Suppressed Ion Chromatography Research Articles

Forbidden ion transport through cation exchange membranes

Cation exchange membranes (CEMs) are widely used in many applications. The fixed anionic groups e.g., COO−, –SO3-, etc. in the polymer matrix ideally allows the passage only of oppositely charged cations, driven by a potential or a concentration gradient. Anions, charged negative, the same as the membrane matrix, cannot pass through the membrane due to electrostatic repulsion. Such “Donnan-forbidden” passage can, however, occur to some degree, if the electrical or concentration gradient is high enough to overcome the “Donnan barrier”. Except for salt uptake/transport in concentrated salt solutions, the factors that govern such Forbidden Ion Transport (FIT) have rarely been studied. In most applications of transmembrane ion transport, whether electrically driven as in electrodialysis, or concentration-driven, it is the transport of the counterion to the fixed charged groups, such as that of the proton through a CEM, that is usually of interest. Nevertheless, CEMs are also of interest in analytical chemistry, specifically in suppressed ion chromatography. As used in membrane suppressors, both transport of permitted ions and rejection of forbidden ions are important. If the latter is indeed governed by electrostatic factors, other things being equal, the primary governing factor should be the charge density of the membrane, tantamount to its ion exchange capacity (IEC). In fabricating microscale suppressors, we found useful to synthesize a new ion exchange polymer that can be easily molded to make tubular microconduits. Despite a high IEC of this material, FIT was also found to be surprisingly high. We measured several relevant properties for thirteen commercial and four custom-made membranes to discover that while FIT is indeed linearly related to 1/IEC for a significant number of these membranes, for very high water-content membranes, FIT may be overwhelmingly governed by the water content of the membrane. In addition, FIT through all CEMs differ greatly among strong acids, they may still be transported as the molecular acids and the extent is in the same order as the expected activity of the molecular acid in the CEM. These results are discussed with the perspective that even for strong acids, the transport does take place as un-ionized molecular acids.

Determination of phosphorus in commercially available milk using ion chromatography with perchloric acid deproteinization.

Suppressed ion chromatography with perchloric acid deproteinization was developed for the determination of phosphorus in commercially available milk. Although the perchloric acid deproteinization method is widely used in the medical field, it sees limited application in the food industry. Herein, the concentration of perchloric acid and hydrolysis conditions were examined, specifically regarding perchloric acid deproteinization, which was used as a deproteinization method in this study. The calibration curve constructed from the peak area of orthophosphoric acid (monohydrogen phosphate ion: HPO42-) was linear, with a correlation coefficient of 0.999. The relative standard deviation of the peak area of 50mg/L of HPO42- from six replicates was 0.35%. The detection and quantitative limits of HPO42-, calculated from its signal-to-noise ratio were 0.033mg/L and 0.100mg/L, respectively. The proposed method was applied to the analysis of phosphorus in commercially available milk. Perchloric acid deproteinization has proved to be useful in the food industry.

