Direct Conductivity Detection Research Articles

A Simple Method for Determination of Homologue Imidazolium Cations in Ionic Liquids Using IC with Direct Conductivity Detection

An investigation was carried out into the fast determination of five homologue imidazolium cations in ionic liquids by ion chromatography using a cation-exchange column and direct conductivity detection. Ethylenediamine, complex organic acid (citric acid, oxalic acid and tartaric acid) and organic modifiers (acetonitrile) were used as mobile phase. The influences of the eluent types, eluent concentration, eluent pH and column temperature on separation of the cations were discussed. Simultaneous separation and determination of the five homologue imidazolium cations in ionic liquids were achieved under an optimum condition. The optimized mobile phase was consisted of 0.25 mmol L−1 ethylenediamine + 0.5 mmol L−1 citric acid + 3% acetonitrile (v/v) (pH 4.1), set at a flow rate of 1.0 mL min−1. The column temperature was 40 °C and detection limits were obtained in the range of 1.1–45.6 mg L−1. The relative standard deviations of the chromatographic peak areas for the cations were <3.0% (n = 5). This method was successfully applied to separate imidazolium cations in ionic liquids produced by organic synthesis. The recoveries of spiked components were 92.5–101.9%.

Determination of Imidazolium Ionic Liquid Cations by Ion-Pair Chromatography Using a Monolithic Column and Direct Conductivity Detection

Determination of four imidazolium ionic liquid cations by ion-pair chromatography was carried out using direct conductivity detection. Chromatographic separations were performed on a silica-based monolithic column with 1-heptanesulfonic acid sodium + acetonitrile + citric acid as eluent. Carbon number rule and influence of acetonitrile on the retention of imidazolium cations were discussed. Detection limits (S/N = 3) for the cations were 2.1–55.9 mg L−1. Relative standard deviations (RSD, n = 5) for peak areas were less than 3.0%. The method has been successfully applied to the determination of two ionic liquids synthesized by organic chemistry lab.

Rapid Analysis of Nitrate and Nitrite by Ion-Interaction Chromatography on a Monolithic Column

Ion-interaction chromatography with direct conductivity detection has been used for analysis of nitrate and nitrite. Chromatographic separation was performed on a monolithic silica-based C18 column dynamically modified with tetrabutylammonium (TBA+). Using the optimized mobile phase, containing 2.0 mmol L−1 TBA+ and 0.8 mmol L−1 citrate (pH 6.0), delivered at a flow rate of 6.0 mL min−1, separation of five anions (chloride, nitrite, bromide, nitrate, and sulfate) was achieved in only 40 s at a column temperature of 30 °C. The detection limits for nitrate and nitrite were 0.74 and 0.92 mg L−1, respectively. The relative standard deviation (RSD, n = 5) of the retention times of nitrate and nitrite was 0.1% and RSD of chromatographic peak areas were 0.4 and 0.2%, respectively. The method was successfully used for analysis of the anions in groundwater. Recovery of nitrate and nitrite was 99.1 and 105%, respectively.

Fast determination of tetrafluoroborate by high-performance liquid chromatography using a monolithic column

Fast determination of tetrafluoroborate by high-performance liquid chromatography using a silica-based monolithic column and direct conductivity detection was carried out. Chromatographic separation was performed on a Chromolith Speed ROD RP-18e column (50 mm × 4.6 mm i.d.) with tetrabutylammonium hydroxide (TBA-OH) + phthalic acid as eluent. The effects of eluent concentration, eluent pH, column temperature and flow rate on retention time of tetrafluoroborate were investigated. The optimized chromatographic conditions for determination of tetrafluoroborate were using 0.5 mM TBA-OH + 0.31 mM phthalic acid (pH 5.5) as eluent, column temperature of 30 °C and flow rate of 6.0 mL/min. Retention time of tetrafluoroborate was less than 1 min under the conditions. Common anions (Cl −, Br −, NO 3 − and SO 4 2−) did not interfere with the determination of tetrafluoroborate. Detection limit (S/N = 2) for tetrafluoroborate was 1.4 mg/L. The linear range of calibration curve between peak area and the concentration of tetrafluoroborate was from 1.4 to 100.0 mg/L. The reproducibility was 0.09% and 1.8% ( n = 5) relative standard deviation (RSD) for retention time and peak area, respectively. The method has been applied to the determination of tetrafluoroborate in ionic liquids. Recoveries of tetrafluoroborate after spiking were 98.2–101.5%.

