Cation-exchange isolation and ICP-AES determination of rare earth elements in geological silicate materials

An ion-exchange ICP-AES method for the determination of 14 rare earth elements (REEs) and Y in geological materials is described. The separation of REEs from Ba using a Dowex 50W-X8 cation resin is especially considered since Ba is an excellent internal standard for REE determination by this technique. Although total recovery with either HCl or HNO₃ may be achieved, it is advantageous to use both acids sequentially. Volume and concentration of the acids are optimized attaining a quantitative separation of REEs from Ba by the introduction of the sample solution in a 1.75 mol/l HCl medium, followed by elution with 2 mol/l HNO₃ to remove matrix elements and with 7 mol/l HNO₃ to elute the analytes. The total elution volume is significantly reduced without decreasing the efficiency. The behaviour of the matrix constituents under the selected conditions is also studied, evaluating their elution percentages in each step. The final solution obtained contains only the REEs and Y, with the bulk of Sc and minor amounts of Cr, Fe, Hf and Ta. Experimental data for 5 geological reference standards (NIM-G, GSP-1, AGV-1, NIM-L and NIM-S) are reported. Good agreement between the present results and previously accepted values by various analytical techniques is observed.     

Sensitive and selective determination of mercury by differential pulse stripping voltammetry after accumulation of mercury vapour on a gold plated graphite electrode

 A selective and sensitive method is proposed for the determination of mercury by anodic stripping voltammetry after its preconcentration from the gas phase. Mercury from the sample solution is reduced to elemental Hg by SnCl₂ and volatilized by the bubbles of a carrier gas. The gas containing mercury vapour is dried and passed through a capillary onto a gold coated graphite electrode. An anodic stripping voltammogram is recorded from 0.1 mol/l HClO₄+3×10^-3 mol/l HCl solution. The calibration curve is linear from 1×10^-9 to 4×10^-8 mol/l Hg(NO₃)₂. The absolute detection limit is 0.46 ng Hg. The relative standard deviations for 4×10^-9 mol/l and 2×10^-8 mol/l Hg(NO₃)₂ are 9.8% and 6.1%, respectively (n=5). Received: 18 December 1995/Revised: 16 April 1996/Accepted: 20 April 1996     

Determination of traces of uranium and thorium in (Ba, Sr) TiO3 ferroelectrics by inductively coupled plasma mass spectrometry

 Traces of uranium and thorium in barium(II), strontium(II) titanate ((Ba, Sr)TiO₃) ferroelectric materials were determined by inductively coupled plasma mass spectrometry (ICP-MS). Samples were completely dissolved by a mixture of 1.4% H₂O₂ and 1.0 mol⋅l^-1 HNO₃. For a complete separation of the analytes from the matrix elements, a two step separation technique involving leaching and anion-exchange was applied. By the leaching step with HNO₃ more than 90% of the matrix can be removed whereas the analytes completely remained in the solution. The anion-exchange step was carried out on a BIO⋅RAD AG1-X8 column with a mixture of 1.0 mol⋅l^-1 HF and 0.5 mol⋅l^-1 HNO₃ as eluent. The content of uranium and thorium was subsequently measured by ICP-MS. The detection limits (D.L.) obtained were 0.043 ng g^-1 and 0.035 ng g^-1 for U and Th, respectively. The reproducibility was satisfactory with a relative standard deviation of less than 3% (at the 1 ng g^-1 level, n=5). The matrix concentrations in the final solution were reduced to the sub-μg ml^-1 level which is in the range of the detection limits of USN-ICP-AES (ultrasonic nebulization-ICP-atomic emission spectroscopy). The method was successfully applied to the determination of uranium and thorium in three synthetic (Ba, Sr)TiO₃ samples spiked with the analytes at levels of 1, 5 and 10 ng g^-1 and three (Ba, Sr)TiO₃ ferroelectric samples containing sub-ng g^-1 levels of the analytes. Received: 26 February 1996/Revised: 28 May 1996/Accepted: 5 June 1996     

Determination of traces of uranium and thorium in (Ba, Sr) TiO3 ferroelectrics by inductively coupled plasma mass spectrometry

