Semi-permanent surfactant coatings for inorganic anion analysis in capillary electrophoresis

Dual Contributions of Analyte Adsorption and Electroosmotic Inhomogeneity to Separation Efficiency in Capillary Electrophoresis of Proteins

Improving separation efficiency in capillary electrophoresis (CE) requires systematic study of the influence of the electric field (or solute linear velocity) on plate height for a better understanding of the critical parameters controlling peak broadening. Even for poly(diallyldimethylammonium chloride) (PDADMAC)/poly(sodium styrenesulfonate) (PSS) successive multiple ionic-polymer layer (SMIL) coatings, which lead to efficient and reproducible separations of proteins, plate height increases with migration velocity, limiting the use of high electric fields in CE. Solute adsorption onto the capillary wall was generally considered as the main source of peak dispersion, explaining this plate height increase. However, experiments done with Taylor dispersion analysis and CE in the same conditions indicate that other phenomena may come into play. Protein adsorption with slow kinetics and few adsorption sites was established as a source of peak broadening for specific proteins. Surface charge inhomogeneity was also identified as a contribution to plate height due to local electroosmotic fluctuations. A model was proposed and applied to partial PDADMAC/poly(ethylene oxide) capillary coatings as well as PDADMAC/PSS SMIL coatings. Atomic force microscopy with topography and recognition imaging enabled the determination of roughness and charge distribution of the PDADMAC/PSS SMIL surface.

A Simultaneous Determination Method for the Analysis of Chloride and Nitrate Ions in Air Samples by PLS1

This study describes a multi-ion-selective electrode system for the simultaneous determination of nitrate and chloride ions in air samples by using multivariate calibration methods. The measurement system was constituted of two ion-selective electrodes, an Ag/AgCl double-junction reference electrode and a multi-potentiometer. The measurements were performed at pH 5.0 acetic acid/sodium acetate buffer. The obtained data were evaluated by using Partial Least Squares (PLS1). The system was used to analyze the synthetic samples and fume-hood samples in terms of the amount of chloride and nitrate. The percentage recovery values obtained from fume-hood samples were 93.8% ± 3.8 and 102.4% ± 2.5 for chloride and nitrate, respectively. The presented system could be an easy-to-use approach for monitoring the amount of chloride and nitrate species in the scope of occupational health and safety analysis.

A stationary pseudophase semi-permanent coating for open-tubular capillary liquid chromatography and electrochromatography

We describe the chromatographic and electrochromatographic separation of small neutral and charged analytes using a fused silica capillary with a stationary pseudophase semi-permanent coating of didodecyldimethyl ammonium bromide (DDAB) aggregates. The coating was prepared by flushing the capillary with a DDAB solution that was rinsed out with the mobile phase. Our studies (i.e., electroosmotic flow measurements by capillary electrophoresis, chromatographic retention of a neutral probe and atomic force microscopy) suggested the formation of DDAB patchy admicelle, complete admicelle, or larger aggregates at the solid surface - liquid interface inside the capillary, depending on the concentration of DDAB used in coating the capillary. The analytical figures of merit for open tubular liquid chromatography (OT-LC, pressure driven) and open tubular capillary electrochromatography (OT-CEC, voltage driven) using a capillary coated with 0.5 mM DDAB and mobile phase/background solution of 25 mM borate buffer at pH 9.5 with 10% MeOH were the following: LOD = 3.0-5.0 µg/mL (OT-LC) and 2.5-5.0 µg/mL (OT-CEC); linearity R2 > 0.99 (peak area (OT-LC) and corrected peak area (OT-CEC)), intraday and interday repeatability%RSD < 5% (n = 12) for retention/migration time, peak area (OT-LC) and corrected peak area (OT-CEC). The reversed-phase and anion-exchange property of the stationary pseudophase was studied by the addition of organic solvents and sodium chloride to the mobile phase, respectively. We also demonstrate the increase in the ks of the tested analytes by implementing successive multiple ionic layer (SMIL) coating strategies with DDAB in combination with a cationic and/or anionic polyelectrolyte. The use of a stationary pseudophase coating is potentially an easy alternative way to conduct open-tubular liquid chromatography and electrochromatography.

