Capillary electrokinetic chromatography with charged linear polymers as a nonmicellar pseudostationary phase: determination of capacity factors and characterization by solvation parameters

Physicochemical and chromatographic characteristics of random amphiphilic copolymer aggregation in electrokinetic chromatography

The random amphiphilic polymeric aggregation, self-assembled from poly (methyl methacrylate-co-methacrylic acid) (P(MMA-co-MAA)), was explored as a novel pseudostationary phase (PSP) in electrokinetic chromatography (EKC) in our previous report. This work focused on physicochemical characteristics and PSP performances of the polymeric aggregations. The physicochemical characteristics of polymeric aggregations, including critical aggregation concentration (CAC), zeta potential, hydrodynamic diameter, and micropolarity were determined. Experimental results showed that polymeric aggregations had much lower CAC, which decreased the usage of copolymer in EKC, weakened ionic strength and shortened analysis time. The monomer molar ratio of the copolymer was a key factor for physicochemical characteristics and PSP performances of the polymeric aggregations. With the increase of the hydrophobic monomer molar ratio, CAC, micropolarity and dimension of polymeric aggregation decreased significantly while zeta potentials were similar. Correspondingly, separation window enlarged and methylene selectivity evaluated with six kinds of n-alkylphenone homologous series enhanced. Linear solvation energy relationships (LSER) analysis found that hydrophobic interaction is the most important interaction between analytes and polymeric PSPs. Compared with SDS micelle, polymeric aggregations owned more types of interactions, such as stronger hydrogen bonding and relative larger dipole interaction, which provided a bigger adjustment room to improve PSP selectivity.

Capillary separation: micellar electrokinetic chromatography

Micellar electrokinetic chromatography (MEKC), a separation mode of capillary electrophoresis (CE), has enabled the separation of electrically neutral analytes. MEKC can be performed by adding an ionic micelle to the running solution of CE without modifying the instrument. Its separation principle is based on the differential migration of the ionic micelles and the bulk running buffer under electrophoresis conditions and on the interaction between the analyte and the micelle. Hence, MEKC's separation principle is similar to that of chromatography. MEKC is a useful technique particularly for the separation of small molecules, both neutral and charged, and yields high-efficiency separation in a short time with minimum amounts of sample and reagents. To improve the concentration sensitivity of detection, several on-line sample preconcentration techniques such as sweeping have been developed.

A Method To Reduce Gradient Delay Time of NanoLC

A novel method to dramatically reduce the delay time of a nanoLC gradient is described. The gradient is divided into two parts, and each part is formed at different flow rates. The beginning part is formed and delivered to the inlet of the column at a higher-than-normal flow rate. With the formation of the rest of the gradient at a normal flow rate, the whole gradient is further delivered through the column at the same normal flow rate. To form the gradient with the desired slope, the volumetric gradient slope was kept constant, independent of the flow rate. A gradient delay time reduction of 12.5-16 min was observed with the reported method. The resulting gradient profiles and chromatograms were very similar to those obtained with a conventional method. Comparable retention time reproducibility was observed between the two methods.

Cationic and perfluorinated polymeric pseudostationary phases for electrokinetic chromatography

Separation selectivity in electrokinetic chromatography (EKC) is directly affected by the chemistry and solvent characteristics of the pseudostationary phase (PSP). The chemical selectivity of micellar PSPs has been previously demonstrated to vary significantly between anionic and cationic surfactants as well as between hydrocarbon and fluorocarbon surfactants. Polymeric PSPs have also been demonstrated to provide unique selectivity. In the current study, four cationic polymeric pseudo-stationary phases, two of which have perfluorinated pendant groups, are introduced and characterized as PSPs in EKC. Their performance and selectivity is compared to conventional micellar PSPs with similar structure. The solvation characteristics and selectivity of the four polymers most closely resemble those observed for cationic micelles. The polymers are all more cohesive and more polar than their hydrocarbon micellar counterparts. The fluorocarbon PSPs did show preferential interaction with fluorocarbon solutes, were somewhat more cohesive, and were stronger hydrogen bond donors. However, the presence of fluorocarbon moieties did not have as dramatic an effect on selectivity as was observed and published previously for fluorocarbon micelles. This may result from the selectivity being dominated by the presence of the cationic head groups or from the fluorocarbon character of the pendant groups being moderated by the presence of hydrocarbon functionality on the polymer backbones.

