Retention of alkali,alkaline earth and transition metals on an itaconic acid cation-exchange column. Eluent pH,ionic strength and temperature effects upon selectivity

The unusual selectivity of a methylene succinic (itaconic) acid modified polymeric column was investigated for the separation of alkali, alkaline earth, transition and heavy metals employing non-chelating inorganic eluents. The retention of selected metal ions on the column was investigated with simple HNO3 eluents and eluents prepared from KNO3 and KCl salts of varying pH (adjusted using HNO3). From these studies both the effect of eluent ionic strength and pH upon retention was evaluated for the itaconic acid stationary phase. The results obtained showed that despite slow exchange kinetics causing poor efficiencies, acceptable baseline separations of selected alkaline earth and transitions could be obtained under optimum conditions (the baseline separation of Mg(II), Ca(II), Mn(II), Cd(II), Zn(II) and Co(II) was possible using a 15 mM KNO3-5 mM KCl eluent at pH 3.50 in under 25 min). The use of an simple ionic strength step gradient was shown that facilitated the addition of Pb(II) to the above group of metal ions. An investigation into the effect of temperature upon peak efficiency and retention showed increased column temperature could be used to improve the resolution of closely eluting metal ions such as Ca(II) and Sr(II) and Ca(II) and Mn(II).     

阳离子交换色谱中色谱柱温度对碱金属与碱土金属离子保留的影响

研究了非抑制型阳离子交换色谱中色谱柱温度(25~50℃)对碱金属离子(Li+、Na+、K+、Rb+)和碱土金属离子(Mg2+、Ca2+、Sr2+)以及NH4+保留的影响。在Shim-pack IC-C1磺酸型阳离子交换柱上,以硝酸为流动相分离碱金属离子,以乙二胺或乙二胺-草酸(柠檬酸)为流动相分离碱土金属离子,随着色谱柱温度的升高,碱金属和碱土金属离子的保留时间均增长,其范特霍夫曲线具有良好的线性关系,斜率为负值,表明在此条件下碱金属和碱土金属离子的保留为吸热过程。在Shim-pack IC-SC1羧酸型阳离子交换柱上,以硫酸为流动相同时分离碱金属和碱土金属离子,随着色谱柱温度的升高,Mg2+、Ca2+的保留时间增长,而K+、Rb+的保留时间缩短,Li+、Na+、NH4+的保留时间基本不变。在此条件下,Mg2+、Ca2+、K+和Rb+的范特霍夫曲线具有良好的线性关系,其中Mg2+和Ca2+的曲线斜率为负值,K+和Rb+的曲线斜率为正值,表明Mg2+和Ca2+的保留表现为吸热过程,K+和Rb+的保留表现为放热过程。研究表明在不同固定相和流动相条件下,色谱柱温度对碱金属和碱土金属离子保留行为的影响不同。     

Ion exchange properties of monolithic and particle type iminodiacetic acid modified silica

A 10 cm silica monolith has been modified with iminodiacetic acid (IDA) groups and characterised for its selectivity toward alkali, alkaline earth, and selected transition metal cations. Physical characterisation of the modified monolith found non-homogeneous modification along the length of the monolith, although sufficient capacity was achieved to facilitate significant retention of alkaline earth and transition/heavy metal ions over a range of eluent pH and ionic strength conditions. For alkaline earth and transition/heavy metal ions, selectivity of the 10 cm IDA monolith closely matched that seen with a 25 cm IDA modified silica gel particle packed column, although the separation of alkali metal ions was noticeably poorer on the monolithic column. Peak efficiencies for most metal ions were of a similar order for both column types, except for Zn(II), which showed significant peak broadening on the IDA monolithic column.     

Novel weak acid cation-exchange column

A cation-exchange column with carboxylate cation-exchange sites was developed. Compared with conventional surface-sulfonated or latex agglomerated resins, this weak acid cation exchanger has a high selectivity for hydronium ions. Mono- and divalent cations such as alkali and alkaline earth metals can therefore be determined simultaneously in less than 10 min using a weakly acidic eluent under isocratic conditions. Because the ethylvinylbenzene-divinylbenzene substrate is highly cross-linked, the new separator is solvent compatible, thus allowing the use of organic solvents to alter the selectivity of the separation and to remove organic contaminants from the column. The separation characteristics of this column are presented and various applications are discussed.     

