Ion Capillary Electrophoresis Research Articles

Capillary ion electrophoresis–capacitively coupled contactless conductivity detection of inorganic cations in human saliva on a polyvinyl alcohol-coated capillary

Capillary ion electrophoresis-capacitively coupled contactless conductivity detection (CIE-C4D) with a polyvinyl alcohol chemically coated capillary (PVA capillary) was used to analyze inorganic cations (Na(+), K(+), NH(4)(+), Mg(2+), and Ca(2+)) commonly found in human saliva. The PVA capillary, which was made by our laboratory, minimized electro-osmotic flow in the wide pH range of the background electrolyte (BGE), and the PVA layer adsorbed to capillary wall did not affect the conductimetric background level. In this study, we determined an optimized BGE of 30 mM lactic acid/histidine plus 3 mM 18-crown-6 for the CIE-C4D system using the PVA capillary, which could simultaneously improve the separation of Mg(2+) and Ca(2+) from Na(+) and that of K(+) from NH(4)(+). This system obtained highly reproducible separation of cations in human saliva samples within 8 min at 20 kV without deprotonation. The quantifiability of cations in human saliva samples on the CIE-C4D system was demonstrated through identification by ion chromatography with satisfactory results.

Capillary ion electrophoresis of inorganic anions and uric acid in human saliva using a polyvinyl alcohol coated capillary column and hexamethonium chloride as additive of background electrolyte

A combination of polyvinyl alcohol chemically coated capillary (PVA capillary) and background electrolyte (BGE) with ion-pair reagent (hexamethonium dichloride, HMC) was used on capillary ion electrophoresis-UV detection (CIE-UV) for analysis of Br−, I−, NO2−, NO3−, SCN− and uric acid in human saliva. The PVA capillary prepared in our laboratory minimized electro-osmotic flow (EOF) at the BGE in pH 3–10, and did not affect the UV detection at 210nm by the PVA-layer on capillary wall. Therefore, use of the PVA capillary was suitable for sensitive UV detection for analyte anions, as well as suppression of protein adsorption. In this study, we optimized the BGE of 10mM phosphate plus 10mM HMC with applying a voltage of −15kV. HMC as an additive to BGE could manipulate the electrophoretic mobility of anions, without electrostatic adsorption to the PVA capillary. The CIE-UV could separate and determine analyte anions in human saliva containing proteins by the direct injection without pretreatments such as dilution or deproteinization within 13min. The relative standard deviations (n=10) were ranged of 0.5–1.6% in migration times, 2.2–6.8% in peak heights and 2.8–8.4% in peak areas. The limits of detection (S/N=3) were ranged of 3.42–6.87μM. The peak height of anions in this system was gradually decreased through the successive injections of saliva samples, but the problem was successfully solved by periodically conditioning the PVA capillary. The quantifiability of anions in human saliva samples by the CIE-UV was evaluated through the recoveries by standard addition methods and comparison of other representative analytical methods, as well as identification by ion chromatography (IC). From the anion analyses in 12 different saliva samples, the CIE-UV demonstrated that can obtain obvious differences in concentrations of SCN− between of smoker and non-smoker and those of uric acid between male and female with satisfactory results.

Analytical Methods for Determination of Counter-ions in Pharmaceutical Salts

Inorganic ions as well as organic acids or bases are frequently employed as counter-ions of ionizable drug substances. Therefore, analytical methods are required to confirm production of the correct salt form and to quantify the level of the counter-ion present in each batch of drug substances manufactured. Several options exist for determination of counter-ions in pharmaceutical salts and will be discussed in this review. Ion chromatography (IC) with conductivity detection is probably the most frequently employed method for ion determination. Other techniques such as capillary electrophoresis (CE) represent an alternative to IC for determination of counter-ions in pharmaceutical salts. Due to the lack of chromophores, detection of small ions in CE is typically performed using indirect UV detection. Hydrophilic interaction chromatography (HILIC) with evaporative light-scattering (ELSD) or refractive index detection can also be used for determination of non-UV absorbing ions. Each technique will be described and their advantages highlighted. Current and future trends in measuring counter-ions in pharmaceutical salts will also be discussed. Keywords: Counter-ions, Pharmaceutical, Ion chromatography, Capillary electrophoresis, HILIC, Mixed-mode chromatography

Comparison of Clinpro Cario L‐Pop estimates with CIA lactic acid estimates of the oral microflora

