Microchemical determination of nine rare earth elements in silicate rocks by cation-exchange preconcentration — Ion-interaction chromatography

Summary A method of applying ion-interaction chromatography to the determination of the rare earth elements in silicate rocks on a 100 to 200 mg sample basis has been developed. The rare earths are first separated as a group from matrices by cation-exchange chromatography in hydrochloric acid-thiocyanate media and isolated in a small, defined volume (3.00 ml). Using fractions of this, on-column concentration of the rare earths on a C-18 bonded phase silica coated with 1-octanesulfonate and a subsequent concentration gradient elution with glycolate (0.05 to 0.35 M) at pH 3.5 allows the respective separation of La, Ce, Pr, Nd, and Y (100 l aliquot used) and of Er, Tm, Yb, and Lu (2.00 ml aliquot used). Sm, Eu, Gd, Tb, and Dy elute together, and Ho is not sufficiently well resolved from these middle rare earth elements. The eluted rare earth elements are detected and quantified by post-column reaction with Arsenazo III photometrically, using a UV-VIS spectrophotometer at a wavelength of 650 nm. The method is shown to be capable of determining nine of the rare earth elements in a variety of international reference rock samples with good precision and accuracy.     

Selective determination of yttrium in geological materials by ion-interaction chromatography

Selective separation and determination of yttrium in rare earth ores have been achieved by high performance ion-interaction chromatography. Ores are decomposed by sulfuric acid and the rare earths are precipitated in a group as oxalates. Yttrium is then separated from the other rare earths on a C-18 bonded phase silica column modified with 1-octanesulfonate by linear concentration gradient elution for 20 min with 0.15 to 0.40M glycolic acid(pH 3.5). Yttrium elutes at about 10 min between samarium and neodymium, being separated selectively from all the rare earths as well as scandium, thorium and uranium. Post-column reaction detection and quantitation with Arsenazo III [2,7-bis(2-arsonophenylazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid] are carried out at 650 nm. Quantitative results are quoted for yttrium in sophisticated, synthetic rare earth mixtures, monazite and xenotime.     

On‐line solid phase extraction CZE for the simultaneous determination of lanthanum and gadolinium at picogram per liter levels

A non‐specific on‐line method is presented for the extraction and preconcentration of two rare earth elements using a microcartridge containing C₁₈‐derivatized silica particles prior to their analysis by CZE. The microcartridge, named analyte concentrator, was coupled on‐line to the inlet of the separation capillary (fused‐silica (FS) capillary, 75 μm id ×12 cm from the inlet to the microcartidge and 37 cm from the microcartridge to the detector). The reversed‐phase sorbent quantitatively retained gadolinium (Gd) and lanthanum (La) as 2‐(5‐bromo‐2‐pyridylazo)‐5‐diethylaminophenol complexes in the presence of non‐ionic micelles of polyethylene glycol tert‐octylphenyl ether, enabling sample clean‐up and concentration enhancement with minimum sample handling. The rare earth elements chelates were released from the sorbent with methanol and then analyzed by CZE with diode array detection. A background electrolyte of 20 mM sodium tetraborate containing 8% ACN, pH 9.0, was found to be optimal for the separation of metal chelates. The concentration limits of detection were lowered to picogram per liter levels (20 pg/L for La and 80 pg/L for Gd). A 1000‐fold improvement in concentration sensitivity for La‐ and Gd‐2‐(5‐bromo‐2‐pyridylazo)‐5‐diethylaminophenol complexes with respect to CZE without preconcentration was reached.     

Dynamic ion-exchange chromatography with post-column reaction detection for the determination of the rare earth elements in monazites

Summary Separation and determination of lanthanum, cerium, praseodymium, neodymium and samarium in monazites have been achieved by dynamic ion-exchange chromatography. The ore samples are decomposed by sulfuric acid and the rare earths are separated in a group as oxalates. The rare earth elements are then separated from each other on a column of bonded phase silica by gradient elution with 0.05 to 0.5 M lactic acid (pH 3.5) in the presence of 0.01 M sodium 1-octanesulfonate. Post-column reaction with Arsenazo III is used for detection and quantification of the individual rare earth elements. Results are quoted for lanthanum, cerium, praseodymium, neodymium and samarium in monazites. Detection limit is 1 μg ml⁻¹ with a S/N ratio of 3. The separation is complete within 27 min valley to valley resolution. Precision of better than 1% can usually be obtained.     