On-column capillary suppressor for open tubular ion chromatography

BackgroundSuppressors with different dead volumes are required to match different suppressed ion chromatography systems. Especially for suppressed open tubular ion chromatography (SOTIC), the dead volume is a critical parameter. Both connection tubes between open tubular (OT) columns and suppressors and the dead volumes of the suppressors should be as short/small as possible to minimize peak dispersion. Suppressors with different dead volumes are required to match the various suppressed ion chromatography systems that operate at low flow rates 20–200 nL/min. ResultsWe describe three designs of on-column capillary suppressors for SOTIC: (A) on-column electrodialytic suppressor prepared by making small cracks on the cycloolefin polymer (COP) capillary at targeted locations, (B) on-column electrodialytic suppressor built on a polyether ether ketone (PEEK) capillary by removing the wall materials at target locations, (C) on-column chemical suppressor based on a single cut on a PEEK capillary at a targeted location a single cut on a PEEK capillary at a targeted location. The on-column electrodialytic suppressors work in two different modes with suppression voltage applied in co-current and counter-current direction to the eluent flow. Because of very narrow column inner diameter (i.d.), up to several hundred volts were required to suppress the hydroxide eluent, but it was found the there was a >90% loss of analytes in the suppressor accompanied with a high noise level after on-column electrodialytic suppression. Theoretical analysis reveals that high suppression voltage significantly affects the retention of specific analytes by electromigration. Further analysis indicated that the electrodialytic on-column suppressor in co-current mode would behave totally different from traditional suppressors. The on-column chemical suppression, with minimum dead volume of 0.27 nL, provides fairly well suppression of low hydroxide eluent without analyte loss in the suppressor. In design C, an efficiency of 47000 ± 1800 plates/m for Cl−, corresponding to a peak volume of 17.9 ± 0.7 nL, was obtained when separating five anion mixture (0.5 mM each) in the 25 μm i.d. AS18 latex coated PEEK OT column with an injection of 7.3 nL. Theoretical calculation revealed that a column efficiency loss of ≤3% would result in a cylindrical chemical suppression channel and thus it is taken as the acceptable dispersion contribution originating from the on-column chemical suppressor. SignificanceDifferent on-column suppressors have been designed on OT columns with i.d.s less than 30 μm. Two electrodialytic on-column suppressor designs with eluent flow parallel to the direction of electric field were proposed and tested. The eluent flow rate, analytes’ retention behavior, resistance of suppression channel, current-voltage relationship, and working principles in both co-current and counter-current were experimentally investigated and comprehensively discussed. It was found that although the on-column electrodialytic suppressions (Design A and B) are not feasible in practice, the electrodialytic on-column suppressor on co-current mode has a potential of being used as an enriching capillary column for analyte ions. Design C provides fairly well chemical suppression. Theoretical calculation indicates that the loss of column efficiency can be controlled within 3%.

Boron quantification using ion chromatography tandem triple quadrupole mass spectrometry. Application to retention analysis in boron-treated wood

Boron compounds play a crucial role in various industries, and accurate quantification of boron is essential for quality control and environmental monitoring. This study presents a simple, rapid, and reliable method for determining boron in aqueous solutions using suppressed ion chromatography coupled to electrospray ionisation-triple quadrupole mass spectrometry (IC-ESI-QqQ-MS). Boric acid (B(OH)3) was retained as the tetrahydroxyborate ion (B(OH)4−) on a CarboPac PA300-4 μm anion-exchange column using isocratic elution with 40 mM KOH. During the neutralization process at the suppressor, B(OH)4− was converted to B(OH)3, which subsequently generated the metaborate ion [BO2]− (m/z 43) within the electrospray ionisation source. By employing a pseudo-selected reaction monitoring (SRM) transition from m/z 43 to m/z 43, the method achieved a limit of detection (LOD) of 2.45 μg/L of boron, the lowest reported in the literature to-date for an IC-based method. The analytical performance of the method demonstrated no carry-over issues, no matrix interferences, and excellent intra- and inter-run repeatability of 2.03% and 3.41%, respectively. The method was applied to the evaluation of boron uptake and retention by Tasmanian Oak timber blocks, treated by dip-diffusion in a boric acid solution of 2.5% Boric Acid Equivalent (BAE, m/m) under controlled laboratory conditions. Quantitative determination of the retained and unretained boron allowed a mass balance evaluation and confirmed the accuracy and reliability of the method, with recoveries ranging from 99.3% to 100.2%.