 Traces of uranium and thorium in barium(II), strontium(II) titanate ((Ba, Sr)TiO₃) ferroelectric materials were determined by inductively coupled plasma mass spectrometry (ICP-MS). Samples were completely dissolved by a mixture of 1.4% H₂O₂ and 1.0 mol⋅l^-1 HNO₃. For a complete separation of the analytes from the matrix elements, a two step separation technique involving leaching and anion-exchange was applied. By the leaching step with HNO₃ more than 90% of the matrix can be removed whereas the analytes completely remained in the solution. The anion-exchange step was carried out on a BIO⋅RAD AG1-X8 column with a mixture of 1.0 mol⋅l^-1 HF and 0.5 mol⋅l^-1 HNO₃ as eluent. The content of uranium and thorium was subsequently measured by ICP-MS. The detection limits (D.L.) obtained were 0.043 ng g^-1 and 0.035 ng g^-1 for U and Th, respectively. The reproducibility was satisfactory with a relative standard deviation of less than 3% (at the 1 ng g^-1 level, n=5). The matrix concentrations in the final solution were reduced to the sub-μg ml^-1 level which is in the range of the detection limits of USN-ICP-AES (ultrasonic nebulization-ICP-atomic emission spectroscopy). The method was successfully applied to the determination of uranium and thorium in three synthetic (Ba, Sr)TiO₃ samples spiked with the analytes at levels of 1, 5 and 10 ng g^-1 and three (Ba, Sr)TiO₃ ferroelectric samples containing sub-ng g^-1 levels of the analytes. Received: 26 February 1996/Revised: 28 May 1996/Accepted: 5 June 1996     

Optimized microwave preparation procedure for the elemental analysis of aquatic sediment

 This paper summarizes several key points in applying the microwave preparation technique to the elemental analysis of aquatic sediments and reports systematic experiments in searching for an optimal microwave preparation procedure for element analysis in sediment samples. The determination of the elements Cu, Pb and Cd in a standard reference aquatic sediment sample (CRM 280, COMEUR) was achieved by first digesting the samples in a microwave oven equipped with PFA advanced composite vessels, followed by AAS measurement. The influence of microwave power, digestion time, various dissolution reagents and the HF removing conditions was studied. It has been shown that for a 0.1 g sediment sample the optimal microwave preparation conditions are: 4–5ml HNO₃/HF/H₂O₂ as solvent, digesting time 30 min with 100% microwave power and evaporating the residual acid within 8 min in an open vessel at 80 °C. The element recovery rates with AAS measurement can reach up to 92.4–100.6%. Received: 23 July 1996/Revised: 23 September 1996/Accepted: 25 September 1996     

Novel dissolution procedure using a mixture of manganese dioxide and hydrochloric acid for the matrix-independent determination of gold

 A mixture of manganese dioxide and hydrochloric acid has been used for the accurate and precise determination of gold in various gold-bearing matrices. Results of intermethod comparison studies, F-test on variances, Mann-Whitney U-test, Spearman rank correlation and regression analyses are presented. The recommended method can be applied to a quality control programme and for the evaluation of reference materials. Various standard reference materials (SRM) of diverse matrices have been examined to check the validity of the method and the results were found to be in very good agreement with the certified data. Sample decomposition is straight forward [1–10 g sample (roasted at 600–700 °C)+5 g of MnO₂+30 mL of 2 mol/L HCl]. Gold is coprecipitated with potassium tellurite using tin (II) chloride solution. The precipitate is extracted into toluene and finally stripped back into aqua-regia solution for final nebulization into an air-acetylene flame for atomic absorption spectrophotometry. The method is very simple and easily adaptable, and more convenient than conventional methods involving aqua regia or a hydrobromic acid-bromine water mixture. Received: 23 October 1995/Revised: 18 March 1996/Accepted: 23 March 1996     

Graphite furnace atomic absorption spectrometric (GFAAS) determination of Cu, Cd, Cr, Mn, Ni and Pb in tellurium metal using precipitation and ion-exchange procedures

