Eluent-Induced Separation of Inorganic Cations in Capillary Liquid Chromatography with Contactless Conductivity Detector

A non-suppressed contactless conductivity detector has been used as a capillary detector in a capillary ion chromatograph, combining a reversed-phase C30 column permanently modified with ionic surfactant. The C30 column (100 × 0.32 mm. id) was modified with sodium dodecyl sulfate (SDS) for the separation of inorganic cations. Monovalent cations could be separated by the proposed system, in which methanesulfonic acid (MSA) and SDS were employed as the mobile phase component, but divalent cations could not be eluted under this condition. As for the case of SDS used as the eluent, an H⁺-cation-exchange column was placed before the sample injector to convert the Na⁺ from the eluent into H⁺, and when the mixture of MSA and dodecyl sulfuric acid was used as the eluent, the retention of cations was improved and baseline separation of the cations was achieved within 23 min. The effect of the eluent composition on the retention behavior of inorganic cations was investigated. The repeatability of retention time and peak height varied from 0.39 to 0.58 and 2.21 to 3.25 % as relative standard deviation, respectively.

     

Preparation of a hybrid monolithic stationary phase with allylsulfonate for the rapid and simultaneous separation of cations in capillary ion chromatography

A hybrid monolithic column with sulfonate functionality was successfully prepared for the simultaneous separation of common inorganic cations in ion‐exchange chromatographic mode through a simple and easy single‐step preparation method. The strong cation‐exchange moieties were provided directly from allylsulfonate, which worked as an organic monomer in the single‐step reaction. Inorganic cations (Li⁺, Na⁺, K⁺, NH₄⁺, Cs⁺, Rb⁺, Mg²⁺, Ca²⁺, and Sr²⁺) were separated satisfactorily by using CuSO₄ as the eluent with indirect UV detection. The allysulfonate hybrid monolith showed a better performance in terms of speed and pressure drop than the capillary packed column. The number of theoretical plates achieved was 19 017 plates/m (in the case of NH₄⁺ as the analyte). The relative standard deviations (n = 6) of both retention time and peak height were less than 1.96% for all the analyte cations. The allysulfonate hybrid monolithic column was successfully applied for the rapid and simultaneous separation of inorganic cations in groundwater and the effluent of onsite domestic wastewater treatment system.     

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

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

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.     

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

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

Portable,lightweight, low power,ion chromatographic system with open tubular capillary columns

Basic operation principles of a lightweight, low power, low cost, portable ion chromatograph utilizing open tubular ion chromatography in capillary columns coated with multi-layer polymeric stationary phases are demonstrated. A minimalistic configuration of a portable IC instrument was developed that does not require any chromatographic eluent delivery system, nor sample injection device as it uses gravity-based eluent flow and hydrodynamic sample injection adopted from capillary electrophoresis. As a detection device, an inexpensive commercially available capacitance sensor is used that has been shown to be a suitable substitute for contactless conductivity detection in capillary separation systems. The built-in temperature sensor allows for baseline drift correction typically encountered in conductivity/capacitance measurements without thermostating device. The whole instrument does not require any power supply for its operation, except the detection and data acquisition part that is provided by a USB port of a Netbook computer. It is extremely lightweight, its total weight including the Netbook computer is less than 2.5 kg and it can be continuously operated for more than 8 h. Several parameters of the instrument, such as detection cell design, eluent delivery systems and data treatment were optimized as well as the composition of eluent for non-suppressed ion chromatographic analysis of common inorganic cations (Na⁺, NH₄⁺, K⁺, Cs⁺, Ca²⁺, Mg²⁺, transition metals). Low conductivity eluents based on weakly complexing organic acids such as tartaric, oxalic or pyridine-2,6-dicarboxylic acids were used with contactless capacitance detection for simultaneous separation of mono- and divalent cations. Separation of Na⁺ and NH₄⁺ cations was optimized by addition of 18-crown-6 to the eluent. The best separation of 6 metal cations commonly present in various environmental samples was accomplished in less than 30 min using a 1.75 mM pyridine-2,6-dicarboxylic acid and 3 mM 18-crown-6 eluent with excellent repeatability (below 2%) and detection limits in the low micromolar range. The analysis of field samples is demonstrated; the concentrations of common inorganic cations in river water, mineral water and snow samples were determined.     

