Pressurized-flow anion-exchange capillary electrochromatography using a polymeric ion-exchange stationary phase

Flow splitting at the inlet electrode as a method for decreasing the electric current in electric field assisted liquid chromatography

The combination of pressurized flow and electric field offers, with the use of capillary columns, several options for retention control. However, it has been shown that the utility of this technique is strongly limited by the high electric current that is generated at the high electric field strengths that are needed. We have earlier shown that the high current is a result of locally increased mobile phase ion concentration in the electric field, particularly around the inlet electrode. In this paper, we report that by splitting the mobile phase flow around the inlet electrode a relatively constant ion concentration around the electrode can be obtained and the high currents are there by reduced.

Separation of small peptides by electrochromatography on silica-based reversed phases and hydrophobic anion exchange phases

Peptide separations are regarded as a promising application of capillary electrochromatography (CEC) and, at the same time, a suitable model to elucidate its mixed separation mechanism when charged analytes are involved. In this paper, studies on the separation of small peptides (2-4 amino acids) on a Spherisorb octadecyl silane (ODS) phase at acidic pH and on a strong anion exchange (SAX)/C18 mixed mode phase at weakly basic pH are reported. For the ODS phase a comparison of CEC, capillary zone electrophoresis (CZE) and high-performance liquid chromatography (HPLC) under identical buffer/eluent conditions is presented. The predicted retention factors for CEC under the assumption of simple superposition of HPLC retention and CZE migration matched the measured results for the peptides that had small retention factors in HPLC. For both types of stationary phases, a variation of the acetonitrile content in the mobile phase led to a wide range of retention factors, including negative values when co-electroosmotic migration was dominant. Though both the ODS and the SAX/C18 phase offer unique advantages, the SCX/C18 phase at pH 9 provides more flexibility to alter separation selectivity for the selected peptides.

Controlling the retention in capillary LC with solvents, temperature, and electric fields

Once a suitable stationary phase and column dimensions have been selected, the retention in liquid chromatography (LC) is traditionally adjusted by controlling the mobile phase composition. Solvent gradients enable achievement of good separation selectivity while decreasing the separation time as compared to isocratic elution. Capillary columns allow use of other programming parameters, i.e. temperature and applied electric fields, in addition to solvent gradient elution. This paper presents a review of programmed separation techniques in miniaturized LC, including retention modeling and method transfer from the conventional to micro- and capillary scales. The impact of miniaturized instrumentation on retention and the limitations of capillary LC are discussed. Special attention is focused on the gradient dwell volume effects, which are more important in micro-LC techniques than in conventional analytical LC and may cause significant increase in the time of analysis, unless special instrumentation and (or) pre-column flow-splitting is used. The influence of temperature upon retention is also discussed, and applications where the temperature has been actively used for retention control in capillary LC are included together with the instrumentation utilized. Finally the possibilities of additional selectivity control by applying an electric field over a packed capillary LC column are discussed.

Deviation from Ohm’s law in electric field assisted capillary liquid chromatography

Earlier studies of electric field assisted LC (EF-LC) have shown that the effect on charged analytes of the application of an electric field over a capillary LC column is relatively small. Charged analytes can only be affected by the electric field while present in the mobile phase, which makes the effective time for influence of the electric field t(0) independent of retention time. Because the charged analytes only can be affected for a short time the electric field strength ought to be high in order to increase the impact of the electric field on the separation. We have, however, found that only a relatively low electric field strength can be used in EF-LC when pressure is used as main driving force. The useful field strength was limited by a dramatic increase in the current. This increase in current was found to origin from an increased concentration of buffer ions that have an electrophoretic mobility towards the pumped flow.

The importance of capillary electrophoresis, capillary electrochromatography, and ion chromatography in separations of inorganic ions

The importance of capillary electrophoresis (CE), capillary electrochromatography (CEC), and ion chromatography (IC) in inorganic ion analyses is outlined. Methods for improving the reliability of the CE measurements are briefly described. Selectivity optimization in CE analyses of inorganic cations and anions is discussed. Using the Peakmaster program, CE system peaks (system zones, eigenmobilites) and some important CE parameters, such as effective mobilities, electromigration dispersion, indirect UV, and direct conductivity signals, are predicted and compared with experimental analyses.