Separation of purine and pyrimidine bases by ion chromatography with direct conductivity detection

A simple, rapid and accurate method for the simultaneous determination of four purine and pyrimidine bases (cytosine, 5-methylcytosine, adenine and N6-methyladenine) has been developed. The quantitative determination of these bases was accomplished by ion chromatography (IC) with direct conductivity detection (CD) based on their ionization in acidic medium without chemical suppression. The recovery of cytosine, 5-methylcytosine, and adenine in calf thymus DNA was more than 98% ( n = 3) and the relative standard deviation (RSD, n = 5) less than 2.4%. In a single chromatographic run, the four bases could be separated and determined in less than 10 min. The detection limits were found to be 0.05 μg/mL for cytosine, 0.08 μg/mL for 5-methylcytosine, 0.07 μg/mL for adenine, and 0.07 μg/mL for N6-methyladenine. Linear ranges were 0.2–95.1 μg/mL for cytosine ( r 2 = 0.9996), 0.3–196.6 μg/mL for 5-methylcytosine ( r 2 = 0.9994), 0.3–105.5 μg/mL for adenine ( r 2 = 0.9998), and 0.3–159.1 μg/mL for N6-methyladenine ( r 2 = 0.9999). With the proposed method, purine and pyrimidine bases could be successfully detected in calf thymus DNA. We also determined these bases in calf thymus DNA using RP-HPLC. Compared to RP-HPLC, the IC method offers advantages such as high selectivity and simple mobile phase.

Conductivity detection in capillary zone electrophoresis: Inspection by PeakMaster

A simple rule stating that the signal in conductivity detection in capillary zone electrophoresis is proportional to the difference between the analyte mobility and mobility of the background electrolyte (BGE) co-ion is valid only for systems with fully ionized electrolytes. In zone electrophoresis systems with weak electrolytes both conductivity signal and electromigration dispersion of analyte peaks depend on the conductivity and pH effects. This allows optimization of the composition of BGEs to give a good conductivity signal of analytes while still keeping electromigration dispersion near zero, regardless of the injected amount of sample. The demands to achieve minimum electromigration dispersion and high sensitivity in conductivity detection can be accomplished at the same time. PeakMaster software is used for inspection of BGEs commonly used for separation of sugars (carbohydrates, saccharides) at highly alkaline pH. It is shown that the terms direct and indirect conductivity detection are misleading and should not be used.

Determination of β 2-agonists by ion chromatography with direct conductivity detection

A simple method for the simultaneous detection of four β 2-agonists (salbutamol, fenoterol, clorprenaline, and clenbuterol) using ion chromatography (IC) with direct conductivity detection (CD) based on their ionization in acidic medium without chemical suppression is presented. The mixture of 1.8 mM HNO 3 and 2% (v/v) acetonitrile was used as eluent. The method could be applied to the determination of the β 2-agonists in pharmaceutical preparations. The recovery of salbutamol and clenbuterol in tablets was more than 97% ( n = 3) and the relative standard deviation ( n = 11) less than 2.8%. With the proposed method, salbutamol could also be successfully detected in human plasma. In a single chromatographic run, the four β 2-agonists can be separated and determined in less than 8 min. The linear ranges were of 7.0–1.4 × 10 3 ng/ml for salbutamol, 34–7.8 × 10 3 ng/ml for fenoterol, 8.0–1.6 × 10 3 ng/ml for clorprenaline, and 25–7.5 × 10 3 ng/ml for clenbuterol. The detection limits were 2.0 ng/ml for salbutamol, 10 ng/ml for fenoterol, 3.0 ng/ml for clorprenaline, and 10 ng/ml for clenbuterol.

