Extraction-ability and -selectivity of tetra-aza-crown ethers for transition metal cations

In order to investigate the relative effects of the differences between the structures and lipophilicities of 1, 10-dioxa-4, 7, 13, 16-tetra-azacyclo-octadecane (TA-18-crown-6) and the tetrabenzyl derivative of 1,10-dioxa-4, 7, 13, 16-tetra-azacyclo-octadecane (TBTA-18-crown-6) on their extraction-abilities and -selectivities for transition metal cations, constants of the overall extraction (logK _ex) of 1:1 (M:L) complexes, the distribution (K _D) for two diluents (CH₂Cl₂ and CHCl₃) with different dielectric constants have been determined at 25 ± 0.1 °C. The magnitude of logK _ex is largely determined by that of K _D. The equilibrium constants of TA-18-crown-6 have been compared with those of TBTA-18-crown-6. It is found that:(i) logK _ex sequences of TA-18-crown-6 and TBTA-18-crown-6 for transition metals in CH₂Cl₂ lie in order: Fe³⁺ > Cu²⁺ > Mn²⁺ > Co²⁺ > Cd²⁺ > Ni²⁺ > Zn²⁺ and Fe³⁺ > Cu²⁺ > Co²⁺ > Mn²⁺ > Ni²⁺ > Cd²⁺ > Zn²⁺ respectively; (ii) the stability sequences of two types of tetra-aza-crown ethers with the transition metal cations in CHCl₃ are the same as follows: Fe³⁺ > Ni²⁺ > Cu²⁺ > Co²⁺ > Zn²⁺ > Cd²⁺ > Mn²⁺, and (iii) unusual selectivities are observed for transition metal-tetra-aza-crowns, e.g. the high Fe³⁺/M ⁿ⁺ selectivity factors (S_f) of TA-18-crown-6, except for the competitive-extractions for the special case in CHCl₃ of TBTA-18-crown-6, it was found that the Mn²⁺/M ⁿ⁺ values were relatively higher according to the other transition metal cations. A systematic sequence in these two types of solvents is not found for a given transition metal cation in terms of the variation of selectivity with the tetra-aza-crown ethers. The results provide alternatives for the rational design of other specific ligands on the transition metal cations.     

Analysis of phosphorus herbicides by ion-pairing reversed-phase liquid chromatography coupled to inductively coupled plasma mass spectrometry with octapole reaction cell

A reversed phase ion-pairing high performance liquid chromatographic (RPIP-HPLC) method is developed for the separation of two phosphorus herbicides, Glufosinate and Glyphosate as well as Aminomethylphosphonic acid (AMPA), the major metabolite of Glyphosate. Tetrabutylammonium hydroxide is used as the ion-pairing reagent in conjunction with an ammonium acetate/acetic acid buffering system at pH 4.7. An inductively coupled plasma mass spectrometer (ICP-MS) is coupled to the chromatographic system to detect the herbicides at m/z = 31P. Historically, phosphorus has been recognized as one of the elements difficult to analyze in argon plasma. This is due to its relatively high ionization potential (10.5 eV) as well as the inherent presence of the polyatomic interferences 14N16O1H+ and 15N16O+ overlapping its only isotope at m/z = 31. An octapole reaction cell is utilized to minimize the isobaric polyatomic interferences and to obtain the highest signal-to-background ratio. Detection limits were found to be in the low ppt range (25-32 ng/l). The developed method is successfully applied to the analysis of water samples collected from the Ohio River and spiked with a standard compounds at a level of 20 microg/l.     

