Indirect conductimetric detection of cyclodextrins in liquid chromatography

Indirect conductimetric detection of cyclodextrins (CDs) was investigated in liquid chromatography (LC). The mobile phase contained an electrolytic substance which maintained high background, and CDs were indirectly detected owing to a depression of the background signal due to dilution of the mobile phase by the analyte molecules. The dynamic reserve, defined as the ratio of the background to its noise level, was (1–2) × 10⁵, and the detection limits at S/N = 3 were 23–40 ng for CDs when a microcolumn LC system was employed.     

Indirect conductimetric detection of nonelectrolytes in microcolumn liquid chromatography

Summary Indirect conductimetric detection of nonelectrolytes was investigated in microcolumn liquid chromatography. The mobile phase contained an electrolytic substance which maintained a high background and analytes were indirectly detected owing to a depression of the background signal. The dynamic reserve, defined as the ratio of the background to its noise level, was 9.3×10⁴, and the detection limits at S/N=3 were 10–32 ng for alcohols examined.     

Determination of tetraethyl ammonium by ion-pair chromatography with indirect ultraviolet detection using 4-aminophenol hydrochloride as background ultraviolet absorbing reagent

A method was developed for the determination of tetraethyl ammonium (TEA) by reversed-phase ionpair chromatography with indirect ultraviolet detection. Chromatographic separation was achieved on a reversed-phase C18 column using background ultraviolet absorbing reagent - ion-pair reagent - organic solvent as mobile phase. The effects of the background ultraviolet absorbing reagents, detection wavelength, ion-pair reagents, organic solvents and column temperature on the determination method were investigated and the retention rules discussed. Results found that TEA could be successfully analyzed by using 0.7 μmol/L 4-aminophenol hydrochloride and 0.15 μmol/L 1-heptanesulfonic acid sodium mixed with 20% (v/v) methanol asmobile phase at a UV detection wavelength of 230 nm. Under these conditions, the retention time of tetraethyl ammonium was 2.85 min. The detection limit (S/N = 3) for TEA was 0.06 mg/L. The relative standard deviations (n = 5) for peak area and retention time were 0.35% and 0.02%, respectively. The method has been successfully applied to the determination of synthesized tetraethyl ammonium bromide. Recovery of tetraethyl ammonium after spiking was 99.1%.     

Determination of selenoamino acids using two-dimensional ion-pair reversed phase chromatography with on-line detection by inductively coupled plasma mass spectrometry

Analytical methods for the speciation of nine selenium species (selenite, selenate, selenourea, trimethylselenonium ion, selenocystamine, selenocystine, selenocysteine, selenomethionine and selenoethionine) that are commonly encountered in biological and environmental samples were developed. Good separation was achieved by either a mixed ion-pair reversed phase chromatography (LiChrosorb RP 18, 2.5 mM 1-butanesulfonate-8 mM tetramethylammonium hydroxide-4 mM malonic acid-0.05% methanol, pH 4.5) or a conventional ion-pair reversed phase chromatography (Inertsil ODS, 10 mM tetraethylammonium hydroxide-4.5 mM malonic acid, pH 6.8) with on-line ICP-MS detection. Using a 20-μl sample loop, low detection limits around 1 ng ml⁻¹ expressed as Se were achieved for the examined selenium species. The methods were used for the determination of selenoamino acids in a selenium nutritional supplement. The developed methods were found to be rather robust. No alteration of the separation was observed when the protease enzymatic extracts were analyzed without dilution. Both water extracts and enzymatic extracts were chromatographed first with the mixed ion-pair reversed phase chromatographic system, then the major chromatographic peaks were collected and analyzed by the second ion-pair reversed phase chromatographic system for a further verification of their identity. Selenomethionine was found to be the major selenium species in the supplement. A major unknown species, probably Se-adenosylhomocysteine, could be determined in the extracts. A biological reference material, Dolt-2, was also examined for the selenoamino acids. Selenocystine and selenomethionine could be detected in its enzymatic extract, suggesting that Dolt-2 may be used as a reference material for the identification of selenoamino acids in biological and environmental samples. As selenoethionine does not occur naturally in the investigated samples, it is added as an internal standard in this study.     

