Amphiphilic silica nanoparticles as pseudostationary phase for capillary electrophoresis separation

Amphiphilic silica nanoparticles surface-functionalized by 3-aminopropyltriethoxysilane (APTES) and octyltriethoxylsilane (OTES) were successfully prepared and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectrometry (FT-IR) and thermogravimetry (TG) techniques. The potential use of these bifunctionalized nanoparticles as pseudostationary phases (PSPs) in capillary electrophoresis (CE) for the separation of charged and neutral compounds was evaluated in terms of their suitability. As expected, fast separation of representative aromatic acids was fulfilled with high separation efficiency, because they migrate in the same direction with the electroosmotic flow (EOF) under optimum experimental conditions. Using a buffer solution of 30mmol/L phosphate (pH 3.0) in the presence of 0.5mg/mL of the synthesized bifunctionalized nanoparticles, the investigated basic compounds were baseline-resolved with symmetrical peaks. Due to the existence of amino groups on the surface of nanoparticles, "silanol effect" that occurs between positively charged basic analytes and the silanols on the inner surface of capillary was greatly suppressed. Furthermore, the separation systems also exhibited reversed-phase (RP) behavior when neutral analytes were tested.     

Vesicles as pseudostationary phase for enantiomer separation by capillary electrophoresis

The suitability of noncovalently bilayer-coated capillaries for the analysis of proteins by capillary electrophoresis (CE) at medium pH was investigated. Fused-silica capillaries were coated simply by successively flushing with a polybrene (PB) and a poly(vinyl sulfonate) (PVS) solution. A protein test mixture was used to evaluate the performance of the coated capillaries. Comparisons with bare fused-silica capillaries were made. Several background electrolytes (BGEs) were tested in combination with the PB-PVS coating, showing that optimum performance was obtained for the proteins using high BGE concentrations. With a 300 mM Tris phosphate buffer (pH 7.0), good plate numbers (150,000-300,000), symmetrical peaks, and favorable migration-time repeatabilities (RSDs below 0.8%) were obtained for the proteins. Using bare fused-silica capillaries, the protein peaks were significantly broadened and the migration-time RSDs often exceeded 5%. It is concluded that the PB-PVS coating effectively minimizes adverse protein adsorption and provides a very stable electroosmotic flow (EOF). We also investigated the potential of a commercially available bilayer coating (CEofix) for protein analysis. It is demonstrated that with this coating, good plate numbers and peak symmetries for proteins can be achieved when the CEofix BGE ("accelerator") is replaced by a common BGE such as sodium or Tris phosphate. Apparently, the negatively charged polymer present in the "accelerator" interacts with the proteins causing band broadening. The utility of the bilayer coatings is further illustrated by the separation of proteins such as interferon-alpha 2b, myoglobin and carbonic anhydrase, by the analysis of a degraded insulin sample in time, and by the profiling of the glycoprotein ovalbumin. In addition, it is demonstrated that even in the presence of concentrations of human serum albumin in the sample of up to 60 mg/mL, the PB-PVS coating still provides reproducible protein separations of good performance.     

Silica nanoparticles as pseudostationary phase for protein separation

This paper investigated the potential use of silica nanoparticles (SNPs) as pseudostationary phase (PSP) for protein separation. The wall adsorption of SNPs as well as the influences of SNPs and poly(ethylene oxide) (PEO) were studied. The SNPs showed selectivity toward the proteins, and the concentration ratio of SNPs to PEO influenced the separation. The proteins that could not be baseline-resolved under conventional CZE mode were separated in a buffer consisting of 30 mM phosphoric acid, 0.05% PEO, and 0.05% SNPs at pH 2.37, with detection limits ranging from 2 to 45.5 ppm. The intraday and interday RSDs of the migration times were in the ranges of 2.1-2.8% (n = 5) and 2.5-3.4% (three days, n = 3 x 5 = 15), respectively.     

