Determination of alkylphosphonic acids by capillary zone electrophoresis using indirect UV detection

Capillary zone electrophoresis with indirect UV detection was used for the determination of a series of alkylphosphonic acids. For this purpose, a few UV-absorbing background electrolytes were tested and phenylphosphonic acid, which has a mobility close to that of the analysed compounds, was shown to be the most suitable. The influence of several parameters such as concentration of the UV-absorbing background electrolyte and concentration of borate on both sensitivity and efficiency was investigated. An increase in the borate concentration produced an improvement of the signal-to-noise ratio. Conversely, the sensitivity decreased with increasing concentration of the phenylphosphonic acid. The reproducibility of the method was very satisfactory and limits of detection were less than 0.21 pmol injected.     

Analysis of low molecular weight organic acids in water with capillary zone electrophoresis employing indirect photometric detection

A capillary zone electrophoretic method for the analysis of low molecular weight carboxylic acids in water was developed. The application uses the chromophore trimellitic acid that allows for indirect photometric detection at 254 nm. Selectivity control by the addition of Ca2+ to the carrier electrolyte made possible a baseline resolved separation of 14 carboxylic acids. Good linearity and repeatability were obtained for both electrokinetic and hydrostatic injection modes. The usefulness of the method is demonstrated by the analysis of a snow sample.     

Analysis of low molecular weight organic acids in water with capillary zone electrophoresis employing indirect photometric detection

A capillary zone electrophoretic method for the analysis of low molecular weight carboxylic acids in water was developed. The application uses the chromophore trimellitic acid that allows for indirect photometric detection at 254 nm. Selectivity control by the addition of Ca²⁺ to the carrier electrolyte made possible a baseline resolved separation of 14 carboxylic acids. Good linearity and repeatability were obtained for both electrokinetic and hydrostatic injection modes. The usefulness of the method is demonstrated by the analysis of a snow sample.     

Determination of dissociation constants of amino acids by capillary zone electrophoresis.

A method is proposed for the determination of dissociation constants of amino acids by capillary zone electrophoresis. According to the dissociation equilibrium of amino acids and the basic theory of electrophoresis, the nonlinear relationship between the pH value of the buffer and the effective electrophoretic mobilities of the analyte was obtained. The dissociation constants can be calculated from the pH values and the corresponding effective electrophoretic mobilities using the program written in C++. The dissociation constants, pKa1 and pKa2, of 11 kinds of amino acids were determined successfully by the proposed method. The determined dissociation constants were compared with values in the literature; the differences between them are in the range of -0.03 to 0.06. No significant differences were observed between the determined dissociation constants and the corresponding literature values.     

Determination of low molecular weight organic acids in soil,plants, and water by capillary zone electrophoresis

Determination of low molecular weight organic acids in soils and plants by capillary zone electrophoresis was accomplished using a phthalate buffer and indirect UV detection mode. The influence of some crucial parameters, such as pH, buffer concentration and surfactant were investigated. A good separation of seven organic acids was achieved within 5 min using an electrolyte containing 15 mmol L(-1) potassium hydrogen phthalate, 0.5 mmol L(-1) myristyltrimethylammonium bromide (MTAB), and 5% methanol (MeOH) (v/v) at pH 5.60, separation voltage -20 kV, and temperature 25 degrees C. The relative standard deviation (n=5) of the method was found to be in range 0.18-0.56% for migration time and 3.2-4.8% for peak area. The limit of detection ranged between 0.5 micro mol L(-1) to 6 micro mol L(-1) at a signal-to-noise ratio of 3. The recovery of standard organic acids added to real samples ranged from 87 to 119%. This method was simple, rapid and reproducible, and could be applied to the simultaneous determination of organic acids in environmental samples.     

Determination of organic acids of low molecular weight and phosphate in soil by capillary electrophoresis

Capillary zone electrophoresis with indirect UV detection at 254 nm was found to be suitable for the determination of organic acids and phosphate in aqueous extracts of soil. The best support electrolyte solution was found to be 10 mM p-hydroxybenzoic acid with 0.5 mM tetradecyltrimethylammonium bromide to reverse electroosmotic flow. This methodology was tested with 9 analytes found in soils: acetate, citrate, formate, phosphate, lactate, oxalate, pyruvate, succinate, and tartrate. The results obtained show that the methodology is adequate for most of the analytes. The sensitivity to oxalate and citrate was low, and the high concentrations of major inorganic anions interfered with the detection of the former. The methodology was applied to the analysis of aqueous extracts of soil samples. Formate, phosphate, lactate, and acetate anions were detected in most of the samples.     