Determination of ephedrine and related compounds in pharmaceutical preparations by ion chromatography with direct conductivity detection

An ion chromatographic method with conductivity detection for the simultaneous determination of ephedrine, pseudoephedrine and norephedrine was developed. A mixture of 2.0 mmol/L HNO3 and 2% (v/v) acetonitrile was used as eluent. The three ephedrine-like compounds were separated and determined within 20 min. The linear ranges were 0.08-50 microg/mL for ephedrine, 0.08-40 microg/mL for pseudoephedrine and 0.06-40 microg/mL for norephedrine. The detection limits were 0.03 microg/mL for ephedrine and pseudoephedrine, and 0.02 microg/mL for norephedrine. The method has been applied successfully to the determination of these sympathomimetics in pharmaceutical preparations and in Ephedra herbs.

A simple method for the study of salbutamol pharmacokinetics by ion chromatography with direct conductivity detection

A simple method for the study of the pharmacokinetics of salbutamol using ion chromatography with direct conductivity detection without chemical suppression is presented in this paper. Baseline separation of salbutamol from plasma components was achieved by using diluted HNO 3 (2 mmol l −1) as mobile phase with acetonitrile (6%, v/v) as modifier. Atenolol was utilized as internal standard. Under the optimal conditions, the linear regression coefficients of the calibration curves are 0.996 within the concentration range of 1000–3 ng ml −1 salbutamol. The detection limit (signal-to-noise ratio = 3) was 1 ng ml −1 salbutamol. The noncompartmental pharmacokinetic parameters for salbutamol following administration of 8 mg salbutamol to 18 subjects were tested. The results coincided most satisfactorily with the results obtained by using reversed-phase high performance liquid chromatography (HPLC) with fluorescence detection.

Rapid, low pressure, and simultaneous ion chromatography of common inorganic anions and cations on short permanently coated monolithic columns.

Short permanently coated reversed-phase silica based monolithic columns have been investigated for the rapid separation of inorganic anions and cations. One 2.5 x 0.46 cm column was permanently coated with didodecyldimethylammonium (DDAB), for anion analysis; and a second 5.0 x 0.46 cm column was coated with dioctylsulphosuccinnate (DOSS), for cation analysis. The use of a single combined eluent of 2.5 mM phthalate/1.5 mM ethylenediamine, at flow rates of between 4.0 and 8.0 mL/min, resulted in the rapid separation of 8 anions (in under 100 s) and 5 cations (in under 100 s) on the above columns when used individually, with detection limits for common anions ranging from approximately 0.25 to 5 mg/L, and between 2.5 and 50 mg/L for alkaline earth metals, by direct and indirect conductivity detection, respectively. However, with both columns subsequently connected in parallel, with the eluent delivered using a flow splitter from a single isocratic pump, the simultaneous analysis of anions and cations was also possible, based on a single conductivity detector. The potential of this system for the rapid, complete screening of water samples for multiple common anions and cations is shown.

Application of a contactless conductometric detector for the simultaneous determination of small anions and cations by capillary electrophoresis with dual-opposite end injection

A contactless conductometric detection (CCD) system for capillary electrophoresis (CE) with a flexible detection cell was applied for the simultaneous determination of small anions and/or cations in rain, surface and drainage water samples. The applied frequency, the amplitude of the input signal, the electrolyte conductivity and electrode distance were found to be the most significant factors affecting the detection sensitivity. 2-( N-Morpholino)ethanesulfonic acid/histidine-based (MES/His) electrolytes were used for direct conductivity detection of anions and cations, while ammonium acetate was selected for indirect conductivity determination of alkylammonium salts. For the simultaneous separation procedure, involving dual-opposite end injection, an electrolyte consisting of 20 m M MES/His, 1.5 m M 18-crown-6 and 20 μ M cetyltrimethylammonium bromide provided baseline separation of 13 anions and cations in less than 6 min. The detection limits achieved were 7–30 μg/l for direct conductometric detection of various common inorganic cations and anions, excluding F − (62 μg/l) and H 2PO 4 − (250 μg/l), and 35–178 μg/l for indirect conductometric detection of alkyl ammonium cations. The developed electrophoretic method with conductometric detection was compared to ion chromatography.