In situ one-pot synthesis of polydopamine/octadecylamine co-deposited coating in capillary for open-tubular capillary electrochromatography

Mussel-inspired polydopamine (PDA) based materials are attractive as stationary phase for open-tubular capillary electrochromatography (OT-CEC) due to their many fascinating properties. However, all of the existing strategies for fabricating PDA based OT-CEC columns are limited in aqueous solutions. Consequently, it is a challenge work to directly immobilize the hydrophobic functional materials onto the inner wall of PDA modified capillary. Herein, by using the organic amine-inducing co-deposition strategy, a novel preparative method was developed for in situ one-pot synthesis of PDA/octadecylamine (ODA) co-deposited coating inside capillary as OT-CEC stationary phase. The formation and morphology of the PDA/ODA co-deposited coating were characterized by field emission scanning electron microscopy, atomic force microscope, attenuated total reflectance Fourier transform infrared spectroscopy and contact angle measurements. The separation performance of the fabricated PDA/ODA modified columns was validated by the separation of alkylbenzenes and steroids, which could achieve baseline separation with high separation efficiency. Their separation was found to follow the reversed phase chromatographic retention mechanism. The co-deposited column showed good stability and long lifetime. The repeatability of the PDA/ODA co-deposited column was also evaluated, with the relative standard deviations for intra-day and inter-day runs less than 5% and column-to-column runs less than 6%.

Hybrid phospholipid bilayer coatings for separations of cationic proteins in capillary zone electrophoresis

Protein separations in CZE suffer from nonspecific adsorption of analytes to the capillary surface. Semipermanent phospholipid bilayers have been used to minimize adsorption, but must be regenerated regularly to ensure reproducibility. We investigated the formation, characterization, and use of hybrid phospholipid bilayers (HPBs) as more stable biosurfactant capillary coatings for CZE protein separations. HPBs are formed by covalently modifying a support with a hydrophobic monolayer onto which a self-assembled lipid monolayer is deposited. Monolayers prepared in capillaries using 3-cyanopropyldimethylchlorosilane (CPDCS) or n-octyldimethylchlorosilane (ODCS) yielded hydrophobic surfaces with lowered surface free energies of 6.0 ± 0.3 or 0.2 ± 0.1 mJ m(-2) , respectively, compared to 17 ± 1 mJ m(-2) for bare silica capillaries. HPBs were formed by subsequently fusing vesicles comprised of 1,2-dilauroyl-sn-glycero-3-phosphocholine or 1,2-dioleoyl-sn-glycero-3-phosphocholine to CPDCS- or ODCS-modified capillaries. The resultant HPB coatings shielded the capillary surface and yielded reduced electroosmotic mobility (1.3-1.9 × 10(-4) cm(2) V(-1) s(-1) ) compared to CPDCS- and ODCS-modified or bare capillaries (3.6 ± 0.2 × 10(-4) cm(2) V(-1) s(-1) , 4.8 ± 0.4 × 10(-4) cm(2) V(-1) s(-1) , and 6.0 ± 0.2 × 10(-4) cm(2) V(-1) s(-1) , respectively), with increased stability compared to phospholipid bilayer coatings. HPB-coated capillaries yielded reproducible protein migration times (RSD ≤ 3.6%, n ≥ 6) with separation efficiencies as high as 200 000 plates/m.

Electrophoretic separations in poly(dimethylsiloxane) microchips using a mixture of ionic and zwitterionic surfactants

The use of mixtures of ionic and zwitterionic surfactants in poly(dimethylsiloxane) (PDMS) microchips is reported. The effect of surfactant concentration on electroosmotic flow (EOF) was studied for a single anionic surfactant (sodium dodecyl sulfate, SDS), a single zwitterionic surfactant (N-tetradecylammonium-N,N-dimethyl-3-ammonio-1-propanesulfonate, TDAPS), and a mixed SDS/TDAPS surfactant system. SDS increased the EOF as reported previously while TDAPS showed an initial increase in EOF followed by a reduction at higher concentrations. When TDAPS was added to a solution containing SDS, the EOF decreased in a concentration-dependent manner. The EOF for all three surfactant systems followed expected pH trends, with increasing EOF at higher pH. The mixed surfactant system allowed tuning of the EOF across a range of pH and concentration conditions. After establishing the EOF behavior, the adsorption/desorption kinetics were measured and showed a slower adsorption/desorption rate for TDAPS than SDS. Finally, the separation and electrochemical detection of model catecholamines in buffer and reduced glutathione in red blood cell lysate using the mixed surfactant system were explored. The mixed surfactant system provided shorter analysis times and/or improved resolution when compared to the single surfactant systems.

