pH Of Background Electrolyte Research Articles

PH programming in capillary electrophoresis by means of temperature programming

When using a background electrolyte with a buffer having strong temperature dependence of p K, different operating temperatures result in different operating pH values, using the same background electrolyte (BGE). It has been shown that this can be used to fine-tune selectivities of sample mixtures of weak analytes. Capillary electrophoresis (CE) equipment is designed to operate under isothermal conditions, but by proper programming, a very reproducible temperature and thus BGE pH program during the analysis can be realized. This was experimentally verified and illustrated by computer simulations. Equipment characteristics have been determined, and possibilities and restrictions to make use of this feature are presented.

The Quantitative Structure–Retention Relationship (QSRR) Analysis of Some Centrally Acting Antihypertensives and Diuretics

AbstractThe complete separation of 15 guanidine/imidazoline derivatives, acting as antihypertensive drugs, was achieved by capillary electrophoresis employing 30 mM phosphate background electrolyte (pH 1.5) containing 15 mM β‐Cyclodextrin (BCD). Here the Quantitative Structure–Retention Relationship (QSRR) models of the inclusion complexes between the analyzed compounds (ligands) and BCD were performed to investigate the correlations between electrophoresis migration order and the constitutional, geometrical, physico‐chemical, and electronical properties of the molecular models. The ChemPro, Marvin 4.0.5 ChemAxon, and CS Gaussian 98 [B3LYP/6–31G+(d,p) and HF/3–21G(d) basis sets] programs were applied for molecular parameters computation of the optimized ligands and ligand–BCD complexes. Total charge of the analyzed compounds at experimental pH 1.5, HOMO (BCD–ligand) energy, and Solvent‐Accessible Surface (SAS) (BCD–ligand) area account for the electrophoresis retention parameter log(t). The multiple linear regression models with three variables, log(t)=f [Total Charge (ligand), SAS (BCD–ligand), HOMO (BCD–ligand)], were obtained with R2=0.914 and crossvalidation parameter of prediction $\rm{ q_{{\rm{pre}}}^{\rm{2}} }$=0.778. The developed QSRR approach can help in understanding the structural features that contribute to the electrophoresis retention parameter [log(t)] of the investigated antihypertensives. Therefore, the theoretical method presented could be used as a fast, easy, and reliable tool for electrophoretic migration parameters prediction of other related antihypertensives.

Separation of nucleoside phosphoramidate diastereoisomers by high performance liquid chromatography and capillary electrophoresis

Separations of five diastereoisomers of nucleoside phosphoramidate derivatives (pronucleotides) were performed by both HPLC method using derivatized cellulose and amylose chiral stationary phases and CE method using anionic cyclodextrins added in the background electrolyte (BGE). An optimal baseline separation ( R s > 1.5) was readily obtained with all silica-based celluloses and amyloses using in a normal-phase methodology. Capillary electrophoresis was used as an alternative technique to HPLC for the separation of pronucleotides. The diastereoisomers were fully resolved with sulfated cyclodextrins at both BGE pH (2.5 and 6.2). Limits of detection and limits of quantification, calculated for both methods, are up to 200 times higher in CE separations than in HPLC separations. The analytical HPLC method was then applied in a preliminary study for the pronucleotide 1 quantification in cellular extract.

Capillary electrophoretic chiral separation of Cinchona alkaloids using a cyclodextrin selector

A new capillary electrophoretic method for the chiral separation of four major Cinchona alkaloids (quinine/quinidine and cinchonine/cinchonidine) was developed using heptakis-(2,6-di-O-methyl)-beta-cyclodextrin as the chiral selector. The inner walls of the separation capillary were modified with a thin polyacrylamide layer, which substantially reduced the electroosmotic flow and improved the chiral resolution and the reproducibility of the migration time of the analytes. Various operation parameters were optimised, including the pH, the capillary temperature, the concentration of the background electrolyte, and the concentration of the chiral selector. Baseline separation of the two diastereomer pairs was achieved in 12 minutes in ammonium acetate background electrolyte pH 5.0 with addition of cyclodextrin in a concentration of 3 mM or higher.

