Optimisation of selectivity in the separation of metallo-cyanide complexes by ion-interaction liquid chromatography

A study into the optimisation and selectivity of a reversed-phase ion-interaction liquid chromatographic method for the separation of metallo-cyanide complexes is described. A stable ion-interaction system was developed in which a C₁₈ stationary phase was equilibrated with a 60 mM solution of tetrabutylammonium hydroxide ion-interaction reagent in order to saturate the stationary phase and to minimise retention changes caused by adsorption and desorption of this material. The effects on retention of the metallo-cyanide complexes caused by changes in pH and ionic strength were minimised through the addition of a high concentration of a phosphate buffer (150 mM)to the mobile phase. Perchlorate (0.32–5.62 mM) was then added to the mobile phase as a competing anion and its effect upon the capacity factors of each complex determined. A linear relationship between the logarithm of capacity factor and the logarithm of the concentration of perchlorate was observed, although the slopes of these plots were not accordance with those predicted by a simple ion-exchange model. However, the linearity of the data allowed a simple optimisation procedure to be applied and the concentration of perchlorate could be used to manipulate the separation selectivity of the system. Three differing elution orders of metallo-cyanide complexes were achieved by varying the concentration of perchlorate in the mobile phase within the range 0.94–5.62 mM.     

Creation and characteristics of phosphatidylcholine stationary phases for the chromatographic separation of inorganic anions

New stationary phases for chromatographic separation of anions, obtained by loading liposomes made from dimyristolyphosphatidylcholine (DMPC) onto reversed-phase packed columns (C18 and C30) are reported. Mono- and divalent anions were used as model analyte ions and retention data for these species were obtained using the DMPC stationary phases and used to elucidate the separation mechanisms involved in this chromatographic system. The DMPC stationary phases can separate anions by either a solvation-dependent mechanism or an electrostatic ion-exchange mechanism, depending upon the relative magnitudes of the negative electrostatic potential (Psi(-)) of the phosphate moiety (P-) and the positive electrostatic potential (Psi(+)) of the quaternary ammonium groups (N+) on the headgroup of DMPC. If Psi(+) > Psi(-), such as in case where Psi(-) has been reduced either by binding of eluent cations (e.g., H+ or divalent cations) onto the P- group of DMPC or by steric screening when a C30 reversed-phase material was used to support the DMPC, then the overall electrostatic surface potential (and hence also the effective anion-exchange capacity) was generally large and the anions were separated on the basis of an electrostatic mechanism. However, if Psi(+) was similar to Psi(-), such as in the case of using a C18 reversed-phase support and monovalent cations as eluent cations, then the overall electrostatic surface potential and the effective anion-exchange capacity were very small and the analyte anions were separated on the basis of a solvation-dependent mechanism. The DMPC stationary phases were found to be suitable for the direct determination of iodide and thiocyanate in highly saline water samples, such as seawater samples.     

Poly(tetrafluoroethylene) separation capillaries for capillary electrophoresis. Properties and applications

Poly(tetrafluoroethylene) (PTFE) is a material widely known for its inertness and excellent electrical properties. It is also transparent in the UV region and has a reasonable thermal conductivity. These properties make PTFE a suitable material for the separation capillary in capillary electrophoresis. Differences in the chemistry of the capillary wall compared to fused silica (FS) can make PTFE an interesting alternative to FS for some special applications. In this work, properties of a commercial PTFE capillary of approx. 100 microm i.d. were investigated, including the dependence of electroosmotic flow (EOF) on pH for unmodified and dynamically modified PTFE, optical properties, and practical aspects of use. The main problems encountered for the particular PTFE capillary used in this study were that it was mechanically too soft for routine usage and the crystallinity of the PTFE caused light scattering, leading to high background absorbance values in the low UV region. The profile of the EOF versus pH for bare PTFE surprisingly showed significantly negative EOF values at pH < 4.2, with an EOF of -30 x 10(-9) m2 V(-1) s(-1) being observed at pH 2.5. This is likely to be caused by either impurities or additives of basic character in the PTFE, so that after their protonation at acidic pH they establish a positive charge on the capillary wall and create a negative EOF. A stable cationic semi-permanent coating of poly(diallyldimethylammonium chloride) (PDDAC) could be established on the PTFE capillary and led to very similar magnitudes of EOF to those observed with FS. A hexadecanesulfonate coating produced a cathodic EOF of extremely high magnitude ranging between +90 and +110 x 10(-9) m2 s(-1) V(-1), which are values high enough to allow counter-EOF separation of high mobility inorganic anions. In addition, pH-independent micellar electrokinetic capillary chromatography (MEKC) separations could be easily realised due to hydrophobic adsorption of sodium dodecylsulfate (used to form the micelles) on the wall of the PTFE capillary. The use of polymers that would be mechanically more robust and optically transparent in the low-UV region should make such CE capillaries an interesting alternative to fused silica.     