On-line sample stacking of peptides in capillary electrophoresis for the study of prebiotic reactions between alpha,alpha-dialkylated amino acids and amino acid N-carboxyanhydrides

The reaction between alpha,alpha-dialkylated amino acids and amino acid N-carboxyanhydrides is slow leading to low concentrations of products (peptides). The detection by capillary electrophoresis of the analytes contained in such samples is therefore a challenging issue. In this work, on-line sample pre-concentration methods based on field-amplified sample stacking have been implemented and compared. Because of the high ionic strength present in the sample matrix, samples were diluted with an organic solvent prior to analysis to decrease the sample conductivity. Different modes of sample injection (field amplified sample injection (FASI), hydrodynamic normal sample stacking (NSS) or large volume sample stacking (LVSS)) were compared. Pre-concentration factors of 20 for FASI, about 30-40 for NSS and 60 for LVSS were obtained for the analysis of (l,l) dipeptide of valine in a large excess of isovaline and 0.2M of ionic strength. For LVSS application and resolution optimisation, a new non-covalent coating based on the partial modification of the capillary surface was used to tune the electroosmotic flow magnitude and to pump the sample matrix out of the capillary. This on-line sample pre-concentration step allowed confirming that oligopeptides including alpha,alpha-dialkylated amino acids are formed during the reaction between alpha,alpha-dialkylated amino acids and N-carboxyanhydride amino acids.

Polymer Entrapment in Polymerized Silicate for Preparing Highly Stable Capillary Coatings for CE and CE−MS

An easy-to-implement capillary coating strategy based on polymer entrapment in the network of polymerized silicate is described. In this manner, cationic polymers are tightly fixed onto the inner wall of the capillary for electroosmotic flow control without necessitating complex surface modification chemistries. The resulting coated capillary exhibited good stability over a wide range of pH, good reproducibility, strong endurance in more than 300 electrophoretic runs, and tolerance of commonly employed organic solvent additives in CE. Applications in CE-MS analysis of biologically important anions as well as sample enrichment are shown. Additionally, it was used as a durable base for attachment of multiple layers of charged polymers on the wall, via electrostatic interaction with the preceding layer. Thus, two novel types of highly stable coated capillaries, one with anodic EOF and the other cathodic, were developed.

Capillary zone electrophoresis on chemically bonded imidazolium based salts

In the present paper, fused-silica capillaries were chemically modified with an analogue of the imidazole-based ionic liquid and zwitterionic salt. The coated capillaries were examined for the behavior of the electroosmotic flow in both aqueous and non-aqueous electrolytes. The electroosmotic flow in the capillary coated with an ionic liquid analogue was found to be anodic (reversed) and dependent on the pH of the separation buffer. In the case of a zwitterionic capillary, the electroosmotic flow was cathodic and its velocity remained almost constant in the pH range of 4-7. The zeta-potentials of the modified surfaces were also calculated. The effectiveness of coating was investigated by comparing a separation of five inorganic ions and seven alkylphosphonic acids/monoesters in the modified and uncoated capillaries. All separations were successfully carried out in simple buffers and completed during a short analysis time. Finally, the run-to-run and day-to-day reproducibility of the coated capillaries in terms of the migration time of a neutral marker was determined.