Developments in the use of soluble ionic polymers as pseudo-stationary phases for electrokinetic chromatography and stationary phases for electrochromatography

This article reviews the development, characterization and application of soluble ionic polymeric materials as pseudo-stationary phases for electrokinetic chromatography and as stationary phases for electrochromatography since 1997. Polymeric pseudo-stationary phases for electrokinetic chromatography, including cationic polymers, anionic siloxane and acrylamide polymers, polymerized surfactants (micelle polymers), and chiral polymers are reviewed. Also reviewed are suspended molecularly imprinted polymer micro-particles. Application of polymeric pseudo-stationary phases with electrospray ionization mass spectrometric detection is presented. Recent progress in the development and characterization of physically adsorbed stationary phases for electrochromatography using polymers of the same or similar chemistry is also reviewed.

Metabolic Labeling of Mammalian Organisms with Stable Isotopes for Quantitative Proteomic Analysis

To quantify proteins on a global level from mammalian tissue, a method was developed to metabolically introduce 15N stable isotopes into the proteins of Rattus norvegicus for use as internal standards. The long-term metabolic labeling of rats with a diet enriched in 15N did not result in adverse health consequences. The average 15N amino acid enrichments reflected the relative turnover rates in the different tissues and ranged from 74.3 mpe in brain to 92.2 mpe in plasma. Using the 15N-enriched liver as a quantitative internal standard, changes in individual protein levels in response to cycloheximide treatment were measured for 310 proteins. These measurements revealed 127 proteins with altered protein level (p < 0.05). Most proteins with altered level have previously reported functions involving xenobiotic metabolism and protein-folding machinery of the endoplasmic reticulum. This approach is a powerful tool for the global quantitation of proteins, is capable of measuring proteome-wide changes in response to a drug, and will be useful for studying animal models of disease.

Modelling and optimization of the electrokinetic chromatographic separation of mixtures of organic anions and cations using poly(diallydimethyl- ammonium chloride) and hexanesulfonate as mixed pseudostationary phases

The separation of a series of aromatic carboxylic acids, sulfonates and opiates using electrokinetic chromatography employing a mixture of the soluble cationic polymer poly(diallydimethylammonium chloride) (PDDAC) and the amphiphilic anion hexanesulfonate as pseudostationary phases is described. In this system, the PDDAC pseudostationary phase interacts with the anionic analytes, whereas the hexanesulfonate pseudostationary phase interacts with the cationic analytes. A migration model has been derived which takes into account the ion-exchange (IE) interactions between the anions and the cationic PDDAC as well as the ion-pair (IP) interactions between the opiates and the hexanesulfonate. A further interaction between the combined PDDAC-hexanesulfonate complex and the more hydrophobic analytes is also evident and is accounted for in the model. Constants obtained by applying the model agreed well with the expected trends in IE affinities of the anions for PDDAC and also corresponded with the hydrophobic natures of the analytes. Optimization of the PDDAC and hexanesulfonate concentrations was performed using the normalized resolution product and minimum resolution product criteria. The minimum resolution product criterion proved to be most successful. An advantage of the described system is the improvement in peak shapes obtained after addition of hexanesulfonate to the electrolyte, resulting in increased plate numbers and better resolution. The system was very robust with mobilities varying by less than 2% over a period of days and on using different capillaries.

Automation of nanoscale microcapillary liquid chromatography-tandem mass spectrometry with a vented column

To fully automate the sample introduction step for nanoscale microcapillary liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses, 75 microm i.d. x 14 cm capillary columns were interfaced with a commercial autosampler instrument using a novel procedure which allowed dilute peptide samples to be transferred from the AS loop injector to the nanoscale column at flow rates up to 5 microL min(-1). On-column enrichment and desalting was demonstrated for large sample volumes (>40 microL) by constructing a vent 2 cm after the entrance to the packed bed of 5-microm ODS-AQ modified silica. Salts and nonretained solutes were removed via the vent, which allowed for column washing independent of the continuation of the bed into the electrospray source. Separations of test peptide mixtures demonstrated 50-nL elution peak volumes with low- to subfemtomole detection levels. In addition, a highly complex peptide mixture (outer membrane preparation from Psuedemonas aeruginosa) was efficiently separated with more than 100 proteins identified from a single reversed-phase LC-MS/MS analysis. Finally, the vented column (V-column) was utilized for on-line separations in a multidimensional chromatography/tandem MS experiment where large numbers of strong cation exchange chromatography fractions from a trypsinized yeast lysate were desalted, concentrated, and analyzed in a completely automated fashion. The procedures for constructing and using a V-column require minimal changes in current methods and equipment for nano-LC-MS analyses using columns of 100-microm diameter and smaller.