Separation of metal ions via capillary electrophoresis using a pseudostationary phase microfunctionalized with carbon nanotubes

Microchimica Acta - This study reports on the use of dispersed multi-walled carbon nanotubes (MWCNTs) as a novel pseudostationary phase in capillary electrophoresis for the separation and...     

Separation of nonionic compounds by electrokinetic chromatography using an inorganic layered compound as a pseudostationary phase

The use of an inorganic layered compound as a pseudostationary phase (PSP) in EKC was investigated. A synthetic smectite, which is the most typical swellable clay mineral, with an average diameter of 130 nm was selected as the PSP. The retention characteristics of the smectite and on-line sample concentration by sweeping were examined for the analysis of polyoxyethylene mono phenyl ethers (PPEs) with different degrees of ethoxylation. The retention factor was increased with increase in the number of ethylene oxide groups and a good separation of the PPE homologs was achieved by smectite-EKC. The RSD of the migration time, plate number, and peak area were 0.60, 8.3, and 2.7% (n = 5), respectively. The developed method can be applied to the analysis of PPEs in commercially available consumer products without any sample pretreatments. In addition, ca. 100-fold sensitivity enhancements for the PPEs with high retention factors were obtained by sweeping.     

Separation of amino acids in plant tissue extracts by capillary zone electrophoresis with indirect UV detection using aromatic carboxylates as background electrolytes

Summary Amino acids in extracts of plant tissue were separated and detected by capillary zone electrophoresis (CZE) with indirect UV detection. Various aromatic carboxylates such as salicylate, benzoate, phthalate and trimellitate were investigated as background electrolytes (BGEs). A BGE of benzoate gave the best resolution and detector response. Amino acids were separated at a highly alkaline pH to charge amino acids negatively. Separation was achieved by the co-electroosmotic flow (Co-EOF) by the addition of the cationic surfactant myristyltrimethyl-ammonium bromide (MTAB) to the electrolyte. The condtions affecting the separation of amino acids, including electrolyte pH, concentrations of both benzoate and MTAB, were investigated and optimised. Separation of a mixture of 17 amino acids at pH 11.20 with indirect UV detection at 225 nm was achieved with a BGE of 10 mM benzoate containing 1.0 mM MTAB at pH of 11.20. Detection limits ranged between 10 and 50 μM. The proposed method was demonstrated by the determination of amino acids in extracts of Eucalypt leaves with direct injection of samples.     

Diamino moiety functionalized silica nanoparticles as pseudostationary phase in capillary electrochromatography separation of plant auxins

A novel and simple method for the preparation of silica nanoparticles having surface-functionalized diamino moiety (dASNPs) was reported in our paper and characterized using scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectrometry, and thermogravimetry techniques. To test this method practically, in this contribution we describe the enhanced separation of four plant auxins - indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), 2,4-dichlorophenoxyacetic acid (dCPAA), and 2-(1-naphthyl) acetic acid (NAA) - by capillary electrochromatography using diamino moiety functionalized silica nanoparticles as pseudostationary phase (PSP) in the running buffer. The effect of pH, buffer concentration, and diamino moiety functionalized silica nanoparticles concentration on the selectivity of separation was investigated. A combination of the nanoparticles and running buffer reversed the electroosmotic direction making possible the rapid and efficient separation of the auxins from the auxins migrated in the same direction with the EOF under optimum experimental conditions. A good resolution of four auxins was obtained within 5.5 min under optimum experimental conditions. The precision (RSD, n = 5) was in the range of 0.72-0.91% and 1.89-2.23% for migration time and peak area response, respectively. The detection limits were 0.48, 0.44, 0.46, and 0.42 μM for NAA, IBA, IAA, and dCPAA, respectively. Furthermore, the method was successfully tested for the determination of IAA in the grapes.     