Semipermanent capillary coatings in mixed organic-water solvents for CE

This report describes the creation of semipermanent capillary coatings that are compatible with organic-water solvent systems in CE. The coatings are created by simply rinsing the fused-silica capillary with long double-chain cationic surfactants, such as dimethyl-ditetradecyl ammonium bromide (2C(14)DAB), dihexadecyldimethyl ammonium bromide (2C(16)DAB), and dimethyldioctadecyl ammonium bromide (2C(18)DAB). These surfactants generate semipermanent bilayer coatings on the capillary surface, which display a high degree of stability in buffers containing up to 60% v/v of organic solvents, such as methanol and ACN. The coating stability increases with increasing hydrophobicity of the surfactant, i.e., with increasing chain length. For instance, the EOF changes by only 1.2% in a 2C(18)DAB-coated capillary after 130 capillary volumes of rinsing with 60% v/v methanol containing buffer. The bilayer coatings allow separations to be performed without the need to regenerate the coating between runs or to maintain the EOF modifier in the run buffer. Rapid separations (<2 min) of anions and basic drugs with migration time reproducibility of less than 0.5% RSD and efficiencies of 0.4-0.6 million plates/m are obtained. In addition, selectivity changes for small anions and cationic drugs are also observed when the organic solvent content is adjusted.

Control of the EOF in CE using polyelectrolytes of different charge densities

The control of the EOF direction and magnitude remains one of the more challenging issues for the optimization of separations in CE. In this work, we investigated the possibility to use variously charged polyanions for a fine-tuning of the EOF using polyelectrolyte multilayers. For that purpose, polyanions of poly(acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonate) (PAMAMPS) with different chemical charge rates varying between 3 and 100% were used. These copolymers are statistic hydrophilic copolymers of acrylamide (AM) and 2-acrylamido-2-methyl-1-propanesulfonate (AMPS). The study of the influence of the chemical charge rate (AMPS molar proportion in the copolymer) on the electroosmotic mobility (mu(eo)) of a capillary modified by a polyelectrolyte bilayer (polycation/PAMAMPS) revealed that the fine-tuning of the EOF was possible, at least for cathodic or slightly anodic EOF (micro(eo) from -5 x 10(-5) to +35 x 10(-5) cm(2)V(-1)s(-1)). Electroosmotic mobility values were compared with the free-draining electrophoretic mobilities of the PAMAMPS constituting the last layer of the capillary coating. The stability of the EOF is discussed in detail on the basis of successive determinations of electroosmotic mobility and migration times. The application to the separation of a model peptide mixture demonstrated the interest (and the simplicity) of this approach for optimizing resolution and analysis time. Experimental resolutions were compared to the theoretical ones that we would obtain on a fused-silica capillary having the same EOF as the coated capillary.

Characterization of discontinuous buffer junctions using pH indicators in capillary electrophoresis for protein preconcentration

An effective sample preconcentration technique for proteins and peptides was recently developed using capillary electrophoresis (CE) with discontinuous buffers [C.A. Nesbitt, J.T.-M. Lo, K.K.-C. Yeung, J. Chromatogr. A 1073 (2005) 175]. Two buffers of different pH created a junction to trap the sample molecules at their isoelectric points and resulted in over 1000-fold preconcentration for myoglobin within 30 min. To study the formation of pH junctions in CE, a pH indicator, bromothymol blue, is used in this work to reveal the pH changes at the discontinuous buffer boundary. Bromothymol blue (BTB) exhibits a drastic change in its visible absorption spectrum (300-600 nm) going from the acidic to basic pH conditions, and is therefore ideal for visualizing the changes in pH at the junctions created by various buffer combinations. Preconcentration of myoglobin was performed in discontinuous buffers containing BTB. Major differences in the BTB absorption profiles were identified from buffer systems that differ significantly in preconcentration performance, which in turn, allowed for the identification of ideal buffers for sample preconcentration. Up to 2000-fold preconcentrations of myoglobin were achieved in the buffer systems studied in this work. In addition, the role of the electroosmotic flow (EOF) on the preconcentration performance was investigated. A low EOF was found to be desirable, as the pH junction could stay longer in the capillary for accumulation of proteins. The pH junction also displayed characteristics to resist bandbroadening. Potential laminar flow resulted from the mismatched residual EOFs under the two pH conditions within the discontinuous buffers appeared to have minimal effect on the preconcentration. In fact, external applied pressure can be used to control the migration of the pH junction without compromising the protein preconcentration.