Determination of acid-base dissociation constants of azahelicenes by capillary zone electrophoresis

CZE was employed to determine acid-base dissociation constants (pK(a)) of ionogenic groups of azahelicenes in methanol (MeOH). Azahelicenes are unique 3-D aromatic systems, which consist of ortho-fused benzene/pyridine units and exhibit helical chirality. The pK(a) values of pyridinium groups of the studied azahelicenes were determined from the dependence of their effective electrophoretic mobility on pH by a nonlinear regression analysis. The effective mobilities of azahelicenes were determined by CZE at pH range between 2.1 and 10.5. Thermodynamic pK(a) values of monobasic 1-aza[6]helicene and 2-aza[6]helicene in MeOH were determined to be 4.94 +/- 0.05 and 5.68 +/- 0.05, respectively, and pK(a) values of dibasic 1,14-diaza[5]helicene were found to be equal to 7.56 +/- 0.38 and 8.85 +/- 0.26. From these values, the aqueous pK(a) of these compounds was estimated.     

Determination of various inorganic anions and organic acids in the atmospheric particulate matter using capillary zone electrophoresis with indirect UV detection

Capillary zone electrophoresis with indirect UV detection was developed for the simultaneous analysis of inorganic anions and organic acids using a mixed solution of 2,6-pyridinedicarboxylic acid and cetyltrimethylammonium hydroxide as the background electrolyte (BGE). The parameters which influence the separation, such as indirect UV detection wavelength, BGE conditions, applied voltage and extraction conditions were investigated. Thirteen inorganic anions and organic acids were detected within 20 min. The calibration curves of each analyte were linear with correlation coefficients greater than 0.991. The relative standard deviations (n = 10) of the peak areas ranged from 0.6% to 3.3%. The detection limits for these species ranged from 0.4 to 1.4 mg/L at a signal-to-noise ratio of 3. The recovery rate of each analyte was more than 80% under optimised extraction conditions, except for nitrite. The proposed method was applied towards the analysis of inorganic anions and organic acids in the atmospheric particulate matter using an Andersen sampler. The particle size of the particulate matter was determined, but not the size of the anions.     

Determination of acid dissociation constants of flavin analogues by capillary zone electrophoresis

Acid dissociation constants (pK_a) of nine kinds of flavin analogues as molecular catalyst candidates were determined by CZE. Although some of the analogues are instable and degradable under the light exposure or in alkaline aqueous solutions, the effective electrophoretic mobility of the flavin analogue of interest has been measured with the residual substance. The pK_a values of the flavin analogues were analyzed through the changes in the effective electrophoretic mobility with varying pH of the separation buffer. One or two steps pK_a values were determined by the analysis. One of the degraded species from the flavin analogues, lumichrome, was also detected in the CZE analysis, and its pK_a values were also determined. While coexisting impurities generated over the storage conditions were found in some analogues, the pK_a values of the target analogues were successfully determined with the help of the CZE separations. A pressure-assisted CZE was utilized for the determination or the estimation of the pK_a values of such analogues as possessing carboxylic acid moiety.     

Optimization of cation detection by capillary zone electrophoresis with indirect UV absorbance

Summary The indirect UV detection of cations by capillary zone electrophoresis gives peaks of very different height or area between cations. We show that the height depends on the electrophoretic mobility of the cation compared with the electrophoretic mobility of the chromophore used. Moreover, the limit of detection can be easily improved by about 4-fold by adjusting the concentration of complexing agent or by using a chromophore with the same velocity as the studied cation. Using a new parameter termedsensitization power we can optimize the limit of detection and have determined the best chromophores for each family of cations.Sensitization power is a maximum for: alkali metals and alkali earths with imidazole and UV Cat 2^?, for transition metal with pyridine, for light rare earth with ephedrine and UV Cat 1^?, for heavy lanthanides with 1-naphthylamine and phenyl-trimethyl-ammonium bromide. Corrected areas of all cations studied have normal distributions directly proportional to their ionic charge. Certain behaviour can be predicted for elements such as actinides. Detection can also be enhanced because the relation of proportionality between ionic charge and corrected area allows comparison of the performance of all chromophores.     