Novel stationary phases based on epoxidized polystyrene-divinylbenzene for three modes of liquid chromatography

Mixed-mode stationary phases based on epoxidized copolymer of styrene and divinylbenzene have been obtained by amination with methylamine, further alkylation with 1,4-butanediol diglycidyl ether and opening the terminal oxirane rings with dimethylethanolamine. To evaluate the effect of the number of anchor amino and diol groups on the degree of hydrophilization of resins, the quantity of reagents was varied. Polymerization of glycidol in the functional layer at an increased pH of the reaction medium was carried out for additional shielding of the substrate. It was found that increasing the number of anchor amino groups is promising for increasing hydrophilicity in suppressed ion chromatography and hydrophilic interaction liquid chromatography modes, while polymerization of glycidol increases the degree of substrate shielding. The applicability of the most hydrophilic adsorbent in three HPLC modes was demonstrated. Novel mixed-mode stationary phase allows the separation of six nucleosides and nitrogenous bases by hydrophilic interaction liquid chromatography, 7 alkylbenzenes by reversed phase liquid chromatography, and 20 organic and inorganic anions by suppressed ion chromatography.

Resins Based on Polystyrene–Divinylbenzene with Attached Hydrophilized Polyethyleneimine for Ion and Hydrophilic Interaction Liquid Chromatography

To expand the field of application of anion exchangers based on a copolymer of styrene and divinylbenzene with attached polyethyleneimine, quaternized with glycidol, the following conditions for their synthesis were varied: the amount of the added glycidol, temperature, and duration of synthesis. The influence of these factors on the capacity, selectivity, and efficiency of the resins in the mode of suppressed ion chromatography was studied; in addition, the stationary phases were studied in the mode of hydrophilic interaction liquid chromatography using the Tanaka test. It was shown that the synthesis conditions under study ensure the control of the capacity and selectivity of anion exchangers, while their hydrophilicity changes insignificantly. The behavior of oxoanions on covalently attached resins in the ion chromatography mode was studied for the first time, and the applicability of phases with quaternized polyethyleneimine in the hydrophilic interaction liquid chromatography mode to the separation of sugars, amino acids, water-soluble vitamins, nucleosides, and nitrogenous bases was demonstrated for the first time.

A fast and simple extraction method for analysing levoglucosan and its isomers in sediments by ion chromatography tandem mass spectrometry.

The ability to trace current and past biomass burning events is important for understanding the links between human activity, fire frequency, and climate. One method of tracing biomass burning is to measure the concentrations of certain monosaccharides anhydrides (MAs), specifically levoglucosan (LEV) and its isomers, mannosan (MAN) and galactosan (GAL), which are products of cellulose and hemicellulose pyrolysis. This work presents a simple extraction method allowing for the rapid, sensitive, and selective determination of MAs in sediments. MAs detection was performed using suppressed ion chromatography with electrospray - triple-stage quadrupole tandem mass spectrometry (IC-TSQ-MS). The extraction method involves ultrasound probe sonication using water as the solvent. Extraction time, amplitude, and sonication mode were optimised. Recoveries higher than 86% for all MAs tested were achieved by applying 70% amplitude in continuous mode for 60 s. Analytical performance of the method included instrumental LODs of 0.10, 0.12 and 0.50 μg L-1 for LEV, MAN and GAL, respectively. No carryover issues, no matrix effect and no co-elution of targeted MAs with other sugars likely present in sediments samples were observed. The developed extraction method was further validated by the analysis of LEV and MAN in NIST® 1649b urban dust reference material and the resulting concentrations were in excellent agreement with previously reported values. MAs quantification in 70 lake sediment samples were carried out with concentrations found to range from 0.009 to 0.390 μg g-1 for LEV and from 0.009 to 0.194 μg g-1 for MAN. Plotting MAs concentrations versus approximate sediment age allowed the reconstruction of recent fire events impacting two locations in the Central Highlands of Tasmania, Australia.