Highly efficient protein separations in capillary electrophoresis using a supported bilayer/diblock copolymer coating

A surfactant/polymer wall coating consisting of the doubly chained cationic surfactant dimethyldioctadecylammonium bromide (DODAB) and polyoxyethylene (POE) 40 stearate is investigated. The coating is formed by simply rinsing a capillary with a solution containing DODAB and POE 40 stearate. The resultant coating is semi-permanent--demonstrating stable electroosmotic flow (EOF) even after a 60 min high pressure rinse with buffer. The EOF (-0.45+/-(0.23) x 10(-4) cm(2) V(-1) s(-1) at pH 7.4) is suppressed by more than a factor of ten compared to that observed for DODAB alone. Model protein mixtures were separated over a pH range of 3-10 with efficiencies of up to greater than 1 million plates/m for the basic proteins cytochrome c, lysozyme, ribonuclease A and alpha-lactalbumin, and the acidic proteins insulin chain A, trypsin inhibitor, and alpha-chymotrypsinogen A. Migration time reproducibility was 0.5-4.0% from run to run and 0.6-4.3% from day to day. Protein recoveries with this coating ranged from 84% to 97%.

Preparative capillary zone electrophoresis using a dynamic coated wide-bore capillary

Preparative capillary zone electrophoresis separations of cytochrome c from bovine and horse heart are performed efficiently in a surfactant-coated capillary. The surfactant, dimethylditetradecylammonium bromide (2C(14)DAB), effectively eliminated protein adsorption from the capillary surface, such that symmetrical peaks with efficiencies of 0.7 million plates/m were observed in 50-microm id capillaries when low concentrations of protein were injected. At protein concentrations greater than 1 g/L, electromigration dispersion became the dominant source of band broadening and the peak shape distorted to triangular fronting. Matching of the mobility of the buffer co-ion to that of the cytochrome c resulted in dramatic improvements in the efficiency and peak shape. Using 100 mM bis(2-hydroxyethyl)imino-tris(hydroxymethyl)methane phosphate buffer at pH 7.0 with a 100-microm id capillary, the maximum sample loading capacity in a single run was 160 pmol (2.0 microg) of each protein.

Selection of surfactants for cell lysis in chemical cytometry to study protein-DNA interactions

Protein-DNA interactions play a defining role in many cellular processes. Studying such interactions at the single-cell level is important and challenging. Here we make the first step toward achieving this goal with chemical cytometry. Chemical cytometry utilizes capillary separation for detailed chemical analyses of single cells. The cell is injected into a capillary, lysed, and its components are analyzed by CE or capillary chromatography with highly sensitive detection. In order to apply chemical cytometry to studies of protein-DNA interactions, cell lysis must not destroy protein-DNA complexes. Surfactants represent the most practical means of cell lysis inside the capillary. This work aimed at finding surfactants and lysis conditions that do not destroy protein-DNA complexes. We studied three groups of surfactants--ionic, zwitterionic, and nonionic--with respect to their ability to lyse the cell membrane without significantly influencing the stability of protein-DNA complexes. Nonequilibrium CE of equilibrium mixtures with surfactants in the equilibrium mixtures and in the run buffer was used to measure the equilibrium constant, K(d), and rate constant, k(off), of protein-DNA complex dissociation. We found that nonionic surfactants worked best: they lyse the plasma membrane without significantly influencing K(d), k(off), or the EOF. This work creates the foundation for studies of protein-DNA interactions in single cells by chemical cytometry.

Suppression of electroosmotic flow and its application to determination of electrophoretic mobilities in a poly(vinylpyrrolidone)-coated capillary

