Comparison of liquid chromatographic behaviors on N-methylimidazolium functionalized ZrO(2)/SiO(2)-4 and N-methylimidazolium functionalized SiO(2) stationary phases

A new stationary phase N-methylimidazolium functionalized ZrO(2)/SiO(2)-4 (Zr/SilprMim) has been prepared. The chromatographic property of this stationary phase is investigated by ion chromatography (IC) with inorganic and organic anions, and normal phase HPLC with basic compounds and hydroxybenzenes. The effects of pH and the strength of Lewis base of eluent on separation of anions are studied. This new stationary phase is also compared with a N-methylimidazolium functionalized SiO(2) stationary phase (SilprMim). The results show that this new stationary phase can be used in analysis of inorganic anions with great prospects. At the same time, successful separations of some organic anions can be obtained by using phosphate buffer solution as mobile phase. This new stationary phase also has a distinct advantage in the separation of basic compounds in normal phase. But due to the presence of Lewis acid sites on the surface of ZrO(2)/SiO(2)-4, Lewis bases such as hydroxybenzenes adsorb very strongly on this new stationary phase, and Lewis acid sites can be masked or modified by the eluent that contain Lewis base groups.     

HPLC combined with ICP-MS for the determination of trace amounts of rare earth impurities in high-purity La2O3 by using 2-ethylhexyl hydrogen-2-ethylhexylphosphonate resin as a stationary phase

A new method for the determination of trace rare earth impurities in high-purity La₂O₃ by HPLC combined with ICP-MS is proposed. The chromatographic retention behavior of La and Ce in HPLC was studied with 2-ethylhexyl-hydrogen-2-ethylhexylphosphonate (P507) resin as the stationary phase and inorganic acid (HNO₃) as the mobile phase. The effects of acidity of the mobile phase, flow rate of eluent and column oven temperature on the separation of La and Ce were investigated in detail. The results of the effect of different ratios of La / Ce showed that a favorable separation of La and Ce could be obtained easily and rapidly with the method presented. Compared with the conventional open column method, the proposed separation system shows a good performance with regard to short separation time, low reagent consumption, and simple operation. The method has been applied to the analysis of real samples of high-purity La₂O₃ with satisfactory results. Received: 8 September 1999 / Revised: 5 January 2000 / Accepted: 7 January 2000     

Anion separations for liquid chromatography using propylpyridinium silica as the stationary phase

This work describes the characterization and potential applications of a silica-based anion-exchange phase prepared by a two-step modification process that incorporates a propylpyridinium group. The effects of pH and eluent concentration on anion separation were examined using 150 mm × 3.9 mm HPLC columns packed with the new phase. The mobile phase pH values ranged from 3.8 to 6.6 using phthalic acid/Tris solutions. The best separation was achieved using 2.5 mmol L⁻¹ phthalate/2.4 mmol L⁻¹ Tris solution at pH 4.2 as mobile phase with non-suppressed conductivity detection. The new stationary phase was used for the separation of some inorganic and organic anions showing good resolution. The stability of the silica-based anion exchange phase was also evaluated.Analytical curves, for concentrations ranging from 0.25 to 10 mg L⁻¹ for the inorganic anions chloride, nitrite, bromide and nitrate, showed good linear correlations (r > 0.998). The method was tested with certified rainwater samples. The measured and certified values were in good agreement, indicating that the new phase holds significant promise for the analysis of these anions in environmental samples.     

One-Step Solvothermal Synthesis of Sub-2-µm Sea Urchin-Like TiO2 Microspheres for High-Performance Liquid Chromatography Stationary Phase

Sub-2-µm sea-urchin-like TiO₂ microspheres were prepared through the one-step solvothermal method. Results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that there are a lot of nanorods coming out from the center of the TiO₂ microsphere, making it look like a sea urchin. The morphology and structure of these microspheres are very suitable for chromatographic packing. During the separation of inorganic anions on this new stationary phase, the retention mechanism is electrostatic interaction, and the pH of the mobile phase plays an important role in retention behaviors. With this new stationary phase for separation of organic anions and nucleobases, Lewis acid–base interaction is more outstanding, which can cause peak broadening and tail for larger conjugated structure compounds. The separation of aromatic hydrocarbons on this new stationary phase demonstrates that Lewis acid–base interaction has a favorable influence on the separation of electron-rich aromatic hydrocarbons under normal phase chromatographic conditions.