Fast analysis of thiocyanate by ion-pair chromatography with direct conductivity detection on a monolithic column

Fast analysis of thiocyanate by ion-pair 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 using tetrabutylammonium hydroxide （TBA）-phthalic acid-acetonitrile as eluent. The effects of eluent concentration, eluent pH value, column temperature and flow rate on retention time of thiocyanate were investigated. The optimized chromatographic conditions for the determination of thiocyanate were as follows： 0.25 mmol/L TBA-0.18 mmol/L phthalate-7% acetonitrile （pH 5.5） as eluent, column temperature of 30 ℃, and flow rate of 6.0 mL/min. Retention time of thiocyanate was less than 1 min under the conditions. Common anions （Cl^-, NO3 , SO42 and I^- ） did not interfere with the determination of thiocyanate. Detection limit （S/N = 3） of thiocyanate was 0.96 mg/L. Calibration graph between peak area and the concentration of thiocyanate was linear in the range of 2.0- 100.0 mg/L. Relative standard deviation （RSD） of chromatographic peak area was 1.4% （n = 5）. This method has been applied to the determination of thiocyanate in ionic liquids. Recoveries of thiocyanate after spiking were 100.5%.     

Determination of aromatic sulfonates in coastal water by on-line Ion-pair solid-phase extraction/ion-pair liquid chromatography with UV detection

Summary A method for determining aromatic sulfonates in sea water is presented. Ion-pair solid phase extraction is coupled on-line to ion-pair liquid chromatography with UV detection. In the enrichment step, the recoveries from 100 mL of sea water were higher than 65% for most analytes. Linearity was good and detection limits were between 0.02 and 1 μgL⁻¹. The repeatability of the method, expressed as % of relative standard deviation (n=3) was between 1 and 15%. The method was checked in coastal water near the commercial port of Tarragona, where effluents from the petrochemical industry are discharged. All the samples taken were found to contain 2-naphthalenesulfonate.     

硅胶整体柱离子对色谱快速分析碘离子

建立了用硅胶整体柱和直接电导检测的离子对色谱快速分析碘离子的方法。采用Chromolith Speed ROD RP-18e色谱柱,以氢氧化四丁铵(离子对试剂)-邻苯二甲酸+乙腈(有机改进剂)为淋洗液,讨论了离子对试剂浓度、有机改进剂浓度、pH、流速和色谱柱温度对碘离子保留的影响。确定最佳色谱条件为:0.25 mmol/L氢氧化四丁铵-0.18 mmol/L邻苯二甲酸+体积分数7%乙腈(pH5.5)作为淋洗液,流速6.0 mL/min,色谱柱温30℃。在此条件下,碘离子的保留时间在0.5 min之内,其它常见阴离子(Cl-、NO3-、SO42-)及SCN-、ClO4-不干扰测定。方法的检出限为0.86 mg/L,标准曲线的线性范围为1.6～85.0 mg/L,峰面积的相对标准偏差为2.3%。将方法应用于测定地下水和果汁中的碘离子,加标回收率为98.5%～104.2%。     

Microcolumn ion chromatography with conductimetric detection

Summary A miniaturized flow cell for an electrical conductivity detector has been made and applied to the ion chromatography of inorganic anions. It consisted of stainless steel tubes (0.13 mm ID×0.31 mm OD) and PTFE tubes (0.25 mm ID×2 mm OD). The detection limit for chloride at S/N=3 was 36 pg or 0.33 ppm for a 0.11 l injection.     

Determination of iodine anion in dried kelp and iodized throat tablets by reversed-phase ion-pair liquid chromatography with direct UV detection

Summary In this paper, the amount of iodine anion in dried kelp and iodized throat tablets was determined by reversed-phase ion-pair chromatography. A 300×4 mm I.D. columns packed with -Bondapak-C18 (5 m) was used and distilled water containing 10 mmol/l trimethylphenyl ammonium bromide was used as the eluent. UV detection at 231 nm was selected to monitor the iodine anion.     