Clinpro Cario L-Pop (CCLP) is a semiquantitive test claimed to determine the general potential for caries development and to monitor the individual caries risk. This test translates the capacity of the tongue microflora to produce lactic acid into a score of 1-9, indicating a low, medium or high risk for caries development. The aim of this randomized crossover, clinical trial was to evaluate the CCLP on its variation over time and its capacity to monitor the effect of three different oral hygiene procedures. The CCLP readings were compared with measurements of lactic acid in tongue biofilm and plaque samples by capillary ion electrophoresis (CIA). After four washout periods, the distribution of scores in the low-, medium-, and high-risk categories was 10%, 16%, and 74%, respectively. Out of 30 subjects, 11 scored consistently in the same category. The coefficients of variance of lactic acid concentrations were 31% for tongue samples and 25% for plaque samples. After using antimicrobial toothpaste and mouthwash, the number of high-risk scores was reduced to 33%; reduced acidogenicity was also found in tongue and plaque samples. We conclude that CCLP can be used to monitor and stimulate compliance to an antimicrobial oral hygiene protocol.

Theoretical and empirical approaches to express the mobility of small ions in capillary electrophoresis

A discussion is given about the concepts of the ion mobility, the analyte property which governs migration and thus separation selectivity in CE. It deals with small organic and inorganic ions, not with charged polymers or large particles like colloids. The discussion is directed to two main concepts. (i) The first is based on physico-chemistry of ion conductance in solution, and distinguishes three types of mobility. The absolute mobility is the limiting mobility at zero ionic strength; it depends on the solvent and the temperature. It is obtained by extrapolation of the actual mobilities, those of the fully charged particles at finite ion concentration. The observed reduction of the absolute mobility with ionic concentration is related to an ion cloud, and is formulated by the established theories of ion conductance. It explains the actual mobility as function of (beside other factors) the ionic strength, the viscosity and relative permittivity of the solvent, the temperature, the relaxation time of solvent polarisation and the distance of closest approach between ion and counterion. The effective mobility, finally, is the mobility when association and dissociation equilibria play a role. Most important are acid-base reactions, but complexation, ion pairing and homo- and heteroconjugation were considered as well. (ii) The second approach treats mobility data with different mathematical methods, and formulates their dependence on variables like solvent composition with appropriate algorithms. These empirical methods mainly include least squares and neural network-based methods. The least square methods ranges from the simplest model, which uses only the molecular weight of the analyte, to more complicated model requiring three-dimensional structural descriptors of the solutes. Neural networks have been applied to model the mobility using different input variables and various architectures. Work comparing the accuracy of least squares and neural network methods was discussed; the results showed that the neural network method leads to a more accurate mobility calculation. However, the least squares methods could give some information to the factors affecting the mobility of the analytes. The resulting methods allow the prediction of mobilities under different experimental conditions with certain accuracy. It has been shown that using such models, it is possible to predict mobility of analytes after training the models by a minimum number of data to speed up the method development stage.

Sweeping via Dynamic Complexation with Cyclohexane-1,2-diaminetetraacetic Acid for Trace Metal Analysis in Capillary Electrophoresis

To improve the detection sensitivity of metal ions in capillary electrophoresis (CE), combination of on-line complexation and sweeping was explored using cyclohexane-1,2-diaminetetraacetic acid (CDTA). CDTA has two roles, as a dynamic complexation agent to form a UV-absorbing complex and a carrier for sweeping. Six metal ions were used as test analytes. The effect of pH, CDTA concentration, composition of BGS and sample matrix, injection length of sample solution were examined and optimized to obtain high performance of this method. Under optimized conditions, the technique was validated in terms of the limit of detection, repeatability, and sensitivity enhancement. Improved detection responses were 60- to 160-fold in terms of peak heights for metal ions in comparison with conventional CZE of precapillary complexed metal ions. The limits of detection were in the range of 0.4 – 4.2 × 10−7 M.

Derivatization of inorganic ions in capillary electrophoresis.

This review gives a short overview of the main approaches to the derivatization of inorganic ions in capillary electrophoresis (CE) with emphasis on the most recent works. Various derivatization procedures and detection methods are discussed. A brief account of their advantages and limitations is given. More specific areas such as microchip CE, simultaneous separation of anions and cations, and speciation analysis are also briefly discussed.

Capillary electrophoresis as a nanoreactor of separation capability: on-capillary reaction catalyzed by transition metal ions.