 Two independent procedures have been developed for the determination of Cd, Cu, Ni, Mn, Cr and Pb in high-purity tellurium by GFAAS. In the first, tellurium is precipitated as TeO₂ in the presence of EDTA at pH 4.5±0.2 to remove of 99.4% of tellurium as tellurium dioxide. The supernatant is analysed for impurities. In the second procedure, the trace impurities are preconcentrated on a Chelex-100 resin column at pH 9.4±0.2, and eluted by two column volumes of 2 mol/L HNO₃. In this case a matrix separation coefficient (k_s) of 1500 is achieved. The analyses are carried out by GFAAS. The relative standard deviations of the analyte concentrations in the samples are 4–8%. Separation of the matrix and recovery of the analytes are essentially complete as confirmed by standard addition of the analytes to the matrix. Received: 30 July 1996/Revised: 7 November 1996/Accepted: 13 November 1996     

In-line flow injection extraction-preconcentration through a passive hydrophilic membrane. Determination of total phenols in oil by flow-injection analysis

 An in-line flow injection extraction-preconcentration procedure for the determination of total phenols in oil is described. The reaction between phenolic compounds and 4-aminoantipyrine in the presence of K₂S₂O₈ as oxidizing reagent was used. The phenols were extracted and preconcentrated from a xylene solution by using a more selective passive hydrophilic Spectrapor membrane which also removed interferences. The phenols deprotonated after diffusion to the basic acceptor stream and the preconcentrated phenolate was injected into a carrier stream containing 4-aminoantipyrine as colour reagent. The carrier stream then merged with the oxidant stream, followed by detection at 500 nm. The system was suitable for the determination of total phenols in oil at a sampling rate of 12 samples per hour with an RSD of better than 1.3%. The detection limit was 0.09 mg/l for phenol, 0.18 mg/l for o-cresol and 0.02 mg/l for m-cresol. The results of the proposed system compared favourably with a standard manual 4-AAP method and a standard GC procedure. Received: 30 July 1996/Accepted: 25 August 1996     

Determination of copper by anodic stripping voltammetry on a glassy carbon electrode using a continuous flow system

 A successful flow-through system was developed for trace analysis of copper using DPASV with a glassy-carbon electrode. Periodical chemical regeneration of the electrode with a 1 mol/L NaOH solution increased sensitivity and precision. The method was shown to be applicable with a detection limit of 0.56 μg/L, with a determination time of less than 7 min per measurement (without deaeration time). The drawback of the system is the 10 min deaeration time. The system gave an accuracy of 0.090±0.005% for a certified reference material of low alloy steel containing 0.090±0.004% Cu. Applicability to various fresh water samples with a Cu content between 1.57 and 13.11 μg/L with an RSD<2.36% is illustrated. Received: 11 March 1996/Revised: 1 July 1996/Accepted: 4 July 1996     

Solvent extraction of rare-earth metals with bis(2-ethylhexyl)phosphinic acid

 The extraction behavior of tervalent rare-earth metals (Ln) using a heptane solution containing bis(2-ethylhexyl)phosphinic acid (PIA-8, HR) from 0.1 mol/dm³ sodium perchlorate media was studied. The pH_0.5 values and separation factors obtained were compared among the metals. The stoichiometry of the extracted species and the extraction constants for the present aqueous/heptane system were determined by slope analysis. It is demonstrated that the rare-earth metals were extracted as monomers LnR₃⋅mHR (m=3, 4, 5 or 6), and the extracted species could be stripped into a relatively low concentrated hydrochloric acid. PIA-8 was found to be the most selective extractant for the mutual separation of rare-earth metals among the other phosphinic acids reported. Received: 27 February 1996/Revised: 13 May 1996/Accepted: 21 May 1996     

Separation of trace heavy metals from silver matrix by solid – liquid extraction for graphite-furnace atomic absorption spectrometry

 A silver sample (50–300 mg) was dissolved in 6 mol/l nitric acid and mixed with 6 mol/l hydrochloric acid to form silver chloride. The solution was evaporated to dryness and the residue was treated ultrasonically in 0.1 mol/l nitric acid for the extraction of trace impurities from the silver chloride. Traces of iron, copper, lead and bismuth were recovered in greater than 94% yields, though considerable amounts of cadmium were not extracted due to the strong occlusion in the silver chloride. The separation factor for the silver was about 10^-4; hence the trace heavy metals were immediately determined by graphite-furnace atomic absorption spectrometry without any interference. The validity of the method was confirmed by the analysis of certified reference material. The proposed method is simple and allows the detection of as little as 10 ng/g levels in silver samples. Received: 13 January 1995/Accepted: 29 March 1995     

