co-workers evaluated the effect of a Z-shaped cell with a 3-mm path length on linear dynamic range, efficiency, and resolution (374). They reported minimum detectable concentrations in the range of 10-8 M. Heiger and co-workers (375) and Flint and co-workers (376) discussed optimized optical designs for on-column diode array and variable-wavelength detection schemes, respectively. Xue and Yeung reported on a laser-based UV absorption detector that addresses the significant noise normally associated with UV lasers (377). A postcolumn flow cell in which the outlet of the capillary was connected vertically to the middle of a 3-mm microchannel that served as the optical path was also described (378). A 44% loss in resolution was reported with this design. Wu and Pawliszyn recently described detectors for CIEF (for individual capillaries and arrays) using a CCD camera (379). One detector utilized in this study had a two-dimensional CCD camera to record absorption as a function of distance along the capillary and wavelength simultaneously. A detection limit of 1.5 × 10-3 AU was reported. Derivatization Procedures. Mechref and co-workers introduced a derivatization reaction that focused on the attachment of a UVabsorbing tag, sulfanilic acid, or a UV/fluorescent tag, 7-aminonapthalene-1,3-disulfonic acid, to acidic monosaccharides (380) and sialogangliosides (381). This derivatization occurred between the weak carboxyl group of the sugar and the amino group on the derivatizing agent and also yielded an analyte that is anionic over a wide pH range. Motomizu and co-workers utilized the visible light-absorbing chelating agent 2-(5-bromo-2-pyridylazo)-5-[N-propyl-N-(sulfopropyl)amino]phenol (5-Br-PAPS) to separate and detect 22 metal ions (382). A detection limit for Fe2+ and Co2+(Co3+) of 5 × 10-8 mol/dm3 was reported. The chelating agent was in the injection solution and in the CE buffer. The utilization of on-column complexation with 4-(2-pyridylazo)resorcinol, following a sample stacking procedure, yielded a detection limit of 1 × 10-8 M for Co2+, Fe2+, and Zn2+ (383). Indirect UV and Visible Detection. In this procedure, a carrier ion, capable of absorbing light and having the same charge as the analyte being measured is added to the buffer. When an analyte zone, with a lower molar absorptivity (due to displacement of the carrier ion) passes through the detector, a reduction in signal over background (less absorbance) is noted. The popularity of this technique was evident over this review period. The utility of this technique for the analysis of cations (298, 384) and amino acids (385) was explored. Several studies focused on the evaluation of the carrier electrolyte for indirect UV detection of low molecular weight anions (386, 387). In particular, Cousins and co-workers evaluated indirect detection as related to the transfer ratio, which is a measure of the effectiveness with which the analyte can displace the UV-absorbing carrier ion (387). They reported that when the carrier ion had a higher mobility than the analyte ion, the transfer ratio was higher. They suggested that the chromate ion was the best electrolyte for indirect detection of high-mobility inorganic anions. Shamsi and Danielson utilized naphthalenesulfonates as additives for the detection of inorganic anions, organic acids, and aliphatic anionic surfactants (388). Detection limits as low as 16 µg/L for inorganic anions and 40 µg/L for organic acids were reported. 578R
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Mala and co-workers reported on the use of organic dye additives and a visible light source for indirect detection of anions and cations (389). They claimed a detection sensitivity for cations that was 2 orders of magnitude lower than the best previously reported. Refractive Index and Thermooptical Absorbance Detection. Refractive Index. Several new detectors based on interferometric principles were developed. Specifically, detectors based on offaxis capillary focusing (390) and holographic optical elements (391) were reported. An optical system designed to accommodate a capillary flared in the detection region in order to minimize the Joule heating contribution to background signal was also reported (392). Using a holographic-based system and field-amplified injection, Krattiger and co-workers reported detection limits in the low-nanomolar range for selected cations (391). Thermooptical Absorbance Detection. Krattiger and co-workers (393) described a system where analytes were optically pumped with a frequency-doubled Ar ion laser to induce absorption of UV light. This event was followed by nonradiative relaxation (heat release). This heat generation and the change in refractive index it produced were then probed with a laser diode or He-Ne laser that was guided to the detection region by a holographic optical system. They reported nanomolar concentration detection limits and subfemtomole (1 fmol ) 10-15 mol) mass detection limits for a nucleotide monophosphate. Using the same system, Saz and co-workers (394) reported limits of detection for proteins comparable to that obtained with laser-induced fluorescence and 2 orders of magnitude lower than possible with absorbance detection. The utility of the procedure for analysis of proteins lacking native fluorescence was highlighted. Fluorescence Detection. Derivatization of Amines. Interest in the use of reactive aldehyde derivatives as derivatization agents has increased significantly over this review period. Arriaga and co-workers reported on the use of 3-(p-carboxybenzoyl)quinoline2-carboxaldehyde (CBQ) which yielded a detection limit for arginine of 9.0 zmol (1 zmol ) 10-21 mol) (395). Novel precolumn derivatization of proteins (396), on-column derivatization of the cellular contents of a single mammalian cell (397), and postcolumn derivatization of amino acids and proteins (398, 399) with reactive aldehyde analogs were reported. The utility of two new aldehyde analogs was also explored (400, 401). The utility of fluorescein and fluorescein analogs for fluorescent derivatization of amines continued to be explored (402, 403). Derivatization of Nucleic Acids. The use of intercalating dyes for the detection of duplex DNA was examined (404, 405). In particular, Figeys and co-workers examined the use of the dyes POPO-3, YOYO-3, and YOYO-1. They reported that with one dye molecule incorporated every 10 base pairs, detection limits of a few yoctomoles (1 ymol ) 10-24 mol) were possible (404). Addition Derivatization Schemes. Fluorescent derivatization of carbohydrates (381, 406), short-chain dicarboxylic acids (407), phosphoserine residues (408), phosphate compounds (409), and a monoclonal antibody for affinity probe CE (410) were reported. The labeling of peptides and proteins (411) and oligonucleotides (412) with near-IR fluorescent dyes was also examined. Semiconductor, Solid-State, and Dye Laser Excitation Sources. While the majority of excitation sources for laser-induced fluorescence detection are either He-Cd, Ar ion, or He-Ne lasers, interest in other laser sources is expanding. Nowhere is this more evident than for semiconductor-based lasers. Semiconductor
lasers have the advantage of lower cost, smaller size, and a longer lifetime when compared to the more traditional laser sources. Semiconductor lasers which provide excitation in the blue (frequency doubled) (413), the visible (414), and the deep-red regions were described (415-419). In particular, Mank and Yeung recently utilized the red-absorbing label dicarbocyanine to detect amino acids at a detection limit on the order of 0.1 amol (1 amol ) 10-18 mol) (419). Flanagan and co-workers used a solid-state Ti/sapphire laser to evaluate the effect of methanol in the running buffer on the fluorescence of amino acids labeled with near-IR dyes (420). Nilsson and co-workers developed a real-time fluorescence imaging system to view the electrophoretic process (421). A substantial portion of a decoated capillary was excited by a dye laser which had been pumped with an XeCl eximer laser tuned for the fluorophors in sample. Emitted light was collected by a CCD camera and processed by a computer to generate real-time moving images. The benefits of the real-time observation of the separation processes were discussed. Sheath Flow Cuvette. The sheath flow cuvette (SFC) is a postcolumn detection cell with a flat detection window (reduced light scatter and background noise) that facilitates very high sensitivity in laser-induced fluorescence. Also, the SFC’s contribution to extracolumn band broadening is minimal. The SFC has become increasingly popular over this review period (422-426). Specifically, Chen and co-workers used an SFC for the detection of 50 ymol of rodamine 6G using a green He-Ne laser (424) and for the detection of 10 ymol (6 molecules) of sulforhodamine 101 using a yellow He-Ne laser (425). Takahashi and co-workers expanded on this theme by developing a multiple SFC that facilitated the simultaneous irradiation of fluorophore-labeled DNA from a 20-gel capillary array (423). They reported minimum detectable concentrations of 10-13 M in a one-color mode, and 2 × 10-12 M in a four-color mode, at a base reading rate of 200 bases/h. Multichannel and Other Scanning Detection Schemes in a SingleCapillary Mode. Carson and co-workers developed a two-laser, two-window illumination scheme coupled to an intensified diode array detector for DNA sequencing using the standard four-dyelabeled primer approach (427). They reported that concentration detection limits for all four dye-labeled primers were in the range of 1 × 10-12 M. Sweedler and co-workers (428) and Timperman and co-workers (429) recently utilized wavelength-resolved fluorescence detection systems. Specifically, Timperman described a detection system that utilized an Ar-Kr mixed-gas ion laser, a holographic grating, and a CCD detector for yoctomole detection limits with the simultaneous collection of the entire fluorescence emission spectra. Two studies were reported on the development of spatialscanning laser fluorescence detection systems (430, 431). Beale and Sudmeier developed and optimized a laser-induced fluorescence detector with epi-illumination that was designed to scan the entire length of the capillary to monitor fluorescence in a CIEF mode or to follow the time course of an ongoing separation (430). Multichannel Detection for Capillary Arrays. The development of array-based systems is a direct consequence of the immense analytical challenges inherent in efforts to sequence the entire human genetic code. Several groups contributed to the development of laser-induced fluorescence detection systems designed to support capillary arrays (423, 432-434). Specifically, Ueno