Graphical abstract

     

Dicationic imidazolium ionic liquid modified silica as a novel reversed‐phase/anion‐exchange mixed‐mode stationary phase for high‐performance liquid chromatography

A dicationic imidazolium ionic liquid modified silica stationary phase was prepared and evaluated by reversed‐phase/anion‐exchange mixed‐mode chromatography. Model compounds (polycyclic aromatic hydrocarbons and anilines) were separated well on the column by reversed‐phase chromatography; inorganic anions (bromate, bromide, nitrate, iodide, and thiocyanate), and organic anions (p‐aminobenzoic acid, p‐anilinesulfonic acid, sodium benzoate, pathalic acid, and salicylic acid) were also separated individually by anion‐exchange chromatography. Based on the multiple sites of the stationary phase, the column could separate 14 solutes containing the above series of analytes in one run. The dicationic imidazolium ionic liquid modified silica can interact with hydrophobic analytes by the hydrophobic C₆ chain; it can enhance selectivity to aromatic compounds by imidazolium groups; and it also provided anion‐exchange and electrostatic interactions with ionic solutes. Compared with a monocationic ionic liquid functionalized stationary phase, the new stationary phase represented enhanced selectivity owing to more interaction sites.     

HPLC combined with ICP-MS for the determination of trace amounts of rare earth impurities in high-purity La2O3 by using 2-ethylhexyl hydrogen-2-ethylhexylphosphonate resin as a stationary phase

A new method for the determination of trace rare earth impurities in high-purity La₂O₃ by HPLC combined with ICP-MS is proposed. The chromatographic retention behavior of La and Ce in HPLC was studied with 2-ethylhexyl-hydrogen-2-ethylhexylphosphonate (P507) resin as the stationary phase and inorganic acid (HNO₃) as the mobile phase. The effects of acidity of the mobile phase, flow rate of eluent and column oven temperature on the separation of La and Ce were investigated in detail. The results of the effect of different ratios of La / Ce showed that a favorable separation of La and Ce could be obtained easily and rapidly with the method presented. Compared with the conventional open column method, the proposed separation system shows a good performance with regard to short separation time, low reagent consumption, and simple operation. The method has been applied to the analysis of real samples of high-purity La₂O₃ with satisfactory results.     

Comparison of Reversed Stationary Phases for the Chromatographic Separation of Inorganic Analytes Using Hydrophobic Ion Mobile Phase Additives

Abstract

Alkyl-modified silica (RSi) and polystyrenedivinylbenzene (PRP-1) stationary phases are compared for the chromatographic separation of inorganic analyte anions and cations using hydro-phobic ions of opposite charge as mobile phase additives. Tetra-alkylammonium salts were used for anion separations and alkyl sulfonate salts for cation separations. Two major equilibria influence the retention of analyte ions on PRP-1. These are: retention of the hydrophobic ion on PRP-1 and an ion exchange selectivity between the hydrophobic counterion and the analyte ion. When using RSi retention is also influenced by ion exchange at residual silanol groups, which act as weak cation exchange sites. Mobile and stationary phase variables that influence analyte retention are identified. Optimization of these provides favorable eluting conditions for the separation of inorganic ionic analytes. Of particular interest is the potential use of PRP-1 and RSi columns for the separation of inorganic cations; conditions for the separation of alkali metals and alkaline earths are discussed.     

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.     