Use of contactless conductivity detection for non-invasive characterisation of monolithic stationary-phase coatings for application in capillary ion chromatography

A capacitively-coupled contactless conductivity detector (C4D) has been utilised as an on-capillary detector within a capillary ion chromatograph, incorporating a reversed-phase monolithic silica capillary column semi-permanently modified with a suitable ionic surfactant. The monolithic capillary column (150 x 0.1 mm i.d.) was modified using sodium dioctyl sulfosuccinate (DOSS), an anionic surfactant, for the separation of small inorganic and organic cations. With the use of the on-capillary conductivity detector, the longitudinal homogeneity and temporal stability of the coating were investigated. The approach allowed a detailed non-invasive observation of the nature of the ion-exchange coating over time, and an example of an application of the technique to produce a longitudinal stationary-phase charge gradient is shown. An investigation of the basis of the measured on-capillary conductivity was carried out with a counter ion study, clearly showing the on-capillary detection technique could also distinguish between chemical forms of the immobilised ion exchanger. The above method was used to produce a stable and homogeneously-modified monolithic ion-exchange capillary column, for application to the separation of inorganic alkaline earth cations and amino acids.     

Simultaneous determination of inorganic cations by capillary ion chromatography with a non-suppressed contactless conductivity detector

A non-suppressed capillary ion chromatographic method with a laboratory-made packed cation-exchange column (100 mm × 0.32 mm i.d.) was developed for the separation and simultaneous determination of five common inorganic cations (sodium, ammonium, potassium, magnesium and calcium). Cation exchangers were prepared by the reaction of the hydroxyl group on the surface of diol-group bonded silica gel with 1,3-propanesultone in methanol. Simultaneous separation of these five common inorganic cations were achieved within 17 min using 1 mM methanesulfonic acid and 0.1 mM 15-crown-5 ether in methanol-water (8:2, v/v) as the eluent. The effects of organic solvents and crown ethers in the eluent on the retention of analytes were investigated. The limits of detection (S/N = 3) of the cations were in the range of 18-124 μg/l, the linear correlation coefficients were 0.9991-0.9998, and the RSD values of retention time and peak height were all smaller than 2.1%. The present analytical method was successfully applied to the rapid and direct determination of inorganic cations in samples of river water and commercial drinks, with satisfactory results.     

Ion-exclusion chromatography with the direct UV detection of non-absorbing inorganic cations using an anion-exchange conversion column in the iodide-form

An ion-exclusion chromatographic method for the direct UV detection of non-absorbing inorganic cations such as sodium (Na⁺), ammonium (NH₄⁺) and hydrazine (N₂H₅⁺) ions was developed by connecting an anion-exchange column in the I⁻-form after the separation column. For example, NH₄⁺ is converted to a UV-absorbing molecule, NH₄I, by the anion-exchange column in the I⁻-form after the ion-exclusion separation on anion-exchange column in the OH⁻-form with water eluent. As a result, the direct UV detection of Na⁺, NH₄⁺ and N₂H₅⁺ could be successfully obtained as well as the well-resolved separation. The calibration graphs of the analyte cations detected with UV at 230 nm were linear in the range of 0.001-5.0 mM. The detection limits at S/N = 3 of the cations were below 0.1 μM. This method was applied to real water analysis, the determination of NH₄⁺ in river and rain waters, or that of N₂H₅⁺ in boiler water, with the satisfactory results. This could be applied also to low- or non-absorbing anions such as fluoride or hydrogencarbonate ions by the combination of a weakly acidic cation-exchange resin in the H⁺-form as the separation column and the anion-exchange conversion column.     