Use of basic amphiprotic organic solvents containing neutral-surfactant aggregates as pseudostationary phase in non-aqueous capillary electrophoresis

A new approach to non-aqueous capillary electrophoresis based on the addition of anionic carboxylic surfactants to the basic amphiprotic organic solvent in which form neutral-surfactant aggregates was developed with a view to improving the electrophoretic resolution of charged substances. These aggregates acts as a new pseudostationary phase. The presence of these aggregates allows the effective separation of four tetracyclines with increased selectivity. The efficiency of sodium caprylate, sodium laurate and sodium palmitate as surfactants was examined. The latter two proved more effective than the former as they provided migration times reproducible to within 7% or better in all cases. The additional use of an alcohol allows peak shape to be controlled, which expands the potential of this electrophoretic technique even further. The proposed method was used to determine tetracyclines in water samples. The sensitivity of the determination was improved by using a flow manifold coupled at-line to the capillary electrophoresis system in order to preconcentrate the analytes. The limits of detection thus achieved ranged from 50 to 90 μg/l. Under optimal operating conditions, recoveries ranged from 97 to 104%, and precision from 5.4 to 7.0%.     

Characterization of dendrimer properties by capillary electrophoresis and their use as pseudostationary phases

The general properties of dendrimers and in particular their electrolytic characteristics that are relevant in electrokinetic separations, are described. In order to confirm theoretical considerations on commercial dendrimer charge and hydrodynamic radius, several capillary zone electrophoresis (CZE) experiments were performed. Electrophoretic mobilities measured at different pH values indicated a sensible increase of dendrimer hydrodynamic radius at pH values lower than 2.5. This was probably due to the Coulombic repulsion of charged amine groups of the inner dendrimer shells. The principal reasons that should address the use of dendrimers as pseudostationary phases in micellar electrokinetic chromatography (MEKC) are discussed. Moreover, a survey of different separations performed utilizing dendrimers in MEKC as well as of several future plausible uses of various classes of dendrimers is presented.     

Graphene nanoparticles as pseudostationary phase for the electrokinetic separation of nonsteroidal anti‐inflammatory drugs

The exceptional properties of graphene (G) were exploited here to facilitate capillary electrokinetic separations. Two types of commercially available G consisting of nanoparticles containing—one to three and—four to six G sheets, respectively, were compared for this purpose. Both proved effective in separating the arylpropyl derivatives of nonsteroidal anti‐inflammatory drugs. The highest resolution and shortest migration times were obtained with G containing high amount of single and double G nanosheets. G affords higher resolution than other types of nanoparticles; stable suspensions can be easily prepared and used as BGE without the need of adding an additional surfactant. This results in a high reproducibility in migration times and stability in background noise. The LOD and LOQ obtained by using G nanoparticles as pseudostationary phases spanned the range 0.29–1.18 mg/L and 0.95–3.95 mg/L, respectively, and the RSD was less than 4.7% in all instances.     

Separation of phenolic acids by capillary electrophoresis with indirect contactless conductometric detection

A new method for the electrophoretic separation of nine phenolic acids (derivatives of benzoic and cinnamic acids) with contactless conductometric detection is presented. Based on theoretical calculations, in which the mobility of the electrolyte co- and counterions and mobility of analytes are taken into consideration, the electrolyte composition and detection mode was selected. This approach was found to be especially valuable for optimization of the electrolyte composition for the separation of analytes having medium mobility. Indirect conductometric detection mode was superior to the direct mode as predicted theoretically. The best performance was achieved with 150 mM 2-amino-2-methylpropanol electrolyte at pH 11.6. The separation was carried out in a counter-electroosmotic mode and completed in less than 6 min. The LODs achieved were about 2.3-3.3 microM and could be further improved to 0.12-0.17 microM by using a sample stacking procedure. The method compares well to the UV-Vis detection.     