Turbidimetric titration and gradient high-performance liquid chromatography of polystyrene samples

Summary Polystyrene samples of narrow molecular-weight distribution have been eluted according to their molecular weight from columns packed with bare silica Si50, phenyl, or C18 bonded phase by gradients of methanol and tetrahydrofuran (THF) or ofiso-octane and THF. Among the six combinations investigated,iso-octane/THF with a silica column formed a proper normal-phase system whereas methanol/THF with a C18 column formed a proper reversed-phase system. The combinations of C18 column andiso-octane/THF or of Si50 column and methanol/THF gradient did not correspond to the approved polarity rules in high-performance liquid chromatography but were nevertheless effective in separating polystyrene mixtures by molecular weight. Methanol andiso-octane are nonsolvents for polystyrene whereas THF is a solvent. The solubility of polystyrene as a function of molecular weight and concentration was determined by means of turbidimetric titration of solutions in THF with the nonsolvents used in the gradients. The solubility and elution characteristics were almost identical on C18 columns or in methanol/THF combinations. The elution from phenyl bonded phase and Si50 columns usingiso-octane/THF gradients required more THF than the solubility experiments. Information is also given on the occurrence of multimodal elution patterns.     

Chromatographic separation of trivalent actinides and rare earth elements by using pyridine type resin

Summary In order to develop the partitioning-transmutation system for the reduction of high-level radioactive wastes, the group and mutual separation of actinides and rare earth elements in spent fuels is required. In the present work, a chromatographic separation of trivalent actinides (Am and Cm) and rare earth elements was examined in hydrochloric acid/methanol mixed solvents by using a pyridine resin embedded in high-porous silica beads. In a 70 vol% hydrochloric acid (11.7 mol HCl/dm³)/30 vol% methanol mixed solvent, the elution curves of the trivalent actinides were separated completely from those of rare earth elements at room temperature. The present paper discusses the separation behavior of trivalent actinides and rare earth elements as a function of flow rate, temperature and solvent compositions.     

Determination of individual rare earth elements in Vietnamese monazite by radiochemical neutron activation analysis

Radiochemical neutron activation analysis (RNAA) has been applied for determination of rare earth elements (REE) in Vietnamese monazite. The chemical separation procedure used is based on the chromatographic elution of rare earth groups, after the separation of²³³Pa(Th) in irradiated monazite samples by coprecipitation with MnO₂, the rare earth elements were retained by Biorad AG1×8 resin column in 10% 15.4M HNO₃-90% methanol solution. The elution of heavy rare earth (HREE) and middle rare earth (MREE) groups was carried out with 10% 1M HNO₃-90% methanol and 10% 0.05M HNO₃-90% methanol solution, respectively; while the light rare earths (LREE) were eluted from the column by 0.1M HNO₃ solution. The accuracy of the method was checked by the analysis of granodiorite GSP-I and the rare earth values were in good agreement.     

Nanocomposites based on opal matrices and silica sols doped with rare earth compounds

Ordered 3D composites based on opal matrices and silica sols doped with rare earth elements have been prepared using colloidal chemistry methods. A uniform distribution of rare earth elements (which is important for avoiding luminescence concentration quenching) was achieved by means of repeated filling of the opal matrix interstitial space with silica sols doped with salts or oxides of rare earth elements. Trace amounts (10–30 ppm) of europium in a composite were shown to strongly affect optical properties of the material.     

Rare earth impurities in high purity lanthanum oxide determined by neutron activation analysis

Individual rare earth impurities in high purity La₂O₃ (99.9%) have been determined by NAA after pre-separation of the matrix (La). The separation is carried out on an anion exchanger (Dowex 1×8) using different mixtures of methanol/nitric acid as eluants. The rare earth elements from Dy to Lu are eluted quantitatively using a 10% 1M HNO₃-90% methanol mixture, while the light rare earths from Ce to Gd are eluted quantitatively using a 10% 0.05M HNO₃-90% methanol mixture. La, which is retained on the column, is eluted using 0.1M HNO₃. The recoveries of the various rare earth elements have been checked using radiotracers and also by spiking the sample with known amount of elements, and the recoveries are found to be quantitative. Results obtained on a typical high purity lanthanum oxide are reported here.     

Lipophilicity determination of some nitrostyrene derivatives on RP-2, PR-8 and RP-18 layers

Summary The lipophilicity of 33 nitrostyrene derivatives was determined by reversed-phase thin-layer chromatography on RP-2, RP-8 and RP-18 layers using methanol as the organic phase. The R_M value of each compound linearly decreased with increasing concentration of the organic solvent. The retention strength of RP-2 layer was lower than that of RP-8 and RP-18 layers. Between the retention strength of RP-8 and RP-18 layers no significant difference was detected. Minor differences among the selectivities of the layers were also observed. The impact of the change of methanol concentration on the retention was higher on RP-2 than on RP-18 layers probably due to the sterically favourable interaction of methanol with the shorter alkyl groups on the silica surface.     

