Separation of the positional isomers of phthalic acids in hydroorganic solvents using capillary electrophoresis

The positional isomers of phthalic acids (ortho-, meta-, and para-) and benzoic acid could be completely separated by capillary electrophoresis (CE). A simple CE method employing direct detection in mixed methanol/water buffers is presented. The effect of the electrolyte buffer system, including pH, buffer concentration, and organic solvent on the electrophoretic mobility of the analytes, is investigated. The electroosmotic flow is reversed using cationic surfactant and cetyltrimethylammonium bromide so that anions are separated under the co-EOF mode. The resolution of the analytes and selectivity could be improved by the adjustment of the methanol content. Ion association with the surfactant in methanol/water buffer is discussed. The validity of the method in terms of sensitivity, reproducibility, and linearity is also reported. The text was submitted by the authors in English.     

Determination of Phenols,Inorganic Anions,and Carboxylic Acids in Kraft Black Liquors by Capillary Electrophoresis

Two methods are presented for the quantitative capillary electrophoretic (CE) determination of phenolic lignin degradation compounds as well as of inorganic anions and organic acids in Kraft black liquors. Important phenolic lignin degradation compounds can be rapidly separated by co-electroosmotic CE after acidification of the liquors and subsequent extraction of the compounds with chloroform. A capillary electrophoretic separation of phenolic compounds is performed by using a phosphate/borate electrolyte system and UV detection at 214 nm. In addition, a HPLC method using a gradient with water, methanol, and acetic acid is also developed. Inorganic ions which are of importance to the pulping process can be determined by simply diluting the black liquors after sampling and subsequent analysis with a chromate electrolyte system and indirect UV detection at 185 nm. In addition, the concentration of low molecular aliphatic carboxylic acids can be determined simultaneously within the same run. By method optimization it is possible to separate the anions within one minute and, at the same time, to increase the resolution of the solutes. The electrolyte systems for the CE separations were optimized by varying the pH value and by adding organic solvents. Short separation times are obtained by adding a polycationic EOF modifier (hexadimethrine bromide) to the electrolyte which reverses the electroosmotic flow. A migration of the anionic analytes in the same direction as the electroosmotic flow is thus established.     

Use of 1-alkyl-3-methylimidazolium-based ionic liquids as background electrolytes in capillary electrophoresis for the analysis of inorganic anions

Separation of inorganic anions in CE is often a challenging task because the electrophoretic mobilities of inorganic anions are comparable to or even greater than the EOF mobility. In this study, we present the use of ionic liquids (ILs) as background electrolytes (BGEs) in CE of inorganic anions. The 1-alkyl-3-methylimidazolium-based ILs as BGEs dynamically coated the capillary wall and induced a reversed EOF. This allowed the anions to comigrate with the EOF and yielded a rapid separation. Increasing the alkyl chain length of the ILs and BGE concentration can significantly improve the separation resolution. With 40 mM 1-butyl-3-methylimidazolium tetrafluoroborate as BGE, good separations of five model anions (Br-, I-, NO2(-), NO3(-), and SCN-) were achieved in a range of buffer pH values. The separation efficiency was as high as 34 600-155 000, and the RSDs of the migration times were less than 0.8% (n = 5).     

Light-emitting diode-compatible probes for indirect detection of anions in CE

A range of compounds were evaluated as probes for the indirect detection of inorganic ions using CE and light-emitting diodes (LEDs) as the light source. Emphasis was placed on examining probes likely to absorb strongly in the UV-Vis region near 350-430 nm as compounds, which absorb at longer wavelengths tend to be bulkier and adsorb onto the capillary wall. These probes should act as a replacement for the very effective but carcinogenic probe chromate. Two probes were identified and evaluated: p-nitrophenol and 4-hydroxy-3,5-dinitrobenzoic acid. The former showed the most potential with low-mobility anions, while the later had a moderate electrophoretic mobility and was more suitable for a wider mobility range of analytes. However, neither could match the efficiencies and LOD of chromate for the separation of the fast inorganic ions such as chloride, nitrate and sulphate. Nevertheless, application of the 4-hydroxy-3,5-dinitrobenzoic acid system to the determination of oxalate in Bayer liquors showed excellent sensitivity and selectivity.     

