Capillary electrophoresis for short-chain organic acids and inorganic anions in different samples

This review article is a comprehensive survey of capillary electrophoresis methods developed for the measurement of short-chain organic acids and inorganic anions in a wide variety of matrices, such as food and beverages, environmental, industry, and other applications, as well as clinical applications in body fluids such as urine, plasma or cerebrospinal fluid. Details of sample pretreatment and of electrophoretic conditions have been collected in tables, arranged by the type of matrix. Strategies employed for method development for the analysis of these compounds by capillary electrophoresis in real samples are discussed.     

Capillary electrophoresis of inorganic anions in hydro-organic media. Influence of ion-pairing and solvation phenomena

The capillary electrophoresis separation of four inorganic anions (NO3-, I-, Br- and SCN-) was investigated over the whole range of methanol-water mixture composition. As the separation selectivity was strongly dependent on the solvent composition, the influence of ion-pairing and solvation phenomena was examined in depth in an attempt to explain this modification. First, a series of experiments was performed in methanolic background electrolytes, with counter-ions of different size. Ion-pair formation involving electrolyte ions was assessed to allow for a correction for free electrolyte ion concentration. Ion-pair formation constants between each inorganic anion and electrolyte counter-ion were next determined from the variations of the anion mobility as a function of the free counter-ion concentration. In view of the low values obtained, ion-pair formation alone failed to explain the selectivity variations. Solvation phenomena were then investigated with the help of a theoretical quantum model, the density functional theory (DFT), coupled with a polarizable continuum model to mime non-specific solvent effects. Whereas this model proved successful at predicting the mobility order at infinite dilution in water, it failed to predict the correct order in methanol.     

Quantitative capillary zone electrophoresis of inorganic anions

Despite the availability of commercial capillary electrophoresis systems for over ten years, where quantitative analysis is required, capillary zone electrophoresis (CZE) has often failed to replace ion chromatography as the method of choice for a large number of analytes, not least inorganic anions. To investigate the reasons for this apparent failing, a review is presented of work that has been carried out to-date involving the quantitative application of CZE to the determination of inorganic anions in industrial and environmental samples. This review summarizes work both investigating and improving the quantitative aspects of the CZE of inorganic anions. A complete survey of how CZE has been applied to the determination of inorganic anions in real samples is given, including what, if any, analytical performance parameters were investigated and quoted, and if quality assurance data and validation methods were briefly considered, thoroughly investigated or simply ignored.     

Capillary electrophoresis of inorganic ions: an update

This review as a sequel of three earlier similar reports gives a summary of the progress and significant methodological developments, starting from 2002, in the use of capillary electrophoresis (CE) for inorganic ion analysis. As substantiated by the illustrative number of relevant references, improvements in sensitivity achieved both in and outside a CE system, advances in manipulating the separation selectivity, novel hardware configurations, and system performance innovations are continually being reported over the review period. Specifically viewed are the recent advancements in elemental (bio)speciation analysis, which remains one of the most fertile areas of CE research, as well as in three recently booming research topics: contactless conductivity detection, separations on microchips, and transient isotachophoretic preconcentration. A state-of-the-art picture of technique's potentialities within the field of interest presented here demonstrates that CE has become recognized and is growing in acceptance as a reliable alternative to traditional analytical methods such as high-performance liquid chromatography (HPLC).     

Factors affecting selectivity of inorganic anions in capillary electrophoresis

Capillary zone electrophoretic separations of inorganic anions are largely governed by the intrinsic (infinite dilution) mobility of the anion. This in turn is a function of the hydrodynamic friction caused by the size of the ion and the dielectric friction caused by the charge density of the anion re-orienting the surrounding solvent. The influence of these factors on the mobility of anions is examined in both water and nonaqueous solvents. The influence of other experimental parameters, such as ionic strength, ion association, electroosmotic flow modifier concentration, and the addition of complexing agents such as polymeric cations, cyclodextrins, crown ethers and cryptands are also reviewed. From this discussion, some rules of thumb as to when different approaches will be most effective are drawn.     