Electrokinetic chromatography utilizing two pseudostationary phases providing ion-exchange and hydrophobic interactions

The electrokinetic chromatographic separation of a series of inorganic and organic anions was achieved by utilizing an electrolyte system comprising a cationic soluble polymer (poly(diallydimethylammonium chloride, PDDAC) and a neutral beta-cyclodextrin (beta-CD) as pseudostationary phases. The separation mechanism was a combination of electrophoresis, ion-exchange (IE) interactions with PDDAC, and hydrophobic interactions with beta-CD. The extent of each chromatographic interaction was independently variable, allowing for control of the separation selectivity of the system. IE interactions could be varied by changing either the PDDAC concentration or the concentration of a competing ion (e.g., chloride) in the BGE, while the hydrophobic interactions could be varied by changing the concentration of beta-CD. The separation system was very robust, with the reproducibility of the migration times being <0.7% RSD. A mathematical model that predicted the mobilities of analytes under varying experimental conditions was derived and was shown to give good correlation (r2 = 0.9804) between predicted and experimental migration times. Parameters derived from the model were in good agreement with the ion-exchange and hydrophobic characteristics of the analytes. The model was also applied successfully to the optimization of conditions for the separation of a mixture of analytes or for conditions under which particular analytes migrated in a desired order. That is, the opportunity to tune the separation selectivity has been demonstrated.

Automatic identification of proteins with a MALDI-quadrupole ion trap mass spectrometer

A matrix-assisted laser desorption/ionization (MALDI) ion trap mass spectrometer of new design is described. The instrument is based on a commercial Finnegan LCQ ion trap mass spectrometer to which we have added a MALDI ion source that incorporates a sample stage constructed from a compact disk and a new ion transmission interface. The ion interface contains a quadrupole ion guide installed between the skimmer and the octapoles of the original instrument configuration, allowing for operation in both MALDI and electrospray ionization modes. The instrument has femtomole sensitivity for peptides and is capable of collecting a large number of MALDI MS and MALDI MS/MS spectra within a short period of time. The MALDI source produces reproducible signals for 10(4)-10(5) laser pulses, enabling us to collect MS/MS spectra from all the discernible singly charged ions detected in a MS peptide map. We describe the different modes of the instrument operation and algorithms for data processing as applied to challenging protein identification problems.

Spatial control of cellular measurements with the laser micropipet

Continued progress in understanding cellular physiology requires new strategies for biochemical measurements in solitary cells, multiple cells, and subcompartments of cells. Large spatial gradients in the concentrations of molecules and presumably the activities of enzymes can occur in cells. Consequently, there is a critical need for measurement techniques for mammalian cells with control over the numbers or regions of cells interrogated. In the present work, we developed a strategy to rapidly load the cytoplasmic contents of either multiple cells or a subregion of a single cell into a capillary. A single, focused pulse from a laser created a mechanical shock wave which disrupted a group of cells or a portion of a cell in the path of the shock wave. Simultaneously, the cytoplasm was loaded into a capillary for electrophoretic separation. The size of the region of cellular disruption (and therefore the volume of cytoplasm collected) was controlled by the amount of energy in the laser pulse. Higher energies could be used to sample groups of cells while much lower energies could be utilized to selectively sample the tip of a neuronal process. The feasibility of performing measurements on subcellular compartments was also demonstrated by targeting reporter molecules to these compartments. A reporter localized to the nucleus was detected on the electropherogram following laser-mediated disruption of the cell and the nucleus. Finally, we demonstrate that this method terminated cellular reactions with sufficient rapidity that cellular membrane repair mechanisms were not activated during cytoplasmic collection. The combined ability to preselect a spatial region of a cell or cells and to rapidly load that region into a capillary will greatly enhance the utility of CE in the biochemical analysis of cells.

Effect of pendant chain lengths and backbone functionalities on the chemical selectivity of sulfonated amphiphilic copolymers as pseudo-stationary phases in electrokinetic chromatography

Amphiphilic copolymers of AMPS (2-acrylamido-2-methyl-1-propanesulfonic acid) and hydrophobic monomers with various chemical structures were synthesized, characterized and used as novel electrokinetic chromatography polymeric pseudo-stationary phases, showing significant chemical selectivity differences from that of the conventional monomeric pseudo-stationary phase, sodium lauryl sulphate. Copolymers of AMPS and methacrylates with different pendant chain lengths (C8, C12 and C18) were investigated and no significant difference in chemical selectivity was observed among them. However, the spacer bonding chemistry was shown to contribute to significant chemical selectivity difference, e.g. poly(AMPS-lauryl methacrylate) showed different chemical selectivity from poly(AMPS-lauryl methacrylamide). Linear solvation energy relationship analysis of 20 solutes by eight different polymeric pseudo-stationary phases was employed to investigate the solute molecule structural contributions to the retention. Hydrogen-bonding properties (described by system constants b and a) of poly(AMPS-alkyl methacrylamide) were found stronger than those of poly(AMPS-alkyl methacrylate).