Electroosmotic flow reversal for the determination of inorganic anions by capillary electrophoresis with methanol-water buffers

Manipulation of the electroosmotic flow (EOF) is essential for achieving optimized separations of small anions by capillary electrophoresis (CE). In this work, efficient suppression or reversal of EOF is achieved upon addition of small amounts of the cationic surfactants, cetyltrimethylammonium bromide (CTAB) or didodecyldimethylammonium bromide (DDAB) to the electrophoretic buffer. Highly stable and reversed EOF are achieved using the surfactants in the presence of up to 50% MeOH. In aqueous and low methanol containing solutions (up to 30%, v/v) surface aggregation of the surfactants at the capillary wall occurs at a concentration below the critical micelle concentration (CMC). The impact of MeOH on reversed EOF is predominantly a function of the diminished zeta potential of the silica, and to a lesser extent on the CMC in the bulk solution of the surfactant. Fast baseline separation and selectivity changes for small inorganic anions are observed when mixed aqueous-organic buffers are employed. Changes in EOF, micellar properties of the surfactant and selectivity for inorganic anions upon addition of various percent of methanol are also discussed.     

Simultaneous separation of 17 anions by capillary electrophoresis with the addition of an organic solvent

Seventeen inorganic and organic anions, that normally are insufficiently separated via ion chromatography, were completely separated by the addition of an organic solvent to a solution of BGE combined with an adjustment of the apparent pH via CE in combination with indirect UV absorbance detection. Methanol, ethanol, and acetonitrile were examined for their utility in manipulating the selective separation of anions. Methanol and acetonitrile were better modifiers than ethanol at enhancing the resolution of anions comigrating in an aqueous solution of BGE. Methanol was selected as the modifier that provided the largest separation window that could achieve a complete separation of the target analytes. Via the use of methanol, manipulation of the selectivity between inorganic anions and that between inorganic and organic anions was enhanced, but the separation between organic anions remained difficult when only methanol was used. By varying the apparent pH of the BGE in the presence of 10% v/v methanol, however, the separation selectivity between organic anions was substantially improved. Eventually, 7 inorganic and 10 organic anions were simultaneously separated using BGE at a pH of 6.3 in the presence of 10% v/v methanol.     

Nonionic surfactant enhanced semipermanent coatings for capillary electrophoresis of inorganic anions without use of organic additives

Separation of inorganic anions by capillary electrophoresis (CE) is usually conducted in co-electroosmotic mode due to the large electrophoretic mobilities of inorganic anions. Semipermanent surfactant coatings have been shown to be effective for CE of inorganic anions due to their strong capability of electroosmotic flow (EOF) manipulation. However, semipermanent coatings often suffer from their unsatisfactory stability. In addition, organic solvent additives are usually required to adjust the selectivity, which also aggravate the degradation of coating. In this work, a novel semipermanent coating consisting of cationic Gemini surfactant 18-10-18 and nonionic surfactant Tween 20 was developed to separate inorganic anions in CE. This coating is easy to prepare and more stable than pure Gemini coating. The introduction of nonionic surfactant in the coating not only suppresses the reversed EOF but can also adjust the selectivity of separation. Good separations of six model anions were achieved, the separation efficiency was as high as 65040-169700 plates/m and the RSDs of the migration times were less than 0.5 and 2.5% for run-to-run and day-to-day assays, respectively. Calibration curves were linear in the range of 0.05-5.0 mM; the detection limits ranged from 20 to 50 μM. More importantly, no organic solvents are required in the background buffer to achieve the satisfactory separations. This guarantees the coating stability and makes the method greener than most of other methods for CE of inorganic anions.     

Parameters influencing separation and detection of anions by capillary electrophoresis

Electrolyte composition is critical in optimizing separation and detection of ions by capillary electrophoresis. The parameters which must be considered when designing an electrolyte system for capillary electrophoresis include electrophoretic mobility of electrolyte constituents and analytes, detection mode, and compatibility of electrolyte constituents with one another. An electrolyte system based on pyromellitic acid is well suited for use with indirect photometric detection, and provides excellent separations of anions. The ability to modify the electrophoretic mobility of pyromellitic acid as a function of ph provides flexibility in matching electrophoretic mobilities of analytes. Additionally, the use of alkyl amines as electroosmotic flow modifiers allows the rapid separation of anions by reversing the direction of electroosmotic flow in a fused-silica capillary. The optimization of a capillary electrophoresis electrolyte for anion analysis is also discussed in terms of pH, ionic strength and applied voltage. The effect of organic solvent on separation selectivity is also discussed.     

