Suppressed anion chromatography using mixed zwitter-ionic and carbonate eluents

Effect of mixed eluents which contain zwitterions such as 2-(N-morpholino)ethanesulfonic acid (MES), 3-(N-morpholino)propanesulfonic acid (MOPS), 2-(cyclohexylamino)ethanesulfonic acid (CHES) and 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS) and carbonate for suppressed conductivity ion chromatography (IC) were studied. The retention behaviors of all species were affected with different anion exchange columns when adding some zwitterion into carbonate eluent. The retention time of all species, especially those of strong retetion, was substantially shortened in AS14A column with either Na(2)CO(3)/CHES or Na(2)CO(3)/CAPS as eluent while a general increase in retetion time was noticed in AS9-HC column. Low pH of eluent was achieved when zwitterion was added. Without much changes in the background conductivity after suppressed, CHES could be used as suppressed conductivity ion chromatography eluent for determination of species unstable in strong alkaline, such as determination of phosphate in heteropoly acid (HPA) samples in AS14 column. The mixed eluent could also affect the selectivity when it was applied to the study of simultaneous separation of anions and cations in AS9-HC column by suppressed ion chromatography.     

Efficient separation of acetate and formate by ion chromatography: application to air samples in a cultural heritage environment

A method for the separation of acetate and formate anions by ion chromatography has been optimized under various measurement conditions (e.g. the composition of the mobile phase, and the flow rate of the eluent). For this purpose, two different analytical columns were examined: the IonPac AS14 (250 mm x 4 mm i.d.; designed mostly for the separation of inorganic anions) and the Allsep A-2 (150 mm x 4.6 mm i.d.; designed for the separation of low-molecular mass organic acids). However, nearly baseline separation of acetate and formate has been found on each column using the following conditions: (i) IonPac AS14 column and 2.0 mM Na2B4O7 solution as an eluent with a flow rate of 1.0 ml/min, or (ii) Allsep A-2 column and an eluent containing a mixture of 1.2 mM Na2CO3 plus 1.5 mM NaHCO3 with a flow rate of 1.3 ml/min. Additionally, the separation of fluoride from acetate and formate on both columns was studied. On the IonPac AS14 column it was possible to separate all three investigated anions. However, on the Allsep A-2 column, when the concentration of fluoride was comparable to, or higher than acetate, it was impossible to achieve good separation of these two anions, even using the optimized elution procedure. Therefore, the measurements of real samples were carried out with the use of IonPac AS14 column. The concentrations of acetate and formate have been determined in the air samples of the Cathedral of Cologne (Germany), after sampling the corresponding acids by passive diffusion tubes. Average concentrations of 122 and 9 microg/m(3) for acetic and formic acids were found, respectively, inside the Cathedral and in a depot with medieval stained glass panels.     

Critical comparison of retention models for the optimisation of the separation of anions in ion chromatography: II. Suppressed anion chromatography using carbonate eluents

Seven theoretical retention models, namely the linear solvent strength model (using the dominant equilibrium approach and competing ion effective charge approach), the dual eluent species model, the Kuwamoto model, the extended dual eluent species model, the multiple species eluent/analyte model and the empirical end-points model, were used to describe the retention behaviour of anions in suppressed ion chromatography (IC). An extensive set of experimental retention data was gathered for 24 anions (fluoride, formate, bromate, chloride, hexanesulfonate, bromide, chlorate, nitrate, iodide, thiocyanate, perchlorate, sulfite, succinate, sulfate, tartrate, selenate, oxalate, tungstate, phthalate, molybdate, chromate, thiosulfate and phosphate) on a Dionex AS4A-SC column using carbonate eluents of varying concentration and HCO₃⁻:CO₃²⁻ ratios. Statistical comparison of the predicted and experimentally obtained retention factors showed that the performance of the theoretical models improved with the complexity of the model. However the empirical model (in which a linear relationship is assumed between the logarithm of retention factor and the logarithm of eluent strength, but the slope is determined empirically) gave the most consistent performance across the widest range of anions. The empirical end-points model was also shown to be the most satisfactory model due to its low knowledge requirements and easy solution. Compared with non-suppressed IC (see Part I), the retention behaviour in suppressed IC was found to be easier to model by all retention models.     