Capillary electrophoresis method for analysis of inorganic and organic anions related to habitability and the search for life

In situ missions of exploration require analytical methods that are capable of detecting a wide range of molecular targets in complex matrices without a priori assumptions of sample composition. Furthermore, these methods should minimize the number of reagents needed and any sample preparation steps. We have developed a method for the detection of metabolically relevant inorganic and organic anions that is suitable for implementation on in situ spaceflight missions. Using 55 mM acetic acid, 50 mM triethylamine, and 5% glycerol, more than 21 relevant anions are separated in less than 20 min. The method is robust to sample ionic strength, tolerating high concentrations of background salts (up to 900 mM NaCl and 300 mM MgSO₄). This is an important feature for future missions to ocean worlds. The method was validated using a culture of Escherichia coli and with high salinity natural samples collected from Mono Lake, California.     

Capillary electrophoretic analysis of inorganic anions in atmospheric hailstone samples

Micellar electrokinetic capillary chromatography (MEKC) coupled with sample stacking and polarity switching was investigated for the determination of Viagra (sildenafil citrate, SC) and its metabolite (UK-103,320, UK) in human serum in the concentration range of clinical interest. Human serum samples spiked with SC and UK were eluted with methanol from a C18 cartridge, the extract was evaporated and regenerated in a solution that contained 1 mM phosphate buffer (pH 12.3) and 20% methanol. The MEKC separation was performed using an injection time of 275 s, a polarity switching time of 93 s, a phosphate buffer, (pH 12.3, 15 mM) containing 25 mM sodium dodecyl sulfate as separation electrolyte and a fused-silica capillary. The analysis takes about 6 min and gives satisfactory inter-day precision with respect to migration times and linear responses over the 80-900 ng/ml concentration range investigated for SC and UK. Intra-day RSDs (n=4 graphs) for the slopes of the calibration graphs were 4.86% for SC and 3.50% for UK. Inter-day RSDs for the slopes were 4.37% for SC and 5.39% for UK. Detection limits (S/N=3) were about 17 ng/ml for both compounds in human serum. A 1-ml volume of blood serum was necessary to do this determination.     

Separation of inorganic anions in acidic solution by capillary electrophoresis

Inorganic anions are almost always determined by capillary electrophoresis (CE) at an alkaline pH, so the analytes will be fully ionized. However, a long-chain quaternary ammonium salt usually must be added as a flow modifier to the carrier electrolyte to reverse the direction of the electroosmotic flow. By working at a sufficiently acidic pH, the electroosmotic flow in fused-silica capillaries is virtually eliminated, and anions can be separated simply by differences in their electrophoretic mobilities. Excellent separations were obtained for AuCl₄⁻ and the chloro complexes of platinum group elements in HCl solution at pH 2.0 to 2.4. No additional buffer or flow modifier was needed. This CE technique is an excellent way to follow slow hydrolytic reactions in which one or more of the chloride ligands is replaced by water. Sharp peaks and good separations were also obtained for MnO₄⁻, VO₃⁻, chromate, molybdate, ferrocyanide, ferricyanide and stable complex ions such as chromium oxalate (CrO₃³⁻).     

毛细管电泳定量检测环境水样中无机阴离子

详细讨论了胶束浓度、间接吸收背景物质的浓度及有机添加剂组成等对部分卤素及其含氧酸根的毛细管电泳分离影响;在优化的条件下,3.5min内高效、快速地完成了Cl-、Br-、ClO3-、BrO3-四种无机阴离子的分离分析,分离的理论塔板数在1.6×106～2.8×105/m之间,检出限在11.2～23.3mg/L之间,迁移时间的相对标准偏差小于1%,峰面积的相对标准偏差在5.2%～2.2%之间;并将方法用于环境水样的分析。     

Simultaneous separation of inorganic anions and cations by capillary zone electrophoresis

A new capillary electrophoretic approach for simultaneous separation of fast anions and cations is demonstrated. Indirect UV detection at 214 nm in conjunction with electromigration sampling from both ends of the capillary was developed. Two electrolyte systems based on imidazole-nitrate and copper(II)-ethylenediamine-nitrate were investigated for the simultaneous separation of chloride, sulphate, hydrocarbonate, potassium, ammonium, calcium, sodium and magnesium ions. Experimental parameters that were evaluated included a nature of UV chromophore, pH of electrolyte, a nature of complexing agent. The method permits the excellent separation of three anions and five cations in only 4 min using electrolyte system containing 2.5 mmol l⁻¹ Cu(NO₃)₂, 5 mmol l⁻¹ ethylenediamine and 1 mmol l⁻¹ fumaric acid at pH 8.5 adjusted with tetraethylammonium hydroxide.     

