A comparison of gradient capacity anion chromatography using macrocycles D-2.2.2 and D-2.2.1 in constant or variable temperature mode

The ability of macrocyclic ligands to complex alkali metal cations has been exploited to perform chromatographic separations of anions. Macrocycles adsorbed to reversed phase columns can complex eluent cations, thus generating anion exchange sites. Gradient separations of anions can be performed by changing the column capacity during the course of the separation, either by changing the eluent cation or by changing the column temperature. Gradient anion separations are performed by changing the eluent from sodium hydroxide to lithium hydroxide with the cryptand D-2.2.2, while similar anion separations are achieved with D-2.2.1 by a KOH-LiOH gradient. Since the complexation of cations by macrocycles is exothermic, increasing the column temperature decreases the anion column capacity, allowing temperature gradient separations. The experimentally measured DeltaH values for D-2.2.1 are higher than for D-2.2.2, leading to steeper gradients and thus better separations with D-2.2.1.     

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.     

Simultaneous separation of inorganic anions and cations by using anion-exchange and cation-exchange columns connected in tandem in ion chromatography

Inorganic anions and cations in environmental waters were determined by ion chromatography. Stationary and mobile phases were examined for the simultaneous separation of both anions and cations. Cations detection by UV detection requires a mobile phase with a UV absorbing additive, which indirectly visualizes cations as negative peaks. Simultaneous separation of anions and cations were achieved when using an eluent that consists of inorganic acid with weak basic amino acid as additives. It was convenient to separate both anions and cations by coupling anion-exchange and cation-exchange columns in tandem. The order of the separation columns connected affected the elution profiles. When the eluent comprises of multiple anions and a single cation, the anion-exchange column should be connected in the upper stream, whereas when the eluent comprises multiple cations and a single anion, the cation-exchange column should be connected in the upper stream. Use of switching valves also allowed simultaneous separation of anions and cations in a single chromatographic run. In the present work, operating conditions were optimized for the simultaneous separation of anions and cations.     

Application of retention modelling to the simulation of separation of organic anions in suppressed ion chromatography

The ion-exchange separation of organic anions of varying molecular mass has been demonstrated using ion chromatography with isocratic, gradient and multi-step eluent profiles on commercially available columns with UV detection. A retention model derived previously for inorganic ions and based solely on electrostatic interactions between the analytes and the stationary phase was applied. This model was found to accurately describe the observed elution of all the anions under isocratic, gradient and multi-step eluent conditions. Hydrophobic interactions, although likely to be present to varying degrees, did not limit the applicability of the ion-exchange retention model. Various instrumental configurations were investigated to overcome problems associated with the use of organic modifiers in the eluent which caused compatibility issues with the electrolytically derived, and subsequently suppressed, eluent. The preferred configuration allowed the organic modifier stream to bypass the eluent generator, followed by subsequent mixing before entering the injection valve and column. Accurate elution prediction was achieved even when using 5-step eluent profiles with errors in retention time generally being less than 1% relative standard deviation (RSD) and all being less than 5% RSD. Peak widths for linear gradient separations were also modelled and showed good agreement with experimentally determined values.     

Retention behavior of alkali, alkaline earth and transition metal cations in ion chromatography with an unmodified silica gel column

An unmodified silica gel (Develosil 30-5) column (150×4.6 mm I.D.) has been applied to the ion chromatographic separation of alkali, alkaline earth and transition metal cations. The retention behavior of the above cations on the bare substrate was investigated using a number of weak inorganic and organic acid eluents. During this investigation, several separations were achieved and the most suitable eluent conditions were identified. It was concluded that: (a) 1.5 mM HNO₃-0.5mM pyridine-2,6-dicar☐ylic acid eluent was the most effective for the simultaneous separation of common alkali and alkaline earth metal cations, (b) 1.5 mM oxalic acid eluent resulted in the best separation of alkali, alkaline earth, and transition metal cations, (c) 0.5 mM CuSO₄ eluent could be used for the separation of alkali metal cations alone and (d) 0.5 mM ethylenediamine-oxalic acid eluent at pH 5.5 resulted in themost efficient separation of both alkaline earth and transition metal cations.     