The catalytic oxidation of 1,2-dihydroxy-benzene-3,6-disulfonate (tiron) by metal ions has been studied for detection of the metal ions in capillary electrophoresis (CE). Although Co(2+) shows the strongest catalytic capability, some other metal ions also catalyze this reaction. If metal ions encounter a H(2)O(2 )zone after electrophoretic separation in the running buffer containing tiron, tiron is catalytically oxidized while the metal ion passes through the H(2)O(2) zone. Anionic tiron radicals produced by the reaction are finally measured by the detector; in this scheme, the capillary acts as a nano- or microreactor as well as a microseparator. The effective capillary length can be controlled by changing the interval between metal ion and H(2)O(2) injections. This scheme has been successfully applied to the detection of Co(2+), Cu(2+), Mn(2+), and VO(2+). The detectability is discussed from several viewpoints, such as the intrinsic catalyst ability of metal ions, the kinetics of the catalytic reaction, and reaction times determined by the mobility of the zone of the metal ion. Some strange behaviors, which cannot be predicted by batch experiments, are also reported.

High-voltage contactless conductivity detection of metal ions in capillary electrophoresis.

The detection of alkali, alkaline earth and heavy metal ions with a high-voltage capacitively coupled contactless conductivity detector (HV-C(4)D) was investigated. Eight alkali, alkaline earth metal ions and ammonium could be separated in less than 4 min with detection limits in the order of 5 x 10(-8) M. The heavy metals Mn2+, Pb2+, Cd2+ Fe2+, Zn2+, Co2+, Cu2+ and Ni2+ could also be successfully resolved with a 10 mM 2-(N-morpholino)ethanesulfonic acid/DL-histidine (MES/His)-buffer. Zn2+, Co2+, Cu2+ and Ni2+ showed an indirect response. The detection limits for the heavy metals were determined to range from about 1 to 5 microM.

Head-column field-amplified sample stacking in presence of siphoning: Application to capillary electrophoresis–electrospray ionization mass spectrometry of opioids in urine

Capillary electrophoresis (CE) with head-column field-amplified sample stacking (FASS) in presence of a water plug inserted at the capillary tip is a robust approach providing a more than 1000-fold sensitivity enhancement when applied to low-conductivity samples that are analyzed in an integrated instrument. Employing modular systems comprising a small hydrodynamic buffer flow (siphoning) towards the capillary end and featuring UV absorption or electrospray ionization mass spectrometric (MS) detection, insertion of a water plug is demonstrated to deteriorate the performance of head-column FASS or making it unfunctional. Electroinjection in the absence of the water plug can be employed instead and is shown to provide a ng/ml sensitivity when applied to low conductivity samples. With some suction of sample into the capillary during electroinjection, contamination of the sample vial with buffer is thereby largely avoided. Electroinjection applied to the CE–ion trap MS–MS and MS–MS–MS analysis of twofold diluted urines, urinary solid-phase extracts and urinary liquid–liquid extracts is shown to provide much improved sensitivity compared to hydrodynamic injection of these samples. With electroinjection from diluted urine and urinary solid-phase extracts, the presence of free opioids and their glucuronic acid conjugates can be unambiguously confirmed in urines that were collected after single-dose administration of small amounts of opioids (tested with about 7 mg codeine and 25 mg dihydrocodeine, respectively). Thus, CE–multiple MS with direct electroinjection of opioids from untreated urines could prove to become a rapid and simple approach for unambiguous urinary testing of drug abuse. Procedures leading to the reduction of siphoning in modular CE setups are briefly discussed as well.

Capillary Ion Electrophoresis of Endogenous Anions and Anionic Adulterants in Human Urine

Normal human urine contains many anions and cations. Ionic concentrations in urine have classically been determined by spectrophotometry of color reactions, flame emission spectrophotometry, atomic absorption spectrophotometry, high performance liquid chromatography, or potentiometry with ion-specific electrodes. Capillary ion electrophoresis (CIE) is a form of capillary electrophoresis which uses the differential electrophoretic mobility of ions to perform a separation of an ionic mixture. Various salts can be added to urine specimens to abnormally elevate ionic concentrations and interfere with either immunoassay urine drug screening procedures or gas chromatographic/mass spectrometric confirmation techniques. Application of CIE for the direct detection of endogenous anions and anionic adulterants in human urine specimens was the purpose of this investigation. CIE was performed using a Waters Quanta 4000 Capillary Electrophoresis System with either direct or indirect ultraviolet absorption detection at 254 nm. CIE of 30 random normal urine specimens and 21 urine specimens suspected of adulteration was performed. Duplicate aliquots were assayed by CIE and by colorimetric technique for nitrite. Sixteen specimens had elevated concentrations of nitrite and/or nitrate. The correlation coefficient between nitrite CIE and colorimetric results was 0.9895. Three specimens had detectable concentrations of chromate and were suspected of being adulterated with "Urine Luck," an adulterant found to contain chromate. Two specimens suspected of being adulterated with bleach were found to only contain chloride, sulfate, and phosphate. CIE is applicable to forensic analysis of urine anion concentrations. CIE can easily quantitate numerous endogenous anions and offers a method to detect and/or confirm anion adulteration of urine specimens.