离子色谱法测定高氯、高钠油田回注水中的阴、阳离子及有机酸

建立了高氯、高钠油田回注水中痕量无机阴、阳离子和有机酸的离子色谱分析方法。对高钠基质中痕量阳离子的测定,选用IonPac CS12A分析柱、H2SO4溶液梯度淋洗、电导检测器检测;对高氯基质中阴离子及有机酸的测定,选用对OH-具有高选择性的高容量的IonPac AS11-HC柱、KOH梯度淋洗、电导检测器检测。在优化的梯度淋洗条件下,高氯或高钠的存在不影响痕量阴离子或阳离子的测定。该方法具有良好的线性(r=0.9926~0.9990)和精密度(测定组分峰面积的相对标准偏差(n=7)在8.0%以下),回收率     

Optimization of separation in two-dimensional high-performance liquid chromatography by adjusting phase system selectivity and using programmed elution techniques

The overall peak capacity in comprehensive two-dimensional liquid chromatographic (LC x LC) separation can be considerably increased using efficient columns and carefully optimized mobile phases providing large differences in the retention mechanisms and separation selectivity between the first and the second dimension. Gradient-elution operation and fraction-transfer modulation by matching the retention and the elution strength of the mobile phases in the two dimensions are useful means to suppress the band broadening in the second dimension and to increase the number of sample compounds separated in LC x LC. Matching parallel gradients in the first and second dimension eliminate the necessity of second-dimension column re-equilibration after the independent gradient runs for each fraction, increase the use of the available second-dimension separation time and can significantly improve the regularity of the coverage of the available retention space in LC x LC separations, especially with the first- and second-dimension systems showing partial selectivity correlations. Systematic development of an LC x LC method with parallel two-dimensional gradients was applied for separation of phenolic acids and flavone compounds. Several types of bonded C18, amide, phenyl, pentafluorophenyl and poly(ethylene glycol) columns were compared using the linear free energy relationship method to find suitable column combination with low correlation of retention of representative standards. The phase systems were optimized step-by-step to find the mobile phases and gradients providing best separation selectivity for phenolic compounds. The optimization of simultaneous parallel gradients in the first and second dimension resulted in significant improvement in the utilization of the available two-dimensional retention space.     

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.     

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.     

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.     

Organic solvent and temperature-enhanced ion chromatography-high resolution mass spectrometry for the determination of low molecular weight organic and inorganic anions

There has recently been increased interest in coupling ion chromatography (IC) to high resolution mass spectrometry (HRMS) to enable highly sensitive and selective analysis. Herein, the first comprehensive study focusing on the direct coupling of suppressed IC to HRMS without the need for post-suppressor organic solvent modification is presented. Chromatographic selectivity and added HRMS sensitivity offered by organic solvent-modified IC eluents on a modern hyper-crosslinked polymeric anion-exchange resin (IonPac AS18) are shown using isocratic eluents containing 5–50 mM hydroxide with 0–80% methanol or acetonitrile for a range of low molecular weight anions (<165 Da). Comprehensive experiments on IC thermodynamics over a temperature range between 20–45 °C with the eluent containing up to 60% of acetonitrile or methanol revealed markedly different retention behaviour and selectivity for the selected analytes on the same polymer based ion-exchange resin. Optimised sensitivity with HRMS was achieved with as low as 30–40% organic eluent content. Analytical performance characteristics are presented and compared with other IC-MS based works. This study also presents the first application of IC-HRMS to forensic detection of trace low-order anionic explosive residues in latent human fingermarks.     

亲水作用色谱柱与阳离子交换树脂柱结合分离无机离子（英文）

A combination of hydrophilic interaction chromatographic(HILIC) column and a weakly acidic cation-exchange resin(WCX) column was used for simultaneous separation of inorganic anions and cations by ion chromatography(IC).Firstly,the capability of HILIC column for the separation of analyte ions was evaluated under acidic eluent conditions.The columns used were SeQuant ZIC-HILIC(ZIC-HILIC) with a sulfobetaine-zwitterion stationary phase(ZIC-HILIC) and Acclaim HILIC-10 with a diol stationary phase(HILIC-10).When using tartaric acid as the eluent,the HILIC columns indicated strong retentions for anions,based on ion-pair interaction.Especially,HILIC-10 could strongly retain anions compared with ZIC-HILIC.The selectivity for analyte anions of HILIC-10 with 5 mmol/L tartaric acid eluent was in the order of I-> NO-3 > Br-> Cl-> H2PO-4.However,since HILIC-10 could not separate analyte cations,a WCX column(TSKgel Super IC-A/C) was connected after the HILIC column in series.The combination column system of HILIC and WCX columns could successfully separate ten ions(Na+,NH+4,K+,Mg2+,Ca2+,H2PO-4,Cl-,Br-,NO-3 and I-) with elution of 4 mmol/L tartaric acid plus 8 mmol/L 18-crown-6.The relative standard deviations(RSDs) of analyte ions by the system were in the ranges of 0.02%-0.05% in retention times and 0.18%-5.3% in peak areas through three-time successive injections.The limits of detection at signal-to-noise ratio of 3 were 0.24-0.30 μmol/L for the cations and 0.31-1.2 μmol/L for the anions.This system was applied for the simultaneous determination of the cations and the anions in a vegetable juice sample with satisfactory results.     