Prediction of the effects of methanol and competing ion concentration on retention in the ion chromatographic separation of anionic and cationic pharmaceutically related compounds

The mixed-mode separation of a selection of anionic and cationic pharmaceutically related compounds is studied using ion-exchange columns and eluents consisting of ionic salts (potassium hydroxide or methanesulfonic acid) and an organic modifier (methanol). All separations were performed using commercially available ion-exchange columns and an ion chromatography instrument modified to allow introduction of methanol into the eluent without introducing compatibility problems with the eluent generation system. Isocratic retention prediction was undertaken over the two-dimensional space defined by the concentration of the competing ion and the percentage of organic modifier in the eluent. Various empirical models describing the observed relationships between analyte retention and both the competing ion concentration and the percentage of methanol were evaluated, with the resultant model being capable of describing the separation, including peak width, over the entire experimental space based on six initial experiments. Average errors in retention time and peak width were less than 6% and 27%, respectively, for runs taken from both inside and outside of the experimental space. Separations performed under methanol gradient conditions (while holding the competing ion concentration constant) were also modelled. The observed effect on retention of varying the methanol composition differed between analytes with several analytes exhibiting increased retention with increased percentage methanol in the eluent. An empirical model was derived based on integration of the observed t_R vs. %methanol plot for each analyte. A combination of the isocratic and gradient models allowed for the prediction of retention time using multi-step methanol gradient profiles with average errors in predicted retention times being less than 4% over 30 different 2- and 3-step gradient profiles for anions and less than 6% over 14 different 2- and 3-step gradient profiles for cations. A modified peak compression model was used to estimate peak widths under these conditions. This provided adequate width prediction with the average error between observed and predicted peak widths being less than 15% for 40 1-, 2- and 3-step gradients for anions and less than 13% over 14 1-, 2- and 3-step gradients for cations.     

Optimization of alkali-alkalkine earth cation separation on weak cation-exchangers

Summary The elution behavior of alkali and earth alkaline cations has been studied on a newly synthesized weak cationexchanger (WCX). Applying this stationary phase isocratic separation of these cations is possible. The dependence of retention on competing ion concentration is described via a simple model. The influence of crown ethers as well as organic eluent additives on selectivity and peak efficiency is demonstrated. With optimized separation such cations can be determined with detection limits of about 0.5 ppm using a conductivity detector without ion suppression.     

Macrocyclic Ligands in Separations

The selectivity of macrocyclic ligands such as crown ethers and cryptands in binding metal and other cations in aqueous and nonaqueous solvents can be exploited to make ion separations. Cations are usually separated by direct interaction with the ligand. In addition, anions associated with the positively charged macrocyclic complexes can be separated in novel separations systems. We have incorporated macrocyclic ligands into high performance ion chromatography, liquid membranes, and solvent extraction separation systems involving coalescence extraction.     

Novel imidazolium stationary phase for high-performance liquid chromatography

A new imidazolium anion-exchange phase immobilized on silica is synthesized. HPLC separations of common inorganic anions (IO3-, Cl-, NO2-, Br-, NO3-, I-, SCN-) have been performed using a HPLC column (200 mm x 4.6 mm I.D.) packed with this stationary phase, with a phosphate buffer solution as the mobile phase and UV detection at 200 nm. The effects of pH and concentration of eluent on the separation of anions have been studied. Chromatographic parameters are calculated and the results show that the new stationary phase is of significant potential for the analysis of these anions. Successful separations of some ordinary organic anions have also been achieved with the said stationary phase. Meaningfully, organic and inorganic anions can be determined simultaneously and satisfactorily with several neutral compounds using the column. The separation of some organic compounds including hydroxybenzenes, bases and amines by this stationary phase with only water as the eluent has been investigated.     

Determination of sodium and ammonium ions in disproportionate concentration ratios by ion chromatography

In ion chromatography, samples of very different ammonium-to-sodium concentration ratios are difficult to quantify since these two cations have similar selectivities for stationary phases containing commonly used sulfonate or carboxylate cation-exchange functional groups. The IonPac CS15 cation-exchange column, with carboxylate and phosphonate functional groups as well as a crown ether group, was developed to address this limitation. Selectivity for the common inorganic cations on this column is different from that of conventional cation-exchange columns in that the separation between sodium and ammonium ions has been greatly increased, allowing for determinations of low levels of one in the presence of high levels of the other with an isocratic eluent. For larger than 4000:1 sodium-to-ammonium concentration ratios, an eluent step change or gradient elution is needed. For moderate ratios, combinations of this column with a carboxylate column, containing no crown ether group, can be used at room temperature with an isocratic eluent containing no organic solvent.     