Method development and validation for the determination of various sulfur-containing anions and other anions in the corrosion process by capillary ion electrophoresis with indirect detection.

A method of capillary ion electrophoresis with indirect detection is developed for the simultaneous determination of the sulfur-containing anions S2O4(2-), S2O3(2-), SO4(2-), SO3(2-), and S2- and other anions (Cl-, Br-, NO2-, NO3-, (COO)2(2-), F-, and PO4(3-)) in the corrosion process. The effects of pH, tetradecyltrimethylammonium hydroxide, chromate, 2-[n-cyclohexylamino]-ethane sulfonate, calcium gluconate, and acetonitrile on the migration and resolution of the anions and the stability of sulfur-containing anions are systematically investigated. The detection limits, repeatability, and linearity for the anions are comparatively studied at 374, 274, and 254 nm, and the results show that 374 nm is the optimal length. The simultaneous multiwavelength detection at 374, 254, 214, and 195 nm can assist in confirming the identification of UV-absorbing anions.

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

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.

Determination of Inorganic Cations by Capillary Ion Electrophoresis in Ilex paraguariensis (St. H.), a Plant Used to Prepare Tea in South America

A practical and economical capillary ion electrophoresis method with indirect UV detection at 214 nm was developed for determination of inorganic cations in plants of Ilex paraguariensis(St. H.) and their infusion known as mate tea, a very popular beverage in South America. A microwave digestion procedure was used to prepare the herbal plants, but the infusion was only diluted. The background electrolyte contained 6mM imidazole and 10mM alpha-hydroxyisobutyric acid, pH 4.0. The running voltage was 20 kV and temperature was 25 degrees C. K, Na, Ca, Mg, and Mn ions were quantitated, and linearity was demonstrated between 0.6 and 120 ppm. The results were in good agreement with those obtained by flame atomic absorption and emission spectrometry. Accuracy of the method was verified by comparison with Beech leaves CRM 100, a standard reference material. The high content of minerals and several oligoelements, especially Mn in mate tea, is considered to be of nutritional interest.

Analysis of small amines in pharmaceuticals by capillary ion electrophoresis with conductivity detection

Capillary ion electrophoresis (CIE) with direct conductivity detection is a simple, fast, and versatile method for analysis of ions. In this study CIE with conductivity detection was used to analyze for hydroxylamine which is often used in the synthesis of pharmaceutical intermediates and drug substances. The conductivity response of hydroxylamine was investigated by using different buffer types and buffer concentrations. Other small amines were also studied with this method and were correlated with the molecular weight of the amines. The response of hydroxylamine increased significantly with MES–glycylglycine buffer as compared to MES–histidine buffer (LOD from 10 to 1 ppm). The detection of trace level of hydroxylamine in a drug substance proved to be possible with this method. The developed method was also tested for the linearity range, reproducibility, and selectivity with several small amines. The method developed in this study was demonstrated as a sensitive and reliable method for detection and quantification of small amines in pharmaceutical substances.