A hydrophilic polymer, poly(vinylpyrrolidone) (PVP), was employed for suppressing the electroosmotic flow (EOF). A capillary was filled with aqueous PVP solution for coating the capillary wall with PVP; the PVP solution was then replaced by a migration buffer solution containing no PVP. Three types of PVP with different molecular weights were examined. The EOF was suppressed more effectively as the molecular weight of PVP increased. The EOF in the coated capillary was approximately 10-fold smaller than that of a bare capillary and was constant in the pH range of 6-8. The suppressed EOF was stable even when no PVP was added to the migration buffer. However, the EOF increased significantly when sodium dodecyl sulfate was added into the migration buffer. The method was applied for determining the electrophoretic mobilities of inorganic anions that have negative electrophoretic mobilities larger than the electroosmotic mobility of the bare capillary. A novel method for determining the electrophoretic mobilities was proposed based on the linear relationship between electric current and electrophoretic mobility. The electrophoretic mobility was proportional to the electric current. Therefore, the intercept of the regression equation represents the electrophoretic mobility at room temperature. The electrophoretic mobilities were in good agreement with the absolute electrophoretic mobilities.

Enhancement of electroosmotic flow using zwitterionic additives

Zwitterionic additives provide a means of altering the EOF without increasing conductivity. The magnitude of the EOF in a bare silica capillary increased by as much as 69% upon addition of 500 mM of zwitterion to the running buffer. The EOF enhancement increases linearly with the zwitterion concentration. With zwitterionic additives of the form +NH3-(CH2)n-COO-, the magnitude of the EOF increase is directly related to the number of methylene groups, (n), which ranges from n = 1 to 7. The endgroups on the zwitterions also affect the EOF enhancement. The effect of Z1-methyl (+N(CH3)3CH2CH2CH2SO3-) on EOF was not a function of either the buffer cation or pH. The EOF enhancement is a function of the dielectric increment of the additive and the nature of the amine functionality.

Selective Sampling of Phosphopeptides for Detection by MALDI Mass Spectrometry

The analysis of phosphopeptides by mass spectrometry (MS) is one of the most challenging tasks in proteomics. This is due to the lower isoelectric point (pI) of phosphopeptides, which leads to inefficient sample ionization in MS, particularly when competing with other peptides. The problem is compounded by the typical low abundance of phosphopeptides in biological samples. We describe here a simple nonsorptive method to isolate phosphopeptides based on their pI. A voltage is applied to selectively migrate the phosphopeptides into a capillary, which are negatively charged at acidic pH. The selectively sampled fraction is directly deposited onto MALDI sample target in nanoliter volumes (7-35 nL) for highly sensitive MS detection. No significant sample loss is evident in this procedure; hence, the MS was able to detect the isolated phosphopeptides at trace quantity. In this case, attomole-level detection limit is achieved for synthetic phosphopeptides (nM concentration and nL volume), from a mixture containing other peptides at up to 1 million times higher in concentration. Selective sampling was also applied to the tryptic digest of beta- and alpha-caseins to reveal the multiple phosphorylated peptides at the low-femtomole level using MALDI MS. Knowledge of pI based on the rejection/injection of peptides was found to be useful in peak assignment. To confirm the sequence of the selectively sampled peptides, fraction collection was performed for offline ESI MS/MS analysis.

Oligomerized Phospholipid Bilayers as Semipermanent Coatings in Capillary Electrophoresis

Double-chained surfactants form semipermanent coatings that prevent protein adsorption in capillary electrophoresis (CE). To make such coatings more permanent, vesicles of the unsaturated phospholipid 1,2-dioleoyl-sn-glycero-3-phosphocholine were prepared and subjected to free-radical-initiated polymerization, both inside the capillary and in free solution. The latter generated oligomers of 2-5 units based on ESI-TOF MS, and formed the more stable coating in CE. Rinsing the capillary with a solution of the ex situ oligomerized DOPC suppressed EOF (0.8 x 10(-)(8) m(2)/V.s) for more than 20 h, whereas in situ oligomerized electroosmotic flow (EOF) suppressed the EOF for only 10 h. Mixtures of anionic and cationic proteins were separated under neutral pH and low ionic strength buffer with efficiencies of 480,000-930,000 plates/m and recoveries of 75-99%.