Determination of inorganic anions in Kraft pulping liquors by capillary electrophoresis

Various sulfur containing anions (sulfate, sulfite, and thiosulfate) in Kraft pulping process liquors are determined by capillary electrophoresis. In addition, other inorganic anions (hydroxide, chloride, oxalate, carbonate) are analyzed with the developed method. Through optimization of the separation conditions it is possible to simultaneously determine these anionic species in pulping liquors with direct and indirect UV detection at 185, 214, and 254 nm. To ensure short separation times a migration of the anionic analytes in the same direction as the electroosmotic flow (co-electroosmotic CE) is established by reversal of the electroosmotic flow with 1,5-dimethyl-1,5-diazaundecamethylene polymethobromide (hexadimethrine bromide; HDB; polybrene™) which is added to the electrolyte as EOF modifier. The impact of acetonitrile as organic modifier to improve the selectivity of the anionic analytes is also investigated. The developed method is then applied to analyze and quantify various anions in pulping liquors (white and black liquors). By simultaneously determining the hydroxide concentration it is possible to calculate effective alkalinity and sulfidity with the measured concentrations without the need of volumetric methods.     

Comparison between capillary electrophoresis and ion-chromatography for analysis of inorganic anions

Separation and detection of some selected inorganic anions with capillary electrophoresis are shown. The anions are separated in microbore capillaries (25 m* 0.2 m) and detected with a UV-detector. Results are compared with the method of ion exchange chromatography. In consideration of the most important physical and chemical parameters an easy kind of computer simulation for such electropherograms was developed.To get optimal results of separation and UV-detection in capillary electrophoresis some parameters of the device HPE 100, i.e. loading time in the electrokinetic sample injection mode and the running voltage are varied. The behaviour of absorption in the UV region of the chosen anions as well as the influence of pH values in retention behaviour are investigated. There is a simple way to calculate the electrophoretic mobilities from known retention times. Approximate limits of detection for all anions and for each technique are given.     

Capillary electrophoresis of organic and inorganic cations with indirect UV detection

Summary The determination of alkali and alkaline earth metal ions by capillary electrophoresis using indirect UV detection is described. With the identical system it is also possible to determine short chain aliphatic amines and alkanol amines within 4 minutes. Indirect UV detection is achieved at 214 nm with a background electrolyte containing 5 mmol L^–1 imidazole. Linear calibration curves could be obtained for peak areas between 0.5 and 10 ppm. The detection limits are around 0.1 ppm (corresponding to 10 fmol in about 10 nL sample volume) for all cations and amines and 0.05 ppm for lithium. Practical applications demonstrate the applicability of this system in routine analysis.     

Determination of inorganic anions in human saliva by zwitterionic micellar capillary electrophoresis

Capillary electrophoresis (CE) using sulfobetaine-type zwitterionic micelles as the background electrolyte (BGE) has been used to determine inorganic anions in human saliva. The zwitterionic micelles resulted in unique migration behavior for the separation of inorganic anions. They also prevented adsorption of proteins on the inner wall of the capillary. These properties of the zwitterionic micelles enabled the direct determination of inorganic anions in human saliva. Three species of inorganic anions, NO₂ ^–, NO₃ ^–, and SCN^–, were found in real samples and the analysis was achieved within 3 min. Direct UV-absorption was used as the detection method and the detection limits for these anions were 2.0, 1.0, and 5.0 μmol L^–1, respectively (0.09, 0.06, and 0.30 μg mL^–1).     