CAPILLARY ION ELECTROPHORESIS OF INORGANIC CATIONS IN STANDARD REFERENCE MATERIALS FROM VEGETABLE SOURCE

Taking into account the growing interest in applying capillary ion electrophoresis (CIE) for the determination of inorganic ions in real samples, this work focuses on CIE analysis of K+, Na+, Ca2+, Mg2+, and Mn2+ in botanical and food reference materials. Microwave-Assisted Digestion for sample preparation has been chosen ensuring a rapid, accurate, and reproducible treatment. A fused-silica capillary, 75 μm × 60 cm, a background electrolyte consisting of 6 mM imidazole and 10 mM α-hydroxy-isobutyric acid (HIBA). and indirect UV detection at 214 nm were used. Certified reference materials as Apple leaves SRM 1515 (NIST, USA), Beech leaves CRM 100, Rye flour CRM 381, Haricot beans CRM 383, and the green algae Ulva lactuca CRM 279 (BCR, Belgium) were selected. The optimal experimental conditions for microwave digestion and operational parameters in the electrophoresis system were studied. The calibration curves for K, Ca, Mg, and Mn ions were linear over the range of 0.6 ppm to 120 ppm and the detection limits (CLOD) were between 0.05 ppm and 0.20 ppm depending on the ions analyzed. Accuracy and precision within and between days were determined. The results were in good agreement with those reported in certified materials and they were also comparable with those obtained by atomic absorption and emission spectrometry.

CE-MS-MS of peptides using a novel orthogonal CE ESI sprayer and ion trap MS

Capillary electrophoresis (CE) coupled with mass spectrometry (MS) has proven a powerful alternative to conventional chromatographic separation techiques. Here the analysis of various compounds with a novel CE ESI sprayer and an ion trap mass analyzer is described.

Mixed-mode capillary electrochromatographic separation of anionic analytes

In this work, mixed-mode capillary electrochromatography is introduced as a method for selectivity manipulation in the separation of charged analytes and is investigated for a number of analytes. This concept involves utilising a component of the eluent to permit the chromatographic and capillary electrophoresis (CE) separation mechanisms to contribute in varying proportions to the separation. This approach was first investigated using a combination of CE with reversed-phase liquid chromatography (RP-LC) for hydrophobic, charged analytes (aliphatic sulfonates), and using the concentration of organic modifier in the eluent to control the contributions of CE and RP-LC. However, the use of reversed-phase columns was found to be problematic for mobile phases with less then 50% organic modifier due to the hydrophobicity of the stationary phase causing the column bed to overheat and dry, and low electroosmotic flow (EOF) values (µ ⩽ 17.8 × 10–9 m2 V–1 s–1) caused additional restrictions. In a second case, ion-exchange stationary phases were used, with the type and concentration of a competing anion in the eluent being used to control the contributions of ion chromatography (IC) and CE to the separation. Nine common inorganic anions were separated using a silica based anion-exchange column and phosphate (pH 7.20) or sulfate (pH 8.2) as eluent with direct UV detection at 214 nm and 17 inorganic and small organic anions were separated using a nitrate eluent (pH 6.80) with indirect UV detection at 214 nm. The separation selectivity was shown to be a combination of IC and CE.

Capillary ion electrophoresis screening of nerve agent degradation products in environmental samples using conductivity detection

A method of detecting signature methylphosphonic acid (MPA) breakdown products of V and G nerve agents in environmental samples was developed using capillary ion electrophoresis with conductivity detection. The electrolyte (30 m M l-histidine, 30 m M 2-(N-morpholino)ethanesulfonic acid, 0.7 m M tetradecyltrimethylammonium hydroxide, and 0.03 weight% Triton X-100) allowed baseline separation of MPA, ethyl methylphosphonic acid (EMPA), isopropyl methylphosphonic acid (IMPA), and pinacolyl methylphosphonic acid (PMPA) in less than 10 min. Detector response was linear in the 6–60 μg/ml concentration range (correlation coefficient=0.99) with a detection limit around 6 μg/ml. The application of this method for screening MPA, EMPA, IMPA, and PMPA in surface water, groundwater, and soil extracts is demonstrated.

Analysis of anions and cations in drinking water samples by Capillary Ion Analysis

The use of capillary ion electrophoresis (CIE, Waters’ tradename: Capillary Ion Analysis, CIA) for the analysis of four anions and four cations in drinking water samples available on the Malaysian market, such as natural mineral water, bottled drinking water and tap water, was investigated. In addition, zam-zam water (an underground water, much sought-after by Muslims and only available in Mekah, Saudi Arabia) was also analyzed. The anions analyzed were chloride, sulphate, nitrate and fluoride while the cations analyzed were potassium, calcium, sodium and magnesium. Results of this determination generally show a low content of anions and high content of calcium and magnesium in natural mineral water and non-detectable amounts of anions and cations in bottled drinking water. Out of the 15 mineral waters of different brands that were analyzed, four brands show anionic and cationic levels almost similar to that of tap water. With the exception of fluoride, an abnormally high level of both anions and cations were detected in all the zam-zam water samples analyzed, as compared to the other drinking waters.