Determination of inorganic anions in environmental waters with a hydroxide-selective column

US Environmental Protection Agency Method 300.0 specifies the use of an IonPac AS4A anion-exchange column with a carbonate-hydrogencarbonate eluent and suppressed conductivity detection for the determination of inorganic anions in environmental waters by ion chromatography. Hydroxide eluents have not typically been used for the routine analysis of common inorganic anions due to the lack of an appropriate hydroxide selective column and the difficulty in preparing contaminant free hydroxide eluents. The use of ion chromatography with a hydroxide-selective IonPac AS17 column, automated eluent generation and potassium hydroxide gradient represents a new approach to the routine determination of inorganic anions in environmental waters. This new approach, which is a modification of Method 300.0, allows equivalent method performance with improved linearity, precision, and method detection limits. The AS17 column provides superior retention of fluoride from the column void volume and improved resolution from small organic acids, such as formate and acetate, compared to the AS4A column. Quantitative recoveries were obtained for all the common inorganic anions spiked into typical environmental waters using this new approach, and the Performance Based Measurement System Tier 1 method validation quality control acceptance criteria are well within the acceptable ranges defined by Method 300.0. In addition, the EG40 eluent generator eliminates the need to manually prepare eluents, increasing the level of automation and ease-of-use of the ion chromatography system.     

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.     

In-line respeciation: an ion-exchange ion chromatographic method applied to the separation of degradation products of chemical warfare nerve agents in soil

The natural background of anions encountered when analyzing soil samples by ion chromatography (IC) present significant problems in the separation, detection and quantification of isopropyl methylphosphonic acid (IMPA) and methylphosphonic acid (MPA), the degradation products of sarin, a chemical warfare nerve agent. Using chemically-suppressed IC with conductivity detection, a commercially available ion-exchange column, and an isocratic binary eluent system, IMPA and MPA were determined in aqueous extracts of soil at sub-ppm (μg/g) concentrations without the need for gradient elution or organic solvent eluent modifiers. Common soil anions such as chloride, nitrate, sulfate and phosphate do not interfere with the analysis method due to the composition of the binary eluent allowing for greater mobilization of multivalent anions (e.g., MPA, carbonate, and sulfate) while monovalent anions (e.g., IMPA and nitrate) are relatively unaffected. Carbonate is selectively removed by in-line respeciation to bicarbonate.     

二维离子色谱法同时测定4种无机阴离子和葡萄糖酸根离子

建立了一种新的二维离子色谱分析模式,应用阀切换技术并联抑制电导和脉冲安培双检测体系,同时测定Cl^-、NO₂^-、SO₄^2-、NO₃^-和葡萄糖酸根离子。第一维色谱采用Ionpac AG18+Ionpac AS18阴离子分析柱,分别以5和20 mmol/L的NaOH溶液等度淋洗,流速为1.0 mL/min,进样量为25 μL,抑制电导检测Cl^-、NO₂^-、SO₄^2-和NO₃^-。第二维色谱采用CarboPac PA1+CarboPac PA20两保护柱串联,以90 mmol/L NaOH溶液、0.8 mL/min的流速洗脱,由AG15柱分离富集葡萄糖酸根,脉冲安培检测器检测。结果表明:无机阴离子在0.1~5.0 mg/L、葡萄糖酸根在0.0856~4.2825 mg/L范围内有良好的线性关系,RSD在1.05%~1.94%之间,相关系数(R²)在0.9945以上;无机阴离子的方法检出限为0.615~2.17 μg/L,葡萄糖酸根的方法检出限为24.24 μg/L;回收率在90.3%~102.8%之间。该方法并联两种检测模式,有良好的准确度和精密度,适用于复杂样品的分离分析。     

Comparison of High-Temperature Gradient Heart-Cutting and Comprehensive LC × LC Systems for the Separation of Phenolic Antioxidants

Natural phenolic antioxidants were separated using comprehensive 2D HPLC on a Purospher Star RP-18e column in the first dimension and on two parallel Zirconia Carbon columns working in alternating cycles in the second dimension. The combination of the two columns provides great differences in separation selectivity in each dimension and an almost orthogonal 2D system. Temperature and solvent gradients were compared for the separation of the first-dimension fraction in the stop-flow heart-cutting 2D setup. Temperature gradients provide shorter separation times in comparison with solvent gradients. However, the time required for post-run column equilibration is too long for comprehensive LC × LC. High-temperature isocratic separation was employed in the second dimension of the comprehensive setup, allowing improvement of the fraction transfer frequency between the two dimensions and shorter 2D separation time in comparison to the earlier published method. The approach was applied to the analysis of beer and wine.     