Microcolumn ion chromatography of inorganic anions using bovine serum albumin stationary phase with indirect photometric detection

Summary Microcolumn ion chromatography of inorganic anions has been studied using bovine serum albumin immobilized on silica gel as a stationary phase. Several mobile phase solutions were examined, involving sodium iodide, potassium hydrogen phthalate, 2,6-anthraquinone disulfonate (2,6-AQDS) and sodium salicylate. 2,6-AQDS achieved better separation of the analytes tested. Chloride, nitrate, iodide, thiocyanate and sulphate could be separated within 8 min. Detection limits were in the range of 0.9–2.9 M, corresponding to mass detection limits of 0.18–0.59 pmol. The system was applied to the determination of inorganic anions in environmental water and biological samples.     

Determination of ethambutol by ion-pair reversed phase liquid chromatography with UV detection

An ion-pair reversed phase liquid chromatography method for the antituberculosis drug ethambutol hydrochloride was developed using sodium 1-heptanesulfonate (4.0 mg/ml) as an ion-pairing (IP) reagent. To enable detection of the ethambutol with a UV detector without sample pretreatment, the pH 4.5 aqueous tetrahydrofuran (THF) (25%, v/v) mobile phase contained 1.0 mM Cu(II), which forms a UV-absorbing complex with the analyte. At a column temperature of 35 °C, ethambutol gives a symmetrical peak with a retention time of 5 min. Chromatographic conditions were optimized through study of the effects of mobile phase composition and pH, Cu(II) and IP reagent concentration, and column temperature. The method is shown to be simple, precise, efficient, robust, linear up to at least 0.25 mg/ml, and to have a limit of quantitation of 6 μg/ml.     

Indirect detection in reversed-phase liquid chromatography: Ion-pair retention models for cations on polystyrene polymers

Summary The distribution equilibria of cationic compounds in reversed-phase chromatographic systems (ion-pair chromatography) have been studied on the basis of their effect on a detectable mobile phase component. The solid phase was a polystyrene-divinylbenzene copolymer and the detectable component, a quaternary ammonium ion, 1-methylpyridine. The solutes were mono- and divalent amines and quaternary ammonium ions. The cations can be retained by ion-pair adsorption and ion exchange. Expressions for the ion-pair retention of the solutes and the mobile phase cation (system peak) have been developed assuming Langmuir distribution of ion pairs to a solid phase with one kind of binding site. The validity of the expressions has been tested by evaluation of ion-pair distribution constants using non-linear curve fitting techniques. Good agreement for the constants of common ion pairs was obtained from different kinds of capacity ratio expressions. Ion exchange retention can appear beside ion-pair retention, and it has been observed in the pH range 1.6–6.1. The effect depends not only on cations in the mobile phase, but also on the nature of the buffering systems.     

Rapid and simultaneous determination of imidazolium and pyridinium ionic liquid cations by ion-pair chromatography using a monolithic column

A method for rapid and simultaneous determination of imidazolium and pyridinium ionic liquid cations by ion-pair chromatography with ultraviolet detection was developed.Chromatographic separations were performed on a reversed-phase silica-based monolithic column using 1-heptanesulfonic acid sodium-acetonitrile as mobile phase.The effects of ion-pair reagent and acetonitrile concentration on retention of the cations were investigated.The retention times of the cations accord with carbon number rule.The method has been successfully applied to the determination of four ionic liquids synthesized by organic chemistry laboratory.     

Determination of tylosin and related macrolides in fermentation broth by gradient elution ion-pair liquid chromatography

Summary A gradient elution ion-pair HPLC assay was developed for the analysis of tylosin and related macrolides in fermentation broth. The effect of the ion-pair agent concentration and identity was studied to optimize the reproducibility of the separation. The assay compares favorably with the biological assay and is sufficiently durable for the analysis of many samples per column.     

Retention behaviour of an homologous series of oligodeoxythymidilic acids using reversed-phase ion-pair chromatography

Summary The retention behaviour of an homologous series of phosphorylated oligodeoxythymidylic acid (pd(T)_5–18) oligonucleotides was studied using reversed-phase ion-pair chromatography with isocratic elution conditions. The effects of temperature, pH, eluent ionic strength, percentage organic modifier, concentration and alkyl chain length of the ion-pairing reagent were investigated. The retention behaviour was generally explicable by current theoretical models of ion-pair chromatography. However, the marked effect of mobile phase pH on the retention of the oligonucleotides was unexpected, and this was ascribed to the presence of ionisable residual silanols on the surface of the reversed-phase packing material.     