Micellar Electrokinetic Chromatography for the Quantitative Analysis of Flavonoids in the Radix of Astragalus membranaceus var. mongholicus

The separation and determination of eight flavonoids in the radix of Astragalus membranaceus (Fisch.) Bge var. mongholicus (Bge.) Hsiao was achieved by using micellar electrokinetic chromatography (MEKC) with a diode array detector (DAD). The influence of background electrolyte (BGE) concentrations and pH, surfactant concentrations, organic solvents, temperature, and voltage on the separate procedure was systematically investigated. The optimal resolution was obtained with a micellar phase consisting of 100 mM sodium cholate, 25 % (v/v) acetonitrile, 20 mM Na2B4O7, and 20 mM H3BO3 buffer at pH 9.2 by using a fused silica capillary at + 25 kV and 25 degrees C. A high reproducibility and good linearity ( R2 > 0.9978) were obtained over the concentration range tested. The relative standard deviations (R.S.D.s) from intra-day and inter-day precision for injection were less than 4.42 %. Six plant samples from different locations were then analyzed for the flavonoids by this newly developed MEKC method.

Comparison of On‐Line Concentration Methods in Capillary Zone Electrophoresis for Analysis of Water‐Soluble Vitamins

The separation of water‐soluble vitamins by capillary zone electrophoresis was developed, in which on‐line concentration methods, namely field‐enhanced sample stacking and dynamic pH junction, were utilized to improve the detection sensitivity. The effects of some critical parameters, including pH and concentration of background electrolyte, sample matrix pH and concentration, and injection volume were examined. The effects of field‐enhanced sample stacking and dynamic pH junction on the separation resolution and concentration efficiency were compared. The limits of detection of the vitamins were from 6 to 119 ng ml−1 (2.7×10−8 to 53.4×10−8 M) based on the signal‐to‐noise ratio of 3 and the relative standard deviations of migration time and peak area for each vitamin (1 µg ml−1) were less than 3.5% using the field‐enhanced sample stacking as an on‐line concentration method. The developed method was applied to the analysis of water‐soluble vitamins in corns.

Application of 1-alkyl-3-methylimidazolium-based ionic liquids as background electrolyte in capillary zone electrophoresis for the simultaneous determination of five anthraquinones in Rhubarb

A capillary zone electrophoresis method using only 1-alkyl-3-methylimidazolium-based ionic liquids as background electrolyte for the simultaneous determination of five anthraquinone derivatives including aloe-emodin, emodin, chrysophanol, physcion and rhein in Rhubarb species was described. Ion association constants, K ass, between anthraquinone anions and imidazolium cations were determined by analyzing the electrophoretic mobility change of anthraquinone anions using a non-linear least-squares method and factors contributing to ion associability were systematically clarified. For method optimization, several parameters such as ionic liquids concentration, background electrolyte pH and applied voltage, on the separation were evaluated and the optimum conditions were obtained as follows: 90 mM 1-butyl-3-methylimidazolium tetrafluoroborate (pH 11.0) with an applied voltage of 20 kV. Under these conditions, the method has been successfully applied to the determination of anthraquinones in extracts of two kinds of Rhubarb plants ( R. palmatum and R. hotaoense) within 12 min. The method proposed herein was shown to be much simpler than the previously reported methods.

Determination of doxycycline in pharmaceuticals and human urine by micellar electrokinetic capillary chromatography

Micellar electrokinetic capillary chromatography (MEKC) was performed at 25 degrees C and 30 kV (under a pressure of 15 mbar), using 30 mM borate buffer containing 60 mM sodium dodecysulfate (SDS) and 5% (v/v) methanol as background electrolyte (pH 9.0) to determine doxycycline. UV detection was at 350 nm. The method was shown to be specific, accurate (recovery was 100.3 +/- 1.0%), linear over the tested range (correlation coefficient 0.9995) and precise (RSD <1.9%). The method was used to determine doxycycline in tablets, capsules and human urine after oral application.

Direct sample injection for capillary electrophoretic determination of organic acids in cerebrospinal fluid

Organic acids in cerebrospinal fluid (CSF) are potential diagnostic markers for neurological diseases and metabolic disorders. A capillary electrophoretic (CE) method for the direct analysis, i.e., without any sample preparation, of six organic acids in CSF was developed. A capillary coating consisting of a triple layer of charged polymers (polybrene-dextran sulfate-polybrene) was used in combination with a negative separation voltage, providing fast and efficient analysis of acidic compounds. Separation conditions, such as background electrolyte (BGE) concentration and pH were optimized, and the influence of albumin and sodium chloride was systematically studied using a set of test compounds. With injection volumes of ca. 44 nL, plate numbers of up to ca. 150,000 were obtained with a BGE of 200 mM sodium phosphate (pH 6.0). It appeared that high sodium chloride concentrations in the sample hardly affected the peak width and shape of the organic acids, most probably due to transient isotachophoresis effects occurring in the sample zone. Adverse effects of CSF proteins, which frequently compromise the CE performance, could be effectively minimized by the triple layer coating in combination with rinses of 0.1 M hydrochloric acid. Overall, the developed CE system allowed direct injections of CSF samples, yielding good separation efficiencies and stable migration times (RSDs<2%) for organic acids. Validation of the method with artificial and real CSF samples showed good linear responses (r>0.99), and LODs for the organic acids were in the range of 2-8 microg/mL when applying UV detection. RSDs for migration times and peak areas were <2% and <7%, respectively. The applicability of the CE system is shown for the determination of organic acids in CSF samples.