Determination of inorganic anions in human saliva by zwitterionic micellar capillary electrophoresis

Capillary electrophoresis (CE) using sulfobetaine-type zwitterionic micelles as the background electrolyte (BGE) has been used to determine inorganic anions in human saliva. The zwitterionic micelles resulted in unique migration behavior for the separation of inorganic anions. They also prevented adsorption of proteins on the inner wall of the capillary. These properties of the zwitterionic micelles enabled the direct determination of inorganic anions in human saliva. Three species of inorganic anions, NO2-, NO3-, and SCN-, were found in real samples and the analysis was achieved within 3 min. Direct UV-absorption was used as the detection method and the detection limits for these anions were 2.0, 1.0, and 5.0 micromol L(-1), respectively (0.09, 0.06, and 0.30 microg mL(-1)).     

Capillary zone electrophoresis for the determination of light-absorbing anions in environmental samples.

A rapid and simple method for separation and determination of inorganic anions by capillary zone electrophoresis was described. The detection was carried out directly with a diode array detector. The experimental conditions, such as concentration of carrier electrolyte, capillary length, voltage, and temperature were optimized. In order to improve selectivity, different organic modifiers were also investigated. The baseline separation of 10 light-absorbing anions was accomplished within 3.5 min with a background electrolyte consisting of 50 mM sodium tetraborate containing 5% MeOH. Linear plots were obtained in the concentration range of 0.1-10 microg/ml. With sample stacking injection, the quantitation limits of the anions were found to be in the range of 0.02-0.1 microg/ml. The proposed method was successfully applied to the determination of inorganic anions in environmental samples and in effluents of a power plant.     

Analysis of organic acids and inorganic anions in beverage drinks by capillary electrophoresis

To meet the need for a new and validated analytical method for simultaneous determination of inorganic and organic acid anions in beverage drinks, a capillary zone electrophoresis (CZE) procedure had been developed based on a new background electrolyte (BGE) system containing 3 mmol/L 1,3,5-benzenetricarboxylic acid (BTA), 15 mmol/L tris(hydroxymethyl)aminomethane and 1.5 mmol/L tetraethylenepentamine (TEPA) at pH 8.4. Baseline separation of anions commonly found in beverage drinks could be achieved in less than 14 min with indirect UV detection at 240 nm. Comigration problems for hydroxycarboxylic acids could be solved using TEPA as BGE additive. The results indicate excellent repeatability for migration time (RSD, 0.27-0.67%, n = 5) and good precision for both peak height (RSD, 3.2-4.2%, n = 5) and peak area (RSD, 3.1-4.5%, n = 5). Under the optimized conditions and using corrected peak area for quantitation, an excellent linear dynamic range (with correlation coefficient > 0.997 in a concentration range from 0.005 to 2.0 mmol/L) and low detection limit (1-4 micro mol/L) were obtained for all the anions investigated. The applicability and reliability of the CE procedure developed were established by parallel method determination using established ion chromatography procedure for the analysis of inorganic and organic acid anions in orange juice and wine samples. Our CZE procedure provided a sensitive and economic technique for simultaneous determination of inorganic and organic acid anions in orange juice, red and white wine samples.     

Capillary electrophoretic determination of inorganic anions in the drainage and surface water samples.

Capillary electrophoresis was used for separation and quantitation of several inorganic anions in the drainage and surface water samples from the region with extensive use of fertilisers. Baseline separation of 13 small anions including nitrite and nitrate up to the concentrations of 100 mg/l was achieved in less than 5 min. The electrolyte consisted of 3 mM K2CrO4, 30 microM cetyltrimethylammonium bromide and 3 mM boric acid at pH 8. The method yielded precisions of 1.8-7.2% (RSD, n = 10) and detection limits from 4 micrograms/l (Cl-) up to 500 micrograms/l (citrate). The results of the CE method were compared to ion chromatography using water-acetonitrile (86:14) at pH 8.6 adjusted with NaOH as the mobile phase and consistent results were obtained.     

Capillary electrophoresis

The most important characteristics of capillary electrophoresis are studied. Principles of the technique, instrumentation employed and the most important applications are described.