Precipitation synthesis of lanthanide hydroxynitrate anion exchange materials, Ln2(OH)5NO3·H2O (Ln=Y, Eu-Er)

Layered lanthanide hydroxynitrate anion exchange host lattices have been prepared via a room temperature precipitation synthesis. These materials have the composition Ln₂(OH)₅NO₃·H₂O and are formed for Y and the lanthanides from Eu to Er and as such include the first Eu containing nitrate anion exchange host lattice. The interlayer separation of these materials, approximately 8.5 Å, is lower than in the related phases Ln₂(OH)₅NO₃·1.5H₂O which have a corresponding value of 9.1 Å and is consistent with the reduction in the co-intercalated water content of these materials. These new intercalation hosts have been shown to undergo facile anion exchange reactions with a wide range of organic carboxylate and sulfonate anions. These reactions produce phases with up to three times the interlayer separation of the host lattice demonstrating the flexibility of these materials.     

Poly(ethylene oxide)-bonded stationary phase for capillary ion chromatography

Methyl-capped poly(ethylene oxide) moieties were chemically bonded to silica gel using an amine-reactive modification reagent and evaluated as the stationary phase for ion chromatography. In this work, primary amino groups of an aminopropylsilica packing material were reacted with methyl-PEO₁₂-NHS ester (succinimidyl-{[N-methyl]-dodecaethyleneglycol} ester) in phosphate buffer (pH 7.0) at room temperature. The prepared poly(ethylene oxide)-bonded stationary was evaluated for the separation of inorganic anions, and the retention behavior of inorganic anions on the prepared stationary phase was examined. The elution order of the investigated anions was the same as that observed in common ion chromatography. Both cations and anions of the eluent affected the retention of the analyte anions. Ion exchange was involved for the retention of analyte anions, although the present stationary phase does not possess any discrete ion-exchange sites. The stationary phase was applied to the separation of trace anions contained in tap water and a rock salt.     

Preparation and chromatographic performance of polymer-based anion exchangers for ion chromatography: A review

In the last decade the developments in the field of ion chromatography (IC) were aimed at increasing the efficiency, sensitivity and rapidity of analysis, as well as on improving separation selectivity. Since selectivity and efficiency to the large extent depend on the surface chemistry of the stationary phase, the development of novel anion exchangers remains one of the priority tasks in modern IC. The exact chemistry of commercially available resins is not known and not many literature data devoted to the procedures of preparing anion exchangers for IC have become available in the last 10–15 years. However, the knowledge about the surface chemistry of anion exchangers can provide understanding of the trends in selectivity and efficiency changes, as well as help with the choice of the stationary phase type suitable for solving a particular analytical task. The current review is devoted to the methods of preparing anion exchangers based on polystyrene-divinylbenzene (PS-DVB) and ethylvinylbenzene-divinylbenzene (EVB-DVB) for IC of inorganic and small organic anions and is aimed at demonstrating the improvement of their performance over the years, which was brought by the development of the new types of stationary phase architecture.     

High temperature crystal structures and superionic properties of SrCl2, SrBr2, BaCl2 and BaBr2

The structural properties of the binary alkaline-earth halides SrCl₂, SrBr₂, BaCl₂ and BaBr₂ have been investigated from ambient temperature up to close to their melting points, using the neutron powder diffraction technique. Fluorite-structured SrCl₂ undergoes a gradual transition to a superionic phase at 900–1100 K, characterised by an increasing concentration of anion Frenkel defects. At a temperature of 920(3) K, the tetragonal phase of SrBr₂ undergoes a first-order transition to a cubic fluorite phase. This high temperature phase shows the presence of extensive disorder within the anion sublattice, which differs from that found in superionic SrCl₂. BaCl₂ and BaBr₂ both adopt the cotunnite crystal structure under ambient conditions. BaCl₂ undergoes a first-order structural transition at 917(5) K to a disordered fluorite-structured phase. The relationship between the (disordered) crystal structures and the ionic conductivity behaviour is discussed and the influence of the size of the mobile anion on the superionic behaviour is explored.     