EVALUATION OF AN ION EXCLUSION/DIRECT CONDUCTIVITY METHOD FOR QUANTITATING ACETIC AND LACTIC ACIDS IN PHARMA-CEUTICAL LVP BASE FORMULATIONS

Organic acids (e.g., acetic, lactic and gluconic acid) are common components in Large Volume Parenteral (LVP) base solutions. The characterization of such solution products is necessary for production control and product release. A chromatographic method, which couples separation by anion exclusion with direct conductivity detection, has been developed for this purpose. The evaluation of this method, carried out in a manner consistent with United States Pharmacopeia (USP) and International Council for Harmonization (ICH) guidelines, is summarized in this manuscript. The method meets the acceptance criteria for in-process and/or product release testing with respect to accuracy, linearity, range, precision, and specificity.

Ultra-fast HPLC separation of common anions using a monolithic stationary phase.

In this work, a monolithic column was used to perform ultrafast separations of common inorganic anions in as little as 15 seconds. Separations were performed using ion-interaction chromatography with tetrabutylammonium-phthalate as the ion-interaction reagent and were monitored using either direct conductivity or indirect absorbance detection. Detection limits for direct conductivity were in the low ppm range, whereas those for indirect absorbance detection were up to an order of magnitude higher. The reproducibility was 0.4% and 2% RSD for retention time and peak area, respectively, for a one minute separation, and 2.8% and 3-15%, respectively, for the 15 second separation. The proposed method was validated versus standard ion chromatography with suppressed conductivity detection for the analysis of an industrial water sample.

EVALUATION OF AN ION EXCLUSION/DIRECT CONDUCTIVITY METHOD FOR QUANTITATING ACETIC AND LACTIC ACIDS IN PHARMA-CEUTICAL LVP BASE FORMULATIONS

Organic acids (e.g., acetic, lactic and gluconic acid) are common components in Large Volume Parenteral (LVP) base solutions. The characterization of such solution products is necessary for production control and product release. A chromatographic method, which couples separation by anion exclusion with direct conductivity detection, has been developed for this purpose. The evaluation of this method, carried out in a manner consistent with United States Pharmacopeia (USP) and International Council for Harmonization (ICH) guidelines, is summarized in this manuscript. The method meets the acceptance criteria for in-process and/or product release testing with respect to accuracy, linearity, range, precision, and specificity.

Comparison of different conductivity detector geometries on an isotachophoresis PMMA-microchip

Conductivity detection is one of the most often employed means of detection in isotachophoresis. In microanalytical devices, thin-film platinum electrodes can be used for conductivity detection and for other electrochemical methods of detection. The design and the performance of different electrode geometries for on-column contact conductivity detection with thin-film platinum electrodes integrated on an isotachophoresis PMMA-microchip is described. Three different electrode geometries for direct conductivity detection were used for the investigation of isotachophoretic separations. The influence of the width of the electrodes and their positioning relative to the separation channel was investigated. The performance of the different detectors is compared for the analysis of organic carboxylic acid anions.

Anion-exchange capillary electrochromatography with indirect UV and direct contactless conductivity detection.