Determination of acid dissociation constants by capillary electrophoresis

Capillary electrophoresis affords a simple, automated approach for the measurement of pKa values in the range 2-11 at a throughput of less than 1 h per sample per instrument. Agreement with literature values is usually within 0.20 log units with a precision better than 0.07 log units. The attractive features of capillary electrophoresis for pKa measurements are: (1) conventional instrumentation with a high level of automation are suitable for all measurements; (2) because it is a separation method samples need not be of high purity; (3) samples of low water solubility with suitable chromophores are easily handled (detection limits in the microM range); (4) sample consumption per measurement is in the microgram range; and (5) since only mobilities are measured, exact knowledge of concentrations is not needed. The general approach can be extended to pKa measurements in aqueous-organic solvent mixtures and non-aqueous solvents with suitable calibration. The widespread use of absorbance detection in capillary electrophoresis means that the sample must have a suitable chromophore for detection. The main source of controllable error is the accuracy of buffer standardization and their stability in use, and uncontrollable error, the retentive interactions of the sample with the column wall. The latter seems to be a rare problem in practice for typical operating conditions.     

Determination of dissociation constants of flavonoids by capillary electrophoresis

Ionization constants of some flavanols (catechin and epicatechin) and flavonols (kaempherol, fisetin, morin, and quercetin) are determined by capillary zone electrophoresis (CZE). This technique allows the determination of pK(a) values until about 12. The pK(a) values obtained are compared with those calculated by the SPARC computational program. This program predicts the microscopic and macroscopic pK(a) values and the order of deprotonation of the different -OH groups. While for catechin and epicatechin the first ionizable OH group occurs in ring 1 and the second ionizable group in ring 2, in flavonols the first deprotonation occurs in ring 2 and the second in ring 1.     

Determination of organic acids in food samples by capillary zone electrophoresis

A comprehensive survey of the use of capillary zone electrophoresis for the determination of organic acids in food and beverage samples is presented. The analytes discussed in this paper include low-molecular-mass organic acids, amino acids, vitamin related compounds and free fatty acids.     

Determination and prediction of transfer ratios for anions in capillary zone electrophoresis with indirect UV detection

Transfer ratios (i.e. the number of moles of the UV-absorbing probe anion displaced by one mole of analyte anion) were determined for the separation of inorganic and organic anions by capillary zone electrophoresis using indirect UV absorbance detection. When the electrolyte was buffered and contained only the probe anion and a single counter-cation, transfer ratios calculated from Kohlrausch theory were found to agree well with values obtained experimentally from accurately determined mobility data. However, these electrolyte systems gave long analysis times and were therefore considered impractical. More useful electrolytes were obtained by the addition of surfactants to suppress or reverse the electroosmotic flow but the co-anion introduced with the surfactant can reduce the value of the measured transfer ratio and hence adversely affect sensitivity. This problem was overcome by the use of a surfactant in the hydroxide from such as cetyltrimethylammonium hydroxide combined with a suitable buffering counter-cation such as protonated 1,3-bis[tris(hydroxymethyl)-methylamino]-propane or tris(hydroxymethyl)aminoethane. Four buffered electrolytes consisting of chromate, benzoate, phthalate, or trimellitate as probes and a suitable surfactant were used to determine transfer ratios. These systems were shown to give transfer ratios that were close to those calculated from Kohlrausch theory, thereby enabling prediction of experimental conditions giving maximum transfer ratios.     

Analysis of inorganic cations in honey by capillary zone electrophoresis with indirect UV detection

An indirect UV detection method based on capillary electrophoresis was developed to separate eleven metal cations completely, including alkali, alkaline earth and transition metal, which are related to evaluate the quality of honey. The background electrolyte contains 15 mmol/L chromophore imidazole, and acetic acid (pH = 3.7), which functioned as pH adjustor and complexing reagent. The selected cations can be completely separated within 8 min under hydrodynamic mode injection with a running voltage of 20 kV at 25°C. Limit of detection, linearity, reproducibility relative standard deviation of migration time and recoveries are in the range of 0.01-0.21 mg/L, 0.06-60.0 mg/L, 2.1-3.4% and 95.4-104.1%, respectively. The applicability of the method is shown by the analysis of honey samples, including a comparison with results of pretreatment and unpretreatment of eight samples. The results demonstrate that the developed method can conveniently be used in routine analysis of honey.     