Retention parameters of aromatic hydrocarbons with mono-substituted polar groups in binary RP-HPLC systems

Summary Capacity factor (k′) values of aromatic hydrocarbons with mono-substituted polar-groups are correlated for reversed-phase systems involving stationary phases with C₁₈ or C₄ ligands chemically bonded to silica and a binary aqueous eluent containing modifiers: methanol, acetonitrile, tetrahydrofuran, isopropanol, dioxane or dimethoxyethane. The relative retention variations of the solutes are interpreted with special consideration of their interactions with non-polar stationary phases and the molecular structure of the modifiers and solutes. Rules for retention and selectivity optimisation in RP-HPLC systems are given.     

Liquid-solid extraction and HPLC determination of amiodarone metabolites in biological fluids

Summary An improved liquid-solid extraction procedure for the purification and determination of amiodarone and desethylamiodarone in biological fluids is proposed. The sample is passed through a silica-C₁₈ column, and after washing, the analytes are eluted with methanol. The determination is then accomplished by RP HPLC using an octadecyl silica column and a mobile phase of methanol containing 0.0015% of ammonium hydroxide. The effect of ammonia concentration on the capacity factors of the analytes has been used for estimating the acid dissociation constants of the investigated secondary and tertiary amines in methanol.     

High-performance liquid chromatography of biphenols and bis (hydroxyphenyl) propanes (dianes) with voltammetric and UV photometric detection

Summary The retention behaviour of mono- and dihydroxy derivatives of biphenol and bis(hydroxyphenyl)propanes was studied on chemically bonded C-18, CN-, phenyl, phenoxypropyl- and NH₂ stationary phases and on silica gel. The effects of the mobile phase, i.e., of the methanol content, the ionic strength, pH and the contents of cationic and anionic ion-pairing agents on the retention data were investigated. In the reversed-phase systems, all the substances are satisfactorily separated except for 3- and 4-hydroxybiphenyl; these isomers can be separated on silica gel, using a mixture of heptane and propanol as the mobile phase. Sodium dodecylsulphate, present at concentrations higher than the critical micelle concentration (about 10⁻² M, depending on the methanol content), causes a decrease in the retention times and an improvement in the separation. UV absorption spectra, calibration curves and the detection limit values were obtained at two wavelengths and at various salt concentrations in the mobile phase. Hydrodynamic voltammograms of the solutes were measured in a carbon fibre detector and the calibration curves and detection limit values were measured at the optimal voltage (+1.2 V/Ag/AgCl) as a function of the mobile phase composition. For most test substances voltammetric detection is two to three times more sensitive than UV detection. The combination of a UV and a voltammetric detector in series has been utilized to identify the products of microbial degradation of biphenyl.     

Determination of Rare earth elements in environmental samples by solid phase extraction ICP OES

A simple sample pre-treatment method for rare earth elements enrichment from environmental water samples prior to optical emission spectrometry determination with inductively coupled plasma (ICP OES) is proposed based on solid phase extraction with octadecyl silica gel modified with 1,2,5,8-tetrahydroxyanthraquinone. Optimal experimental conditions including pH of sample solution, sample volume, type, concentration and volume of eluent were investigated and established. Rare earth elements ions were quantitatively adsorbed from aqueous solution onto octadecyl silica gel modified with Quinalizarin at pH 7.0. The adsorbed ions were eluted with 1.0 mL of 2 M HNO₃ and simultaneously determined by ICP OES. Under obtained optimum conditions the calibration curves were linear with the coefficient of variation better than 0.99. The limit of detection of the method for the studied elements was in the range of 0.0004–0.01 ng/mL. The proposed method has a pre-concentration factor of 320–450 in water samples, which results in high sensitivity detection of trace ions. The developed method gave recoveries better than 80% and RSDs less than 15%.     

Extraction of rare earth elements with 1-(diphenylphosphorylmethoxy)-2-diphenylphosphoryl-4-ethylbenzene with the use of 1,1,7-trihydrododecafluoroheptanol as a solvent

Extraction of rare earth elements from nitric acid solutions in a 1,1,7-trihydrododecafluoroheptanol-water system with the use of phosphoryl-containing podand 1-(diphenylphosphorylmethoxy)-2-diphenylphosphoryl-4-ethylbenzene (L) was studied. The content of metals in organic phase was shown to be negligible at nitric acid concentration lower 1 mol/L. Distribution ratio sharply increases with nitric acid concentration from 1 mol/L and reaches 5.5 for the yttrium subgroup elements at HNO₃ concentration of 6 mol/L. The rare earth elements of the yttrium subgroup were found to be extracted much better than the rare earth elements of the cerium subgroup under the same conditions, the distribution ratios in both subgroups smoothly rise with atomic number of element. It was shown using the shift of extraction equilibrium that the M: L ratio in extracted complexes is 1: 2 irrespective of the nature of rare earth element. The structure of complex {Yb[η²-(O,O′)-L]₂(η²-O₂NO)₂}(O₂NOHNO₃), whose single crystals were isolated from extraction solution, was established by X-ray diffraction study. The system can be used for the isolation and separation of rare earth elements.     