Indirect photomeric detection of anions in capillary electrophoresis using dyes as probes and electrolytes buffered with an isoelectric ampholyte

The use of highly absorbing anionic dyes as probes and isoelectric ampholytes as buffers in background electrolytes (BGEs) combined with the use of a light emitting diode (LED) as a light source has been studied for ultrasensitive indirect photometric detection in capillary electrophoresis (CE). Potential dyes and buffers were evaluated based on characteristics relevant to indirect photometric detection principles, such as the electrophoretic mobility of the probe dye, its solubility and adsorption behaviour, and the isoelectric point and buffering capacity of the ampholytic buffer. Two dyes, tartrazine and naphthol yellow S, and histidine as the ampholytic buffer, were selected for detailed investigation. Purification of the probes was vital to avoid anionic impurities interfering with the detection. For the electrolytes containing a purified probe (0.5 mM) and histidine as the isoelectric buffer (p/ 7.7), hydroxypropylmethyl cellulose (approximately 0.05%) was effective in suppression of the electroosmotic flow (EOF). Analytical method performance characteristics were determined. For both probes, experimentally determined mobilities were generally close to literature values, excellent peak shapes and separation efficiencies of up to 298 000 theoretical plates were obtained, and detection limits were generally at the sub-microM level. For the naphthol yellow S-histidine BGE, linearity and reproducibility were also evaluated, with excellent linearity being observed over a range of 5-500 microM, and reproducibility (relative standard deviation, RSD) less than 1% for migration times and 2-8% for normalised peak areas. The approach developed was applied successfully to several real samples including tap water, mineral waters, and beer.     

Polymeric ionic liquid as a background electrolyte modifier enhancing the separation of inorganic anions by capillary electrophoresis

A novel and easy method for the separation of inorganic anions by capillary electrophoresis using a polymeric ionic liquid (PIL), poly(1-vinyl-3-butylimidazolium bromide) as a background electrolyte modifier has been developed. The PIL has been proved to generate a reversed electroosmotic flow which reduces the analysis time and improves the separation significantly. Effects of the PIL concentration and buffer composition (pH and concentration) were evaluated on basis of the resolution and efficiency of the sample. Under optimum conditions, good separation of six model inorganic anions was achieved with high efficiency and excellent reproducibility within 3 min. The results obtained indicate that the combination of reversed EOF and the association between the analytes and the PIL on the capillary wall or BGE play a prominent role in the separation of anions. Therefore, the PIL presents a useful alternative for the BGE modifier in the study of inorganic anions by CE.     

Understanding mechanisms of pressure-assisted electrokinetic injection: application to analysis of bromate, arsenic and selenium species in drinking water by capillary electrophoresis-mass spectrometry

The mechanism underlying the enrichment power by pressure-assisted electrokinetic injection (PAEKI) in capillary electrophoresis (CE) was investigated for on-line pre-concentration of arsenic [As(III) and As(V)], selenium [Se(IV) and Se(VI)] and bromate (BrO(3)(-)). Analyte diffusion behaviour from PAEKI sample plugs were evaluated by monitoring peak broadening as a function of stagnant time and position in the capillary. During PAEKI, anionic analytes accumulate at the sample-separation buffer boundary. We proposed that a counter-ion layer formed in PAEKI, where a cation layer was formed at the separation buffer side of boundary. The cation layer served as a soft boundary which impeded zone broadening via electrostatic attraction between layers. This effect likely played an important role in maintaining focused analyte bands by suppressing diffusion. Comparison of analyte behaviour in PAEKI injected sample plugs to behaviour in hydrodynamically injected ones proved the existence of a counter-ion layer. The dependence of analyte diffusion in PAEKI plugs on electrochemical properties (viscosity, conductivity, electrophoretic mobility) further supported the hypothesis. Additionally, it was noted that analytes with low electrophoretic mobility were more efficiently pre-concentrated by PAEKI and were less subject to forces of dispersion than analytes with greater electrophoretic mobility. PAEKI-CE coupled to electrospray tandem mass spectroscopy (ESI-MS/MS) was then optimized and validated for detection of arsenic, selenium and bromate in water samples. On-line enrichment of the target analytes was achieved with 1-3 ng mL(-1) detection limits, which was below the maximum contaminant levels in drinking water for all five anions studied. Noteworthy, the potential of the method for unbiased detection of molecular species in untreated water was demonstrated. No contamination was detected in the water samples tested; however, recovery was 90-118% for spiked samples. The method was demonstrated be comparable to current methods for detection of inorganic contaminants in drinking water and is a good alternative method to ion chromatography/liquid chromatography-MS.     