Development and optimization of an analytical method for the determination of UV filters in suntan lotions based on microemulsion electrokinetic chromatography

Microemulsion electrokinetic chromatography (MEEKC) has been applied to the separation of some UV filters (Eusolex 4360, Eusolex 6300, Eusolex OCR, Eusolex 2292, Eusolex 6007, Eusolex 9020, Eusolex HMS, Eusolex OS, and Eusolex 232) commonly found in suntan lotions. The composition of the microemulsion employed was optimized with respect to the best possible separation of the selected analytes using artificial neural networks (ANNs). Two parameters namely the composition of the mixed surfactant system comprising the anionic sodium dodecyl sulfate (SDS) and neutral Brij 35 and the amount of organic modifier (2-propanol) present in the aqueous phase of the microemulsion were modeled. Using an optimized MEEKC buffer consisting of 2.25 g SDS, 0.75 g Brij 35, 6.6 g 1-butanol, 0.8 g n-octane, 17.5 g 2-propanol, and 72.1 g of 10 mM borate buffer (pH 9.2), eight target analytes could be separated in under 25 min employing a diode-array detector to segregate the overlapping signals obtained for Eusolex 9020 and Eusolex HMS. Detection limits from 0.8 to 6.0 nug/mL were obtained and the calibration plots were linear over at least one order of magnitude. The optimized method could be applied to the determination of Eusolex 6300 and Eusolex 9020 in a commercial suntan lotion.

Predictive non-linear modeling of complex data by artificial neural networks

An artificial neural network (ANN) is an artificial intelligence tool that identifies arbitrary nonlinear multiparametric discriminant functions directly from experimental data. The use of ANNs has gained increasing popularity for applications where a mechanistic description of the dependency between dependent and independent variables is either unknown or very complex. This machine learning technique can be roughly described as a universal algebraic function that will distinguish signal from noise directly from experimental data. The application of ANNs to complex relationships makes them highly attractive for the study of biological systems. Recent applications include the analysis of expression profiles and genomic and proteomic sequences.

Use of ionic polymers as stationary and pseudo-stationary phases in the separation of ions by capillary electrophoresis and capillary electrochromatography

One of the problems with capillary electrophoresis is a lack of versatility regarding manipulation of the separation selectivity. A new and potentially universal concept is to introduce an ion-exchange component into a separation so that the migration of analyte ions is influenced by both their electrophoretic mobilities and their chromatographic properties. This may be accomplished by use of capillaries filled with or coated with solid ion-exchange polymers, or by addition of a soluble ionic polymer to the background electrolyte to create a pseudo-stationary phase. While each of these methods achieves the same result, they are not competitive, but rather complementary as the problems associated by one approach are overcome by the others. Recent highlights in the field are used to illustrate the flexibility that this approach provides to electrophoretic separation of ions.

Recent applications of capillary electrochromatography

A review is presented of the most important recent applications of capillary electrochromatography (CEC) for the analysis of acidic, basic, and neutral compounds, of biomolecules, environmental substances, natural products, pharmaceuticals, and chiral compounds. Packed-column CEC (packed-CEC), open-tubular (OT-CEC), as well as pressure-assisted CEC (pseudo-CEC) are hereby considered. Papers published between July 1999 and April 2001 were taken into account. Applications before July 1999 have been reviewed in Electrophoresis 1999, 20, 3027-3065.

Recent advances in capillary electrophoresis and capillary electrochromatography of pollutants

An overview of major developments in capillary electrophoresis and capillary electrochromatography systems in the environmental field is presented, covering relevant publications between the second half of 1999 and early 2001. Contributions are reviewed in relation to developments in detection, sample preparation/preconcentration, precision and applications. Many interesting examples are shown and the influence of important parameters on the performance of developed methods is discussed.

Application of hydrophobic anion-exchange phases in capillary electrochromatography

Capillary electrochromatography (CEC) requires stationary phases that enable appropriate electroosmotic propel under various conditions. Analyte retention can be controlled through hydrophobic or electrostatic interaction with the packing material. The development and characterization of new strong anion-exchange materials with additional hydrophobic moieties (SAX/C18 mixed-mode phases) is described. The synthesis was based on polymer encapsulation of porous silica. The phases were systematically characterized by means of elemental analyses, HPLC frontal analyses and CEC experiments. The studies focused on the influence of various parameters (e.g., pH, kind of buffer, capillary wall) on the electroosmotic flow (EOF). Phases with high anion-exchange capacity generated a fast and constant EOF over a wide pH range. Long-time stability of EOF and hydrophobic retention under CEC conditions were demonstrated within the course of 100 consecutive injections. The applicability of the SAX/C18 phases in appropriate buffer systems is demonstrated for neutral, acidic and basic compounds.