Poly(1-allylimidazole)-grafted silica,a new specific stationary phase for reversed-phase and anion-exchange liquid chromatography

A new specific stationary phase based on poly(1-allylimidazole)-grafted silica has been synthesized and characterized, by infrared spectra, elemental analysis, thermogravimetric analysis and X-ray photoelectron spectroscopy. The results of test showed that poly(1-allylimidazole) can effectively mask the residual silanol groups and reduce the adverse effect of residual silanol. Using this stationary phase, phenol compounds, aniline compounds, and polycyclic aromatic hydrocarbons were successfully separated with symmetric peak shapes in the reversed-phase chromatography. Inorganic anions (IO₃⁻, BrO₃⁻, Br⁻, NO₃⁻, I⁻, SCN⁻) were also separated completely in the anion-exchange chromatography using sodium chloride solution as the mobile phase. The effects of pH and the concentration of eluent on the separation of inorganic anions were studied. The separation mechanism appears to involve the mixed interactions of hydrogen bonding, hydrophobic, π–π, electrostatic, and anion-exchange interactions.     

Reversed-phase liquid chromatography analysis of alkyl-imidazolium ionic liquids II. Effects of different added salts and stationary phase influence

Two mixtures of four 1-alkyl-3-methylimidazolium ionic liquids (ILs) salts associated to the anions tetrafluoroborate or hexafluorophosphate were analyzed by reversed-phase liquid chromatography with three different stationary phases: Kromasil C(8), Zorbax Extend C(18) and Zorbax Sb-Aq. The effect on retention of various inorganic salts (NaCl, NaH(2)PO(4,) NaBF(4), NaClO(4) and NaPF(6)) added to acetonitrile/water mobile phases was studied. The three columns gave similar separation profiles. In all cases, the retention of ILs increased with the increasing affinity of the inorganic anions for the apolar stationary phases; a phenomenon called chaotropicity. The chaotropic anion order is Cl(-) approximately H(2)PO(4)(-) < BF(4)(-) approximately ClO(4)(-) < PF(6)(-). It is established that the presence of chaotropic anions in the mobile phase do not permit to differentiate between ILs associated to different anions. However, chloride or dihydrogenphosphate added salts do not fully screen the retention differences between ILs associated with different anions. Distorted and even split peaks may appear in the chromatogram depending on the nature and concentration of the injected ILs. In the RPLC analysis of imidazolium-based IL, it is recommended to add to the mobile phase significant amounts of a salt containing a chaotropic anion. This salt addition will improve the IL peak shapes and give reproducible retention factors. LODs in the low microgram range ( approximately 5 nmol) were obtained with the Kromasil C(8) column with a 50/50 acetonitrile-water mobile phase containing 0.01 M NaPF(6) added salt and 230 nm UV detection.     

原子转移自由基聚合法制备新型亲水作用色谱固定相及其性能评价

以自制的6.0μm单分散大孔交联聚氯甲基苯乙烯-二乙烯基苯(Poly(4-vinylbenzylchloride-co-divi-nylbenzene),PCMS/DVB)微球为基质和引发剂,CuCl和自行合成的三[(2-二甲基氨基)乙基]胺(Tris[2-(dimeth-ylamino)ethyl]amine,Me6TREN)组成混合催化体系,使4-乙烯基吡啶(4-Vinyl pyridine,4-VP)在甲苯中进行原子转移自由基聚合,制得4-乙烯基吡啶聚合物,单体4-乙烯基吡啶的接枝率为8.55%。将该聚合物与正溴丁烷反应制得新型亲水色谱固定相。在亲水作用色谱模式下,流速1 mL/min,乙腈-水为流动相可分离5种芳胺化合物和4种酚类化合物。在离子交换色谱模式下,6 mmol/L Na2CO3-5.5 mmol/L NaHCO3为淋洗液可分别分离5种无机阴离子和4种短链有机酸。结果表明,此固定相对极性化合物和无机阴离子具有良好的分离性能,是一种性能优异的亲水作用色谱固定相。     

Retention of anions on silica-based metalloporphyrin stationary phases.