Identification of aromatic sulphonic acids by reaction electrophoresis on paper

A method was elaborated to establish the number of sulphonic groups bound to the aromatic ring of aromatic acids by means of a paper electrophoretic separation of the original compound from the derivative. The difference between the mobilities is directly proportional to the number of substituted sulphonic groups.     

Analysis of aromatic acids by nonaqueous capillary electrophoresis with ionic‐liquid electrolytes

The separation of six kinds of aromatic acids by CZE with 1‐ethyl‐3‐methylimidazolium chloride (EMIMCl) and 1‐ethyl‐3‐methylimidazolium hydrogen sulfate (EMIMHSO₄)_, two kinds of ionic liquids (ILs) as background electrolytes, and acetonitrile as solvent were investigated. The six kinds of aromatic acids can be separated under positive voltage with low IL concentration with either of the two ILs and separation with EMIMHSO₄ is better in consideration of peak shapes and separation efficiency. But the migration order is different when the IL is different. Under negative voltage with high IL concentration, the six analytes can be separated with EMIMCl as background electrolytes and the migration order of the analytes is opposite to those with low concentration of EMIMCl as background electrolyte. The separations are based on the combination effects of heteroconjugation between the anions and cations in the ILs and the analytes, of which the heteroconjugation between the anions in the ILs and the analytes plays a dominant role. The heteroconjugation between the anions of the ILs and analytes is proton sensitive and only a very small amount of proticsolvents added into the electrolyte solution can harm the separation. When EMIMCl concentration is high, the heteroconjugation between the IL anions and the proton in the analytes make the effective mobility of the analytes much higher than the EOF and their migration direction reversed. Finally, the six aromatic acids in water samples were analyzed by nonaqueous CE with low concentration of EMIMHSO₄ as background electrolytes with satisfactory results.     

Separation of perfluorocarboxylic acids using capillary electrophoresis with UV detection

A capillary electrophoretic method with UV detection for separation and quantitation of perfluorocarboxylic acids (PFCAs) from C6-PFCA to C12-PFCA has been developed. The optimization of measurement conditions included the choice of the most appropriate type and concentration of buffer in the background electrolyte (BGE), as well as the type and the content of an organic modifier. The optimal separation of investigated PFCAs was achieved with 50 mM phosphate buffer and 40% isopropanol in the BGE using direct UV detection. The optimum wavelength for direct UV detection was optimized at 190 nm. For indirect detection, several chromophores were studied. Five mM 3,5-Dinitrobenzoic acid (3,5-DNBA) in 20 mM phosphate buffer BGE and indirect UV detection at 280 nm gave the optimal detection and separation performance for the investigated PFCAs. The possibility of on-line preconcentration of solutes by stacking has been examined for indirect detection. The detection limits (LODs) determined for direct UV detection ranged from 2 microg/mL for C6-PFCA to 33 microg/mL for C12-PFCA. The LODs obtained for indirect UV detection were comparable to those obtained for direct UV detection.     

Separation of native amino acids at low pH by capillary electrophoresis

Amino Acids are cations at low PH and can be readily separated by capillarty electrophoresis provided an alkanesulfonic acid is added to the elecrolyte carrier. Formation of a Positive net charge on the bare fused-silica surface at low PH was confirmed by measurement of an anodic electroosmotic flow. The addition of ethanesulfonic acid or octanesulfonic acid to the electrolyte carrier causes a reversal of the EOF. A mechanism is proposed in which the alkanesulfonic acid adsorbs to the positively-charged capillary wall through electrostatci attraction. Adsorption of a second molecule of alkanesulfonate by hudrophobic attraction to the carbon chain forms a negatively-charge coating on the capillary wall. The alkanesuflfonate also imparts selectivety to the system by participation in ionpairing interactions with the native amino acids to improve resolution. The CE separation of a mixture of the twenty common amino acids at PH 2.8 with direct absorabance detection at 185 nm resulted in 17 amino acid peals in 20 minutes with a 30 KV applied voltage. The effect of several variables was studied including electrolyte carrieres containing different alkanesulfonic acids, the influence of PH, applied voltage, and concentration of electrolyte carrier.     