Determination of Inorganic Anions by Capillary IC Using Hexadimethrine Bromide-Modified Silica Stationary Phases

A simple, rapid and highly sensitive capillary ion chromatographic method for direct determination of iodide and thiocyanate is reported. Separation was achieved on a laboratory-made capillary column (100 mm × 0.32 mm i.d.) packed with silica gel, followed by modification with 20 mM hexadimethrine bromide. Sodium perchlorate?Cmethanol (95:5, v/v) was used as the eluent and analyte anions were detected at 225 nm. Iodate, bromate, nitrate, iodide and thiocyanate were eluted within 8 min, with relative standard deviations of the retention time, peak area and peak height smaller than 2.4%. Effects of the eluent composition on the retention behavior were also investigated. The limit of detection (S/N = 3) of iodide was 6.5 ??g L^?1, whereas that of thiocyanate was 16.2 ??g L^?1. The method was successfully applied to the rapid and direct determination of iodide in powdered milk and thiocyanate in human saliva without any pre-concentration. The modified column could be used for about 1 month (8 h operation per day) without loss of hexadimethrine bromide.     

SEC Characterization of Au Nanoparticles Prepared through Seed-Assisted Synthesis

In this paper we report the use of size-exclusion chromatography (SEC) for rapid determination of the sizes and size distributions of Au nanoparticles (NPs) prepared by seed-assisted synthesis. Analytical separation of Au NPs was performed in a polymer-based column of pore size 400 nm. We characterized the sizes and size distributions of the Au NPs by using 10 mM sodium dodecyl sulfate (SDS) as mobile phase and obtained a linear relationship (R ² = 0.986) between retention time and size of Au NPs within the range 9.8–79.1 nm; the relative standard deviations of these retention times were less than 0.3%. These separation conditions were used to characterize the sizes and size distributions of Au NPs prepared by seed-assisted synthesis. In addition to observing the elution times of the Au NPs we also simultaneously characterized their size-dependent optical properties by spectral measurement of the eluting peaks by use of an on-line diode-array detector (DAD), i.e., monitoring of the stability of the Au NP products. By using this approach we found the presence of SDS was beneficial in stabilizing the synthesized Au NPs. We also found that the volume of Au metal ions used affected the sizes of the final products. SEC seems an efficient tool for characterizing the sizes of NPs fabricated by seed-assisted synthesis.     

Rapid separation of inorganic anions by capillary ion chromatography using monolithic silica columns modified with dilauryldimethylammonium ion.

The rapid separation of inorganic anions was determined by capillary ion chromatography using monolithic silica capillary columns modified with dilauryldimethylammonium bromide. The stability of the modified stationary phase was satisfactory owing to a strong hydrophobic interaction between the lauryl groups of the reagent, even if the eluent did not contain dilauryldimethylammonium ion. Bromide in seawater samples could be determined by the present system. The repeatability of a retention time of bromide for six successive measurements was around 1.8% when a 500 mM sodium chloride aqueous solution was used as the eluent. Seawater samples were directly injected onto the prepared column without any interference of matrix ions, because an aqueous solution of high-concentration sodium chloride could be used as the eluent. Bromide in seawater samples could be determined within 2 min.     

Simultaneous separation of inorganic anions and metal-citrate complexes on a zwitterionic stationary phase with on-column complexation

The retention and separation selectivity of inorganic anions and on-column derivatised negatively charged citrate or oxalate metal complexes on reversed-phase stationary phases dynamically coated with N-(dodecyl-N,N-dimethylammonio)undecanoate (DDMAU) has been investigated. The retention mechanism for the metal-citrate complexes was predominantly anion exchange, although the amphoteric/zwitterionic nature of the stationary phase coating undoubtedly also contributed to the unusual separation selectivity shown. A mixture of 10 inorganic anions and metal cations was achieved using a 20 cm monolithic DDMAU modified column and a 1 mM citrate eluent, pH 4.0, flow rate equal to 0.8 mL/min. Selectivity was found to be strongly pH dependent, allowing additional scope for manipulation of solute retention, and thus application to complex samples. This is illustrated with the analysis of an acidic mine drainage sample with a range of inorganic anions and transition metal cations, varying significantly in their concentrations levels.     