Validated Modernized Assay for Foscarnet in Pharmaceutical Formulations Using Suppressed Ion Chromatography Developed through a Quality by Design Approach

Inspired by the United States Pharmacopoeia (USP) “monograph modernization” initiative, we developed and validated an assay for foscarnet sodium injection solution (“foscavir”), following quality by design (QbD) principles, incorporating design of experiments (DoE) and multivariate data analysis to establish the design space and robust setpoint of the method. The resulting analytical procedure was based on ion chromatography (IC) with suppressed conductivity detection, employing an isocratic carbonate–bicarbonate eluent system. The assay was successfully validated at the robust setpoint conditions, according to the guidelines established by the International Council for Harmonization (ICH). The linear range stretched at least from 5 to 100 mg/L with high repeatability (relative standard deviation, RSD ≤ 0.3%) both at the target concentration (60 mg/L) and at 50% and 150% from this level. Special attention was given to establish a rugged assay that would be easily transferable between laboratories, and the recorded recoveries of 98.2–100.5% for both the formulated drug product and the drug substance during intermediate precision evaluation at different analysis situations indicated that this mission was accomplished. A multivariate assessment of intermediate precision data acquired using an experimental design scheme revealed that the assay was not adversely affected by any of the situation variables, including the use of different liquid chromatography instrument types, regardless of if they were constructed from inert materials or stainless steel that had been passivated, even though such problems have been reported in several previous methods for analysis of foscarnet.

Ultra-trace determination of oxyhalides in ozonated aquacultural marine waters by direct injection ion chromatography coupled with triple-quadrupole mass spectrometry

A direct, robust, accurate and highly sensitive method for oxyhalide species in natural waters, including seawater, using suppressed ion chromatography coupled with mass spectrometry (IC-MS) is described. The method utilised a high capacity, high efficiency anion-exchange column (Dionex IonPac AS11-HC, 4 mm, 2 × 250 mm), with the separation achieved using an electrolytically generated potassium hydroxide gradient, delivered at 0.380 mL min−1. Applying the method, detection limits for iodate, bromate, and chlorate in seawater after direct sample injection (20 μL injection volume, samples diluted 10-fold), were 11, 30 and 13 ng L−1 (ppt), respectively. Standard addition calibrations to ozonated seawater samples were linear, in all cases R2 > 0.999 (n = 10), with intra-day repeatability of 3.7, 11.2 and 1.8 % RSD (n = 10) for a low-level standard mixture (0.30 μg L−1 of iodate, 0.15 μg L−1 of bromate, and 1.50 μg L−1 of chlorate). The method was applied to the analysis of seawater samples taken pre- and post-disinfection points within a recirculating aquacultural system. Iodate, bromate and chlorate were detected as the main oxyanionic disinfection by-products, demonstrating the practical utility of the new method as a valuable tool for monitoring changes to seawater composition following disinfection treatments.

Prospects of using hyperbranched stationary phase based on poly(styrene-divinylbenzene) in mixed-mode chromatography

Evaluation of the chromatographic properties of covalently bonded hyperbranched stationary phase based on poly(styrene-divinylbenzene) (PS-DVB) and containing zwitterionic fragments in the structure of functional layer was conducted in suppressed ion chromatography (IC), reversed phase high performance liquid chromatography (RP HPLC), and hydrophilic interaction liquid chromatography (HILIC) modes. Besides the possibility of resolving 20 inorganic anions and organic acids using KOH eluent in suppressed IC, prepared resin provided the separation of alkylbenzenes in RP HPLC, water-soluble vitamins, amino acids, and sugars in HILIC mode. Trends in the retention of hydrophobic and polar analytes on the prepared stationary phase indicated the dominating effect of analyte nature on the retention mechanism and proved satisfactory hydrophilization of PS-DVB surface with hyperbranched functional layer for retaining polar compounds. The obtained results revealed good prospects of using hydrophobic PS-DVB substrate for preparing stationary phases for mixed-mode chromatography.

Gradient nanopump based suppressed ion chromatography using PEEK open tubular columns

The progress of open tubular suppressed ion chromatography is reviewed from a broad perspective. Then a suppressed open tubular ion chromatography system, composed mostly of commercial components, is described: A splitless gradient-capable nanopump providing eluent delivery in the 10–1000 nL/min regime, a 7-nL injector integral to the pump, an anion exchange column based on a 25 µm i.d. poly(ether ether ketone) (PEEK) capillary, and an on-column admittance detector. The membrane suppressor was custom fabricated. Good gradient separation of seven common anions in 9 min is demonstrated using a manually prepared hydroxide eluent.