and Yeung evaluated various excitation schemes for the simultaneous laser illumination of a 100-capillary array, and they discussed future systems capable of accommodating up to 4096 independent channels (432). Indirect Fluorescence. Indirect fluorescence detection strategies operate in a manner similar to indirect absorbance. The utility of the buffer additive fluorescein for the detection of complexed metal ions (435) and anions in single erythrocytes (436) was examined. Fuchigami and Imasaka reported on the use of indirect semiconductor laser fluorescence in a MEKC mode (437). Using a cationic surfactant and the dye, methylene blue, they reported a detection limit for flavin adenine dinucleotide of 100 fmol. Other Fluorescence Detection Strategies. Several additional optimization studies focused on such areas as automated velocity programming (438, 439), MEKC (440), and adjustable detection zones in CGE (441). Arriaga and co-workers reported a detection limit of 200 zmol for fluorescein-labeled amino acids using a 75-W Xe arc lamp (442). Haab and Mathies recently developed a system for single-molecule fluorescence burst detection of DNA fragments that could evolve into a new routine detection technology for DNA analysis via CE (443). Chemiluminescence. The feature common to all chemiluminescence (CL) detection schemes is that the analyte or a derivatized analog undergoes a chemical reaction in the detection region of the system that generates light. Building on established luminescence and bioluminescence reactions, Dadoo and coworkers developed an end-column CL detector in which the visible light generated from the reaction of an analyte (just as it exits the capillary) with CL reaction reagents was delivered by fiber optics to a photomultiplier tube (444). Concentration detection limits of 2 × 10-8 M for luminol and 5 × 10-9 M for ATP were reported. Hara and co-workers conducted several studies on the CL detection of dye-labeled proteins (445-448). Hara (445) demonstrated that bovine serum albumin (BSA) when mixed with the xanthene dye Rose Bengal formed a supramolecular complex. This complex, following electromigration, was then reacted with bis(2,4,6-trichlorophenyl) oxalate and hydrogen peroxide to facilitate CL detection limits for BSA of 4 fmol (S/N ) 3). The utility of acridinium tagging for CL detection of peptides was also reported (449). Gilman and co-workers reported on the development of an electrogenerated chemiluminescence (ECL) process that involved the use of luminol derivatives, hydrogen peroxide, and a carbon or platinum microelectrode (450). They demonstrated MEKC with ECL detection of amines which were derivatized with N-(4aminobutyl)-N-ethylisoluminol coupled to N,N-disuccinimidyl carbonate at detection limits in the low to subfemtomole level. Mass Spectrometry. On-Line Ionization Methods. Electrospray ionization (ESI) methods continue to dominate this category with numerous studies focused on further refinement of the technique. Studies that focused on a liquid sheath (451, 452) and sheathless interfaces (453-455) were reported. Foret and co-workers studied theoretically and experimentally the formation of moving ionic boundaries inside the capillary due to the migration of liquid sheath counterions into the capillary (452). These moving ionic boundaries were reported to alter the migration and resolution of proteins examined in this study. Wahl and Smith conducted a comparison study of buffer systems and interface designs for ESI (455). They reexamined a sheathless Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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interface using a conductive capillary tip. They suggested this design appeared to provide greater sensitivity over the liquid sheath. Kriger and co-workers recently described a simple procedure to prepare a tapered gold-coated capillary tip for ESI (456). Sheppard and co-workers reported the use of ion spray (pneumatically assisted ESI) for the detection of enantiomercyclodextrin inclusion complexes (457). Takada and co-workers recently reported on the development of an atmospheric pressure chemical ionization (APCI) interface for CE that protonated caffeine molecules in a sodium phosphate buffer (458). A new spray ionization procedure, known as sonic spray, in which it is not necessary to apply an electric field to the capillary tip was also reported (459). The interfacing of CE to inductively coupled plasma (ICP) mass spectrometry for elemental analysis was explored, in depth, by several groups over this review period (460-462). In particular, Olesik and co-workers developed an interface to generate a fine aerosol that was delivered from the end of the capillary to the ICP with minimal dead volume, mass detection sensitivity in the amol range, and high peak area and elution time reproducibility (462). Off-Line Ionization Methods. Several studies utilizing matrixassisted laser desorption/ionization (MALDI) (463, 464) and 252Cf plasma desorption/ionization (465, 466) were reported. In particular, Weinmann and co-workers developed a sheath flow interface, similar to that used in ESI, for sample isolation of picomolar amounts of CE-separated proteins which were then subsequently analyzed by MALDI/MS (464). Mass Analyzers. The ion tap mass spectrometer holds great promise for high-sensitivity and low-cost MS detection and was examined in several studies over this review period. Henion and co-workers utilized ion spray coupled to an ion trap to generate full-scan spectra from ∼400 amol of an isoquinoline alkaloid (467). Ramsey and McLuckey recently utilized a sheathless ESI interface (with a gold-tipped capillary) coupled to an ion trap and reported midattomole detection levels for leucine enkephalin under fullscan conditions (468). MS/MS spectra from low-femtomole amounts of this peptide were also generated. Fourier transform ion cyclotron resonance (FTICR) MS provides sensitive and high-precision mass measurements, and its coupling to CE was explored in depth over this review period (469-472). Hofstadler and co-workers used CE/ESI/FTICR MS to collect high-resolution mass spectra from 4.5 fmol of a cellular protein from 10 human red blood cells (471). Fang and co-workers utilized CE/ESI with on-line time-of-flight (TOF) MS to acquire complete mass spectra from peptides at a concentration detection limit of 10-6 M (473). Perkins and Tomer reported on the development of a CE/ESI system coupled to a magnetic sector mass spectrometer and on its utility for highresolution discrimination between peptides with similar mass-tocharge ratios (474). Sample Preconcentration For CE/MS. On-line preconcentration procedures to enhance concentration sensitivity have included CITP (475, 476), chromatographic methods (477), and electrostacking (478). Van der Vlis and co-workers reported nanomolar concentration detection limits after using a combined liquid-liquid electroextraction and CITP system for on-line analyte focusing prior to CZE/ESI/MS (479). 580R
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CEC Coupled to MS. The coupling of CEC to ESI sources (480-482) was reported. Gordon and co-workers described the coupling of CEC to a continuous-flow fast-atom bombardment interface (483). Additional Hyphenated Techniques Incorporating CE/MS. Takada and co-workers reported on the coupling of in vivo microdialysis to CE/MS for the analysis of brain dialysates (484). Van der Hoeven and co-workers recently evaluated the performance of an electrospray interface coupled to a thermospray ion source (485). Additional CE/MS Topics. Tomlinson and co-workers examined the utility of nonaqueous solvents in CE/MS (486, 487). Tetler and co-workers examined the relationship between the physical dimensions of the capillaries that make up a sheath flow interface and sensitivity (488). Varghese and Cole reported on the use of cetyltrimethylammonium chloride as a cationic surfactant additive for reversed-polarity CE/ESI/MS analysis of large cationic molecules (489). Electrochemical Detection (EC). Amperometric: Electrode Materials. Amperometric detection on copper/copper oxide electrodes is related to the complexation of an analyte with Cu2+ ions on the electrode surface which alters the electrode’s steadystate current. This process was used in the detection of such nonelectroactive species as amino acids and peptides (490-492) and carbohydrates (493-495). In particular, Zhou and Lunte demonstrated with a copper electrode the value of low-conductivity zwitterionic buffer additives which contributed to a lower CE current and reduced detector background noise (492). They reported detection limits for amino acids of between 10 and 400 nM. The development of chemically modified microelectrodes for selective EC detection was reported. Microelectrodes incorporating cobalt phthalocyanine for the analysis of thiols (496), glucose oxidase for the detection of glucose (496), mixed-valence ruthenium cyanide for simultaneous detection of thiols and disulfides (497), and platinum particles on carbon for the analysis of hydrazines (498) were reported. Lu and Cassidy conducted a comprehensive evaluation of the detector background noise in CE/EC, with particular focus on the chemical composition of the electrode materials (499). Amperometric: CE/EC Coupling. Research has continued into the development of CE/EC interface systems designed to electrically isolate the high electric field in CE from the EC detector with minimal reduction in separation efficiency (500-502). Recently, Zhou and Lunte described a membrane-based, oncolumn mixer that facilitates the use of a buffer with a different ionic strength and pH in the detector cell than is employed during the CE separation (503). Amperometric: Voltage Scanning and Modulation. Ferris and co-workers developed a scanning EC detector for CE which provided voltammetric information and micromolar-level detection limits for catechols (504). The development of pulsed amperometric detection at gold electrodes for carbohydrate analysis was also discussed (505, 506). Conductometric and Potentiometric Detection. Kaniansky and co-workers developed an on-line CITP/CZE system with conductivity detection that yielded limits of detection in the parts-perbillion to parts-per-trillion range for several inorganic anions (507). Kar and co-workers developed a novel computer-interfaced bipolar pulse conductivity cell for suppressed conductivity detection in CE (508).
Potentiometric detection of anions using ion-selective microelectrodes was reported (509, 510). Specifically, Nann and Pretsch reported a detection limit of 5 × 10-8 M for the perchlorate ion (510). Other Detection Strategies. Spectroscopic and Optical Systems. Wu and co-workers developed an NMR detection cell of ∼5 nL for on-line detection in CE (511, 512). They reported detection limits in nanogram range for amino acids using less than a 1 min acquisition time. Kowalchyk and co-workers reported detection limits in the upper parts-per-billion range for oxy anions using field-amplified injection with CZE and on-line near-Raman spectroscopy with 1 s integration times (513). Recently, Odake and co-workers reported the detection of several amino acids at low-attomole levels using laser-induced capillary vibration detection (514). Radionuclide Detection. The development of postcolumn radionuclide detection for low-energy β emitters continued over this review period (515, 516). Specifically, Tracht and co-workers (516) developed a process in which eluant from the capillary was directed onto a peptide-binding membrane that was coated with a solid scintillator. They reported detection limits of 88 zmol for 32P-labeled analytes, 17 amol for 35S-labeled analytes, and 8 fmol for 3H-labeled analytes. Postcolumn Reactors. Postcolumn reactors that facilitate EC detection (517), that conduct on-line enzyme assays (518, 519), and that are fabricated on a microchip (520) were reported. ACKNOWLEDGMENT
The author expresses his sincere appreciation to Dayna Scarborough of GlaxoWellcome for her technical assistance in conducting the 2-year comprehensive literature search upon which this review is based. In addition, the author recognizes that without the support of Alan Colborn, Ph.D., Department Head of Analytical Sciences, GlaxoWellcome, the preparation of the manuscript would not have been possible. Robert L. St. Claire, III, is Research Leader of Separation Science within the Analytical Sciences Division of GlaxoWellcome. He received a B.S. degree in Biochemistry from Virginia Polytechnic Institute in 1978 and a M.A. degree in Biochemistry from University of Texas Medical Branch in 1981. He earned a Ph.D. degree in Chemistry from the University of North Carolina at Chapel Hill under the direction of James W. Jorgenson in 1986. Following this, he joined the Analytical Chemistry Department of Glaxo Inc. and remained there until 1988 when he became Group Leader of Separation Science in the Glaxo Research Institute Drug Metabolism Department. He continued in this capacity until 1995. Currently, he heads a separation science focus group and matrix team which deals with methods development issues, evaluation and development of new technology, and teaching.
LITERATURE CITED (1) Baker, D. R. Capillary Electrophoresis; Wiley: New York, 1995. (2) Directory of Capillary Electrophoresis; Coleman, D., Ed.; Elsevier: Amsterdam, 1994. (3) Foret, F.; Krivankov, L.; Bocek, P. Capillary Zone Electrophoresis; VCH: New York, 1994. (4) Hartwick, R. A. Introduction to Capillary Electrophoresis; CRC Press: Boca Raton, FL, 1994. (5) Handbook of Capillary Electrophoresis; Landers, J. P., Ed.; CRC Press: Boca Raton, FL, 1994. (6) Monnig, C. A.; Kennedy, R. T. Anal. Chem. 1994, 66, 280R314R. (7) Issaq, H. J.; Janini, G. M.; Chan, K. C.; El Rassi, Z. Adv. Chromatogr. 1995, 35, 101-69. (8) Khaledi, M. G.; Quang, C.; Sahota, R. S.; Strasters, J. K.; Smith, S. C. Chromatogr. Sci. Ser. 1993, 64, 187-260. (9) Corstjens, H.; Billiet, H. A. H.; Frank, J.; Luyben, K. C. A. M. J. Chromatogr., A 1995, 715, 1-11. (10) Dittmann, M. M.; Wienand, K.; Bek, F.; Rozing, G. P. LC-GC 1995, 13, 803-4. (11) Baba, Y. J. Chromatogr., Biomed. Appl. 1993, 618, 41-55. (12) Terabe, S.; Otsuka, K.; Nishi, H. J. Chromatogr., A 1994, 666, 295-319. (13) Vespalec, R.; Bocek, P. Electrophoresis 1994, 15, 755-62.