Focusing of alkali earth metals in ligand step gradient

A capillary electroseparation technique for focusing and selective pre-concentration of metal chelates with subsequent on-line isotachophoresis (ITP) analysis was developed and verified. The ions of alkali earth metals (Mg, Ca, Sr, and Ba) were pre-concentrated from the mixture and analyzed. The focusing of the metals was carried out in a ligand step gradient, which was created by the addition of a convenient ligand agent to the regular stationary pH step gradient. The analytical procedure consisted of three steps. During the first step, the metal ions were electrokinetically continuously dosed into the column where they were selectively trapped on the stationary ligand step gradient in the form of unmoving zones of chelate complexes with effectively zero charge. After a detectable amount of analyte was accumulated, the dosing was stopped. The accumulated zones were mobilized to the analytical column, where they were analyzed by the ITP method with conductivity or photometric detection. The proper electrolyte systems for dosing, mobilizing, and analyzing in isoelectric focusing (IEF), moving boundary electrophoresis (MBE), and ITP modes were consequently developed and put into practice. The trapping selectivity can be regulated by the choice of pH and convenient complexing agents. A mixture of alkali earth metals were used as model analytes. Using a 3000 s dosing time, the proposed method improved the detection limit by 5-29 times in comparison to analysis by ITP with classical injection.     

Some factors affecting separation and detection of amino acids by high-performance anion-exchange chromatography with integrated pulsed amperometric detection

Some factors influencing the separation and detection of amino acids by high-performance anion-exchange chromatography with integrated pulsed amperometric detection were investigated. These factors include eluent concentration, column temperature, and detection waveform. The selectivity changes in weakly retained amino acids are slight with changing sodium hydroxide eluent concentration. When sodium acetate eluent concentration is changed, the selectivity variations between strongly retained amino acids containing two carboxyl groups and containing only one carboxyl group are obviously different. Significant but slight selectivity changes in weakly retained amino acids can be achieved through changing the column temperature. Sodium hydroxide and sodium acetate eluent concentration affect the detection of amino acids. Detection sensitivity of amino acids can be improved by increasing the concentration of sodium hydroxide and sodium acetate in a certain concentration range. The detections of amino acids at two different detection waveforms were compared. The hydroxyl amino acids can be selectively detected by choosing a modified detection waveform. The optimized gradient elution condition and column temperature for analyzing 19 amino acids were obtained. The time for the gradient elution program was 60 min. The column temperature was 35 degrees C. Under the optimized conditions, detection limits for 19 amino acids were 0.15-4.52 pmol. The calibration graphs of peak area for all the analytes were linear for about three orders of magnitude. The RSDs (n=5) of peak area were 0.6-5.6%. The determination of trace amino acid impurities in valine product is shown as an application example.     

Non-trivial temperature effects on the cation exchange chromatography and chelation ion chromatography of metal ions

The effect of column temperature upon the retention of metal ions on sulfonated and mono-, di-, and amino-carboxylated cation exchange columns has been investigated. The retention of alkali, alkaline earth and transition metal ions on each of the above types of cation exchanger was studied over the temperature range 19-65 degrees C. A major difference between the behaviour of mono- and divalent metal ions was shown on each of the above stationary phases, with the monovalent alkali metals exhibiting clearly exothermic behaviour (a decrease in retention with increased temperature) under acidic eluent conditions and an apparent relationship between retention factor and the magnitude of the temperature effect. The effect of temperature upon alkaline earth metal ions was less defined, although strongly endothermic behaviour (increase in retention with temperature) could be seen on all stationary phases through correct choice of eluent. The transition metal ions studied showed endothermic behaviour on all four stationary phases, with the sulfonated column unexpectedly showing the largest increases in retention. The above behaviour can be partially explained through the dominance of the type of solute-stationary phase interaction governing retention. In several of the above columns, both ion-exchange and surface complexation interactions can occur, with the effects of temperature indicating which process dominates under specific eluent conditions.     