Optimisation of the separation of anions by ion chromatography-capillary electrophoresis using indirect UV detection

The separation of a complex mixture of inorganic and organic anions by ion chromatography-capillary electrophoresis using a cationic polymer added to the background electrolyte and indirect UV detection has been studied. The addition of unmodified polymer to an electrolyte suitable for indirect detection resulted in the appearance of a system peak due to the counter-anion on the polymer and while the position of the analytes relative to this system peak could be changed, this was found to be an unacceptable approach for mixtures of large numbers of analytes. Although conversion of the polymer to replace the counter-ion with the indirect UV detection probe ion simplified the system, this approach restricted the flexibility of the system because the probe and polymer concentration were necessarily linked. This limitation could be overcome by selecting the appropriate type of probe ion, with probes having a low ion-exchange selectivity coefficient providing greater retention of analytes than probes with a high ion-exchange selectivity coefficient. Three electrolyte systems with different probes (benzoate, chromate and phthalate) were modelled using a previously derived migration equation and this was used to optimise the electrolyte composition to enable the separation of a mixture of 24 inorganic and organic anions within 7 min. The electrolyte composition was then optimised for the analysis of anions in Bayer liquor with the final separation selectivity being substantially improved for selected key analytes.

Lonene-coated sulfonated silica as a packing material in the packed-capillary mode of electrochromatography

An aliphatic ionene (2-10-ionene) has been selected as a modifier to prepare a novel polymer-coated packing material for capillary electrochromatography. The packing material was produced by dynamical modification of a commercially available sulfonated silica Exsil-100 SCX. Strong ion-exchange interactions in the capillary packed with ionene-modified sulfonated silica have been demonstrated by the example of the retention of p-aminobenzoic acid. The total calculated anion-exchange capacity of the sorbent in the capillary was about 4 x 10(-9) mol. A fast separation (about 15 min) of several aromatic acids was achieved with the packing material. The highest number of theoretical plates obtained was about 120,000. Limits of detection of the aromatic acids were 2-5 microg/ml. The advantages and lacks of the approach are discussed briefly.

Anion-exchange capillary electrochromatography with indirect UV and direct contactless conductivity detection

Conductivity detection is applied to ion-exchange capillary electrochromatography (IE-CEC) with a packed stationary phase, using a capacitively coupled contactless conductivity detector with detection occurring through the packed bed. Columns were packed with a polymeric latex-agglomerate anion-exchanger (Dionex AS9-SC). A systematic approach was used to determine suitable eluants for IE-CEC separations using simultaneous indirect UV and direct conductivity detection. Salicylate and p-toluenesulfonate were identified as potential eluant competing anions having sufficient eluotropic strength to induce changes in separation selectivity, but salicylate was found to be unsuitable with regard to baseline stability. It was also found for both indirect UV and direct conductivity detection that homogenous column packing was imperative, and monitoring of the baseline could be used to assess the homogeneity of the packed bed. Using a p-toluenesulfonate eluant, the separation of eight common anions was achieved in 2.5 min. Direct conductivity detection was found to be superior to indirect UV detection with regard to both baseline stability and detection sensitivity with detection limits of 4-25 microg/L being obtained. However, the calibration for each anion was not linear over more than one order of magnitude. When using conductivity detection, the concentration of the eluant could be varied over a wider range (2.5-50 mM p-toluenesulfonate) than was the case with indirect UV detection (2.5-10 mM), thereby allowing greater changes in separation selectivity to be achieved. By varying the concentration of p-toluenesulfonate in the eluant, the separation selectivity could be manipulated from being predominantly ion-exchange in nature (2.5 mM) to predominantly electrophoretic in nature (50 mM).

Fast separation of pyrimidine derivatives by capillary electrochromatography on ion-exchange/reversed-phase mixed-mode stationary phases

This work describes the use of mixed-mode stationary phases which exhibit both strong ion-exchange (either cation-exchange, SCX, or anion-exchange, SAX) and reversed-phase chromatographic characteristics in capillary electrochromatographic separations of pyrimidine derivatives. Different packing materials, namely C6, SCX/C6 and SAX/C6, were compared and the influence of the composition of the carrier electrolyte (concentration of acetonitrile and pH) on the retention behavior of the selected solutes was investigated. A separation of all eight pyrimidine derivatives could be obtained on a 6.5 cm column packed with the SAX/C6 stationary phase in less than 3 min, with good peak shapes and efficiencies in the range 39,000 to 81,000 plates per meter.