(14) El Rassi, Z.; Nashabeh, W. Chromatogr. Sci. Ser. 1993, 64, 383434. (15) Schomburg, G.; Belder, D.; Gilges, M.; Motsch, S. J. Capillary Electrophor. 1994, 1, 219-30. (16) Vindevogel, J.; Sandra, P. Electrophoresis 1994, 15, 842-7. (17) Jung, M.; Mayer, S.; Schurig, V. LC-GC 1994, 12, 458-66. (18) Wehr, T. LC-GC 1993, 11, 19-20. (19) Wirth, M. J.; Fatunmbi, H. O. LC-GC 1994, 12, 222-8. (20) Manz, A.; Verpoorte, E.; Effenhauser, C. S.; Burggraf, N.; Raymond, D. E.; Widmer, H. M. Fresenius’ J. Anal. Chem. 1994, 348, 567-71. (21) Manz, A.; Effenhauser, C. S.; Burggraf, N.; Verpoorte, E.; Raymond, D. E.; Widmer, H. M. Analusis 1994, 22, M25-8. (22) Manz, A.; Effenhauser, C. S.; Burggraf, N.; Verpoorte, E.; Raymond, D. E.; Widmer, H. M. Kagaku Kogyo 1994, 47, 153641. (23) Janini, G. M.; Issaq, H. J. Chromatogr. Sci. Ser. 1993, 64, 11960. (24) Bruin, G. J. M.; Paulus, A. Anal. Methods Instrum. 1995, 2, 3-26. (25) Kenndler, E. Chromatogr. Sci. Ser. 1993, 64, 161-86. (26) Oefner, P.; Chiesa, C.; Bonn, G.; Horvath, C. J. Capillary Electrophor. 1994, 1, 5-26. (27) Otsuka, K.; Terabe, S. Chromatogr. Sci. Ser. 1993, 64, 61729. (28) Rogan, M. M.; Altria, K. D.; Goodall, D. M. Chirality 1994, 6, 25-40. (29) Bereuter, T. L. LC-GC 1994, 12, 748-66. (30) Fanali, S.; Kilar, F. J. Capillary Electrophor. 1994, 1, 72-8. (31) Heller, C. J. Chromatogr., A 1995, 698, 19-31. (32) Dolnik, V. J. Microcolumn Sep. 1994, 6, 315-30. (33) Bae, Y. C.; Soane, D. J. Chromatogr. 1993, 652, 17-22. (34) Kenndler, E.; Poppe, H. J. Capillary Electrophor. 1994, 1, 14457. (35) Yeung, E. S. Adv. Chromatogr. 1995, 35, 1-51. (36) Yeung, E. S. Chromatogr. Sci. Ser. 1993, 64, 587-603. (37) Rassi, Z. E.; Smith, J. T. J. Chromatogr. Libr. 1995, 58, 60740. (38) Bruno, A. E.; Krattiger, B. J. Chromatogr. Libr. 1995, 58, 43146. (39) Saz, J. M.; Diez-Masa, J. C. J. Liq. Chromatogr. 1994, 17, 499520. (40) Schwartz, H. E.; Ulfelder, K. J.; Chen, F. T. A.; Pentoney, S. L., Jr. J. Capillary Electrophor. 1994, 1, 36-54. (41) Gooijer, C.; van de Nesse, R. J.; Mank, A. J. G.; Ariese, F.; Brinkman, U. A. T.; Velthorst, N. H. Spec. Publ.-R. Soc. Chem. 1994, 154, 153-67. (42) Cannon, J.; Rickman, M. Am. Biotechnol. Lab. 1993, 11, 12. (43) Baeyens, W. R. G.; Ling, B. L.; Imai, K.; Calokerinos, A. C.; Schulman, S. G. J. Microcolumn Sep. 1994, 6, 195-206. (44) Krull, I. S.; Deyl, Z.; Lingeman, H. J. Chromatogr., B: Biomed. Appl. 1994, 659, 1-17. (45) Szulc, M. E.; Krull, I. S. J. Chromatogr. 1994, 659, 231-45. (46) Cui, H.; Leon, J.; Reusaet, E.; Bult, A. J. Chromatogr., A 1995, 704, 27-36. (47) Cai, J.; Henion, J. J. Chromatogr., A 1995, 703, 667-92. (48) Tomer, K. B.; Deterding, L. J.; Parker, C. E. Adv. Chromatogr. 1995, 35, 53-99. (49) Tomer, K. B. Chromatogr. Sci. Ser. 1993, 64, 569-86. (50) Smith, R. D.; Udseth, H. R. Chromatogr. Sci. Ser. 1993, 64, 52567. (51) Allen, M. H.; Shushan, B. I. LC-GC 1993, 11, 112-4. (52) Albert, K. J. Chromatogr., A 1995, 703, 123-47. (53) Ewing, A. G.; Mesaros, J. M.; Gavin, P. F. Anal. Chem. 1994, 66, 527A-37A. (54) Lunte, S. M.; O’Shea, T. J. Electrophoresis 1994, 15, 79-86. (55) Cifuentes, A.; Poppe, H. Electrophoresis 1995, 16, 516-24. (56) Chen, N.; Wang, l.; Zhang, Y. J. Microcolumn Sep. 1995, 7, 1938. (57) Cifuentes, A.; Poppe, H. J. Chromatogr., A 1994, 680, 321-40. (58) Chen, N.; Wang, L.; Zhang, Y. K. Chromatographia 1993, 37, 429-32. (59) Cordier, Y.; Roch, O.; Cordier, P.; Bischoff, R. J. Chromatogr., A 1994, 680, 479-89. (60) Friedl, W.; Reijenga, J. C.; Kenndler, E. J. Chromatogr., A 1995, 709, 163-70. (61) Timerbaev, A. R. J. Capillary Electrophor. 1995, 2, 165-74. (62) Timerbaev, A. R.; Semenova, O. P. J. Chromatogr., A 1995, 690, 141-8. (63) Yang, Q.; Zhuang, Y.; Smeyers-Verbeke, J.; Massart, D. L. J. Chromatogr., A 1995, 706, 503-15. (64) Rowe, R. C.; Wren, S. A. C.; McKillop, A. G. Electrophoresis 1994, 15, 635-9. (65) Jumppanen, J. H.; Riekkola, M. L. Anal. Chem. 1995, 67, 10606. (66) Jumppanen, J. H.; Riekkola, M. L. Electrophoresis 1995, 16, 1441-4. (67) Williams, B. A.; Vigh, G. Anal. Chem. 1995, 67, 3079-81. (68) Lee, T. T.; Dadoo, R.; Zare, R. N. Anal. Chem. 1994, 66, 2694700. (69) Wanders, B. J.; van de Goor, T. A. A. M.; Everaerts, F. M. J. Chromatogr. 1993, 652, 291-4. (70) Rohlicek, V.; Deyl, Z.; Miksik, I. J. Chromatogr., A 1994, 662, 369-73.
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(71) Kuzdzal, S. A.; Hagen, J. J.; Monnig, C. A. J. High Resolut. Chromatogr. 1995, 18, 439-42. (72) Taylor, J. A.; Yeung, E. S. Anal. Chem. 1993, 65, 2928-32. (73) Kasicka, V.; Prusik, Z.; Mudra, P.; Stepanek, J. J. Chromatogr., A 1995, 709, 31-8. (74) Gas, B.; Stedry, M.; Kenndler, E. J. Chromatogr., A 1995, 709, 63-8. (75) Bello, M. S.; Capelli, L.; Righetti, P. G. J. Chromatogr., A 1994, 684, 311-22. (76) Keely, C. A.; van de Goor, T. A. A. M.; McManigill, D. Anal. Chem. 1994, 66, 4236-42. (77) Keely, C. A.; Holloway, R. R.; van de Goor, T. A. A. M.; McManigill, D. J. Chromatogr. 1993, 652, 283-9. (78) Wu, C. T.; Huang, T. L.; Lee, C. S. J. Chromatogr. 1993, 652, 277-81. (79) Potocek, B.; Gas, B.; Kenndler, E.; Stedry, M. J. Chromatogr., A 1995, 709, 51-62. (80) Dickens, J. E.; Gorse, J.; Everhart, J. A.; Ryan, M. J. Chromatogr., B: Biomed. Appl. 1994, 657, 401-7. (81) Jumppanen, J. H.; Siren, H.; Riekkola, M. L. J. High Resolut. Chromatogr. 1994, 17, 537-42. (82) Huang, T. L.; Tsai, P.; Wu, C. T.; Lee, C. S. Anal. Chem. 1993, 65, 2887-93. (83) Tavares, M. F. M.; McGuffin, V. L. Anal. Chem. 1995, 67, 368796. (84) Kitagawa, S.; Tsuda, T. J. Microcolumn Sep. 1995, 7, 59-64. (85) Knox, J. H.; McCormack, K. A. Chromatographia 1994, 38, 207-14. (86) Knox, J. H.; McCormack, K. A. Chromatographia 1994, 38, 215-21. (87) Reijenga, J. C. Chromatographia 1994, 38, 658. (88) Bello, M. S.; Levin, E. I.; Righetti, P. G. J. Chromatogr. 1993, 652, 329-36. (89) Dose, E. V.; Guiochon, G. J. Chromatogr. 1993, 652, 263-75. (90) Liu, K. L. K.; Davis, K. L.; Morris, M. D. Anal. Chem. 1994, 66, 3744-50. (91) Bello, M. S.; de Besi, P.; Righetti, P. G. J. Chromatogr. 1993, 652, 317-27. (92) Knox, J. H.; McCormack, K. A. Chromatographia 1994, 38, 279-82. (93) Lu, H.; Arriaga, E.; Chen, D. Y.; Figeys, D.; Dovichi, N. J. J. Chromatogr., A 1994, 680, 503-10. (94) Cifuentes, A.; Kok, W. T.; Poppe, H. J. Microcolumn Sep. 1995, 7, 365-74. (95) Ermakov, S. V.; Zhukov, M. Y.; Capelli, L.; Righetti, P. G. J. Chromatogr., A 1995, 699, 297-313. (96) Gas, B.; Stedry, M.; Rizzi, A.; Kenndler, E. Electrophoresis 1995, 16, 958-67. (97) Schure, M. R.; Lenhoff, A. M. Anal. Chem. 1993, 65, 3024-37. (98) Pagonabarraga, I.; Bafaluy, J.; Rubi, J. M. Phys. Rev. Lett. 1995, 75, 461-5. (99) Gassner, B.; Friedl, W.; Kenndler, E. J. Chromatogr., A 1994, 680, 25-31. (100) Ermakov, S. V.; Bello, M. S.; Righetti, P. G. J. Chromatogr. 