Separation of alkali and alkaline earth metals by polyethers using extraction chromatography. Effect of diluent

The separation of alkali and alkaline earth metals by means of an acyclic polyether, 1,13-bis(8-chinolinyl)-1, 4, 7, 10, 13-pentaoxatridecane (CPOD), and cyclic polyethers, benzo-15-crown-5 (BC), dibenzo-18-crown-6 (DBC) and dicyclohexyl-18-crown-6 (DCHC), using extraction chromatography has been studied. The alkali metals can be effectively separated using SCN⁻ ions. Different elution sequences for these groups were observed using chloroform and mesitylene and diluents for the polyethers.     

Determination of trace metallic impurities in high-purity quartz by ion chromatography

A method has been developed for the determination of relevant trace impurities (alkali, alkaline and transition metals) in high purity quartz by ion-chromatography. In situ reagent (HF) purification and simultaneous sample dissolution was achieved in a multichannel vapour phase digestion assembly. Twenty-one samples can be digested at a time in this vapour phase system. Significant decrease in the process blank levels for all the analytes was observed. Drastic reduction (250 times) of NH4+ blank was achieved in the described vapour phase digestion, which enables the determination of trace concentration of sodium in high purity quartz. After volatilisation of the matrix and unreacted HF, the clear water leached solutions were injected into an ion-chromatograph equipped with conductivity detector for the determination of alkali and alkaline earth metals. In the case of transition metals, the trace residues were leached with 10 mM HCl and after separation on a mixed bed analytical column (IonPac CS5) were detected by spectrophotometry after post column derivatisation using 4-(2-pyridylazo)resorcinol (PAR). The accuracy of the result was checked by their comparison with those obtained by independent methods like inductively coupled plasma (ICP) MS and ICP atomic emission spectrometry. The achievable detection limits are between 0.4 ng/g (Li) and 22 ng/g (Mn). The application of the method to the determination of the above trace metals in two high-purity-grade quartz samples is demonstrated.     

Polymer coated cation exchanger in ion chromatography: preparation, properties and applications

Summary Stationary phases synthesized by polymer-coating of silica gel were investigated for the separation of cations by ion chromatography. Two kinds of polymer coating (polystyrene, polyglycidylmethacrylate) with sulphonic acid cation-exchange groups on various silica gels were prepared. These low capacity strong acid cation-exchangers were applied to determine alkali and alkaline earth metal ions in tap and mineral water, and grape juice samples. The IC-results were compared with those of AAS-measurement and of EDTA-titration of alkali metals and of alkaline earth metals, respectively.     

Capillary electrochromatography with contactless conductivity detection for the determination of some inorganic and organic cations using monolithic octadecylsilica columns

A fast separation of alkali and alkaline earth metal cations and ammonium was carried out by capillary electrochromatography on monolithic octadecylsilica columns of 15 cm length and 100 μm inner diameter using water/methanol mixtures containing acetic acid as mobile phase. On-column contactless conductivity detection was used for quantification of these non-UV-absorbing species. The method was also extended successfully to the determination of small amines as well as of amino acids, and the separation selectivity was optimized by varying the composition of the mobile phase. Detection limits of about 1 μM were possible for the inorganic cations as well as for the small amines, while the amino acids could be quantified down to about 10 μM. The separation of 12 amino acids was achieved in the relatively short time of 10 min.     

Complexation of silver and co-recovered metals with novel aza-crown ether macrocycles by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) is used to evaluate the metal binding selectivities of an array of novel caged macrocycles for silver, gold, copper, nickel, zinc, iron, lead, manganese and alkali metal ions. It is found that five of the new compounds display silver selectivity, and their relative affinities for various metals depend on the type, number, and arrangement of heteroatoms (N, O), the cavity size, and the presence of aromatic substituents. Alkali metal cation binding studies are used to evaluate the size-selectivities of the cavities of the macrocycles. Electronic structure calculation by B3LYP density function theory methods were used to model the metal complexes. The presence of nitrogen atoms in the macrocyclic ring is essential for silver selectivity over other transition metals and alkali metal ions, and the presence of aromatic groups also enhances silver avidity. Macrocycle 3, a triaza-18-crown-6 analog modified with two phenyl groups and a cage group, is capable of selective extraction of Ag+ from aqueous solutions in the presence of other transition metal ions and the most common alkali and alkaline earth metal ions.     