Capillary electrophoresis of inorganic ions: An update

This review as a sequel of three earlier similar reports gives a summary of the progress and significant methodological developments, starting from 2002, in the use of capillary electrophoresis (CE) for inorganic ion analysis. As substantiated by the illustrative number of relevant references, improvements in sensitivity achieved both in and outside a CE system, advances in manipulating the separation selectivity, novel hardware configurations, and system performance innovations are continually being reported over the review period. Specifically viewed are the recent advancements in elemental (bio)speciation analysis, which remains one of the most fertile areas of CE research, as well as in three recently booming research topics: contactless conductivity detection, separations on microchips, and transient isotachophoretic preconcentration. A state-of-the-art picture of technique's potentialities within the field of interest presented here demonstrates that CE has become recognized and is growing in acceptance as a reliable alternative to traditional analytical methods such as high-performance liquid chromatography (HPLC).

A new method for the determination of the critical micelle concentration of Triton X-100 in the absence and presence of beta-cyclodextrin by resonance Rayleigh scattering technology

A new method for the determination of the critical micelle concentration (CMC) of Triton X-100 in aqueous solution and beta-cyclodextrin solution by resonance Rayleigh scattering (RRS) has been developed. The method is based on the measurement of the RRS intensity of different concentration of Triton X-100 in aqueous solution and beta-cyclodextrin solution (6.0 x 10(-4) mol l(-1)). When the RRS intensities were plotted against the concentration of Triton X-100, an inflection point appeared at the Triton X-100 concentration of 5.0 x 10(-4) mol l(-1) in aqueous solution and 1.1 x 10(-3) mol l(-1) in beta-cyclodextrin solution, respectively. These values of concentration corresponded to the CMC of Triton X-100 in aqueous solution and beta-cyclodextrin solution, which also agreed closely with the results reported by surface tension and UV-Vis absorption spectrophotometry. Therefore, the present RRS method is very convenient, rapid and accurate and can be used as a new technology for the determination of CMC values of surfactants without any probe. The relationship between the RRS intensity and the concentration, aggregate state and the aggregate molecular size of Triton X-100 has been primarily discussed.

Factors Affecting the Temporal Stability of Semipermanent Bilayer Coatings in Capillary Electrophoresis Prepared Using Double-Chained Surfactants

Surfactants such as didodecyldimethylammonium bromide (DDAB) adsorb onto fused-silica capillaries to form semipermanent bilayer coatings. However, such coatings must be regenerated between runs to maintain efficiency and reproducibility. In this paper, chemical and physical factors affecting the stability of DDAB coatings are investigated. Chemical factors such as ionic strength and the nature of the buffer anion (e.g., from acetate to phosphate), which decrease the critical micelle concentration of DDAB, improve the coating stability. Increasing buffer pH also increases the coating stability. Finally, reducing the capillary diameter and reducing the volume of buffer flushed through the capillary enhance the coating stability. Using 50 mM acetate, pH 5.0, in a 25-microm-i.d. capillary, cationic proteins were separated with efficiencies of 1.05 million plates/m and a run-to-run migration time reproducibility of 0.6-0.8% RSD for 10 successive runs without regeneration of the DDAB coating between runs.

Control of electroosmotic flow and wall interactions in capillary electrophoresis capillaries by photografted zwitterionic polymer surface layers

A novel capillary with covalently bonded zwitterionic surface modification was prepared by photograft polymerization of the zwitterionic monomer N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)ammonium betaine, onto the inner surface of a UV-transparent fused-silica capillary. Although the zwitterionic moieties in the resulting polymeric "tentacles" comprise both a positive quaternary ammonium group and a negative sulfonate group, the coating has a net zero charge. The electroosmotic flow (EOF) was therefore extensively suppressed on the grafted capillary compared to the native silica capillary and to the silica capillary that had been activated for graft polymerization by reaction with 3-(methacryloyl)oxypropyltrimethoxysilane. It was also found that the EOF can be varied by adding chaotropic anions or divalent cations such as perchlorate ion and magnesium ion to the running buffer, due to the interaction between these ions and zwitterionic functional group. This provides a new way of altering the EOF and the wall interaction without changing the pH or the overall ionic strength of the separation buffer. The influence of pH and ionic strength of separation buffer on the EOF were also investigated to optimize the separation conditions. Good separations of a mixture containing eight inorganic anions were achieved within 5 min under optimal conditions by capillary zone electrophoresis. The newly prepared capillary was also well suited for the separation of peptides or proteins.