Separation of chlorophenoxyacetic acids and chlorophenols by using capillary zone electrophoresis

In this study, the choice of electrolyte systems for the separation and detection of a range of chlorophenoxyacetic acids and chlorophenols by means of capillary zone electrophoresis (CZE) is discussed. A series of acetate buffers over the buffering capacity pH range 4.03-5.5 were initially chosen for the separation. It was found that chlorophenoxyacetic acids could be separated at pH 4.03 and 4.5 but the most satisfactory separation of chlorophenols was obtained at pH 5.5. The factors affecting separation selectivity, including the addition of organic modifiers, was also studied. The use of 25% 2-butanol, 5% ethylene glycol and 10% acetonitrile as organic solvents resulted in the total separation of both classes of these compounds but poor peak shape of chlorophenols resulted and a number of chlorophenoxyacetic acids were not well separated. A borate-phosphate buffer gave improved peak shape of chlorophenols. Further improved separation of the components of the mixture was obtained by the addition of 2 mM fully methylated-beta-cyclodextrin to the 35 mM borate- 60 mM phosphate buffer at pH 6.5, maintaining good peak shape. In this case, separation of the two compound classes, chlorophenoxyacetic acids and chlorophenols, is achieved, with complete resolution of individual compounds in less than 5 min with high efficiency (of the order of 150,000 plates for the ca. 40 cm column). The method is applied to a commercial 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide mixture.     

Determination of aromatic acids and nitrophenols in atmospheric aerosols by capillary electrophoresis

A capillary zone electrophoresis method is developed for the determination of aromatic organic acids and nitrophenols in atmospheric aerosols. The procedure is based on sampling atmospheric particulate matter on quartz fiber filters and the extraction and analysis of the extracts by capillary electrophoresis. Separation conditions are optimized by varying the pH and acetonitrile content of the electrolyte buffer. Separations in a 20% acetonitrile-20 mM borate mixture (pH 9.9) are able to resolve all of the geometric isomers of hydroxybenzoic acid, phthalic acid, benzenetricarboxylic acid, and nitrophenol as well as 1,2,4,5-benzenetricarboxylic acid, m-toluic acid, and sulfosalicylic acid. A buffer consisting of 11% acetonitrile-20 mM borate (pH 9.9) is found to be most suitable for the analysis of atmospheric aerosol samples. Detection limits are in the order of 40 to 130 ng/mL. Intersample migration time reproducibility is generally better than 1.5%, with day-to-day variations under 3%. A general extraction scheme using diethyl ether-HCl in combination with a preconcentration step is developed. Recoveries of spiked standards range from 59% to 102%, with relative standard deviations ranging from 2% to 17% for five determinations. The method is applied towards the analysis of ambient aerosol samples as well as vehicle emission studies with promising results, thus showing it to be a potential complement to already existing methodology for the analysis of organic acids and nitrophenols in atmospheric aerosols.     

Separation and identification of aromatic acids in soil and the Everglades sediment samples using solid-phase microextraction followed by capillary zone electrophoresis

The separation and identification of aromatic acids in soil and the Everglades sediment samples was carried out using solid-phase microextraction (SPME) followed by capillary zone electrophoresis (CZE). The soil and sediment samples were subject to a series of sample treatments including oxidative hydrolysis with molecular oxygen in a sodium hydroxide solution, acidification and filtration. The aromatic acids in the sample filtrate were extracted using SPME with a polyacrylate-coated fiber. The acids adsorbed on the fiber were subsequently desorbed in methanol. The desorbed acids were then separated by CZE. Several aromatic acids (e.g.. salicylic acid, p-coumaric acid, ferulic acid and vanillic acid) in both soil and sediment samples were separated, identified and quantified. The results of this study show that the combination of SPME with CZE is promising for environmental analysis.