The silica-based Fe(III)-protoporphyrin and Zn-tetraphenylporphyrin stationary phases were examined for the HPLC separation of anions. The retention of nine common inorganic anions as well as benzoate anion (BA) and its hydroxy analogues (HBA) was examined using tartrate, acetate, and succinate eluents. The retention factors of inorganic anions on the FeProP stationary phase were in the order Cl- < NO3- < ClO4- < I- < SCN- and for organic anions benzoate < p-hydroxybenzoate < m-hydoxybenzoate < o-hydroxybenzoate. The retention factors of organic anions examined for a ZnTPP column were in the order p-HBA < m-HBA < BA < o-HBA.     

Dipyridine modified silica--a novel multi-interaction stationary phase for high performance liquid chromatography

A novel multi-interaction stationary phase based on 4,4'-dipyridine modified silica was synthesized and characterized, by infrared spectra, X-ray photoelectron spectroscopy and elemental analysis. Mechanism involved in the chromatographic separation is the multi-interaction including π-π, hydrophobic, hydrogen-bonding, electrostatic and anion-exchange interactions. Based on these interactions, polycyclic aromatic hydrocarbons and phenols were successfully separated respectively in reversed-phase chromatography; inorganic and organic anions were also separated individually in anion-exchange chromatography by using the same column. Furthermore, the simultaneous separation of neutral organics, inorganic and organic anions was obtained on this stationary phase with the appropriate mobile phase. Therefore, such stationary phase has the characteristics of multi-interaction mechanism and multi-modal separation, and has potential application on complex samples.     

Relative efficiencies of substituted aromatic monocarboxylic acids as eluents in ion chromatographic separation of inorganic and organic anions

The eluent strengths of para, ortho and meta substituted hydroxy-, nitro-, amino- and sulfobenzoic acids in single column ion chromatographic separations of inorganic and organic anions have been evaluated and compared with benzoic acid.o-Sulfobenzoic acid turns out to be a stronger and efficient eluent compared to others for the separation and determination of monovalent (Cl^–, NO ₂ ^– , Br^–, NO ₃ ^– ) and divalent (SO ₄ ^2– , SeO ₄ ^2– , S₂O ₃ ^2– , S₂O ₆ ^2– ) inorganic anions. In addition it also functions as an appropriate mobile phase for the detection and quantification of some substituted benzoate ions in an aqueous medium.     

Improvement of the chromatographic separation performance of an imidazolium ionic liquid functionalized silica column by in situ anion‐exchange with dodecyl sulfonate and dodecylbenzene sulfonate anions

The anionic part of ionic liquids can provide additional interactions during chromatographic separations. In this work, the chromatographic separation performance of a silica column functionalized with 1‐propyl‐3‐methylimidazolium chloride ionic liquid was improved by in situ anion‐exchange from chloride anions to dodecyl sulfonate anions and dodecylbenzene sulfonate anions. The separation performances of these ionic liquid functionalized phases were investigated and compared with each other using polycyclic aromatic hydrocarbons, phthalates, parabens, and phenols as model compounds. Results indicated that the new columns presented a better chromatographic separation than the original one. This was ascribed retention mechanism from organic anions. The introduction of dodecyl sulfonate anions increased the hydrophobicity of stationary phase. Furthermore, the phenyl groups of dodecylbenzene sulfonate anions could provide an enhanced selectivity to aromatic compounds such as polycyclic aromatic hydrocarbons by π–π interactions. Analysis repeatability of the new columns was satisfactory (RSD of retention time, 0.10–0.40%; RSD of peak area, 0.66–0.84%).     

Novel ion chromatographic stationary phases for the analysis of complex matrices

Ion chromatography (IC) has a proven track record in the determination of inorganic and organic anions and cations in complex matrices. Recently, application of IC to the separation and determination of bio-molecules such as amino acids, carbohydrates, nucleotides, proteins and peptides has also received much attention. The key to the determination of all of the above species in the most analytically challenging complex matrices is the ability to manipulate selectivity through control of stationary phase chemistry, mobile phase chemistry and the choice of detection method. This Tutorial Review summarises some of the most significant recent advances made in IC stationary phase technology. In particular, the review details stationary phases specifically designed for ion analysis in complex sample matrices, and considers in which direction future stationary phase development might proceed.