Use of Microspherical Sulfonated Hypercrosslinked Polystyrene in Ion Chromatography

The possibility of applying sulfonated cation exchangers on the basis of hypercrosslinked polystyrene (HCPS) of different ion-exchange capacity for the ion-chromatographic separation of alkali metal and ammonium cations was demonstrated. The effect of the nature and concentration of the eluent, the temperature of the chromatographic column, additions of an organic solvent to the eluent, and the ion-exchange capacity of the sorbent on the retention of cations was examined. An unusual selectivity of the sorbent for lithium and ammonium cations was found; the elution order can change for the Li⁺/Na⁺ and Rb⁺/NH⁺ ₄ pairs depending on the nature of the eluent, the temperature, additions of an organic solvent, and the structural characteristics of the sorbent. When dilute solutions of nitric acid are used as the eluent, the following elution order of cations was obtained: Na⁺ < Li⁺ < K⁺ < NH⁺ ₄ < Rb⁺ < Cs⁺ Mg²⁺ < Sr²⁺ < Ca²⁺ < Ba²⁺. Under the optimum conditions of separation (1 mM solution of sulfuric acid, 20°C) on sulfonated HCPS with an ion-exchange capacity of 0.07 mequiv/g, the separation of ammonium and alkali metal cations was achieved within 17 min.     

Rapid Determination of Trifluoromethanesulfonate and p-Toluenesulfonate by Ion-Pair Chromatography Using a Reversed-Phase Silica-Based Monolithic Column: Application to the Analysis of Ionic Liquids

A method of ion-pair chromatography was developed on a reversed-phase silica-based monolithic column for the fast and simultaneous determination of trifluoromethanesulfonate (CF₃SO₃ ⁻) and p-toluenesulfonate (C₇H₇SO₃ ⁻). The analysis was performed using a mobile phase of tetrabutylammonium hydroxide + citric acid + acetonitrile on the Chromolith Speed ROD RP-18e column with direct conductivity detection. The effects of the eluent, column temperature and flow rate on the retention of the anions were investigated. The experimental phenomenon was discussed according to hydrophobic interaction and ion-exchange mechanism in the separation. The optimized chromatographic conditions were selected. The optimized eluent for the separation consisted of 0.2 mmol L⁻¹ tetrabutylammonium hydroxide + 0.10 mmol L⁻¹ citric acid + 9% acetonitrile (pH 5.5). The flow rate was set at 6.0 mL min⁻¹. The column temperature was 25 °C. Under the optimal conditions, the better separation of CF₃SO₃ ⁻ and C₇H₇SO₃ ⁻ was achieved without any interference by other anions (Cl⁻, Br⁻, I⁻, NO₃ ⁻, SO₄ ²⁻, ClO₃ ⁻, BF₄ ⁻ and PF₆ ⁻). The detection limit (S/N = 3) was 0.28 and 0.71 mg L⁻¹ for CF₃SO₃ ⁻ and C₇H₇SO₃ ⁻, respectively. The method has been applied to the determination of CF₃SO₃ ⁻ and C₇H₇SO₃ ⁻ in ionic liquids. The spiked recoveries of CF₃SO₃ ⁻ and C₇H₇SO₃ ⁻ were 101.1 and 100.2%, respectively.