Enantiodifferentiation of N-benzyloxycarbonylaminophosphonic and phosphinic acids and their esters using cyclodextrins by means of capillary electrophoresis

Capillary electrophoresis was successfully applied for separation of the enantiomers of N-benzyloxycarbonyl-α-aminophosphonic and α-aminophosphinic acids as well as their ethyl and phenyl monoesters with the use of a range of commercially available cyclodextrins (α, β and hydroxypropyl-γ-cyclodextrins) as chiral selectors. The dependence of effectiveness of separation on type and concentration of these chiral selectors as well as on pH of background electrolyte was examined in some detail.

Characterization of anacardic acids by micellar electrokinetic chromatography and mass spectrometry

A possibility of using capillary electrophoresis for separation of anacardic acids (6-alkylsalicylic acids) has been studied. Conventional micellar electrokinetic chromatography (MEKC) in non-coated fused silica capillaries and reversed-flow micellar electrokinetic chromatography (RF-MEKC) in capillaries coated with polydimethylacrylamide was applied for separation of anacardic acids extracted from cashew nuts. Influence of the composition of background electrolyte on the resolution of anacardic acid isomers was evaluated. Separations were performed using sodium dodecyl sulphate (SDS) micelles and mixed micelles of SDS and polyoxyethylene lauryl ether as a pseudostationary phase. To further improve the separation in RF-MEKC, β-cyclodextrin and a dual cyclodextrin system of β-cyclodextrin with heptakis-6-sulphato-β-cyclodextrin was added to the working electrolyte. Best separation of anacardic acids were achieved in the polydimethylacrylamide-coated capillary using 10 mM phosphate background electrolyte pH 6.5 with addition of 1 M urea, 20% acetonitrile, 10 mM of β-cyclodextrin and 1 mM of heptakis-6-sulfo-β-cyclodextrin. Mass spectrometry was used for the identification of anacardic acids in the extract from cashew nuts in single and tandem mode using Q-TOF instrument. Nine anacardic acids were identified in the extract form the cashew nuts.

Separation of Water‐Soluble Vitamins by Micellar Electrokinetic Capillary Chromatography in Pharmaceutical Samples

The separation of six water‐soluble vitamins (e.g., B1—thiamine, B2—riboflavin, B3—nicotinic acid, B6—pyridoxine, B12—cyanocobalamin, and C—ascorbic acid) was investigated using capillary electrophoresis. Initially, the most important variables in micellar electrokinetic capillary chromatography were established, such as: background electrolyte pH; surfactant concentration; detection wavelength; applied voltage, and amount of organic modifier. The solutes were separated within 8 min, with excellent efficiency (3 · 105 theoretical plates) and resolution, which means great advantage towards the analysis of real samples. Two pharmaceutical samples were analyzed using the optimized method, which was shown to be adequate for this purpose.

Paper electrophoretic determination of the stability constants of binary and ternary complexes of copper(II) and cobalt(II) with nitrilotriacetate and cysteine

A paper ionophoretic method is described for the study of equilibria in mixed ligand (nitrilotriacetate–cysteine) complex system in solution. The proportion of ionic species of nitrilotriacetate (NTA) and cysteine were varied by changing the pH of background electrolyte. The stability constants of Cu(II)–NTA–cysteine and Co(II)–NTA–cysteine complexes were found to be 6.35 ± 0.05 and 5.45 ± 0.02 (logarithm stability constant values), respectively, at ionic strength 0.1 M and a temperature of 35 °C.

Synthesis and application of mono-6-ammonium-6-deoxy-β-cyclodextrin chloride as chiral selector for capillary electrophoresis

A facile synthetic approach for mono-6-amino-6-deoxy-β-cyclodextrin (β-CD-NH 2) was proposed. Its hydroxy chloride salt, mono-6-ammonium-6-deoxy-β-cyclodextrin chloride (β-CD-NH 3Cl) was further prepared and used for the enantioseparation of various anionic and ampholytic analytes by capillary electrophoresis (CE). The effect of background electrolyte (BGE) pH and selector concentration on the enantioseparation was studied. Results showed that β-CD-NH 3Cl displayed powerful chiral resolution ability towards anionic analytes. In addition, baseline separation of a standard mixture consisting of eight acids was achieved within 35 min.