1994, 661, 265-78. (101) Ermakov, S. V.; Righetti, P. G. J. Chromatogr., A 1994, 667, 257-70. (102) Beckers, J. L. J. Chromatogr., A 1995, 696, 285-94. (103) Beckers, J. L. J. Chromatogr., A 1995, 693, 347-57. (104) Bello, M. S.; Zhukov, M. Y.; Righetti, P. G. J. Chromatogr., A 1995, 693, 113-30. (105) Bullock, J.; Strasters, J.; Snide, J. Anal. Chem. 1995, 67, 324652. (106) Rawjee, Y. Y.; Williams, R. L.; Vigh, G. Anal. Chem. 1994, 66, 3777-81. (107) Song, L.; Ou, Q.; Yu, W.; Xu, G. J. Chromatogr., A 1995, 696, 307-19. (108) Cifuentes, A.; Poppe, H. Chromatographia 1994, 39, 391-404. (109) Slater, G. W.; Mayer, P. Electrophoresis 1995, 16, 771-9. (110) Barberi, R.; Giocondo, M.; Bartolino, R.; Righetti, P. G. Electrophoresis 1995, 16, 1445-50. (111) Reijenga, J. C.; Kenndler, E. J. Chromatogr. 1994, 659, 41726. (112) Reijenga, J. C.; Kenndler, E. J. Chromatogr. 1994, 659, 40315. (113) Wu, J.; Wong, M. K.; Li, S. F. Y.; Lee, H. K.; Ong, C. N. J. Chromatogr., A 1995, 709, 351-9. (114) Jacquier, J. C.; Rony, C.; Desbene, P. L. J. Chromatogr. 1993, 652, 337-45. (115) Pretswell, E. L.; Morrisson, A. R. Anal. Methods Instrum. 1995, 2, 87-91. (116) Virtanen, R. Electrophoresis 1993, 14, 1266-70. (117) Rawjee, Y. Y.; Vigh, G. Anal. Chem. 1994, 66, 619-27. (118) Biggin, M. E.; Williams, R. L.; Vigh, G. J. Chromatogr., A 1995, 692, 319-25. (119) Copper, C. L.; Davis, J. B.; Cole, R. O.; Sepaniak, M. J. Electrophoresis 1994, 15, 785-92. (120) Wren, S. A.; Rowe, R. C.; Payne, R. S. Electrophoresis 1994, 15, 774-8. (121) D’Hulst, A.; Verbeke, N. Electrophoresis 1994, 15, 854-63. (122) Janini, G. M.; Chan, K. C.; Muschik, G. M.; Issaq, H. J. J. Liq. Chromatogr. 1993, 16, 3591-607. (123) Dunn, C. D.; Hankins, M. G.; Ghowsi, K. Sep. Sci. Technol. 1994, 29, 2419-33. (124) Jimidar, M.; Massart, D. L. Anal. Chim. Acta 1994, 294, 16576. 582R
Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
(125) Muijselaar, P. G. H. M.; Claessens, H. A.; Cramers, C. A. J. Chromatogr., A 1995, 696, 273-84. (126) Reijenga, J. C.; Hutta, M. J. Chromatogr., A 1995, 709, 21-9. (127) Muijselaar, P. G. H. M.; Claessens, H. A.; Cramers, C. A. Anal. Chem. 1994, 66, 2799. (128) Muijselaar, P. G. H. M.; Claessens, H. A.; Cramers, C. A. Anal. Chem. 1994, 66, 635-44. (129) Yang, S.; Khaledi, M. G. J. Chromatogr., A 1995, 692, 301-10. (130) Jimidar, M.; Bourguignon, B.; Massart, D. L. Anal. Chim. Acta 1995, 310, 27-42. (131) Pyell, U.; Buetehorn, U. Chromatographia 1995, 40, 175-84. (132) Pyell, U.; Buetehorn, U. Chromatographia 1995, 40, 69-77. (133) Luckey, J. A.; Smith, L. M. Anal. Chem. 1993, 65, 2841-50. (134) Sun, P.; Hartwick, R. A. J. Liq. Chromatogr. 1994, 17, 186175. (135) Slater, G. W.; Mayer, P.; Grossman, P. D. Electrophoresis 1995, 16, 75-83. (136) Barron, A. E.; Blanch, H. W.; Soane, D. S. Electrophoresis 1994, 15, 597-615. (137) Luckey, J. A.; Norris, T. B.; Smith, L. M. J. Phys. Chem. 1993, 97, 3067-75. (138) McGregor, D. A.; Yeung, E. S. J. Chromatogr. 1993, 652, 6773. (139) Manabe, T.; Chen, N.; Terabe, S.; Yohda, M.; Endo, I. Anal. Chem. 1994, 66, 4243-52. (140) Rebscher, H.; Pyell, U. Chromatographia 1994, 38, 737-43. (141) Rudge, S. R.; Basak, S. K.; Ladisch, M. R. AIChE J. 1993, 39, 797-808. (142) Basak, S. K.; Ladisch, M. R. AIChE J. 1995, 41, 2499-507. (143) Mazereeuw, M.; Tjaden, U. R.; Greef, J. v. d. J. Chromatogr., A 1994, 677, 151-7. (144) Reinhoud, N. J.; Tjaden, U. R.; van der Greef, J. J. Chromatogr., A 1994, 673, 239-53. (145) van der Vlis, E.; Mazereeuw, M.; Tjaden, U. R.; Irth, H.; van der Greef, J. J. Chromatogr., A 1994, 687, 333-41. (146) Beattie, J. H.; Self, R.; Richards, M. P. Electrophoresis 1995, 16, 322-8. (147) Tomlinson, A. J.; Benson, L. M.; Oda, R. P.; Braddock, W. D.; Strausbauch, M. A.; Wettstein, P. J.; Naylor, S. J. High Resolut. Chromatogr. 1994, 17, 669-70. (148) Tomlinson, A. J.; Benson, L. M.; Braddock, W. D.; Oda, R. P. J. High Resolut. Chromatogr. 1995, 18, 381-3. (149) Tomlinson, A. J.; Naylor, S. J. High Resolut. Chromatogr. 1995, 18, 384-6. (150) Cole, L. J.; Kennedy, R. T. Electrophoresis 1995, 16, 549-56. (151) Hogan, B. L.; Lunte, S. M.; Stobaugh, J. F.; Lunte, C. E. Anal. Chem. 1994, 66, 596-602. (152) Lada, M. W.; Schaller, G.; Carriger, M. H.; Vickroy, T. W.; Kennedy, R. T. Anal. Chim. Acta 1995, 307, 217-25. (153) Jacobson, S. C.; Hergenroder, R.; Moore, A. W., Jr.; Ramsey, J. M. Anal. Chem. 1994, 66, 4127-32. (154) Licklider, L.; Kuhr, W. G. Anal. Chem. 1994, 66, 4400-7. (155) Zhou, S. Y.; Zuo, H.; Stobaugh, J. F.; Lunte, C. E.; Lunte, S. M. Anal. Chem. 1995, 67, 594-9. (156) Baechmann, K.; Goettlicher, B.; Haag, I.; Hannina, M.; Hensel, W. Fresenius’ J. Anal. Chem. 1994, 350, 368-71. (157) Burgi, D. S. Anal. Chem. 1993, 65, 3726-9. (158) Wojtusik, M. J.; Harrold, M. P. J. Chromatogr., A 1994, 671, 411-7. (159) Dasgupta, P. K.; Liu, S. Anal. Chem. 1994, 66, 3060-5. (160) Hjerten, S.; Liao, J. L.; Zhang, R. J. Chromatogr., A 1994, 676, 409-20. (161) Hjerten, S.; Valtcheva, L.; Li, Y. M. J. Capillary Electrophor. 1994, 1, 83-9. (162) Liao, J. L.; Zhang, R.; Hjerten, S. J. Chromatogr., A 1994, 676, 421-30. (163) Zhang, H. W.; Chen, X. G.; Hu, Z. D. J. Chromatogr., A 1994, 677, 159-67. (164) Fishman, H. A.; Scheller, R. H.; Zare, R. N. J. Chromatogr., A 1994, 680, 99-107. (165) Fishman, H. A.; Amudi, N. M.; Lee, T. T.; Scheller, R. H.; Zare, R. N. Anal. Chem. 1994, 66, 2318-29. (166) Sziele, D.; Brueggemann, O.; Doering, M.; Freitag, R.; Schuegerl, K. J. Chromatogr., A 1994, 669, 254-8. (167) Moore, A. W., Jr.; Jorgenson, J. W. Anal. Chem. 1993, 65, 355060. (168) Moore, A. W., Jr.; Jorgenson, J. W. Anal. Chem. 1995, 67, 346475. (169) Kleparnik, K.; Garner, M.; Bocek, P. J. Chromatogr., A 1995, 698, 375-83. (170) Crabtree, H. J.; Ireland, I. D.; Dovichi, N. J. J. Chromatogr., A 1994, 669, 263-7. (171) Beckers, J. L. J. Chromatogr., A 1994, 662, 153-66. (172) Beckers, J. L. J. Chromatogr., A 1994, 679, 153-65. (173) Ermakov, S. V.; Zhukov, M. Y.; Capelli, L.; Righetti, P. G. Anal. Chem. 1994, 66, 4034-42. (174) Ermakov, S. V.; Zhukov, M. Y.; Capelli, L.; Righetti, P. G. Anal. Chem. 1995, 67, 2957-65. (175) Huang, M.; Plocek, J.; Novotny, M. V. Electrophoresis 1995, 16, 396-401. (176) Liao, J. L.; Abramson, J.; Hjerten, S. J. Capillary Electrophor. 1995, 2, 191-6. (177) Mechref, Y.; El Rassi, Z. Electrophoresis 1995, 16, 617-24. (178) Ng, C. L.; Lee, H. K.; Li, S. F. Y. J. Chromatogr. 1994, 659, 427-34.