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.     

Ion-chromatographic behavior of alkali metal cations and ammonium ion on zirconium-adsorbing silica gel

The preparation and evaluation of zirconium-adsorbing silica gel (Zr-Silica) as an ion-exchange stationary phase in ion chromatography for inorganic anions and cations was carried out. The Zr-Silica was prepared by the reaction of silanol groups on the surface of the silica gel with zirconium butoxide (Zr(OCH2CH2CH2CH3)4) in ethanol. The ion-exchange characteristics of the Zr-Silica were evaluated using 10 mM tartaric acid at pH 2.5 as eluent. The Zr-Silica was found to act as a cation-exchanger under the eluent conditions. The retention behavior of alkali and alkaline earth metal cations was then investigated. The Zr-Silica column was proved to be suitable for the simultaneous separation of alkali metal cations and ammonium ion. Excellent separation of the cations on a 15 cm Zr-Silica column was achieved in 25 min using 10 mM tartaric acid as eluent.     

Two-dimensional optimization using different pairs of variables for the reversed-phase high-performance liquid chromatographic separation of a mixture of acidic compounds

A cryptand-based anion exchanger has been developed in which the capacity and to a lesser degree, selectivity are adjustable simply by the choice of the mobile phase. Although much work has been done in the past using cryptand-based anion exchangers, these stationary phases were based on adsorbed cryptands rather than covalently bound cryptands. These phases suffered from the usual problems associated with adsorbed systems. A novel styrene-based cryptand has been synthesized which can be covalently attached to a solid support. A brief review of cryptands and binding constants as well as comparisons of adsorbed phases versus covalently bound phases will be discussed. Some of the unique chromatographic properties of this prototype column will be illustrated as well.     

Separation of inorganic anions by liquid chromatography with crown ether as eluent additive

Inorganic anions were separated on a reversed-phase stationary phase dynamically modified with crown ether as a selector in capillary ion chromatography. The eluent contained crown ether, acetonitrile and a salt. Free and cation-trapped crown ether molecules in the eluent were adsorbed on a hydrophobic stationary phase such as triacontyl-functionalized silica (C30). The eluent cations trapped on crown ether worked as the ion-exchange sites, where the eluent anions and the analyte anions were competing for electrostatic interaction. The sizes of crown ether and the salt cation affected the retention of analyte anions. The concentrations of acetonitrile and crown ether as well as the eluent anion also affected the retention of analyte anions. An aqueous solution containing 18-crown-6-ether, potassium salt and acetonitrile achieved larger retention for analyte anions. Effects of the eluent conditions on the retention of analyte anions were examined in detail.     

Successive non-suppressed ion chromatography of common cations and anions using dual columns with single eluent

A successive non-suppressed ion chromatography (IC) system for the determination of common cations (Na+, K+, Mg2+, Ca2+) and anions (Cl-, Br-, NO3-, SO4(2-)) was developed, using two separation columns and a single eluent. 5-Sulfoisophthalic acid eluent was very suitable for such separations with a commercially available cation-exchange column for the mono- and di-valent cations and with an ODS column coated with cetyltrimethylammonium for the anions. Both cations and anions were detected with conductimetrically high sensitivity without any suppressor. After injecting an aliquot of sample solution, the solvent front from the cation-exchange column, including most of the anionic species, was firstly accumulated into the additional 2 ml accumulation loop for 60 s, while the cation IC was performed. Subsequently, the accumulated fraction was introduced into the anion-exchange column and chromatographed. Relative standard deviations (RSDs) of retention times and conductimetric area responses for common cations were within 6% and within 4%, respectively. The linear relationships between molar concentration and detector response ranged from 0.01 to 1.00 mM with r2 of 0.9994 for Na+, 0.9992 for K+, 0.9993 for Mg2+, and 0.9988 for Ca2+. The successive anion IC through the accumulating process was also quantitative, with 95% recovery or over for each analyte. The linear ranges were between 0.01 and 1.00 mM with r2 of 0.9996 for Cl-, 0.9997 for Br-, 0.9993 for NO3-, and 0.9984 for SO4(2-). The method was applied to the determination of common cations and anions in several mineral waters and a hot spring water.     