     

Tunable separation of anions and cations by column switching in ion chromatography

A convenient ion chromatography method has been proposed for the routine and simple determination of anions (Cl⁻, SO₄²⁻ and NO₃⁻) and/or cations (Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺) using a single pump, a single eluent and a single detector. The present system used cation-exchange and anion-exchange columns connected in series via two 6-port switching valves or a single 10-port valve. The connection order of the ion-exchange columns could be varied by switching the valve(s). The present system therefore allowed the separation of either cations or anions in a single chromatographic run. While one ion-exchange column is being operated, the other ion-exchange column is being conditioned, i.e., the columns are always ready for analysis at any time. When 2.4 mM 5-sulfosalicylic acid was used as the eluent, the three anions and the five cations could be separated on the anion-exchange column and cation-exchange column, respectively. In order to obtain the separations of the target ions, the injection valve was placed between the two columns. Complete separations of the above anions or cations were demonstrated within 10 min each. The detection limits at S/N = 3 were 19-50 ppb (μg/l) for cations and 10-14 ppb for anions. The relative standard deviations of the analyte ions were less than 1.1, 2.9 and 2.8% for retention time, peak area and peak height, respectively. This proposed technique was applied to the determination of common anions and cations in river water samples.     

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.     

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.     

Separation of Aliphatic and Aromatic Carboxylic Acids by Conventional and Ultra High-Performance Ion-Exclusion Chromatography

An ion-exclusion chromatography (IELC) comparison between a conventional ion-exchange column and an ultra high-performance liquid chromatography (UHPLC) dynamically surfactant modified C18 column for the separation of an aliphatic carboxylic acid and two aromatic carboxylic acids is presented. Professional software is used to optimize the conventional IELC separation conditions for acetylsalicylic acid and the hydrolysis products: salicylic acid and acetic acid. Four different variables are simultaneously optimized including H₂SO₄ concentration, pH, flow rate, and sample injection volume. Thirty different runs are suggested by the software. The resolutions and the time of each run are calculated and feed back to the software to predict the optimum conditions. Derringer’s desirability functions are used to evaluate the test conditions and those with the highest desirability value are utilized to separate acetylsalicylic acid, salicylic acid, and acetic acid. These conditions include using a 0.35 mM H₂SO₄ (pH 3.93) eluent at a flow rate of 1 mL min⁻¹ and an injection volume of 72 μL. To decrease the run time and improve the performance, a UHPLC C18 column is used after dynamic modification with sodium dodecyl sulfate. Using pure water as a mobile phase, a shorter analysis time and better resolution are achieved. In addition, the elution order is different from the IELC method which indicates the contribution of the reversed-phase mode to the separation mechanism.

     

Separation of Aliphatic and Aromatic Carboxylic Acids by Conventional and Ultra High-Performance Ion-Exclusion Chromatography

An ion-exclusion chromatography (IELC) comparison between a conventional ion-exchange column and an ultra high-performance liquid chromatography (UHPLC) dynamically surfactant modified C18 column for the separation of an aliphatic carboxylic acid and two aromatic carboxylic acids is presented. Professional software is used to optimize the conventional IELC separation conditions for acetylsalicylic acid and the hydrolysis products: salicylic acid and acetic acid. Four different variables are simultaneously optimized including H₂SO₄ concentration, pH, flow rate, and sample injection volume. Thirty different runs are suggested by the software. The resolutions and the time of each run are calculated and feed back to the software to predict the optimum conditions. Derringer’s desirability functions are used to evaluate the test conditions and those with the highest desirability value are utilized to separate acetylsalicylic acid, salicylic acid, and acetic acid. These conditions include using a 0.35 mM H₂SO₄ (pH 3.93) eluent at a flow rate of 1 mL min^?1 and an injection volume of 72 μL. To decrease the run time and improve the performance, a UHPLC C18 column is used after dynamic modification with sodium dodecyl sulfate. Using pure water as a mobile phase, a shorter analysis time and better resolution are achieved. In addition, the elution order is different from the IELC method which indicates the contribution of the reversed-phase mode to the separation mechanism.