(179) Tsai, P.; Wu, C. T.; Lee, C. S. J. Chromatogr., B: Biomed. Appl. 1994, 657, 285-90. (180) Huang, M.; Dubrovcakova-Schneiderman, E.; Novotny, M. V.; Fatunmbi, H. O.; Wirth, M. J. J. Microcolumn Sep. 1994, 6, 5716. (181) Cole, R. B.; Varghese, J.; McCormick, R. M.; Kadlecek, D. J. Chromatogr., A 1994, 680, 363-73. (182) Nakatani, M.; Shibukawa, A.; Nakagawa, T. Electrophoresis 1995, 16, 1451-6. (183) Bedia Erim, F.; Cifuentes, A.; Poppe, H.; Kraak, J. C. J. Chromatogr., A 1995, 708, 356-61. (184) Cheng, K.; Zhao, Z.; Garrick, R.; Nordmeyer, F. R.; Lee, M. L.; Lamb, J. D. J. Chromatogr., A 1995, 706, 517-26. (185) Chiu, R. W.; Jimenez, J. C.; Monnig, C. A. Anal. Chim. Acta 1995, 307, 193-201. (186) Liu, Y.; Fu, R.; Gu, J. J. Chromatogr., A 1995, 694, 498-506. (187) Sun, P.; Landman, A.; Hartwick, R. A. J. Microcolumn Sep. 1994, 6, 403-7. (188) Thorsteindottir, M.; Isaksson, R.; Westerlund, D. Electrophoresis 1995, 16, 557-63. (189) Sun, P.; Landman, A.; Barker, G. E.; Hartwick, R. A. J. Chromatogr., A 1994, 685, 303-12. (190) Landman, A.; Sun, P.; Hartwick, R. A. J. Chromatogr., A 1994, 669, 259-62. (191) Chen, G. J.; Lee, N. M.; Hu, C. C.; Liu, C. Y. J. Chromatogr., A 1995, 699, 343-51. (192) Mayer, S.; Schurig, V. Electrophoresis 1994, 15, 835-41. (193) Mayer, S.; Schleimer, M.; Schurig, V. J. Microcolumn Sep. 1994, 6, 43-8. (194) Yoshimoto, Y.; Shibukawa, A.; Sasagawa, H.; Nitta, S.; Nakagawa, T. J. Pharm. Biomed. Anal. 1995, 13, 483-8. (195) Guo, Y.; Colon, L. A. Anal. Chem. 1995, 67, 2511-6. (196) Fujimoto, C.; Matsui, H.; Sawada, H.; Jinno, K. J. Chromatogr., A 1994, 680, 33-42. (197) Slais, K. J. Microcolumn Sep. 1995, 7, 127-35. (198) Nielen, M. W. F. J. High Resolut. Chromatogr. 1993, 16, 62-4. (199) Liu, P. Z.; Malik, A.; Kuchar, M. C. J.; Lee, M. L. J. Microcolumn Sep. 1994, 6, 581-9. (200) Boughtflower, R. J.; Underwood, T.; Paterson, C. J. Chromatographia 1995, 40, 329-35. (201) Boughtflower, R. J.; Underwood, T.; Maddin, J. Chromatographia 1995, 41, 398-402. (202) Li, S.; Lloyd, D. K. Anal. Chem. 1993, 65, 3684-90. (203) Yan, C.; Schaufelberger, D.; Erni, F. J. Chromatogr., A 1994, 670, 15-23. (204) Li, S.; Lloyd, D. K. J. Chromatogr., A 1994, 666, 321-35. (205) Harrison, D. J.; Fan, Z.; Seiler, K.; Manz, A.; Widmer, H. M. Anal. Chim. Acta 1993, 283, 361-6. (206) Fan, Z. H.; Harrison, D. J. Anal. Chem. 1994, 66, 177-84. (207) Effenhauser, C. S.; Manz, A.; Widmer, H. M. Anal. Chem. 1993, 65, 2637-42. (208) Pukl, M.; Prosek, M.; Kaiser, R. E. Chromatographia 1994, 38, 83-7. (209) Jacobson, S. C.; Moore, A. W.; Ramsey, J. M. Anal. Chem. 1995, 67, 2059-63. (210) Jacobson, S. C.; Hergenroeder, R.; Koutny, L. B.; Ramsey, J. M. Anal. Chem. 1994, 66, 2369-73. (211) Woolley, A. T.; Mathies, R. A. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 11348-52. (212) Woolley, A. T.; Mathies, R. A. Anal. Chem. 1995, 67, 3676-80. (213) Burggraf, N.; Manz, A.; Effenhauser, C. S.; Verpoorte, E.; de Rooij, N. F.; Widmer, H. M. J. High Resolut. Chromatogr. 1993, 16, 594-6. (214) Burggraf, N.; Manz, A.; De Rooij, N. F.; Widmer, H. M. Anal. Methods Instrum. 1993, 1, 55-9. (215) Burggraf, N.; Manz, A.; Verpoorte, E.; Effenhauser, C. S.; Widmer, H. M.; de Rooij, N. F. Sens. Actuators, B 1994, 20, 103-10. (216) Mesaros, J. M.; Ewing, A. G. J. Microcolumn Sep. 1994, 6, 48394. (217) Krasensky, S.; Fanali, S.; Krivankova, L.; Bocek, P. Electrophoresis 1995, 16, 968-73. (218) Moore, A. W., Jr.; Jorgenson, J. W. Anal. Chem. 1995, 67, 345663. (219) Moore, A. W., Jr.; Jorgenson, J. W. Anal. Chem. 1995, 67, 344855. (220) Ng, C. L.; Ong, C. P.; Lee, H. K.; Li, S. F. Y. J. Chromatogr. Sci. 1994, 32, 121-5. (221) Strausbauch, M. A.; Madden, B. J.; Wettstein, P. J.; Landers, J. P. Electrophoresis 1995, 16, 541-8. (222) Jones, R. F. D.; Camilleri, P.; Kirby, A. J.; Okafo, G. N. J. Chem. Soc., Chem. Commun. 1994, 11, 1311-2. (223) Shi, Y.; Fritz, J. S. Anal. Chem. 1995, 67, 3023-7. (224) Smith, J. T.; Rassi, Z. E. Electrophoresis 1994, 15, 1248-59. (225) Smith, J. T.; El Rassi, Z. J. Chromatogr., A 1994, 685, 131-43. (226) Smith, J. T.; El Rassi, Z. J. Microcolumn Sep. 1994, 6, 127-38. (227) Tickle, D. C.; Okafo, G. N.; Camilleri, P.; Jones, R. F. D.; Kirby, A. J. Anal. Chem. 1994, 66, 4121-6. (228) Cohen, N.; Grushka, E. J. Capillary Electrophor. 1994, 1, 1125. (229) Shi, Y.; Fritz, J. S. J. High Resolut. Chromatogr. 1994, 17, 7138. (230) Harakuwe, A. H.; Haddad, P. R.; Buchberger, W. J. Chromatogr., A 1994, 685, 161-5. (231) Janini, G. M.; Chan, K. C.; Barnes, J. A.; Muschik, G. M.; Issaq, H. J. J. Chromatogr. 1993, 653, 321-7.
(232) Kaneta, T.; Tanaka, S.; Taga, M. J. Chromatogr. 1993, 653, 3139. (233) Oda, R. P.; Madden, B. J.; Spelsberg, T. C.; Landers, J. P. J. Chromatogr., A 1994, 680, 85-92. (234) Matsubara, N.; Terabe, S. J. Chromatogr., A 1994, 680, 311-5. (235) Greve, K. F.; Nashabeh, W.; Karger, B. L. J. Chromatogr., A 1994, 680, 15-24. (236) Mazzeo, J. R.; Grover, E. R.; Swartz, M. E.; Petersen, J. S. J. Chromatogr., A 1994, 680, 125-35. (237) Nashabeh, W.; Greve, K. F.; Kirby, D.; Foret, F.; Karger, B. L.; Reifsnyder, D. H.; Builder, S. E. Anal. Chem. 1994, 66, 214854. (238) Ahuja, E. S.; Nielsen, K. R.; Foley, J. P.; Little, E. L. Anal. Chem. 1995, 67, 26-33. (239) Emmer, A.; Roeraade, J. J. Liq. Chromatogr. 1994, 17, 383146. (240) Corradini, D.; Cannarsa, G. Electrophoresis 1995, 16, 630-5. (241) Corradini, D.; Cannarsa, G.; Fabbri, E.; Corradini, C. J. Chromatogr., A 1995, 709, 127-34. (242) Song, L.; Ou, Q.; Yu, W. J. Chromatogr. 1993, 657, 175-83. (243) Song, L.; Ou, Q.; Yu, W. J. Liq. Chromatogr. 1994, 17, 195369. (244) Iki, N.; Hoshino, H.; Yotsuyanagi, T. J. Chromatogr. 1993, 652, 539-45. (245) Stathakis, C.; Cassidy, R. M. Anal. Chem. 1994, 66, 2110-5. (246) Stathakis, C.; Cassidy, R. M. J. Chromatogr., A 1995, 699, 35361. (247) Masselter, S. M.; Zemann, A. J. J. Chromatogr., A 1995, 693, 359-65. (248) Hinze, W. L.; Moreno, B.; Quina, F. H.; Suzuki, Y.; Wang, H. Anal. Chem. 1994, 66, 3449-57. (249) Masselter, S. M.; Zemann, A. J. Anal. Chem. 1995, 67, 104753. (250) Ward, T. J.; Nichols, M.; Sturdivant, L.; King, C. C. Amino Acids 1995, 8, 337-44. (251) Zhang, C. X.; von Heeren, F.; Thormann, W. Anal. Chem. 1995, 67, 2070-7. (252) Praus, P.; Dombek, V. Anal. Chim. Acta 1993, 283, 917-21. (253) Esaka, Y.; Yamaguchi, Y.; Kano, K.; Goto, M.; Haraguchi, H.; Takahashi, J. i. Anal. Chem. 1994, 66, 2441-5. (254) Esaka, Y.; Goto, M.; Haraguchi, H.; Ikeda, T.; Kano, K. J. Chromatogr., A 1995, 711, 305-11. (255) Greenaway, M.; Okafo, G.; Manallack, D.; Camilleri, P. Electrophoresis 1994, 15, 1284-9. (256) Kitagawa, S.; Tsuda, T. J. Microcolumn Sep. 1994, 6, 91-6. (257) Bjornsdottir, I.; Hansen, S. H. J. Chromatogr., A 1995, 711, 31322. (258) Jansson, M.; Roeraade, J. Chromatographia 1995, 40, 163-9. (259) Okada, T. J. Chromatogr., A 1995, 695, 309-17. (260) Sahota, R. S.; Khaledi, M. G. Anal. Chem. 1994, 66, 1141-6. (261) Salimi-Moosavi, H.; Cassidy, R. M. Anal. Chem. 1995, 67, 106773. (262) Aturki, Z.; Fanali, S. J. Chromatogr., A 1994, 680, 137-46. (263) Chankvetadze, B.; Endresz, G.; Blaschke, G. Electrophoresis 1994, 15, 804-7. (264) Chankvetadze, B.; Endresz, G.; Blaschke, G. J. Chromatogr., A 1995, 704, 234-7. (265) Dette, C.; Ebel, S.; Terabe, S. Electrophoresis 1994, 15, 799803. (266) Fanali, S. Chromatogr. Sci. Ser. 1993, 64, 731-52. (267) Gahm, K. H.; Stalcup, A. M. Anal. Chem. 1995, 67, 19-26. (268) Lurie, I. S.; Klein, R. F. X.; Dal Cason, T. A.; LeBelle, M. J.; Brenneisen, R.; Weinberger, R. E. Anal. Chem. 1994, 66, 401926. (269) Penn, S. G.; Bergstrom, E. T.; Goodall, D. M.; Loran, J. S. Anal. Chem. 1994, 66, 2866-73. (270) Quang, C.; Khaledi, M. G. Anal. Chem. 1993, 65, 3354-8. (271) Rawjee, Y. Y.; Williams, R. L.; Vigh, G. J. Chromatogr. 1993, 652, 233-45. (272) Rawjee, Y. Y.; Williams, R. L.; Buckingham, L. A.; Vigh, G. J. Chromatogr., A 1994, 688, 273-82. (273) Rawjee, Y. Y.; Williams, R. L.; Vigh, G. J. Chromatogr., A 1994, 680, 599-607. (274) Schmitt, T.; Engelhardt, H. Chromatographia 1993, 37, 47581. (275) Valko, I. E.; Billiet, H. A. H.; Frank, J.; Luyben, K. C. A. M. J. Chromatogr., A 1994, 678, 139-44. (276) Tait, R. J.; Thompson, D. O.; Stella, V. J.; Stobaugh, J. F. Anal. Chem. 1994, 66, 4013-8. (277) Yoshinaga, M.; Tanaka, M. J. Chromatogr., A 1995, 710, 3317. (278) D’Hulst, A.; Verbeke, N. Chirality 1994, 6, 225-9. (279) Kano, K.; Minami, K.; Horiguchi, K.; Ishimura, T.; Kodera, M. J. Chromatogr., A 1995, 694, 307-13. (280) Quang, C.; Khaledi, M. G. J. High Resolut. Chromatogr. 1994, 17, 609-12. (281) Soini, H.; Stefansson, M.; Riekkola, M. L.; Novotny, M. V. Anal. Chem. 1994, 66, 3477-84. (282) Stalcup, A. M.; Agyei, N. M. Anal. Chem. 1994, 66, 3054-9. (283) Arai, T.; Ichinose, M.; Kuroda, H.; Nimura, N.; Kinoshita, T. Anal. Biochem. 1994, 217, 7-11. (284) Arai, T.; Nimura, N.; Kinoshita, T. Biomed. Chromatogr. 1995, 9, 68-74. (285) Yang, J.; Hage, D. S. Anal. Chem. 1994, 66, 2719-25. (286) Kilar, F.; Fanali, S. Electrophoresis 1995, 16, 1510-8.