Determination of copper by anodic stripping voltammetry on a glassy carbon electrode using a continuous flow system

 A successful flow-through system was developed for trace analysis of copper using DPASV with a glassy-carbon electrode. Periodical chemical regeneration of the electrode with a 1 mol/L NaOH solution increased sensitivity and precision. The method was shown to be applicable with a detection limit of 0.56 μg/L, with a determination time of less than 7 min per measurement (without deaeration time). The drawback of the system is the 10 min deaeration time. The system gave an accuracy of 0.090±0.005% for a certified reference material of low alloy steel containing 0.090±0.004% Cu. Applicability to various fresh water samples with a Cu content between 1.57 and 13.11 μg/L with an RSD<2.36% is illustrated. Received: 11 March 1996/Revised: 1 July 1996/Accepted: 4 July 1996     

Extraction and separation of scandium salicylate with triphenylphosphine oxide

Scandium can be extracted from 5.0 × 10^–2 mol/l sodium salicylate solution, adjusted to pH 4.0–5.0 with 0.5% triphenylphosphine oxide dissolved in toluene as an extractant. After stripping from the organic phase with 0.5 mol/l HCl it can be subsequently determined spectrophotometrically with Alizarin Red S. The method permits a separation of Sc(III) from Ti(IV), V(V), Cr(VI), Fe(III), Y(III), La(III), Ce(III), Nd(III) and Sm(III) in synthetic mixtures. The method is fast, simple and selective. Received: 6 May 1996 / Revised: 21 July 1996 / Accepted: 25 July 1996     

Liquid-liquid extraction and separation studies of yttrium

A simple selective method is presented for the solvent extraction of yttrium from salicylate media by using triphenylphosphine oxide. Yttrium is extracted quantitatively from 0.05 mol/l sodium salicylate solution at pH 4.5–5.0 using 1.8% triphenylphosphine oxide dissolved in toluene as an extractant and can be subsequently stripped using water and determined spectrophotometrically with Thoron-I. This precise and accurate method permits the separation of Y from Sc, Ti, V, Cr, Fe, Zr, Hf, Th and U from binary mixtures and multi-component systems. Received: 17 January 1995 / Accepted: 12 May 1995     

Investigation of the separation of scandium and rare earth elements from red mud by use of reversed-phase HPLC

A chromatographic method has been developed for separation and determination of scandium (Sc) and rare earth elements (REEs) in samples from a red mud (RM)-utilization process. Reversed-phase high-performance liquid chromatography (RP-HPLC) with post-column derivatization using 4-(2-pyridylazo)-resorcinol (PAR) and UV–visible detection at 520 nm was tested using different gradient elution profiles and pH values to optimize separation and recovery, primarily for Sc but also for yttrium and the individual lanthanides, from iron present in the samples. The separation was performed in less than 20 min by use of a mobile phase gradient. The concentration of -hydroxyisobutyric acid (-HIBA), as eluent, was altered from 0.06 to 0.4 mol L^–1 (pH 3.7) and 0.01 mol L^–1 sodium salt n-octane sulfonic acid (OS) was used as modifier. Very low detection limits in the nanogram range and a good resolution for Sc and REEs except for Y/Dy were achieved. Before application of the method to the red mud samples and to the corresponding bauxites, Sc and REEs were leached from red mud with 0.6 mol L^–1 HNO₃ and mostly separated, as a group, from the main elements by ion exchange/selective elution (6 mol L^–1 HNO₃) in accordance with a pilot-plant process developed in this laboratory. After evaporation of the eluent to dryness the extracted elements were re-dissolved in the mobile phase. By use of this chromatographic method Sc, which is the most expensive of the elements investigated and occurs in economically interesting concentrations in red mud, could be separated not only from co-existing Fe but also from Y/Dy, Yb, Er, Ho, Gd, Eu, Sm, Nd, Pr, Ce and La. All the elements investigated were individually recovered. Their recoveries were found to be nearly quantitative.     