Anion Exchange Separation of Rare Earths in Methanol-Nitric Acid Mixed Solutions I. Study on the Distribution Coefficients and Column Elution Method

Anion exchange method for the mutual separation of rare earth was studied in methanol-nitric acid mixed solutions. For each of rare earth element, the distribution coefficients in different concentrations of nitric acid and also in various compositions of methanol solution were studied with the batch equilibrium method. From these distribution coefficients, elution of rare earth by gradient variation in the compositions of methonal was devised. That is, by keeping constant nitric acid at 2 N acidity and gradiently diluting methanol from its original 80% by volume. Results were satisfactory for the mutual separation of the lighter rare earths, but only fairly for the heavier earths. The secondary metal effect for this anion exchange was also studied in 80% by volume of methanol solution by keeping total concentration of nitrate at 0.8 N. Distribution coefficients were larger than nitric acid with LiNO₃ but smaller in the case of NH₄NO₃ and NaNO₃. Despite of these difference in distribution coefficients, no remarkable differences were observed for the separation factors of adjacent pairs.     

A solvent-free sampling method for airborne toluene diisocyanate

Summary A useful method of sampling and measurement of toluene diisocyanate concentration in atmosphere is described. The sampler consists of glass-fibre filters impregnated with the reagent 1-(2-methoxyphenyl)piperazine¹ so that the 2,4 and 2,6 isomers of TDI react to form urea derivatives which are analysed by high performance liquid chromatography in isocratic mode on cyan-amino and C₁₈ bonded phases. A dynamic system was used to generate standard atmospheres of TDI and to validate the sampling method within the humidity range of 0% to 75%. A coated filter, a bubbling solution and an impregnated silica gel were compared as samplers requiring the piperazine reagent in performance experiments.     

The effects of pH, ionic strength and buffer concentration of mobile phase onR F of acidic compounds in ion-pair TLC

Summary The effects of some factors important in ion-pair, high-performance liquid chromatography were studied in ion-pair, thin-layer chromatography. Tetramethyl and cetyltrimethylammonium salts were used as ion-pairing reagents. As stationary phases, silica gel and chemically bonded reversed-layers (C₁₈) were used. Layers were impregnated with ion-pairing reagent prior to chromatography. In some cases the stationary phase was treated with buffer at different concentrations. The mobile phase contained methanol and water, in one set of experiments buffer, salt for adjustment of ionic strength and ion-pairing reagent were added. The migration behaviour of different benzoic acids was studied. Several problems of ion-pair thin-layer chromatography are discussed. Passed away on 13^th of April, 1998 Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997.     

Influence of intermolecular interaction with the eluent on the retention of phenol and aniline on polar and non-polar surfaces

Summary The retention order of aniline and phenol on hydroxylated silica gel surface is reversed with the increase of the concentration of a polar component in the eluent. At minor (about 1%) concentrations of isopropanol in hexane aniline emerges first followed by phenol, the elution order being reversed with the increase of isopropanol concentration above 2%. The same behaviour is observed for silica gel with chemically bonded C₁₆ n-alkyl groups and for porous styrene-divinylbenzene copolymer. In all these cases the retention depends to a great extent on substance-eluent intermolecular interactions. At high isopropanol concentration in hexane the rise of column temperature changes the retention order of aniline and phenol on silica gel with hydroxylated surface since the association of these molecules with isopropanol as well as the isopropanol adsorption weaken as the temperature increases.     

大黄酸键合硅胶固定相的简便制备及色谱性能评价

以大黄酸为原料,γ-氨丙基三乙氧基硅烷(KH-550)为偶联剂,简便制备了一种新型羧基键合硅胶固定相(RBSP),并用红外光谱、热重分析及元素分析对其结构进行表征.考察了流动相中甲醇含量对键合固定相色谱性能的影响,并以含酸性、中性和碱性化合物的混合物为溶质,评价了RBSP的色谱性能.以甲醇-水为流动相,用C18柱作参比,研究了该键合硅胶作为HPLC固定相对两种大豆异黄酮化合物和几种生物碱基的分离,并对其色谱分离机理进行了初步探讨.实验结果表明,该固定相(RBSP)具有较好的反相色谱性能,同时由于键合相中含有酚羟基及酰胺基团,能为多种溶质提供作用位点,对极性化合物的分离具有明显优势,且分离速度快,可有效用于极性化合物的分离分析.