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(287) Castelnovo, P.; Albanesi, C. J. Chromatogr., A 1995, 715, 1439. (288) Kuhn, R.; Steinmetz, C.; Bereuter, T.; Haas, P.; Erni, F. J. Chromatogr., A 1994, 666, 367-73. (289) Kuhn, R.; Wagner, J.; Walbroehl, Y.; Bereuter, T. Electrophoresis 1994, 15, 828-34. (290) Schmid, M. G.; Guebitz, G. J. Chromatogr., A 1995, 709, 818. (291) Walbroehl, Y.; Wagner, J. J. Chromatogr., A 1994, 685, 321-9. (292) Walbroehl, Y.; Wagner, J. J. Chromatogr., A 1994, 680, 25361. (293) Armstrong, D. W.; Gasper, M. P.; Rundlett, K. L. J. Chromatogr., A 1995, 689, 285-304. (294) Armstrong, D. W.; Rundlett, K.; Reid, G. L., III Anal. Chem. 1994, 66, 1690-5. (295) Armstrong, D. W.; Rundlett, K. L.; Chen, J. R. Chirality 1994, 6, 496-509. (296) Vespalec, R.; Corstjens, H.; Billiet, H. A. H.; Frank, J.; Luyben, K. C. A. M. Anal. Chem. 1995, 67, 3223-8. (297) Rundlett, K. L.; Armstrong, D. W. Anal. Chem. 1995, 67, 208895. (298) Lee, Y. H.; Lin, T. I. J. Chromatogr., A 1994, 675, 227-36. (299) Lin, T. I.; Lee, Y. H.; Chen, Y. C. J. Chromatogr. 1993, 654, 167-76. (300) Timerbaev, A. R.; Semenova, O. P.; Bonn, G. K. Analyst 1994, 119, 2795-9. (301) Timerbaev, A. R.; Semenova, O. P.; Bonn, G. K.; Fritz, J. S. Anal. Chim. Acta 1994, 296, 119-28. (302) Wiley, J. P. J. Chromatogr., A 1995, 692, 267-74. (303) Stefansson, M.; Novotny, M. J. Am. Chem. Soc. 1993, 115, 11573-80. (304) Stefansson, M.; Novotny, M. Anal. Chem. 1994, 66, 1134-40. (305) Tadey, T.; Purdy, W. C. J. Chromatogr., B: Biomed. Appl. 1994, 657, 365-72. (306) Fang, X. h.; Zhu, T.; Sun, V. H. J. High Resolut. Chromatogr. 1994, 17, 749-52. (307) Yao, Y. J.; Li, S. F. Y. J. Chromatogr., A 1994, 663, 97-104. (308) Okafo, G. N.; Vinther, A.; Kornfelt, T.; Camilleri, P. Electrophoresis 1995, 16, 1917-21. (309) Wu, N.; Barker, G. E.; Huie, C. W. J. Chromatogr. 1994, 659, 435-42. (310) Schuetzner, W.; Caponecchi, G.; Fanali, S.; Rizzi, A.; Kenndler, E. Electrophoresis 1994, 15, 769-73. (311) Soini, H.; Riekkola, M. L.; Novotny, M. V. J. Chromatogr., A 1994, 680, 623-34. (312) Zhang, Y.; Zhang, R.; Hjerten, S.; Lundahl, P. Electrophoresis 1995, 16, 1519-23. (313) Baechmann, K.; Goettlicher, B.; Haag, I.; Han, K. Y.; Hensel, W.; Mainka, A. J. Chromatogr., A 1994, 688, 283-92. (314) Castagnola, M.; Cassiano, L.; Lupi, A.; Messana, I.; Patamia, M.; Rabino, R.; Rossetti, D. V.; Giardina, B. J. Chromatogr., A 1995, 694, 463-9. (315) Shohat, D.; Grushka, E. Anal. Chem. 1994, 66, 747-50. (316) Reid, R. H. P. J. Chromatogr., A 1994, 669, 151-83. (317) Lamb, J. D.; Edwards, B. R.; Smith, R. G.; Garrick, R. Talanta 1995, 42, 109-17. (318) Copper, C. L.; Sepaniak, M. J. Anal. Chem. 1994, 66, 147-54. (319) Sepaniak, M. J.; Copper, C. L.; Whitaker, K. W.; Anigbogu, V. C. Anal. Chem. 1995, 67, 2037-41. (320) Jinno, K.; Sawada, Y. J. Capillary Electrophor. 1994, 1, 10611. (321) Szolar, O. H. J.; Brown, R. S.; Luong, J. H. T. Anal. Chem. 1995, 67, 3004-10. (322) Hjerten, S.; Valtcheva, L.; Elenbring, K.; Liao, J. L. Electrophoresis 1995, 16, 584-94. (323) Zhu, T.; Sun, Y.; Zhang, C.; Ling, D.; Sun, Z. J. High Resolut. Chromatogr. 1994, 17, 563-4. (324) Chen, Y.; Hoeltje, J. V.; Schwarz, U. J. Chromatogr., A 1994, 680, 63-71. (325) Hjerten, S.; Srichaiyo, T.; Palm, A. Biomed. Chromatogr. 1994, 8, 73-6. (326) Kozulic, B. Appl. Theor. Electrophor. 1994, 4, 137-48. (327) Weiss, G. H.; Garner, M.; Yarmola, E.; Bocek, P.; Chrambach, A. Electrophoresis 1995, 16, 1345-53. (328) Chiari, M.; Nesi, M.; Righetti, P. G. J. Chromatogr. 1993, 652, 31-9. (329) Figeys, D.; Renborg, A.; Dovichi, N. J. Electrophoresis 1994, 15, 1512-7. (330) Gelfi, C.; Orsi, A.; Leoncini, F.; Righetti, P. G. J. Chromatogr., A 1995, 689, 97-105. (331) Heller, C.; Viovy, J. L. Appl. Theor. Electrophor. 1994, 4, 3941. (332) Pariat, Y. F.; Berka, J.; Heiger, D. N.; Schmitt, T.; Vilenchik, M.; Cohen, A. S.; Foret, F.; Karger, B. L. J. Chromatogr. 1993, 652, 57-66. (333) Barron, A. E.; Soane, D. S.; Blanch, H. W. J. Chromatogr. 1993, 652, 3-16. (334) Mitnik, L.; Salome, L.; Viovy, J. L.; Heller, C. J. Chromatogr., A 1995, 710, 309-21. (335) Shi, X.; Hammond, R. W.; Morris, M. D. Anal. Chem. 1995, 67, 1132-8. (336) Karim, M. R.; Janson, J. C.; Takagi, T. Electrophoresis 1994, 15, 1531-4. (337) Takagi, T.; Karim, M. R. Electrophoresis 1995, 16, 1463-7. (338) Fung, E. N.; Yeung, E. S. Anal. Chem. 1995, 67, 1913-9. (339) Guttman, A. Electrophoresis 1995, 16, 611-16. 584R
Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
(340) Guttman, A.; Shieh, P.; Hoang, D.; Horvath, J.; Cooke, N. Electrophoresis 1994, 15, 221-4. (341) Guttman, A.; Shieh, P.; Lindahl, J.; Cooke, N. J. Chromatogr., A 1994, 676, 227-31. (342) Shieh, P. C. H.; Hoang, D.; Guttman, A.; Cooke, N. J. Chromatogr., A 1994, 676, 219-26. (343) Benedek, K.; Thiede, S. J. Chromatogr., A 1994, 676, 209-17. (344) Chiari, M.; Nesi, M.; Righetti, P. G. Electrophoresis 1994, 15, 616-22. (345) Fujimoto, C. Anal. Chem. 1995, 67, 2050-3. (346) Simo-Alfonso, E.; Conti, M.; Gelfi, C.; Righetti, P. G. J. Chromatogr., A 1995, 689, 85-96. (347) Singhal, R. P.; Xian, J. J. Chromatogr. 1993, 652, 47-56. (348) Kim, Y.; Morris, M. D. Anal. Chem. 1994, 66, 1168-74. (349) Cheng, J.; Mitchelson, K. R. Anal. Chem. 1994, 66, 4210-4. (350) Baba, Y.; Tsuhako, M.; Sawa, T.; Akashi, M. J. Chromatogr. 1993, 652, 93-9. (351) Sun, P.; Barker, G. E.; Hartwick, R. A.; Grinberg, N.; Kaliszan, R. J. Chromatogr. 1993, 652, 247-52. (352) Sun, P.; Barker, G. E.; Mariano, G. J.; Hartwick, R. A. Electrophoresis 1994, 15, 793-8. (353) Heller, C.; Pakleza, C.; Viovy, J. L. Electrophoresis 1995, 16, 1423-8. (354) Kim, Y.; Morris, M. D. Anal. Chem. 1994, 66, 3081-5. (355) Kim, Y.; Morris, M. D. Anal. Chem. 1995, 67, 784-6. (356) Navin, M. J.; Rapp, T. L.; Morris, M. D. Anal. Chem. 1994, 66, 1179-82. (357) Sudor, J.; Novotny, M. Nucleic Acids Res. 1995, 23, 2538-43. (358) Culbertson, C. T.; Jorgenson, J. W. Anal. Chem. 1994, 66, 95562. (359) Huang, T. L.; Shieh, P. C. H.; Cooke, N. Chromatographia 1994, 39, 543-8. (360) Weinmann, W.; Maier, C.; Baumeister, K.; Przybylski, M.; Parker, C. E.; Tomer, K. B. J. Chromatogr., A 1994, 664, 2715. (361) Eimer, T.; Unger, K. K.; Tsuda, T. Fresenius’ J. Anal. Chem. 1995, 352, 649-53. (362) Bevan, C. D.; Mutton, I. M. J. Chromatogr., A 1995, 697, 5418. (363) Janini, G. M.; Muschik, G. M.; Issaq, H. J. J. High Resolut. Chromatogr. 1994, 17, 753-5. (364) Razee, S.; Tamura, A.; Masujima, T. Chem. Pharm. Bull. 1994, 42, 2376-8. (365) Effenhauser, C. S.; Manz, A.; Widmer, H. M. Anal. Chem. 1995, 67, 2284-7. (366) Fujimoto, C.; Jinno, K. Chromatogr. Sci. Ser. 1993, 64, 50923. (367) Grimm, R.; Herold, M. J. Capillary Electrophor. 