Electrokinetic chromatography with micelles, polymeric and monomeric additives with similar chemical functionality as pseudo-stationary phases

A comparison is made of the retention properties of additives applied as positively charged pseudo-stationary phases for electrokinetic chromatography of neutral analytes. All additives have a quaternary ammonium as functional group. The polymeric additive [poly(N,N,N',N'-tetramethyl-N-trimethylenehexamethylenediammonium), Polybrene] has a concentration of 2% (w/w) in the background electrolyte (acetate, pH 5.2). Monomeric octyltrimethylammonium (OTMA) was used at a concentration below or above its critical micelle concentration (CMC) (140 mmol/l). At a concentration (259 mmol/l) above the CMC the system is that normally used for micellar electrokinetic chromatography with cationic micelles. However, even below the CMC, where OTMA is present as monomer, retention of the neutral analytes is observed as well. In all systems coating of the capillary wall with Polybrene establishes an electroosmotic flow directed towards the anode, counter-migrating to the electrophoretic movement of the additive. Based on the measurement of the mobility of the analytes (15 small, monofunctional aromatic compounds with different functional groups), their capacity factors, k(i), were determined in all systems. Low correlation of the k(i) values is observed between the particular systems, indicating their different selectivity at least for individual pairs of analytes. Based on the log k(i) values, a linear free energy relationship was applied to elucidate the main types of chemical interaction responsible for retention. As a result, cavity formation and n or pi electron interactions were found being significant for the micellar OTMA system, which agrees with findings described in the literature for other (cationic and anionic) micellar systems. For the polymeric system and for the monomeric OTMA system, the significant retention parameter is indicating n and pi electron interactions.

Application of linear solvation energy relationships to polymeric pseudostationary phases in micellar electrokinetic chromatography

Polymerized sodium 11-acrylamidoundecanoate (poly(Na 11-AAU)) was used as a pseudostationary phase (PSP) for micellar electrokinetic chromatography to separate uncharged compounds. The polymer PSP showed signifcantly different solute migration behaviors from conventional micelles including sodium dodecyl sulfate and poly (sodium 10-undecylenate), giving high separation efficiencies (>200000 theoretical plates/m). Linear solvation energy relationships were used to evaluate and characterize the chemical interactions that influence the retention behavior in the poly (Na 11-AAU) micellar system. It was found that the solute volume and solute hydrogen bond basicity mainly influenced the retention. The characteristic feature of the poly (Na 11-AAU) micellar system is that the micelle has a significantly higher capacity for dipole-dipole and dipole-induced dipole interactions as well as a slightly higher capacity for electron pair interactions than the aqueous phase. Due to its unique selectivity, the poly(Na 11-AAU) micellar system would become an attractive new option for selectivity optimization on methods development.

Synthesis and characterization of novel anionic siloxane polymers as pseudostationary phases for electrokinetic chromatography

Four novel siloxane polymeric pseudostationary phases with three different ionic head groups have been synthesized and characterized for electrokinetic chromatography. Siloxane polymers are of interest in this application because of the wide range of chemistries that can be developed based on these backbones, including much of the chromatographic stationary phase chemistry developed in the last thirty years. All four of the siloxanes studied were synthesized by modification of a single methylhydrosiloxane polymer with highly acidic anionic functionalities. One of the siloxanes had both ionic groups and alkane chains attached to the siloxane backbone. The electrophoretic mobilities varied from being somewhat less than sodium dodecyl sulfate (SDS) to being much greater than SDS. The siloxanes substituted with ionic groups at all of the silicon sites showed significant nonequilibrium band broadening, severely limiting the efficiencies of these polymers. Substitution of 20% of the silicon sites with an alkyl group improved the efficiency of the separations and the peak symmetry. The chemical selectivities of the siloxane polymers are very different from SDS, but are similar to each other.