Determination of dissociation constants of compounds with potential cognition enhancing activity by capillary zone electrophoresis

Accurate determination of pK(a) values is important for proper characterization of newly synthesized molecules. In this work we have used CZE for determination of pK(a) values of new compounds prepared from intermediates, 2, 3 and 4-(2-chloro-acetylamino)-phenoxyacetic acids, by substituting chloride for 2-oxo-pyrrolidine, 2-oxo-piperidine or 2-oxo-azepane. These substances are expected to have a cognition enhancing activity and free radicals scavenging effect. Measurements were performed in a polyacrylamide-coated fused-silica capillary of 0.075 mm ID using direct UV detection at 254 nm. Three electrolyte systems were used for measurements to eliminate effects of potential interactions between tested compounds and components of the BGE. In the pH range 2.7-5.4, chloride, formate, acetate and phosphate were used as BGE co-ions, and sodium, beta-alanine and epsilon-aminocaproate as counterions. Mobility standards were measured simultaneously with the tested compounds for calculations of correct electrophoretic mobilities. Several approaches for the calculation of the pK(a) values were used. The values of pK(a) were determined by standard point-to-point calculation using Henderson-Hasselbach equation. Mobility and pH data were also evaluated by using nonlinear regression. Three parameter sigmoidal function fitted the experimental data with correlation coefficients higher than 0.99. Results from CZE measurements were compared with spectrophotometric measurements performed in sodium formate buffer solutions and evaluated at wavelength where the highest absorbance difference for varying pH was recorded. The experimental pK(a) values were compared with corresponding values calculated by the SPARC online calculator. Results of all three used methods were in good correlation.     

Determination of acid-base dissociation constants of very weak zwitterionic heterocyclic bases by capillary zone electrophoresis

Thermodynamic acid-base dissociation (ionization) constants (pK(a)) of seven zwitterionic heterocyclic bases, first representatives of new heterocyclic family (2,3,5,7,8,9-hexahydro-1H-diimidazo[1,2-c:2',1'-f][1,3,2]diazaphosphinin-4-ium-5-olate 5-oxides), originally designed as chiral Lewis base catalysts for enantioselective reactions, were determined by capillary zone electrophoresis (CZE). The pK(a) values of the above very weak zwitterionic bases were determined from the dependence of their effective electrophoretic mobility on pH in strongly acidic background electrolytes (pH 0.85-2.80). Prior to pK(a) calculation by non-linear regression analysis, the CZE measured effective mobilities were corrected to reference temperature, 25°C, and constant ionic strength, 25 mM. Thermodynamic pK(a) values of the analyzed zwitterionic heterocyclic bases were found to be particularly low, in the range 0.04-0.32. Moreover, from the pH dependence of effective mobility of the bases, some other relevant characteristics, such as actual and absolute ionic mobilities and hydrodynamic radii of the acidic cationic forms of the bases were determined.     

Determination of drug-cyclodextrin binding constants by capillary zone electrophoresis

Binding constants of the optical isomers of Deprenyl® (selegiline) and its potential metabolites with (2,6-di-O-methyl)--cyclodextrin were determined using electrophoretic mobility data gained from separations performed by capillary electrophoresis, and absorbancies obtained from spectrophotometric experiments. To calculate equilibrium constants l: l complex formation have been assumed. The comparison of the equilibrium constants calculated from different methods shows similar values in their order of magnitude. Their difference may probably be explained by the different media of the measurements. The effect of the structure of compounds on chiral discrimination were also elucidated.     

Determination of ionization constants of saccharides by capillary zone electrophoresis with amperometric detection

Summary A method based on a linear model enabling the efficient determination of the ionization constants (K _a) of saccharides by capillary zone electrophoresis with amperometric detection has been demonstrated. TheK _a values obtained from the plots of the reciprocal effective mobility against the inverse concentration of sodium hydroxide were in agreement with literature values.     

毛细管电泳电化学检测法同时测定三种氨基酸的电离常数

利用自制微圆盘铜电极,建立了一种毛细管电泳电化学检测同时测定色氨酸、丝氨酸和半胱氨酸pKα值的新方法。在不同pH条件下测定各氨基酸的有效淌度（μeff）,利用Origin软件对μeff-[H^＋]按理论关系式进行非线性拟合,得到其pKα值。该方法简便、快速,测定值与文献值符合良好。