Colorimetric determination of iron in infant fortified formulas by sequential injection analysis

 A procedure is described for the colorimetric determination of iron in infant fortified formulas based on sequential injection analysis (SIA). Iron(III) complexation with thiocyanate is used as colour developing reaction. The system enables the determination of iron in the samples (after digestion by dry ashing and treatment with 0.2 mol/L nitric acid in the range of 0.50–20.0 mg/L, consuming 140 μL of the sample and 8 mg thiocyanate per determination. The reactor geometry and the adjustment of the ionic content of the calibration solutions is important for the accuracy of the results. A regression line according to the equation [Fe(III) (mg/L)]_SIA=−0.3(±0.4)+1.03(±0.04) [Fe(III) (mg/L)]_FAAS was obtained after comparative analysis of a set of 12 samples. The measurement rate was 34 s, thus allowing to analyze 100 samples per hour with a relative standard deviation lower than 2%. Received: 30 July 1996/Revised: 1 October 1996/Accepted: 4 October 1996     

Selenium determination by HG-ICPAES: Elimination of iron interferences by means of an ion-exchange resin in a continuous flow system

 An accurate procedure for the elimination of iron interferences in the determination of selenium in geological materials by the Hydride Generation – Inductively Coupled Plasma Atomic Emission Spectrometry technique (HG-ICPAES) is proposed. A selective removal of iron is achieved by on-line incorporation of a microcolumn filled with strongly acidic cation exchange resin (Dowex 50W-X8). The microcolumn manifold used was interfaced with the hydride generation manifold by a flow injection sample injection valve. After the removal of the iron, a 500 μl sample was injected into a carrier stream of water. This was merged with hydrochloric acid and sodium tetrahydroborate in order to generate the corresponding selenium hydride. The system was found to have a limit of detection of 0.4 ng ml^-1 and a relative standard deviation of 2% for 20 ng ml^-1 selenium. The application of the method on different Geochemical Standard Reference Samples demonstrated that results were statistically indistinguishable from certified values. Received: 7 March 1996 / Revised: 30 May 1996 / Accepted: 4 June 1996     

Separation of ADAM derivatives of carnitine and acyl-carnitines by capillary electrophoresis

 The separation of carnitine, acetylcarnitine and palmitoylcarnitine as ADAM (9-anthryldiazo-methane) derivatives was performed using capillary electrophoresis. A buffer system with 90% methanol and various amounts of phosphoric acid and micelle forming SDS was optimized with respect to the best resolution of the carnitine derivatives. A detection limit of 10 μmol/l or 32 ng carnitine was determined by laser induced fluorescence detection. Under optimized conditions low carnitine contents in acylcarnitine standards have been determined. Received: 30 May 1996/Accepted: 17 June 1996     

Separation of ADAM derivatives of carnitine and acyl-carnitines by capillary electrophoresis

 The separation of carnitine, acetylcarnitine and palmitoylcarnitine as ADAM (9-anthryldiazo-methane) derivatives was performed using capillary electrophoresis. A buffer system with 90% methanol and various amounts of phosphoric acid and micelle forming SDS was optimized with respect to the best resolution of the carnitine derivatives. A detection limit of 10 μmol/l or 32 ng carnitine was determined by laser induced fluorescence detection. Under optimized conditions low carnitine contents in acylcarnitine standards have been determined. Received: 30 May 1996/Accepted: 17 June 1996     

HPLC determination of collectors used for the flotation of heavy metal minerals

 A reversed-phase HPLC-method for the separation of mixtures of collectors for the flotation of heavy metal minerals is described. It is based on a Nucleosil 5C18 column, isocratic elution and UV-detection at 238 nm. The mobile phase is methanol-water-5% phosphoric acid (40:60:4, v/v). The method is applied to the determination of six collectors in aqueous solutions from flotation processes. The relative standard deviations are 1.6–3.2% in the concentration range 2–10 mg/L. The detection limits are 1 μg/L for 8-hydroxyquinoline, dimethylglyoxime and salicylic acid, 2 μg/L for salicylhydroxamic acid and 5 μg/L for benzenetriazol and salicylaldoxime, respectively. Received: 19 April 1996/Revised: 14 August 1996/Accepted: 23 August 1996