A chip-based capillary electrophoresis-contactless conductivity microsystem for fast measurements of low-explosive ionic components

A miniaturized analytical system for separating and detecting inorganic explosive residues, based on the coupling of a micromachined capillary electrophoresis (CE) chip with a contactless conductivity detector is described. The low electroosmotic flow (EOF) of the poly(methylmethacrylate) (PMMA) chip material facilitates the rapid switching between analyses of cations and anions using the same microchannel and run buffer (and without an EOF modifier), and hence offers rapid (< 1 min) measurement of seven explosive-related cations and anions. Experimental parameters relevant to the separation and detection processes have been optimized. Addition of a 18-crown-6 ether modifier has been used for separating the peaks of co-migrating potassium and ammonium ions. The ionic-explosive microchip system combines the distinct advantages of contactless conductivity detection with the attractive features of plastic CE microchips. The new microsystem offers great promise for monitoring explosive-related ions at the sample source, with significant advantages of speed/warning, efficiency, cost, or sample size.     

Enhanced sensitivity and resolution for the analysis of paralytic shellfish poisoning toxins in water using capillary electrophoresis with amperometric detection and field‐amplified sample injection

The profiling of the most lethal paralytic shellfish poisoning toxins (PSTs) in freshwater has increased the need to establish an alternative analytical method with high sensitivity and resolution. In this paper, a coupling technique of field‐amplified sample injection (FASI) and CE with end‐column amperometric detection (CE‐AD) was developed to improve the detection sensitivity and separation of PSTs by electrokinetically injecting a water plug of analytes to the capillary filled with a high‐conductivity BGE. Parameters affecting FASI and CE process were carefully adjusted to achieve the highest response and resolution. Separation selectivity for PSTs, especially for the analogues and epimers, was greatly enhanced by using 40 mM Britton–Robinson buffer (pH 9.5) as BGE, which altered the EOF and mobility of the analytes that interacted with polyborate ions. Satisfactory linear relationship between peak current and concentration of toxins were gained over a wide range of 1.95–254 μg/L. The detection limits (S/N = 3) for five PSTs ranged from 0.63 to 3.11 μg/L, which are below the health alert level in drinking water. In comparison with the up‐to‐date reporting chromatographic methods, the FASI‐CE‐AD method was simple, low‐cost, selective, and sensitive enough for direct quantification of PSTs at very low levels, implying a potential for screening and monitoring of PSTs in surface waters.     

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

Capillary electrophoretic separations of inorganic anions are performed using a capillary coated with a mixture of the cationic surfactant didodecyldimethylammonium bromide (DDAB) and the zwitterionic surfactant 1,2-dilauroyl-sn-phosphatidylcholine (DLPC). These double-chained surfactants form semi-permanent coatings on the capillary wall, which allows the excess surfactant to be removed from the buffer prior to separation. Interactions between surfactant aggregates in the buffer and analyte anions are thus eliminated. The electroosmotic flow (EOF) can be altered from fully reversed (100% DDAB) to near zero (100% DLPC) using different ratios of DDAB and DLPC. Controlling the EOF allows for improved resolution of the anions while maintaining a rapid, co-EOF separation, free from analyte-surfactant additive interactions.     

Sensitive determination of inorganic anions at trace levels in samples of snow water from sierra nevada (Granada, Spain) by capillary ion electrophoresis using calix[4]arene as selective modifier

Summary A highly sensitive and selective capillary ion electrophoretic (CIE) method has been established to show the applicability of this separation technique to the determination of some of the anions usually found in water samples. The ions were monitored by UV spectrophotometry with diode-array detection in presence of sulfonated calixarene compounds as organic modifiers in the electrophoretic buffer medium. The instrumental variables affecting the sensitivity and resolution of the mixture were carefully optimized. Snow water samples were been selected for analytical determination of iodide, chloride, sulfate, nitrite, phosphate, and nitrate anions at ng mL⁻¹ levels. The repeatability and reproducibility were calculated and were in the range 2.1 to 5.4% for the analytes determined.     