Separation of metals by liquid surfactant membranes containing crown ether carboxylic acids

A water-in-oil-in-water emulsion liquid membrane system is used to transport alkali metal and alkaline earth cations from an external alkaline aqueous source phase through an organic membrane containing a crown ether carboxylic acid and into the internal acidic aqueous phase of the emulsion droplet. The influence of varying the crown ether carboxylic acid structure upon the selectivity and efficiency of competitive metal ion transport is examined.     

Thin-layer chromatographic behavior and separation of alkaline earth metals on silica gel in aqueous sodium perchlorate solution

The thin-layer chromatographic behavior of alkaline earth metal ions (Mg2+, Ca2+, Sr2+, and Ba2+) in aqueous sodium perchlorate solutions on silica gel thin-layers impregnated with sodium hydroxide has been surveyed as a function of salt concentration. At salt concentrations above 2 mol 1(-1), the selectivity of the metals increased with a decrease in the crystal ionic radii; with further increases in salt concentration, the selectivity differences among the metals expanded remarkably. In the present systems, it was supposed that the cation exchange, the surface complexation, and the salting-out effect participate simultaneously in the adsorption of the metals on silica gel. Typical chromatograms for the mutual separation of the alkaline earth metals are presented.     

Potentiometric behavior of N,N,N',N'-tetrabenzylmethylenediamine-based hydrogen ion-selective electrodes

H(+)-ion selective electrodes (H(+)-ISE) based on N,N,N',N'-tetrabenzylalkylenediamine (alkylene: methylene, ethylene, propylene, hexylene) are prepared. We are compared with their response potentials to carbon numbers between diamino groups. They were showed linear selective to hydrogen ion in the range of pH 1.5-9.0, 2.5-9.0, 3.5-9.0 and 4.0-9.0, and their Nernstian slopes were 57.3, 53.5, 57.4 and 56.1 mV pH(-1) at 20+/-0.2 degrees C (theoretical value: 58.2 mV pH(-1)), respectively. The interference effect on the cations were measured to alkali metal ions, alkaline earth metals ions. Selectivity coefficients were measured by the mixed-solution method. Among all electrodes the N,N,N',N'-tetrabenzylmethylenediamine (TBMDA)-based electrode has shown the best selectivity in acidic solution.     

Thin-layer chromatographic separation of alkaline earth metals on microcrystalline cellulose

The thin-layer chromatographic behaviour of all the alkaline earth metals except for radium on microcrystalline cellulose in methanol-nitric acid media has been investigated. The R(f) values increase in the order beryllium > magnesium > calcium > strontium > barium. The five metals can be effectively separated from each other with methanol-14M nitric acid (3:1, v/v). The system is also suitable for the separation of mixtures of two metal ions existing in widely varying proportions.     

Automated screening of reversed‐phase stationary phases for small‐molecule separations using liquid chromatography with mass spectrometry

There are various reversed‐phase stationary phases that offer significant differences in selectivity and retention. To investigate different reversed‐phase stationary phases (aqueous stable C₁₈, biphenyl, pentafluorophenyl propyl, and polar‐embedded alkyl) in an automated fashion, commercial software and associated hardware for mobile phase and column selection were used in conjunction with liquid chromatography and a triple quadrupole mass spectrometer detector. A model analyte mixture was prepared using a combination of standards from varying classes of analytes (including drugs, drugs of abuse, amino acids, nicotine, and nicotine‐like compounds). Chromatographic results revealed diverse variations in selectivity and peak shape. Differences in the elution order of analytes on the polar‐embedded alkyl phase for several analytes showed distinct selectivity differences compared to the aqueous C₁₈ phase. The electron‐rich pentafluorophenyl propyl phase showed unique selectivity toward protonated amines. The biphenyl phase provided further changes in selectivity relative to C₁₈ with a methanolic phase, but it behaved very similarly to a C₁₈ when an acetonitrile‐based mobile phase was evaluated. This study shows the value of rapid column screening as an alternative to excessive mobile phase variation to obtain suitable chromatographic settings for analyte separation.     