Enantiomeric purity determination of tamsulosin by capillary electrophoresis using cyclodextrins and a polyacrylamide-coated capillary

The chiral separation of racemic tamsulosin hydrochloride (TH) was carried out using cyclodextrin (CD)-mediated capillary electrophoresis (CE) with DAD at 200 nm. The best separation of enantiomers of the studied compound was achieved at 20 kV with 30 cm × 50 μm I.D. polyacrylamide (PAA)-coated fused-silica capillary (effective length 20 cm) and running buffer with sulfated-β-CD (S-β-CD) as chiral selector. Other selected native or derivatized CDs were also tested: β-CD (5, 15 mmol l −1), carboxymethyl-β-CD (5, 30 mmol l −1), dimethyl-β-CD (15 mmol l −1) and hydroxypropyl-β-CD (5, 30 mmol l −1). Several parameters such as capillary pretreatment, buffer type and concentration, pH of background electrolyte, methanol content, separation temperature and voltage, were optimized. The excellent baseline separation of chiral TH was successfully achieved within 12 min using 100 mmol l −1 phosphate buffer with pH 2.5 containing 1.7 mmol l −1 S-β-CD. Rectilinear calibration range was 50.0–500.0 μmol l −1 of each enantiomer ( r = 0.9993–0.9996). The method was applied to the assay of R-TH in Omnic, capsules (nominal content 0.4 mg per capsule) with R.S.D. 2.75% ( n = 6), recovery 99.3–101.7% and it was suitable for the chiral purity control of the active enantiomer in the pharmaceutical.

Effect of background electrolytes and pH on the adsorption of Cu(II)/EDTA onto TiO 2

Cu(II)/EDTA adsorption onto TiO 2 has been studied with a variation of pH, ionic strength, and type of background electrolytes. Cu(II) adsorption onto TiO 2 increased as ionic strength increased when NaClO 4 was used as a background electrolyte. This can be explained by the increase of exp ( − F Ψ / R T ) as a part of the electrostatic correction within a surface complexation model. Model predictions described experimental adsorption trends. Types of background anions (ClO 4, Cl, NO 2, NO 3, SO 3, and PO 4) did not affect adsorption trends and adsorption amounts of Cu(II) onto TiO 2. However, different trends were observed with various types of background ions used as ionic strength in EDTA and Cu(II)–EDTA adsorption. EDTA adsorption was decreased by using Na 2SO 3 and Na 3PO 4 as background ions, while NaClO 4, NaCl, NaNO 2, and NaNO 3 showed negligible interference on the EDTA adsorption, which matched well with model predictions. The presence Na 2SO 3 and Na 3PO 4 also interfered with Cu(II)–EDTA adsorption, to a somewhat greater extent compared to EDTA adsorption, especially at lower pH. This interference was also noted in Cu(II)–EDTA adsorption with a variation of Cu(II)–EDTA concentration at constant ionic strength ( 3 × 10 −3 M ) by using Na 2SO 3 and Na 3PO 4, especially at lower ratios of Cu(II)–EDTA to Na 2SO 3 and Na 3PO 4. These results suggest that the ratio of Cu(II)–EDTA to Na 2SO 3 and Na 3PO 4 is an important factor for the controlling of competition between these background ions and Cu(II)–EDTA onto TiO 2. Model prediction generally matched well with experimental adsorption using NaClO 4, NaCl, NaNO 2, and NaNO 3 as backgrounds ions, while a severe deviation was observed in the presence of Na 2SO 3 and Na 3PO 4. These results suggest that the mobility of copper ions as Cu(II)–EDTA can be increased from polluted area in the presence of multivalent background ions, especially as the ratio of adsorbates/background ions decreased.

Enantioseparation of acidic enantiomers in capillary electrophoresis using a novel single-isomer of positively charged β-cyclodextrin: Mono-6 A- N- pentylammonium-6 A-deoxy-β-cyclodextrin chloride

The new single-isomer of positively charged β-cyclodextrin, mono-6 A- N-pentylammonium-6 A-deoxy-β-cyclodextrin chloride (PeAM-β-CD), was employed for the first time for the enantioseparation of anionic and ampholytic analytes by capillary electrophoresis (CE). The synthesis and characterization of PeAM-β-CD were reported. The effect of background electrolyte (BGE) pH and selector concentration on the enantioseparation was investigated. Good separation was obtained at low BGE pH (ca. 5.0–6.0). The effective mobilities of all analytes were found to decrease with increasing CD concentration. PeAM-β-CD proved to be an effective chiral selector for most studied anionic analytes.