Conductivity detection is applied to ion-exchange capillary electrochromatography (IE-CEC) with a packed stationary phase, using a capacitively coupled contactless conductivity detector with detection occurring through the packed bed. Columns were packed with a polymeric latex-agglomerate anion-exchanger (Dionex AS9-SC). A systematic approach was used to determine suitable eluants for IE-CEC separations using simultaneous indirect UV and direct conductivity detection. Salicylate and p-toluenesulfonate were identified as potential eluant competing anions having sufficient eluotropic strength to induce changes in separation selectivity, but salicylate was found to be unsuitable with regard to baseline stability. It was also found for both indirect UV and direct conductivity detection that homogenous column packing was imperative, and monitoring of the baseline could be used to assess the homogeneity of the packed bed. Using a p-toluenesulfonate eluant, the separation of eight common anions was achieved in 2.5 min. Direct conductivity detection was found to be superior to indirect UV detection with regard to both baseline stability and detection sensitivity with detection limits of 4-25 microg/L being obtained. However, the calibration for each anion was not linear over more than one order of magnitude. When using conductivity detection, the concentration of the eluant could be varied over a wider range (2.5-50 mM p-toluenesulfonate) than was the case with indirect UV detection (2.5-10 mM), thereby allowing greater changes in separation selectivity to be achieved. By varying the concentration of p-toluenesulfonate in the eluant, the separation selectivity could be manipulated from being predominantly ion-exchange in nature (2.5 mM) to predominantly electrophoretic in nature (50 mM).

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.

Simultaneous determination of catecholamines by ion chromatography with direct conductivity detection

A simple method for simultaneous determination of three catecholamines using ion chromatography (IC) with direct conductivity detection (CD) based on the ionization of catecholamines in acidic medium without chemical suppression is developed in the present paper. The method could be used for the determination of these catecholamines in pharmaceutical preparations for the purpose of drug quality control. The recovery of catecholamines was more than 97% ( n=3) and the relative standard deviation (R.S.D.) ( n=11) was less than 2.1%. In a single chromatographic run, norepinephrine (NE), epinephrine (E) and dopamine (DA) can be determined in less than 10 min. The detection limits were found to be 0.001 μg/ml for NE, 0.01 μg/ml for E and DA respectively. Linear ranges were 0.01–50 μg/ml for NE ( r 2=0.9998), 0.1–50 μg/ml for E ( r 2=0.9995) and DA ( r 2=0.9999), respectively.

Separation of inorganic and organic anions by capillary zone electrophoresis with simultaneous indirect UV and conductivity detection.

The analysis of low-molecular-mass anionic compounds using capillary zone electrophoresis (CZE) with simultaneous direct conductivity and indirect UV detection is described. A number of carrier electrolyte compositions has been investigated with respect to the crucial parameters for their compatibility with both detection principles discussed in this paper (UV absorptivity and electrophoretic mobility of the buffer coion). Additionally, parameters including pH and buffer concentration affecting both peak symmetry and signal-to-noise ratio obtained for the analytes of interest had to be optimized. Best results could be achieved with a carrier electrolyte consisting of 4-aminobenzoic acid, bis(2-hydroxyethyl)imino-tris(hydroxymethyl)aminomethane (BIS-Tris) and a pH of 7.0 adjusted with LiOH. This running buffer was used for the analysis of low-molecular-mass anionic compounds in a sample of white wine.

Determination of low-molecular-mass anionic compounds in beverage samples using capillary zone electrophoresis with simultaneous indirect ultraviolet and conductivity detection

The determination of low-molecular-mass anionic components in beer samples using capillary zone electrophoresis with simultaneous direct conductivity and indirect UV detection is described. Optimization of the carrier electrolyte composition has been performed taking into account parameters such as pH as well as those that were found to be crucial for its compatibility with both detection principles employed in this paper. The latter are UV absorptivity, electrophoretic mobility of the buffer co-ion and the buffer co-ion concentration, which strongly influence the quality (signal-to-noise ratio, chromatographic resolution of adjacent peaks) of the electropherogram. Best results could be achieved with a carrier electrolyte consisting of 4-aminobenzoic acid, tetradecyltrimethylammonium bromide electroosmotic flow reversal and a pH of 5.75, adjusted using histidine. Using this carrier electrolyte, limits of detection ranging from 0.218 mg l −1 for pyroglutamate down to 0.018 mg l −1 for chloride could be obtained.