Separation of olanzapine, carbamazepine and their main metabolites by capillary electrophoresis with pseudo-stationary phases

Conditions were worked out for the separation of carbamazepine, olanzapine, and their main metabolites carbamazepine 10,11-epoxide, 10-hydroxycarbamazepine, and desmethylolanzapine. The separation was based on electrokinetically driven methods in the capillary format. The main difficulty in separating these compounds is related to their different chemical classes. Whereas the carbamazepine members are amides, and are electrically neutral, the olanzapine members have aliphatic amino groups and are thus cationic under most experimental conditions. Different additives were applied as pseudo-stationary phases to implement selectivity. Poly(diallyldimethylammonium), PDADMA, is a polycationic replaceable and soluble polymer, that interacts mainly according to the polarisability of the analyte molecules. The MEKC principle was applied with the common SDS as micelle former. In both systems, only partial resolution of the analytes was obtained. The most favorable system consisted of a charged, oligomeric additive: full separation of all analytes within 4 min was achieved with heptakis-6-sulfato-beta-cyclodextrin (7 mM) in 30 mM borate buffer, pH 8.5.

Polymeric and polymer-supported pseudostationary phases in micellar electrokinetic chromatography: performance and selectivity

Several types of synthetic ionic polymers have been employed as pseudostationary phases in electrokinetic chromatography. The polymers have been shown to have some significant advantages and different chemical selectivity relative to conventional surfactant micelles. Polymeric phases are effective for the separation and analysis of hydrophobic and chiral compounds, and may be useful for the application of mass spectrometric detection. Additionally, the polymeric phases often demonstrate unique selectivity relative to micellar phases, and can be designed and synthesized to provide desired selectivity. This review covers efforts to develop and characterize the performance, characteristics, and selectivity of synthetic polymeric pseudostationary phases since their introduction in 1992. Some ideas for the future development of polymeric pseudostationary phases and the role they may play in electrokinetic separations are presented.

Comparison of separation selectivity in capillary electrokinetic chromatography using a cationic linear polymeric pseudo-stationary phase or monomeric additives of similar structure

The retention properties in electrically driven systems with monomeric additives were compared to an electrokinetic chromatographic system with a linear, charged polymer of similar chemical structure (all additives are quaternary tetraalkyl ammonium ions). The monomeric additives were tetramethylammonium (TMA), tetraethylammonium (TEA) and dimethylpyrrolidinium (DMP), respectively, the polymeric additive was poly(diallyldimethyl)ammonium (PDADMA). The additive concentration in the background electrolyte was 2 and 4% (w/w). The retention characteristics were based on the apparent mobilities of 10 non-charged analytes with different chemical functionality, which were transported by the anodic electroosmotic flow in the dynamically coated capillary, and retained by the counter-flowing cationic additives. From these data capacity factors were derived, which ranged up to 0.8. Association constants were calculated, and were found between 10 and 170. Roughly, the association constants increased for a given analyte in the sequence TMA<TEA<DMP< PDADMA. However, changes in the retention order were observed for some cases, reflecting the different selectivity of the particular systems for certain pairs of analytes. A general advantage of polymeric pseudo-stationary phases compared to monomeric additives is given by the negligible reduction of the mobility of the analyte-polymer associate in relation to the free additive ion, resulting in a broader retention window under most practical conditions.

Recent innovation in capillary electrokinetic chromatography with replaceable charged pseudostationary phases or additives

Recent developments of separation of neutral analytes in capillary systems with the mobile phase driven by the electroosmotic flow (EOF) and charged additives acting as a pseudostationary phase are reviewed. As pseudostationary phases a number of additives are used. Soluble polymers, either anionic or cationic, were applied as alternatives to micelles. Monomeric charged additives are also intended to form associates with the analytes, leading to selective retention and separation in a similar way as the polymeric pseudostationary phases. Dendrimers, spherical macromolecules with highly branched chains and charged terminal groups, are successfully applied for the separation of lipophilic analytes. Polymers with covalently stabilized structures are introduced in the form of permanent micelles and are therefore insensitive to the mobile phase composition, enlarging the applicability of micellar electrokinetic capillary chromatography (MEKC).

Diffusion coefficient and capacity factor in capillary electrokinetic chromatography with replaceable charged polymeric pseudophase

Apparent diffusion coefficients, Dapp,i, were determined in solutions with a polycationic additive -- poly(diallyidimethylammonium) -- acting as a pseudostationary phase for electrokinetic chromatography. They were determined for six small neutral analytes at five concentrations of the polymeric additive (between 0 and 4% w/w) by a stopped migration method. The apparent diffusion coefficients decrease with increasing polymer concentration only within 40% maximum, an effect that cannot be associated with the macroscopic viscosity of the polymer solution (which increases by a factor of 10). The change of the apparent diffusion coefficients is related to the interaction of the neutral analyte molecules with the polyelectrolyte chain. Applying the model of analyte partitioning between "free" solution and polymer, capacity factors and partition constants were derived from the slope of the 1/Dapp,i vs. polymer concentration curves. Partition constants determined by this method (ranging between 40 and 170) agree with those obtained by electrokinetic chromatography.