Fast separation of low molecular weight analytes on structurally optimized polymeric capillary monoliths

The optimization of recently introduced tetrakis(4-vinylbenzyl)silane (TVBS)-derived capillary monoliths to suit the fast separation of small molecules was accomplished by simultaneous changes in both the stoichiometry of the polymerization mixture and in the polymerization temperature. Four types of monoliths were prepared by varying the polymerization temperature within 60 and 65 °C as well as by changing the monomer content between 15.0 and 20.0 wt.%. Changes in the porosity and morphology of the capillary columns were studied applying inverse size exclusion chromatography (ISEC), nitrogen sorption (BET), and scanning electron microscopy (SEM). All monoliths were used for the reversed-phase separation of various sets of low molecular weight analytes such as alkyl benzenes, aryl amines, carboxylic acids, and a mixture of phenols and carbonyl compounds. Comparable efficiency and performance towards the different sets of analytes were observed. With optimized monolith structures, the fast separation of six-alkyl benzenes was accomplished within less than 2 min. This was made possible by a high linear solvent flow (10 mm/s) at low column pressure drops (<15 MPa).     

The importance of cerium substituted phosphates as cation exchanger-some unique properties and related application potentials

Seven different samples of an inorganic ion exchanger, cerium phosphate, suitable for column use have been prepared under varying conditions. The property of these exchangers has been characterized by Inductively Coupled Plasma Spectroscopy. These exchangers are stable in water, dilute mineral acids, ethanol, methanol, acetone and ether. However, in concentrated HCl and HNO(3) they decompose. They retain about 50% of their exchange value after drying at 80 degrees C, and can be regenerated twice without any decrease in exchange capacity. The distribution coefficient measurements for alkaline earth metals, tellurium, iodine and molybdenum using these seven ion exchangers were studied. This revealed the relative affinity for each exchanger, where the sorption in general was most effective at pH 6-8. The titration curves of cerium phosphate (disodium) with alkaline earth metals showed that the selectivity sequence Ba(2+)>Sr(2+)>Ca(2+)>Mg(2+) is observed. Furthermore, it could be deduced that the adsorption of alkaline earth metal cations greatly depends on the cation. These studies have also shown that cerium phosphates with divalent ions are strongly preferred to monovalent ones. Therefore, as for the cerium phosphates with large monovalent ions, the lack of exchange for Ba(2+), Mg(2+) or other alkali earth metal ions should be essentially due to steric hindrance and this could include any one of the following: the large crystalline radius of metal ions or large hydrated ionic radius and high energy of hydration for other divalent ions. Three binary separations of Te(IV)-Mo(VI), Te(IV)-I(I) and Mo(VI)-I(I) has been developed and the recovery ranging from 90 to 100% has been achieved on cerium phosphate (disodium) columns.     

Chromatographic properties of tetramethyl-p-silphenylene-dimethyl,diphenylsiloxane copolymers as stationary phases for gas-liquid chromatography

Seven tetramethyl-p-silphenylene-dimethyl, diphenylsiloxane copolymers were coated on fused-silica capillary columns to evaluate their properties as stationary phases in gas-liquid chromatography. The capillary columns were tested concerning their selectivity, separation efficiency, column bleed, inertness, elution temperatures, and working range. The following characteristic properties of the silphenylene unit were found: (i) the impact of the silphenylene group on the chromatographic selectivity is similar to that of two dimethylsiloxy groups and half of a diphenylsiloxy group; (ii) silphenylene-siloxane copolymers offer reduced column bleed and increased maximum allowable operating temperature in comparison to polysiloxanes, since the backbone stiffing phenylene group enhances thermal stability; (iii) the elution temperatures of analytes are increased by 15-30 degrees C on silphenylene-siloxane copolymers compared to polysiloxanes; (iv) the silphenylene unit increases the glass transition temperature of the polymers resulting in elevated minimum allowable operating temperatures.