反相硅胶整体柱离子对色谱法快速测定吡啶离子液体阳离子

建立了整体柱离子对色谱-紫外检测法梯度淋洗快速分离测定4种吡啶离子液体阳离子的方法。分离采用C18反相硅胶整体柱,以离子对试剂(用柠檬酸调节pH值)-乙腈为淋洗液,并采用多级梯度洗脱程序。实验考察了色谱柱、离子对试剂、乙腈浓度、色谱柱温度及流速对吡啶阳离子保留的影响,并讨论了其保留规律。咪唑阳离子的保留符合碳数规律。最佳色谱条件是:在流速3.0 mL/min,柱温30℃下,以1.0 mmol/L庚烷磺酸钠(pH 4.0)(A)+乙腈(B)为淋洗液进行梯度洗脱。淋洗梯度为0～2.0 min,10%B;2.0～2.5 min,10%～15%B;2.5～4.0 min,15%B;4.0～4.5 min,15%～20%B;4.5～10.0 min,20%B。在此条件下,4种吡啶阳离子可在7 min内基线分离。所测阳离子的检出限(S/N=3)为0.05～0.17 mg/L;峰面积的相对标准偏差(n=5)小于0.6%。将本方法用于实验室合成的离子液体样品和污水样品的分析,加标回收率在95.7%～99.0%之间。本方法准确、快速,具有较好的实用性。     

Peak parking technique for the simultaneous determination of anions and cations

An ion chromatography method is described for the simultaneous determination of anions (Cl^–, NO₃^–, and SO₄^2–) and cations (Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺) using a single pump, a single eluent and a single detector. An anion-exchange column modified with chondroitin sulfate C facilitated the elution of the above three anions using 5 mM tartaric acid as the eluent in isocratic mode, whereas the same eluent facilitated the separation of the above five cations on a commercially-available cation-exchange column. The separation columns were connected in series via two six-port switching valves, so the required cation-exchange or anion-exchange separation could be carried out by selecting the appropriate positions for the switching valves. The separations were completed in 30 min.     

离子色谱-抑制电导法分别测定海水中阴离子和阳离子

采用离子交换-抑制电导法测定海水中阴、阳离子。采用抑制电导可以降低淋洗液的背景电导,又可以增加被测离子的电导值,改善信噪比。采用的电化学自身再生抑制器,由连续电解水产生抑制淋洗液所需要的H^＋或者OH^-,加上电场引力,能用于高客量分离柱所用的淋洗液浓度和梯度淋洗。在试验条件下,利用阴离子和阳离子分离柱,配合抑制电导检测,可以同时分离和测定海水中7种阴离子和6种阳离子。且都可以得到很好的线性和较低的检出限。     

Two-dimensional ion chromatography using tandem ion-exchange columns with gradient-pulse column switching

A two-dimensional ion chromatography (2D-IC) approach has been developed which provides greater resolution of complex samples than is possible currently using a single column. Two columns containing different stationary phases are connected via a tee-piece, which enables an additional eluent flow and independent control of eluent concentration on each column. The resultant mixed eluent flow at the tee-piece can be varied to produce a different eluent concentration on the second column. This allows analytes strongly retained on the first column to be separated rapidly on the second column, whilst maintaining a highly efficient, well resolved separation of analytes retained weakly on the first column. A group of 18 inorganic anions has been separated to demonstrate the utility of this approach and the proposed 2D-IC method provided separation of this mixture with resolution of all analytes greater than 1.3. Careful optimisation of the eluent profiles on both columns resulted in run times of less than 28 min, including re-equilibration. Separations were performed using isocratic or gradient elution on the first column, with an isocratic separation being used on the second column. Switching of the analytes onto the second column was performed using a gradient pulse of concentrated eluent to quickly elute strongly retained analytes from the first column onto the second column. The separations were highly repeatable (RSD of 0.01–0.12% for retention times and 0.08–2.9% for peak areas) and efficient (typically 8000–260,000 plates). Detection limits were 3–80 ppb.     

Simultaneous separation of alkali and alkaline-earth cations on polybutadiene-maleic acid-coated stationary phase by mineral acid eluents

The simultaneous separation of alkali and alkaline-earth cations on polybutadiene-maleic acid-coated silica columns can be achieved with mineral acid eluents. Simple ion-exchange retention mechanisms and the high selectivity of eluent hydronium ions towards the carboxylate group are the basis for the separation. Use of mineral acid eluents allows using this column in both single column and suppressor-based IC systems. Both types of ion chromatography systems provide detection limits in the low-ng/ml range with excellent linearity.