Full-capillary sample stacking/sweeping-MEKC for the separation of naphthalene-2,3-dicarboxaldehyde-derivatized tryptophan and isoleucine

In an attempt to improve the sensitivity of detection in capillary electrophoresis (CE), a novel online sample-concentration method, full-capillary sample stacking (FCSS)/sweeping-micellar electrokinetic chromatography (sweeping-MEKC) mode, is proposed. Naphthalene-2,3-dicarboxaldehyde (NDA)-derivatized tryptophan and isoleucine were selected as model compounds. In the initial step, the weakly acidic compounds, dissolved in a low-conductivity buffer (35.1 microS/cm; apparent ph (pH*) in a mixed solution of acetonitrile/methanol/water, 4.6), fill the entire capillary, two vials of a high-conductivity buffer (2.06 mS/cm; pH* 2.0) are placed on each end, and a negative polarity is then applied. Under these conditions, the direction of the electroosmotic flow (EOF) is toward the inlet. Meanwhile, the anionic analytes move in the reverse direction and are neutralized and stacked at the boundary of a dynamic pH-junction (between the sample matrix and the nonmicellar background solution (BGS)). When the sample concentration is completed, the BGS is quickly changed to solutions containing SDS-BGS for the subsequent separation. Since the mobility of SDS-analytes is then greater than the EOF, the following steps occur by the sweeping (for focusing) and MEKC (for separation) mode. Using these steps, a full-capillary sample injection/separation can be achieved.     

Application of self-organizing maps for the classification of chromatographic systems and prediction of values of chromatographic quantities

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.     

Applications of a sulfonated-polymer wall-modified open-tubular fused-silica capillary in capillary zone electrophoretic separations

A fused-silica capillary that is wall-modified via chemically bonding a sulfonated polymer to the capillary wall has a uniform negative charge density on its surface and produces an electroosmotic flow (EOF) greater than 4 x 10(-4) cm2 V(-1) s(-1) The EOF is nearly independent of buffer pH over the pH range of 2 to 10 and is lower than the EOF obtained for the bare fused-silica capillary at the more basic pH but is higher at the more acidic buffer pH. Optimization of buffer pH can be based on analyte pKa values to improve the overall quality of the capillary zone electrophoresis (CZE) separation of complex mixtures of weak acid and base analytes. Because of the high EOF in an acidic buffer, the capillary is useful for the separation of weak organic bases which are in their cation forms in the acidic buffer. EOF for the sulfonic acid bonded phase capillary can be adjusted via buffer additives such as organic solvent, tetraalkylammonium salts, multivalent cations and alkylsulfonic acids. The advantages of utilizing buffer pH and the EOF buffer modifiers to enhance migration time, selectivity, and resolution in CZE separations with this capillary are illustrated using a series of test analyte mixtures of inorganic anions, carboxylic acids, alkylsulfonic acids, benzenesulfonic acids, sulfas, pyridines, anilines or small-chain peptides.     

Separation of anions by ion chromatography-capillary electrophoresis

Capillary electrophoresis (CE) with a water-soluble ion-exchange polymer in the background electrolyte is very efficient for the separation of organic and inorganic anions because the ion-exchange selectivity, as well as differences in electrophoretic mobility, can be used for separating sample ions. Poly(diallyldimethylammonium chloride) (PDDAC) was employed for this purpose. A very stable electroosmotic flow was obtained between pH 2.3 and 8.5 due to the strong adsorption of PDDAC onto the capillary wall. The effect of ion exchange on the migration of sample anions and their separation was controlled by varying the concentration of PDDAC, the concentration and the type of salt used in the CE background electrolyte. Addition of organic solvent (e.g., acetonitrile) could also modify the sample migration and the separation. Baseline separations were obtained for anions with very similar mobilities, such as bromide and iodide, naphthalenesulfonates, and bi- and tricarboxylic acids. Typical separation efficiencies were between 195,000 and 429,000 theoretical plates per meter. Ten replicate separations gave an average RSD of 1.0% for migration times of the sample anions studied. Excellent separations were obtained for a variety of samples, including a separation of 17 inorganic and organic anions in less than 6 min.     

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.