Chiral separation of aminoalcohols by ion-pair chromatography

Summary A new chiral counter ion, N-benzoxycarbonyl-glycyl-L-proline (ZGP), added to the organic mobile phase (dichloromethane) has been used for separation of enantiomers of aminoalcohols with LiChrosorb DIOL as the solid phase. Separation factors of 1.2 to 1.4 for enantiomers of -adrenergic blocking agents (e.g., alprenolol, metoprolol and propranolol) were obtained. Some structural requirements in the solutes and the counter ion essential for chiral resolution were observed. The retention was regulated by the concentration of counter ion or by the addition of triethylamine to the mobile phase. The chiral counter ion was utilized to determine the enantiomeric impurity of less than 0.1% in S-alprenolol and for the analysis of propranolol enantiomers in plasma samples.Presented at the 9th International Symposium on Column Liquid Chromatography, Edinburgh, July, 1985.     

Determination of melamine in milk-based products and other food and beverage products by ion-pair liquid chromatography-tandem mass spectrometry

This paper describes a fast method for the sensitive and selective determination of melamine in a wide range of food matrices, including several milk-based products. The method involves an extraction with aqueous 1% trichloroacetic acid before the injection of the 10-fold diluted extract into the liquid chromatography-electrospray tandem mass spectrometry (LC-ESI-MS/MS) system, using labelled melamine as the internal standard. As melamine is present in aqueous media in the cationic form, the chromatographic separation in reversed-phase LC requires the use of anionic ion-pair reagents, such as tridecafluoroheptanoic acid (THFA). This allows a satisfactory chromatographic retention and peak shape in all the types of food samples investigated. The method has been validated in six food matrices (biscuit, dry pasta and four milk-based products) by means of recovery experiments in samples spiked at 1 and 5 mg kg⁻¹. Average recoveries (n = 5) ranged from 77% to 100%, with excellent precision (RSDs lower than 5%) and limits of detection between 0.01 and 0.1 mg kg⁻¹. In addition, accuracy and robustness of the method was proven in different soya-based matrices by means of quality control (QC) sample analysis. QC recoveries, at 1 and 2.5 mg kg⁻¹, were satisfactory, ranging from 79% to 110%. The method developed in this work has been applied to the determination of melamine in different types of food samples. All detections were confirmed by acquiring two MS/MS transitions (127 > 85 for quantification; 127 > 68 for confirmation) and comparing their ion intensity ratio with that of reference standards. Accuracy of the method was also assessed by applying it to a milk-based product and a baking mix material as part of an EU proficiency test, in which highly satisfactory results were obtained.     

Determination of paraquat in human blood plasma using reversed-phase ion-pair high-performance liquid chromatography with direct sample injection

This report describes the determination of paraquat (PQ) in human blood plasma samples by a direct-injection reversed-phase ion-pair chromatographic method. Blood plasma filtrate was injected directly into the LiChrospher® RP-18 alkyl-diol silica (ADS) precolumn integrated in a column switching system using a mixture of 3% 2-propanol and 10 mM sodium octane sulfonate (SOS) in a 0.05 M phosphate buffer (pH 2.8). After washing with this phase, the ADS precolumn was back-flushed with the analytical mobile phase consisting of 40% of methanol and 10 mM SOS in a 0.05 M phosphate buffer (pH 2.8) at a flow rate of 1.0 ml min⁻¹, in order to carry the analyte to a conventional reversed-phase analytical column, where the separation of PQ was achieved and finally detected by UV at 258 nm. The recoveries of PQ from human blood plasma samples ranged between 95.0 and 99.5% at nine different concentrations (from 0.05 to 3.00 μg of PQ ml⁻¹) with coefficients of variation <2.5% (n=3). The precision expressed as relative standard deviation was below 3.5% for between-day and below 4.3% for within-day measurements (n=5). The detection limit (signal-to-noise ratio, S/N>3) was 0.005 μg ml⁻¹ with an injection volume of 200 μl. The proposed method is promising for the identification and quantification of PQ at low concentration levels and is suitable for its analysis in human blood plasma samples from intentional or accidental poisonings cases with a sample throughput of 5 samples per hour.