Application of magnetic nanoparticles for the extraction of radium-226 from water samples

Bare (unmodified) and crown ether (CE)-modified Fe₃O₄ magnetic nanoparticles (MNPs) were investigated for the rapid extraction of ²²⁶Ra from water samples. It involved synthesizing the MNPs, introducing them into the sample solutions, ultrasonicating and agitating the suspension, magnetically separating the nanoparticles from solution, and measuring the ²²⁶Ra content in the supernatant. Experimental parameters such as salt choice, salt concentration and pH were optimized to achieve maximum extraction of ²²⁶Ra onto the MNPs. ²²⁶Ra content was determined using a Hidex 300SL liquid scintillation counter with α/β separation capability, or a gamma spectrometric detection system. The bare Fe₃O₄ nanoparticles showed significant pH dependence for the extraction of ²²⁶Ra from an aqueous solution over a pH range of 2–10. They gave an extraction of 95 ± 1 and 98 ± 1 % at pH 9 in 0.1 M NaCl and 0.1 M NaClO₄, respectively, whereas an extraction of 8–24 % was obtained, over the pH ranges from 2 to 5. The CE-modified MNPs yielded extraction efficiencies as high as 99 ± 1 % in the presence of 0.01 M picric acid at pH 4. This study demonstrates that the surface functionalization of Fe₃O₄ MNPs with suitable ligand modification can offer a selective mode of extraction for ²²⁶Ra in the presence of its daughter progenies.     

Rapid determination of 226Ra in drinking water samples using dispersive liquid-liquid microextraction coupled with liquid scintillation counting

A new radioanalytical method was developed for rapid determination of ²²⁶Ra in drinking water samples. The method is based on extraction and preconcentration of ²²⁶Ra from a water sample to an organic solvent using a dispersive liquid-liquid microextraction (DLLME) technique followed by radiometric measurement using liquid scintillation counting. In DLLME for ²²⁶Ra, a mixture of an organic extractant (toluene doped with dibenzo-21-crown-7 and 2-theonyltrifluoroacetone) and a disperser solvent (acetonitrile) is rapidly injected into the water sample resulting in the formation of an emulsion. Within the emulsion, ²²⁶Ra reacts with dibenzo-21-crown-7 and 2-theonyltrifluoroacetone and partitions into the fine droplets of toluene. The water/toluene phases were separated by addition of acetonitrile as a de-emulsifier solvent. The toluene phase containing ²²⁶Ra was then measured by liquid scintillation counting. Several parameters were studied to optimize the extraction efficiency of ²²⁶Ra, including water immiscible organic solvent, disperser and de-emulsifier solvent type and their volume, chelating ligands for ²²⁶Ra and their concentrations, inorganic salt additive and its concentration, and equilibrium pH. With the optimized DLLME conditions, the accuracy (expressed as relative bias, B _r ) and method repeatability (expressed as relative precision, S _B ) were determined by spiking ²²⁶Ra at the maximum acceptable concentration level (0.5 Bq L⁻¹) according to the Guidelines for Canadian Drinking Water Quality. Accuracy and repeatability were found to be less than −5% (B _r ) and less than 6% (S _B ), respectively, for both tap water and bottled natural spring water samples. The minimum detectable activity and sample turnaround time for determination of ²²⁶Ra was 33 mBq L⁻¹ and less than 3 h, respectively. The DLLME technique is selective for extraction of ²²⁶Ra from its decay progenies.     

Characterizing student mathematics teachers’ levels of understanding in spherical geometry

This article presents an exploratory study aimed at the identification of students’ levels of understanding in spherical geometry as van Hiele did for Euclidean geometry. To do this, we developed and implemented a spherical geometry course for student mathematics teachers. Six structured, task-based interviews were held with eight student mathematics teachers at particular times through the course to determine the spherical geometry learning levels. After identifying the properties of spherical geometry levels, we developed Understandings in Spherical Geometry Test to test whether or not the levels form hierarchy, and 58 student mathematics teachers took the test. The outcomes seemed to support our theoretical perspective that there are some understanding levels in spherical geometry that progress through a hierarchical order as van Hiele levels in Euclidean geometry.     