Developments in ion chromatography using monolithic columns

The focus of this review is on current status and on-going developments in ion chromatography (IC) using monolithic phases. The use and potential of both silica and polymeric monoliths in IC is discussed, with silica monoliths achieving efficiencies upwards of 10(5) plates/m for inorganic ions in a few minutes or less. Ion exchange capacity can be introduced onto the monolithic columns through the addition of ion interaction reagents to the eluent, coating of the monolith with ionic surfactants or polyelectrolyte latexes, and covalent bonding. The majority of the studies to date have used surfactant-coated columns, but the stability of surfactant coatings limits this approach. Applications of monolithic IC columns to the separation of inorganic anions and cations are tabulated. Finally, a discussion on the recent commercialization of monolithic IC columns and the use of monolithic phases for IC peripherals such as preconcentrator columns, microextractors and suppressors is presented.     

Long-chain alkylimidazolium ionic liquids, a new class of cationic surfactants coated on ODS columns for anion-exchange chromatography

Separations of common inorganic anions were carried out on ODS columns coated with two long-chain alkylimidazolium ionic liquids ([C(12)MIm]Br and [C(14)MIm]Br) as new cationic surfactants for ion chromatography. With phthalate buffer solution as the mobile phases and non-suppressed conductivity detection, high column efficiencies and excellent selectivity were obtained in the separation of inorganic anions. Chromatographic parameters are calculated and the results show that the coated column possesses significant potential for the analysis of some inorganic anions such as CH(3)COO(-), IO(3)(-), Cl(-), BrO(3)(-), NO(2)(-), Br(-), NO(3)(-), SO(4)(2-), I(-), BF(4)(-), and SCN(-). The effect of eluent pH values on the separation of anions has been studied on the column coated with [C(12)MIm]Br. The stability of the coated columns was also examined.     

Anion-exchange chromatography on short reversed-phase columns modified with amphoteric (N-dodecyl-N,N-dimethylammonio)alcanoates

Short reversed-phase columns (50 mm x 4.6 mm Gemini C(18)) were dynamically coated with carboxybetaines of the general structure, C(12)H(25)N(+)(CH(3))(2)(CH(2))(n)COOH, namely (N-dodecyl-N,N-dimethylammonio)undecanoate, DDMAU (n=10) and (N-dodecyl-N,N-dimethylammonio)butyrate, DDMAB (n=3), and investigated for the separation of inorganic anions in ion chromatography. The role of the ionic strength of coating surfactant solutions on their adsorption and resultant column capacity was studied. The retention of inorganic anions was investigated with different eluents at various concentrations and pH. Interestingly, no retention for anions was found with pure water as the eluent, but the addition of small amounts of electrolytes, up to 0.1 mM, caused a sharp increase in the retention of analytes. The effect of increasing anion retention with an increase in eluent cation charge was also observed. Based on this effect a new cation charge gradient concept was proposed and applied to the separation of a standard mixture of anions.     

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.     

Multidimensional LC x LC analysis of phenolic and flavone natural antioxidants with UV-electrochemical coulometric and MS detection

A comprehensive 2-D LC x LC system was developed for the separation of phenolic and flavone antioxidants, using a PEG-silica column in the first dimension and a C(18) column with porous-shell particles or a monolithic column in the second dimension. Combination of PEG and C18 or C8 stationary phase chemistries provide low selectivity correlations between the first dimension and the second dimension separation systems. This was evidenced by large differences in structural contributions to the retention by -COOH, -OH and other substituents on the basic phenol or flavone structure. Superficially porous columns with fused core particles or monolithic columns improve the resolution and speed of second dimension separation in comparison to a fully porous particle C(18) column. Increased peak capacity and high orthogonality in different 2-D setups was achieved by using gradients with matching profiles running in parallel in the two dimensions over the whole 2-D separation time range. Multi-dimensional set-up combining the LC x LC separation on-line with UV and multi-channel coulometric detection and off-line with MS/MS technique allowed positive peak identification. The Coularray software compensates for the effects of the baseline drift during the gradient elution and is compatible with parallel gradient comprehensive LC x LC technique. Furthermore, it provides significant improvement in the sensitivity and selectivity of detection in comparison to both UV and MS detection. The utility of these systems has been demonstrated in the analysis of beer samples.