Determination of melatonin and monoamines in rat pineal using reversed-phase ion-interaction chromatography with fluorescence detection

A method is reported for the ion-interaction, reversed-phase separation of 24 compounds (chiefly monoamines) arising from the metabolism of tyrosine and tryptophan. These compounds were separated as two groups. The first group comprised 3,4-dihydroxyphenylethylene glycol, tyrosine, 3-methoxy-4-hydroxyphenyl glycol, 5-hydroxytryptophan, norepinephrine, 3,4-dihydroxyphenylacetic acid, epinephrine, 5-hydroxyindole-3-acetic acid, homovanillic acid, 5-hydroxytryptophol, dopamine, tryptophan. N-acetylserotonin, N-acetyltryptophan, 5-methoxytryptophan and serotonin. The mobile phase consisted of a 6.8:93.2 (v/v) mixture of acetonitrile and an aqueous solution containing 0.16 M ammonium phosphate, 0.06 M citric acid, 0.15 mM disodium EDTA, 10 mM dibutylamine and 6 mM sodium 1-octanesulphonate at pH 4.50. The second group of compounds comprised 6-hydroxymelatonin, 5-methoxyindole-3-acetic acid, indole-3-acetic acid, 5-methoxytryptamine, tryptamine, 5-methoxytryptophol, melatonin and tryptophol. The mobile phase consisted of a 16:84 (v/v) mixture of acetonitrile and an aqueous solution containing 0.05 M ammonium phosphate, 0.05 M citric acid, 0.15 mM disodium EDTA, 25 mM dibutylamine and 5 mM sodium 1-octanesulphonate at pH 5.30. Detection was by fluorescence measurement (lambda ex = 280 nm, lambda em = 340 nm). The proposed method exhibited linear calibration over the biochemically significant concentration range, with detection limits in the 10-200 pg range. Excellent precision for peak areas and retention times was observed, even over a period of 24 h. The applicability of amperometric detection (at 0.72V) is also demonstrated. The method is applied to the determination of monoamines in individual rat pineals. Low nanogram levels of tyrosine, norepinephrine, 5-hydroxyindole-3-acetic acid, tryptophan, serotonin and 6-hydroxymelatonin, and picogram levels of 5-hydroxytryptophan, 5-hydroxytryptophol, 5-methoxyindole-3-acetic acid, indole 3-acetic acid, 5-methoxytryptophol and melatonin were indicated in most of the samples.     

New Triterpene Saponins from Acanthophyllum pachystegium

Four new triterpenoid saponins, pachystegiosides A ( 1 ), B ( 2 ), C ( 3 ), and D ( 4 ), were isolated from the roots of Acanthophyllum pachystegium K. H. Their structures were elucidated by means of a combination of homo‐ and heteronuclear 2D‐NMR techniques (COSY, TOCSY, NOESY, HSQC, and HMBC) and by FAB‐MS. The new compounds were characterized as 3‐O‐{O‐β‐D ‐galactopyranosyl‐(1→2)‐O‐[β‐D ‐xylopyranosyl‐(1→3)]‐β‐D ‐glucuronopyranosyl}quillaic acid 28‐{O‐β‐D ‐xylopyranosyl‐(1→3)‐O‐β‐D ‐xylopyranosyl‐(1→4)‐O‐α‐L ‐rhamnopyranosyl‐(1→2)‐O‐[3,4‐di‐O‐acetyl‐β‐D ‐quinovopyranosyl‐(1→4)]‐β‐D ‐fucopyranosyl}ester ( 1 ), 3‐O‐{O‐β‐D ‐galactopyranosyl‐(1→2)‐O‐[β‐D ‐xylopyranosyl‐(1→3)]‐β‐D ‐glucuronopyranosyl}quillaic acid 28‐{O‐β‐D ‐xylopyranosyl‐(1→3)‐O‐β‐D ‐xylopyranosyl‐(1→4)‐O‐α‐L ‐rhamnopyranosyl‐(1→2)‐O‐[4‐O‐acetyl‐β‐D ‐quinovopyranosyl‐(1→4)]‐β‐D ‐fucopyranosyl} ester ( 2 ), 3‐O‐{O‐β‐D ‐galactopyranosyl‐(1→2)‐O‐[β‐D ‐xylopyranosyl‐(1→3)]‐β‐D ‐glucuronopyranosyl}quillaic acid 28‐{O‐β‐D ‐xylopyranosyl‐(1→4)‐O‐α‐L ‐rhamnopyranosyl‐(1→2)‐O‐[4‐O‐acetyl‐β‐D ‐quinovopyranosyl‐(1→4)]‐β‐D ‐fucopyranosyl} ester ( 3 ), and gypsogenic acid 28‐[O‐β‐D ‐glucopyranosyl‐(1→2)‐O‐β‐D ‐glucopyranosyl‐(1→6)‐O‐β‐D ‐glucopyranosyl‐(1→3)‐β‐D ‐galactopyranosyl] ester ( 4 ).     