Atomic absorption spectrometric determination of Cd(II), Mn(II), Ni(II), Pb(II) and Zn(II) ions in water,fertilizer and tea samples after preconcentration on Amberlite XAD-1180 resin loaded with l-(2-pyridylazo)-2-naphthol

A new chelating resin, 1-(2-pyridylazo)-2-naphthol (PAN) coated Amberlite XAD-1180 (AXAD-1180), was prepared and used for the preconcentration of Cd(II), Mn(II), Ni(II), Pb(II) and Zn(II) ions prior to their determination by flame atomic absorption spectrometry (FAAS). The optimum pH for simultaneous retention of the elements and the best elution means for their simultaneous elution were pH 9.5 and 3 M HNO₃, respectively. The sorption capacity of the resin was found to be 5.3 mg/g for Cd and 3.7 mg/g for Ni. The detection limits for Cd(II), Mn(II), Ni(II), Pb(II) and Zn(II) were 0.7, 10, 3.1, 29 and 0.8 μg/L, respectively. The effects of interfering ions for quantitative sorption of the metal ions were investigated. The preconcentration factors of the method were in the range of 10–30. The recoveries obtained were quantitative (≥95%). The standard reference material (GBW07605 Tea sample) was analysed for accuracy of the described method. The proposed method was successfully applied to the analysis of various water, urea fertilizer and tea samples. The article is published in the original.     

Synthesis and characterization of graft copolymers based on polyepichlorohydrin via reversible addition-fragmentation chain transfer polymerization

In this study, synthesis of poly(epichlorohydrin-g-methyl methacrylate) graft copolymers by reversible addition-fragmentation chain transfer (RAFT) polymerization was reported. For this purpose, epichlorohydrin was polymerized by using HNO₃ via cationic ring-opening mechanism. A RAFT macroinitiator (macro-RAFT agent) was obtained by the reaction of potassium ethyl xanthogenate and polyepichlorohydrin. The graft copolymers were synthesized using macro-RAFT agent as initiator and methyl methacrylate as monomer. The synthesis of graft copolymers was conducted by changing the time of polymerization and the amount of monomer-initiator concentration that affect the RAFT polymerization. The effects of these parameters on polymerization were evaluated via various analyses. The characterization of the products was determined using ¹H-nuclear magnetic resonance (¹H-NMR), Fourier-transform infrared spectroscopy, gel-permeation chromatography, thermogravimetric analysis, elemental analysis, and fractional precipitation techniques. The block lengths of the graft copolymers were calculated by using ¹H-NMR spectrum. It was observed that the block length could be altered by varying the monomer and initiator concentrations.     

RNA-sensitive N-isopropylacrylamide/vinylphenylboronic acid random copolymer

Random copolymers of N-isopropylacrylamide (NIPA) and 4-vinylphenylboronic acid (VPBA) were obtained by solution polymerization using 2,2′-azobisizobutyronitrile as the initiator in ethanol at 65 °C. NIPA-co-VPBA copolymer exhibited both temperature- and pH-sensitivity. Thermally reversible phase transitions were observed both in the acidic and alkaline pH region for the copolymers produced with different VPBA/NIPA feed ratios. The pH dependency of the lower critical solution temperature (LCST) was stronger for the copolymers produced with higher VPBA feed concentrations. RNA was selected as a model biomolecule having vicinal-diol and amino groups that were potentially reactive with the boronic acid groups of NIPA-co-VPBA copolymer. The effect of RNA concentration on the LCST of NIPA-co-VPBA copolymer was investigated in aqueous media at different pHs. Although no significant effect was observed at pH 4, 7 or 10.5, the LCST decreased linearly with increasing RNA concentration at a pH approximately equal to the pK_a of boronic acid. This behavior was explained by considering the binding of RNA onto the copolymer chains to occur via two types of complex formation. For the formation of these complexes, the amino and vicinal-diol groups of RNA should react with the boronic acid groups of the copolymer in the tetrahedral anionic form. The results indicated that NIPA-co-VPBA copolymer could be utilized as a new reagent for the determination of RNA concentration in aqueous media. The proposed method was valid for the RNA concentration range of 0–4 g · mL⁻¹.

The schematical representation of the possible interactions between NIPA-co-VPBA copolymer and RNA. (A) A typical structure of single-stranded RNA. (B) Tetrahedral anionic form of boronic acid groups. (C) The interaction between the amino groups of the unpaired bases of RNA and the boronic acid groups of the copolymer. (D) Cyclic borate ester formation by the interaction between vicinal diol groups located at the 3′-end of RNA and boronic acid groups of the copolymer.