Manipulating selectivity of covalently-bonded hyperbranched anion exchangers toward organic acids. Part IV: General algorithm based on the variation of external part of the functional layer

Novel anion exchangers with mono- and dialkanolamines in the external part of covalently-bonded hyperbranched functional layer were synthesized. Comparison of the chromatographic properties of the prepared stationary phases in suppressed ion chromatography (IC) mode using hydroxide eluent allowed us to evaluate the effect of the number of substituents at nitrogen atom in alkanolamine on selectivity of anion exchangers toward organic acids. Obtained anion exchangers were also examined together with previously described hyperbranched stationary phases with different mono- and diamines in the external part of the layer for evaluating the influence of various parameters on their selectivity. Effects of hydrophilicity, functionality of amine, and the number of substituents at nitrogen atom of amine used in the last modification cycle were established independently from each other, which provided the possibility to tailor selectivity toward organic acids when preparing anion exchanger for solving particular analytical task. Predominance of hydrophilicity as a key factor affecting the separation of weakly retained organic acids over other studied parameters was demonstrated.

Determination of full organic acid profiles in fruit juices and alcoholic beverages using novel chemically derivatized hyperbranched anion exchanger

Novel chemically derivatized anion exchanger with hyperbranched functional layer was prepared and successfully applied to the analysis of blueberry, mango, and grape juices as well as light beer and red wine by suppressed ion chromatography (IC). Unique selectivity of the prepared anion exchanger toward organic acids was provided by the structure of the functional layer prepared using both secondary and primary amines for hyperbranching and allowed the determination of full organic acid profiles together with inorganic anions in beverages in one run without using additional analytical methods or organic solvent additives in the mobile phase. The proposed method was validated with respect to linearity, recovery, limits of detection, and intra-day and inter-day precision.

Determination of Bromide and Bromate Ions in the Presence of Standard Ions by Suppressed Ion Chromatography

An ion chromatographic method for determination of bromide and bromate in the presence of standard anions in drinking water was described. The method was based on separation on Metrosep A Supp 7-250 (250 x 4 mm) column with 3.6 mmol/L Na2CO3 as eluent and conductivity detection after sequential chemical and carbon dioxide suppression. The influence of flow rate and column temperature was studied and optimal experimental conditions for simultaneous determination of eight anions were chosen. Analytical characteristics of the ion chromatographic method were assessed for simultaneous determination of bromide, bromate, fluoride, chloride, nitrate, nitrite, phosphate and sulfate. The calibration curves were linear (r2=1, N=7) in the concentration ranges: 0.4-12 mg/L BrO3- and 0.3-11 mg/L Br-. Recovery test was performed on a spiked certified reference material for soft drinking water. The obtained recoveries for bromate and bromide were 96.0 and 101.0 %, respectively. The repeatability and intermediate precision were between 5.1 and 0.14 % (RSD) depending on the analytes concentration. The limits of detection were 20 μg/L BrO3- and 10 μg/L Br-

Quantification of inorganic anions and organic acids in apple and orange juices using novel covalently-bonded hyperbranched anion exchanger with improved selectivity

Chromatographic analysis of orange and apple juices is provided using novel covalently-bonded poly(styrene-divinylbenzene)-based (PS-DVB) hyperbranched anion exchanger for suppressed ion chromatography (IC) with improved selectivity toward inorganic anions and organic acids. The obtained stationary phase provides baseline resolution of weakly retained organic acids such as glycolate, acetate, lactate, and formate, which are not separated to baseline with modern commercially available anion exchangers. The proposed method is validated with respect to linearity, recovery, limits of detection, and intra-day and inter-day precision.