1994, 1, 7982. (368) Herold, M.; Wu, S., III LC-GC 1994, 12, 531-3. (369) Lee, H. G.; Desiderio, D. M. J. Chromatogr., A 1994, 686, 30917. (370) Muller, O.; Foret, F.; Karger, B. L. Anal. Chem. 1995, 67, 297480. (371) Warren, W. J.; Cheng, Y. F.; Fuchs, M. LC-GC 1994, 12, 268. (372) Xue, Y.; Yeung, E. S. Anal. Chem. 1994, 66, 3575-80. (373) Liu, S.; Dasgupta, P. K. Anal. Chim. Acta 1993, 283, 747-53. (374) Moring, S. E.; Reel, R. T.; van Soest, R. E. J. Anal. Chem. 1993, 65, 3454-9. (375) Heiger, D. N.; Kaltenbach, P.; Sievert, H. J. Electrophoresis 1994, 15, 1234-47. (376) Flint, C. D.; Grochowicz, P. R.; Simpson, C. F. Anal. Proc. 1994, 31, 117-21. (377) Xue, Y.; Yeung, E. S. Appl. Spectrosc. 1994, 48, 502-6. (378) Kim, S.; Kim, W.; Hoon Hahn, J. J. Chromatogr., A 1994, 680, 109-16. (379) Wu, J.; Pawliszyn, J. Analyst 1995, 120, 1567-71. (380) Mechref, Y.; El Rassi, Z. Electrophoresis 1994, 15, 627-34. (381) Mechref, Y.; Ostrander, G. K.; El Rassi, Z. J. Chromatogr., A 1995, 695, 83-95. (382) Motomizu, S.; Oshima, M.; Kuwabara, M.; Obata, Y. Analyst 1994, 119, 1787-92. (383) Regan, F. B.; Meaney, M. P.; Lunte, S. M. J. Chromatogr., B: Biomed. Appl. 1994, 657, 409-17. (384) Beck, W.; Engelhardt, H. Fresenius’ J. Anal. Chem. 1993, 346, 618-21. (385) Lee, Y. H.; Lin, T. I. J. Chromatogr., A 1994, 680, 287-97. (386) Buchberger, W.; Cousins, S. M.; Haddad, P. R. TrAC, Trends Anal. Chem. 1994, 13, 313-9. (387) Cousins, S. M.; Haddad, P. R.; Buchberger, W. J. Chromatogr., A 1994, 671, 397-402. (388) Shamsi, S. A.; Danielson, N. D. Anal. Chem. 1994, 66, 375764. (389) Mala, Z.; Vespalec, R.; Bocek, P. Electrophoresis 1994, 15, 152630. (390) Kasyutich, V. L.; Mahnach, I. I. Proc. SPIE-Int. Soc. Opt. Eng. 1995, 2208, 94-102. (391) Krattiger, B.; Bruin, G. J. M.; Bruno, A. E. Anal. Chem. 1994, 66, 1-8. (392) Saz, J. M.; Krattiger, B.; Bruno, A. E.; Maysre, F.; Widmer, H. M. Anal. Methods Instrum. 1994, 1, 203-7. (393) Krattiger, B.; Bruno, A. E.; Widmer, H. M.; Daendliker, R. Anal. Chem. 1995, 67, 124-30. (394) Saz, J. M.; Krattiger, B.; Bruno, A. E.; Diez-Masa, J. C.; Widmer, H. M. J. Chromatogr., A 1995, 699, 315-22.
(395) Arriaga, E. A.; Zhang, Y.; Dovichi, N. J. Anal. Chim. Acta 1995, 299, 319-26. (396) Gump, E. L.; Monnig, C. A. J. Chromatogr., A 1995, 715, 16777. (397) Gilman, S. D.; Ewing, A. G. Anal. Chem. 1995, 67, 58-64. (398) Gilman, S. D.; Ewing, A. G. Anal. Methods Instrum. 1995, 2, 133-41. (399) Gilman, S. D.; Pietron, J. J.; Ewing, A. G. J. Microcolumn Sep. 1994, 6, 373-84. (400) Oguri, S.; Uchida, C.; Miyake, Y.; Miki, Y.; Kakehi, K. Analyst 1995, 120, 63-8. (401) Pinto, D. M.; Arriaga, E. A.; Sia, S.; Li, Z.; Dovichi, N. J. Electrophoresis 1995, 16, 534-40. (402) Banks, P. R.; Paquette, D. M. Bioconjugate Chem. 1995, 6, 44758. (403) Mattusch, J.; Dittrich, K. J. Chromatogr., A 1994, 680, 27985. (404) Figeys, D.; Arriaga, E.; Renborg, A.; Dovichi, N. J. J. Chromatogr., A 1994, 669, 205-16. (405) Srinivasan, K.; Morris, S. C.; Girard, J. E.; Kline, M. C.; Reeder, D. J. Appl. Theor. Electrophor. 1993, 3, 235-9. (406) Evangelista, R. A.; Liu, M. S.; Chen, F. T. A. Anal. Chem. 1995, 67, 2239-45. (407) Schneede, J.; Mortensen, J. H.; Kvalheim, G.; Ueland, P. M. J. Chromatogr., A 1994, 669, 185-93. (408) Fadden, P.; Haystead, T. A. J. Anal. Biochem. 1995, 225, 818. (409) Wang, P.; Giese, R. W. Anal. Chem. 1993, 65, 3518-20. (410) Shimura, K.; Karger, B. L. Anal. Chem. 1994, 66, 9-15. (411) Soper, S. A.; Legendre, B. L., Jr.; Flanagan, J. H., Jr.; Williams, D. C.; Hammer, R. P. Proc. SPIE-Int. Soc. Opt. Eng. 1994, 2136, 244-54. (412) Williams, D. C.; Soper, S. A. Anal. Chem. 1995, 67, 3427-32. (413) Jansson, M.; Roeraade, J.; Laurell, F. Anal. Chem. 1993, 65, 2766-9. (414) Kawazumi, H.; Song, J. M.; Inoue, T.; Ogawa, T. Anal. Sci. 1995, 11, 587-90. (415) Chen, F. T. A.; Tusak, A.; Pentoney, S., Jr.; Konrad, K.; Lew, C.; Koh, E.; Sternberg, J. J. Chromatogr. 1993, 652, 355-60. (416) Fuchigami, T.; Imasaka, T.; Shiga, M. Anal. Chim. Acta 1993, 282, 209-13. (417) Fuchigami, T.; Imasaka, T.; Shiga, M. Proc. SPIE-Int. Soc. Opt. Eng. 1993, 435-8. (418) Mank, A. J. G.; Lingeman, H.; Gooijer, C. J. High Resolut. Chromatogr. 1994, 17, 797-8. (419) Mank, A. J. G.; Yeung, E. S. J. Chromatogr., A 1995, 708, 30921. (420) Flanagan, J. H., Jr.; Legendre, B. L.; Hammer, R. P.; Soper, S. A. Anal. Chem. 1995, 67, 341-7. (421) Nilsson, S.; Johansson, J.; Mecklenburg, M.; Birnbaum, S.; Svanberg, S.; Wahlund, K. G.; Mosbach, K.; Miyabayashi, A.; Larsson, P. O. J. Capillary Electrophor. 1995, 2, 46-52. (422) Timperman, A. T.; Oldenburg, K. E.; Sweedler, J. V. Anal. Chem. 1995, 67, 3421-6. (423) Takahashi, S.; Murakami, K.; Anazawa, T.; Kambara, H. Anal. Chem. 1994, 66, 1021-6. (424) Chen, D. Y.; Dovichi, N. J. J. Chromatogr., B: Biomed. Appl. 1994, 657, 265-9. (425) Chen, D. Y.; Adelhelm, K.; Cheng, X. L.; Dovichi, N. J. Analyst 1994, 119, 349-52. (426) McGregor, D. A.; Yeung, E. S. J. Chromatogr., A 1994, 680, 491-6. (427) Carson, S.; Cohen, A. S.; Belenkii, A.; Ruiz-Martinez, M. C.; Berka, J.; Karger, B. L. Anal. Chem. 1993, 65, 3219-26. (428) Sweedler, J. V.; Timperman, A.; Oldenburg, K.; Cruz, L.; Shippy, S.; Jankowski, J. A. Anal. Spectrosc. Libr. 1995, 6, 385-98. (429) Timperman, A. T.; Khatib, K.; Sweedler, J. V. Anal. Chem. 1995, 67, 139-44. (430) Beale, S. C.; Sudmeier, S. J. Anal. Chem. 1995, 67, 3367-71. (431) Kim, S.; Yoo, H. J.; Nam, H. G.; Hahn, J. H. Chromatographia 1995, 40, 345-9. (432) Ueno, K.; Yeung, E. S. Anal. Chem. 1994, 66, 1424-31. (433) Lu, X.; Yeung, E. S. Appl. Spectrosc. 1995, 49, 605-9. (434) Li, Q.; Yeung, E. S. Appl. Spectrosc. 1995, 49, 825-33. (435) Desbene, P. L.; Morin, C. J.; Desbene Monvernay, A. M.; Groult, R. S. J. Chromatogr., A 1995, 689, 135-48. (436) Xue, Q.; Yeung, E. S. J. Chromatogr., A 1994, 661, 287-95. (437) Fuchigami, T.; Imasaka, T. Anal. Chim. Acta 1994, 291, 1838. (438) Shear, J. B.; Colon, L. A.; Zare, R. N. Anal. Chem. 1993, 65, 3708-12. (439) Shear, J. B.; Dadoo, R.; Fishman, H. A.; Scheller, R. H.; Zare, R. N. Anal. Chem. 1993, 65, 2977-82. (440) Nie, S.; Dadoo, R.; Zare, R. N. Anal. Chem. 1993, 65, 3571-5. (441) Clark, B. K.; Vo-Dinh, T.; Sepaniak, M. J. Anal. Chem. 1995, 67, 680-3. (442) Arriaga, E.; Chen, D. Y.; Cheng, X. L.; Dovichi, N. J. J. Chromatogr. 1993, 652, 347-53. (443) Haab, B. B.; Mathies, R. A. Anal. Chem. 1995, 67, 3253-60. (444) Dadoo, R.; Seto, A. G.; Colon, L. A.; Zare, R. N. Anal. Chem. 1994, 66, 303-6. (445) Hara, T.; Yokogi, J.; Okamura, S.; Kato, S.; Nakajima, R. J. Chromatogr. 1993, 652, 361-7. (446) Hara, T.; Kayama, S.; Nishida, H.; Nakajima, R. Anal. Sci. 1994, 10, 223-5.