Ground water treatment by enhanced ultrafiltration

The efficiency of heavy metal removal from simulated ground water containing humic substances has been investigated by means of enhanced ultrafiltration (UF) with chitosan. The rejection coefficient of Cu (II), Pb (II), Ni (II) and Co (II) in the presence of humic, fulvic acids and polysaccharide biopolymer chitosan was studied for singlemetal and multi-component systems. The different experimental parameters such as time contact, initial concentration of metal ions, background electrolyte concentration, pH solutions, and diverse cellulose acetate UF membranes of different molecular weight cut-off on the extraction kinetic were investigated. Humic substances rejection increased with pH due to conformational transformation of ionizable molecules. The interactions of humic substances with multi-charged metals do not practically affect the retention of organic matter. Involvement of heavy metal ions into complexation with chitosan and humic substances provides effective removal of trace metals from solutions as a result of complex retention. In conformity with this principle, the degree of heavy metal rejection improved with an increase in the ratio between chitosan or humic substances and metal due to an increase in completeness of metal binding. This ratio is also responsible for the phase-dispersion state of the solution initiating a transition of intra-molecular complexes to inter-molecular ones at higher concentrations of metal ions with respect to a higher molecular weight of complex formation substances. This leads to an aggregation of humic acids-metal complexes and a decrease in the transmembrane flux. Fulvic acids do not tend to form intermolecular aggregates, and chitosan-metal complexes allow metal ions to be rejected without any substantial reduction of membrane productivity.

Separation and determination of lignans from seeds of Schisandra species by micellar electrokinetic capillary chromatography using ionic liquid as modifier

In this paper, a micellar electrokinetic chromatographic (MEKC) method using ionic liquid as modifier for the quantification of the active components of lignans found in the medicinal herbs Schisandra species was developed for the first time. Preliminary investigations employing sodium dodecyl sulfate (SDS) as surfactant did not lead to the necessary resolution of the studied compounds, the addition of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF 4) to the SDS micellar system resulted in the complete separation of all the compounds. The effects on the separation by several parameters such as BMIM-BF 4 and SDS concentration, applied voltage, background electrolyte pH and concentration, were evaluated. Under the optimal conditions (5 mM borate-5 mM phosphate buffer in the presence of 20 mM SDS and 10 mM BMIM-BF 4, pH 9.2, applied voltage 25 kV and detection at 254 nm), the method successfully applied to the determination of lignans in extracts of Schisandra chinensis (Turcz.) Baill. and Schisandra henryi C.B. Clarke in less than 13 min. The separation mechanism was also discussed.

Chiral capillary electrophoresis‐mass spectrometry of amino acids in foods

In this work, the development of a new chiral capillary electrophoresis-mass spectrometry (CE-MS) method to separate D- and L-amino acids is shown. On-line coupling between CE and MS is established through an electrospray-coaxial sheath flow interface. Enantiomer separation is achieved by using a cheap, nonvolatile, chiral selector as beta-cyclodextrin in the background electrolyte (BGE) together with a physically coated capillary that is aimed to prevent contamination of the electrospray. The capillary coating is simple and easy to obtain as it only requires flushing of the capillary with a polymer aqueous solution for 3 min. Optimization of CE parameters (pH of BGE, type and concentration of chiral selector, and capillary inner diameter) and electrospray-MS parameters (nature and flow rate of the sheath liquid, nebulizer pressure) is carried out. Two different derivatization protocols of amino acids using dansyl chloride (DNS) and fluorescein isothiocyanate (FITC) are compared in terms of MS sensitivity and chiral resolution. Under optimum CE-MS conditions it is observed that the MS sensitivity obtained for FITC- and DNS-amino acids is similar (with limit of detection (LOD) in the microM range, corresponding to amounts injected in the fmol range) while chiral resolution is better for FITC-amino acids. The optimized method is demonstrated to provide the simultaneous analysis of 15 selected amino acids (i.e., FITC-D/L-Asp, -Glu, -Ser, -Asn, -Ala, -Pro, -Arg, and FITC-gamma-aminobutyric acid (GABA) in a single chiral CE-MS run, corresponding to the main amino acids that can be found in orange. Moreover, as a result of the high resolution achieved, it is possible to detect down to 2% of D-Asp in the presence of 98% of L-Asp. The good possibilities of chiral CE-MS in food analysis are corroborated through the detection of the main amino acids in a commercial orange juice (i.e., FITC-L-Asp, -Glu, -Ser, -Asn, -Pro, -Arg, and the nonchiral FITC-GABA) as well as the determination of the fraudulent addition of synthetic amino acids (containing D- and L-forms) to a fresh orange juice.