Total Sulfur Dioxide Determination in Red Wine by Suppressed Ion Chromatography with In-Sample Oxidation and SPE

Suppressed ion chromatography (IC) based on in-sample oxidation and SPE is described for total sulfur dioxide determination in red wines. In this method, sulfur dioxide was converted to stable sulfate ion through a simple H2O2 oxidation step and then the impurities were removed with C-18 micro-column SPE pretreatment. Finally, the sulfate was determined by suppressed ion chromatography coupled with conductivity detection and the content of total sulfur dioxide could be obtained through sulfate content by blank deduction method. By using a mixture of NaHCO3 and Na2CO3 as mobile phase, the analysis of one sample with nine anions could be completed within 25 min. For sulfate detection, a linear calibration curve with correlation coefficient of 0.9993 was obtained from the peak area with low detection limit (0.45 mg L−1, 3σ) and excellent repeatability (RSD = 1.12%, n = 6). This method was applied to sulfur dioxide determination in real wine samples and compared with conventional iodometry.

Assuring precision and calibration linearity in ultrasensitive suppressed ion chromatography by using deliberately contaminated hydroxide eluent

Experimental: and theoretical studies were conducted to investigate low and non-linear responses in sub-micro molar-level suppressed ion chromatography with a hydroxide eluent. A calculated response was derived using experimentally determined detector effluent ion composition data and compared with measured experimental responses. The calibration curve was non-linear, and its slope varied considerably with different instrumental setups and operating conditions. The non-linearity of the solution conductivity response was determined by two acid-base equilibria of water and carbonic acid, and fluoride ion. By using eluent contaminated with para-toluene sulfonate at micro molar level, the non-linear response was greatly alleviated.

Hyperbranched anion exchangers prepared from thiol-ene modified polymeric substrates for suppressed ion chromatography

The covalent modification of polymeric particles has been a challenge due to their chemical inertness. Herein we describe a facile and neat method via thiol-ene reaction to immobilize cysteamine/cysteine onto allylmethacrylate-divinylbenzene (AMA-DVB) and ethylvinylbenzene-divinylbenzene (EVB-DVB). The introduced functionalities were verified by elementary analysis and scanning electron microscope coupled with energy dispersive spectroscopy. This modification enabled the further grafting of hyperbranched condensation polymers (HBCPs), which converted the particles into anion exchangers. The anion exchangers demonstrated good separation with carbonate/bicarbonate eluents and hydroxide eluents for suppressed anion exchange chromatography. In the case of a 3-layered anion exchanger, simultaneous separation of nine model anions was achieved by isocratic elution in 17 min with efficiencies up to 16,100 plates m−1. High stability and dependable durability was exhibited in long-term tests.

Simultaneous determination of chromate and common inorganic anions using suppressed ion chromatography

Simultaneous determination of chromate and common inorganic anions using suppressed ion chromatography

Indirect amperometric detection of non-redox ions using a ferrocene-functionalized and oriented mesoporous silica thin film electrode

Vertically oriented and ferrocene functionalized mesoporous silica thin films have been prepared by combining the electrochemically assisted self-assembly (EASA) method with click chemistry. The resulting films are electroactive, involving an electron hopping mechanism between adjacent ferrocene moieties. The electrochemical oxidation of ferrocene into ferricinium ion generates positive charges that are compensated by the ingress of anions into the film, opening the door to possible indirect amperometric detection of non-electroactive anions by flow injection analysis. Operating in an electrolyte-free flow, each injection of an anion (e.g., NO3−) at an electrode biased at a suitable positive potential (i.e., +0.5V) gave rise to an amperometric response proportional to the anion concentration. However, to avoid the decrease of the electrochemical signal due to the progressive consumption of ferrocene in multiple successive analyses, it was necessary to regenerate the electrode by reduction of ferricinium moieties, which can be achieved in-situ by square wave amperometry. The feasibility to apply such indirect amperometric detection scheme in suppressed ion chromatography (for detecting anions in mixture) was also demonstrated.