Crystal structure of a new polyoxometalate (POM) compound with a high level of crystallographic disorder

The crystal structure, Hirshfeld surface analysis and spectroscopic analysis of a new polyoxometalate (POM) compound, namely, nonakis(2‐methoxyaniline) bis(diphosphopentamolybdate) trihydrate, (C₇H₉NO)₉[P₂Mo₅O₂₃]₂·3H₂O, is reported. The title compound was synthesized using the solution method and was structurally characterized by single‐crystal X‐ray diffraction, which revealed P symmetry. A study of the intermolecular interactions using Hirshfeld surface analysis confirmed that the hydrogen‐bonding interactions play the dominant role in the stability of the crystal structure. The refinement was complicated by extensive disorder affecting 11 of the 16 ions and molecules in the asymmetric unit. IR and UV–Vis spectroscopic techniques were used to identify the vibrational modes and to classify this compound as an insulator.     

The determination of lactate,acetate, chloride and phosphate in an intravenous solution by non-suppressed ion chromatography

Summary The determination of acetate, lactate, chloride and phosphate in an intravenous solution is investigated using non-suppressed ion chromatography with indirect UV absorption detection. When phthalate eluents are used with low capacity anion-exchange columns, the above solute species cannot be resolved unless acetonitrile is added to the eluent. Optimum results are obtained with 0.3 mM phthalate (pH 6.0) containing 30% acetonitrile as eluent. The improved resolution with this eluent is attributed to the existence of a partial reversed-phase retention mechanism operating on the unfunctionalised portions of the styrene-divinylbenzene polymeric ion-exchange material.     

Energetics and structural consequences of axial ligand coordination in nonplanar nickel porphyrins

The effects of ruffling on the axial ligation properties of a series of nickel(II) tetra(alkyl)porphyrins have been investigated with UV-visible absorption spectroscopy, resonance Raman spectroscopy, X-ray crystallography, classical molecular mechanics calculations, and normal-coordinate structural decomposition analysis. For the modestly nonplanar porphyrins, porphyrin ruffling is found to cause a decrease in binding affinity for pyrrolidine and piperidine, mainly caused by a decrease in the binding constant for addition of the first axial ligand; ligand binding is completely inhibited for the more nonplanar porphyrins. The lowered affinity, resulting from the large energies required to expand the core and flatten the porphyrin to accommodate the large high-spin nickel(II) ion, has implications for nickel porphyrin-based molecular devices and the function of heme proteins and methyl-coenzyme M reductase.     

Performance of a simple UV LED light source in the capillary electrophoresis of inorganic anions with indirect detection using a chromate background electrolyte

Light emitting diodes (LEDs) are known to be excellent light sources for detectors in liquid chromatography and capillary electromigration separation techniques, but to date only LEDs emitting in the visible range have been used. In this work, a UV LED was investigated as a simple alternative light source to standard mercury or deuterium lamps for use in indirect photometric detection of inorganic anions using capillary electrophoresis with a chromate background electrolyte (BGE). The UV LED used had an emission maximum at 379.5 nm, a wavelength at which chromate absorbs strongly and exhibits a 47% higher molar absorptivity than at 254 nm when using a standard mercury light source. The noise, sensitivity and linearity of the LED detector were evaluated and all exhibited superior performance to the mercury light source (up to 70% decrease in noise, up to 26.2% increase in sensitivity, and over 100% increase in linear range). Using the LED detector with a simple chromate-diethanolamine background electrolyte, limits of detection for the common inorganic anions, Cl-, NO3-, SO4(2-), F- and PO4(3-) ranged from 3 to 14 microg L(-1), using electrostatic injection at -5 kV for 5 s.     

Response characteristics of a potentiometric detector with a copper metal electrode for flow-injection and chromatographic determinations of metal ions

The potentiometric response characteristics of a copper metal indicator electrode are reported in a flow-injection system when metal ions are injected into ligands of differing complexing strengths in buffered carrier streams. Theoretical Nernstian derivation of equations relating peak heights to both the injected metal ion concentration and the ligand concentration are shown to give good agreement with experimental peak height measurements for Ca²⁺, Al³⁺, Pb²⁺, Cd²⁺, Co²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Zn²⁺ and UO²⁺₂. A study of injections into buffered ligand streams containing EDTA, ethylenediamine, triethylenetetramine, iminodiacetate, citrate, or glutamate shows advantages for the use of the more weakly complexing ligands in the carrier stream. Linear response are obtained at low (10^?3–10^?4 M) metal ion concentrations over narrow ranges. Some chromatographic applications are outlined.