(447) Hara, T.; Nishida, H.; Kayama, S.; Nakajima, R. Bull. Chem. Soc. Jpn. 1994, 67, 1193-5. (448) Hara, T.; Nishida, H.; Nakajima, R. Anal. Sci. 1994, 10, 8235. (449) Ruberto, M. A.; Grayeski, M. L. J. Microcolumn Sep. 1994, 6, 545-50. (450) Gilman, S. D.; Silverman, C. E.; Ewing, A. G. J. Microcolumn Sep. 1994, 6, 97-106. (451) Banks, J. F., Jr. J. Chromatogr., A 1995, 712, 245-52. (452) Foret, F.; Thompson, T. J.; Vouros, P.; Karger, B. L.; Gebauer, P.; Bocek, P. Anal. Chem. 1994, 66, 4450-8. (453) Lu, W.; Yang, G.; Cole, R. B. Electrophoresis 1995, 16, 487-92. (454) Wahl, J. H.; Gale, D. C.; Smith, R. D. J. Chromatogr. 1994, 659, 217-22. (455) Wahl, J. H.; Smith, R. D. J. Capillary Electrophor. 1994, 1, 6271. (456) Kriger, M. S.; Cook, K. D.; Ramsey, R. S. Anal. Chem. 1995, 67, 385-9. (457) Sheppard, R. L.; Tong, X.; Cai, J.; Henion, J. D. Anal. Chem. 1995, 67, 2054-8. (458) Takada, Y.; Sakairi, M.; Koizumi, H. Anal. Chem. 1995, 67, 1474-6. (459) Hirabayashi, A.; Sakairi, M.; Koizumi, H. Anal. Chem. 1994, 66, 4557-9. (460) Liu, Y.; Lopez-Avila, V.; Zhu, J. J.; Wiederin, D.; Beckert, W. F. Anal. Chem. 1995, 67, 2020-5. (461) Lu, Q.; Bird, S. M.; Barnes, R. M. Anal. Chem. 1995, 67, 294956. (462) Olesik, J. W.; Kinzer, J. A.; Olesik, S. V. Anal. Chem. 1995, 67, 1-12. (463) Alexander, A. J.; Hughes, D. E. Anal. Chem. 1995, 67, 362632. (464) Weinmann, W.; Parker, C. E.; Deterding, L. J.; Papac, D. I.; Hoyes, J.; Przybylski, M.; Tomer, K. B. J. Chromatogr., A 1994, 680, 353-61. (465) Kostiainen, R.; Lasonder, E.; Bloemhoff, W.; van Veelen, P. A.; Welling, G. W.; Bruins, A. P. Biol. Mass Spectrom. 1994, 23, 346-52. (466) Weinmann, W.; Parker, C. E.; Baumeister, K.; Maier, C.; Tomer, K. B.; Przybylski, M. Electrophoresis 1994, 15, 228-33. (467) Henion, J. D.; Mordehai, A. V.; Cai, J. Anal. Chem. 1994, 66, 2103-9. (468) Ramsey, R. S.; McLuckey, S. A. J. Microcolumn Sep. 1995, 7, 461-9. (469) Hofstadler, S. A.; Wahl, J. H.; Bruce, J. E.; Smith, R. D. J. Am. Chem. Soc. 1993, 115, 6983-4. (470) Hofstadler, S. A.; Wahl, J. H.; Bakhtiar, R.; Anderson, G. A.; Bruce, J. E.; Smith, R. D. J. Am. Soc. Mass Spectrom. 1994, 5, 894-9. (471) Hofstadler, S. A.; Swanek, F. D.; Gale, D. C.; Ewing, A. G.; Smith, R. D. Anal. Chem. 1995, 67, 1477-80. (472) Wahl, J. H.; Hofstadler, S. A.; Smith, R. D. Anal. Chem. 1995, 67, 462-5. (473) Fang, L.; Zhang, R.; Williams, E. R.; Zare, R. N. Anal. Chem. 1994, 66, 3696-701. (474) Perkins, J. R.; Tomer, K. B. Anal. Chem. 1994, 66, 2835-40. (475) Lamoree, M. H.; Reinhoud, N. J.; Tjaden, U. R.; Niessen, W. M. A.; van der Greef, J. Biol. Mass Spectrom. 1994, 23, 339-45. (476) Locke, S. J.; Thibault, P. Anal. Chem. 1994, 66, 3436-46. (477) Tomlinson, A. J.; Braddock, W. D.; Benson, L. M.; Oda, R. P.; Naylor, S. J. Chromatogr., B: Biomed. Appl. 1995, 669, 67-73. (478) Wolf, S. M.; Vouros, P. Anal. Chem. 1995, 67, 891-900. (479) van der Vlis, E.; Mazereeuw, M.; Tjaden, U. R.; Irth, H.; van der Greef, J. J. Chromatogr., A 1995, 712, 227-34. (480) Dekkers, S. E. G.; Tjaden, U. R.; van der Greef, J. J. Chromatogr., A 1995, 712, 201-9. (481) Lane, S. J.; Boughtflower, R.; Paterson, C.; Underwood, T. Rapid Commun. Mass Spectrom. 1995, 9, 1283-7. (482) Schmeer, K.; Behnke, B.; Bayer, E. Anal. Chem. 1995, 67, 3656-8. (483) Gordon, D. B.; Lord, G. A.; Jones, D. S. Rapid Commun. Mass Spectrom. 1994, 8, 544-8. (484) Takada, Y.; Yoshida, M.; Sakairi, M.; Koizumi, H. Rapid Commun. Mass Spectrom. 1995, 9, 895-6. (485) van der Hoeven, R. A. M.; Buscher, B. A. P.; Tjaden, U. R.; van der Greef, J. J. Chromatogr., A 1995, 712, 211-8. (486) Tomlinson, A. J.; Benson, L. M.; Gorrod, J. W.; Naylor, S. J. Chromatogr., B: Biomed. Appl. 1994, 657, 373-81. (487) Tomlinson, A. J.; Benson, L. M.; Naylor, S. LC-GC 1994, 12, 122-30. (488) Tetler, L. W.; Cooper, P. A.; Powell, B. J. Chromatogr., A 1995, 700, 21-6. (489) Varghese, J.; Cole, R. B. J. Chromatogr. 1993, 652, 369-76. (490) Guo, Y.; Colon, L. A.; Dadoo, R.; Zare, R. N. Electrophoresis 1995, 16, 493-7. (491) Ye, J.; Baldwin, R. P. Anal. Chem. 1994, 66, 2669-74. (492) Zhou, J.; Lunte, S. M. Electrophoresis 1995, 16, 498-503. (493) Huang, X.; Kok, W. T. J. Chromatogr., A 1995, 707, 335-42. (494) Ye, J.; Baldwin, R. P. Anal. Chem. 1993, 65, 3525-7. (495) Ye, J.; Baldwin, R. P. J. Chromatogr., A 1994, 687, 141-8. (496) O’Shea, T. J.; Lunte, S. M. Anal. Chem. 1994, 66, 307-11. (497) Zhou, J.; O’Shea, T. J.; Lunte, S. M. J. Chromatogr., A 1994, 680, 271-7. (498) Zhou, W.; Xu, L.; Wu, M.; Xu, L.; Wang, E. Anal. Chim. Acta 1994, 299, 189-94. (499) Lu, W.; Cassidy, R. M. Anal. Chem. 1994, 66, 200-4.
Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
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(500) Kaniansky, D.; Havasi, P.; Ivanyi, F.; Marak, J. J. Chromatogr., A 1995, 709, 69-79. (501) Matysik, F. M.; Meister, A.; Werner, G. Anal. Chim. Acta 1995, 305, 114-20. (502) Park, S.; Lunte, S. M.; Lunte, C. E. Anal. Chem. 1995, 67, 9118. (503) Zhou, J.; Lunte, S. M. Anal. Chem. 1995, 67, 13-8. (504) Ferris, S. S.; Lou, G.; Ewing, A. G. J. Microcolumn Sep. 1994, 6, 263-8. (505) Lu, W.; Cassidy, R. M. Anal. Chem. 1993, 65, 2878-81. (506) Roberts, R. E.; Johnson, D. C. Electroanalysis 1994, 6, 269-73. (507) Kaniansky, D.; Zelensky, I.; Hybenova, A.; Onuska, F. I. Anal. Chem. 1994, 66, 4258-64. (508) Kar, S.; Dasgupta, P. K.; Liu, H.; Hwang, H. Anal. Chem. 1994, 66, 2537-43. (509) Hauser, P. C.; Renner, N. D.; Hong, A. P. C. Anal. Chim. Acta 1994, 295, 181-6. (510) Nann, A.; Pretsch, E. J. Chromatogr., A 1994, 676, 437-42. (511) Wu, N.; Peck, T. L.; Webb, A. G.; Magin, R. L.; Sweedler, J. V. Anal. Chem. 1994, 66, 3849-57.
586R
Analytical Chemistry, Vol. 68, No. 12, June 15, 1996
(512) Wu, N.; Peck, T. L.; Webb, A. G.; Magin, R. L.; Sweedler, J. V. J. Am. Chem. Soc. 1994, 116, 7929-30. (513) Kowalchyk, W. K.; Walker, P. A., III; Morris, M. D. Appl. Spectrosc. 1995, 49, 1183-8. (514) Odake, T.; Kitamori, T.; Sawada, T. Anal. Chem. 1995, 67, 1458. (515) Sweedler, J. V.; Fuller, R.; Tracht, S.; Timperman, A.; Toma, V.; Khatib, K. J. Microcolumn Sep. 1993, 5, 403-12. (516) Tracht, S.; Toma, V.; Sweedler, J. V. Anal. Chem. 1994, 66, 2382-9. (517) Cassidy, R. M.; Lu, W.; Tse, V. P. Anal. Chem. 1994, 66, 257883. (518) Emmer, A.; Roeraade, J. Chromatographia 1994, 39, 271-8. (519) Emmer, A.; Roeraade, J. J. Chromatogr., A 1994, 662, 375-81. (520) Jacobson, S. C.; Koutny, L. B.; Hergenroeder, R.; Moore, A. W., Jr.; Ramsey, J. M. Anal. Chem. 1994, 66, 3472-6.
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