Sensitive indirect photometric detection of inorganic and small organic anions by capillary electrophoresis using Orange G as a probe ion

This study addresses the two major problems in the use of dyes as highly absorbing probes for indirect photometric detection in capillary electrophoresis (CE). First, effective electroosmotic flow (EOF) modification or suppression to allow separation and detection of a wide mobility range of analytes is not straightforward when electrolytes containing increased dye concentrations are used. The suppression of EOF to less than + 5x10(-9) m(2)V(-1)s(-1) was achieved with a combination of a poly(ethylenimine) (PEI)-coated capillary and the addition of the neutral polymer hydroxypropylmethylcellulose (HPMC) to the background electrolyte. Second, the deterioration of baselines due to adsorption of the dye probe to the capillary wall is generally a problem. In this work, baseline quality at higher probe concentrations was significantly improved by a rather unusual but highly effective combination of a simultaneous application of a slight overpressure (25 mbar) at the injection end during the separation, and the use of a relatively narrow capillary of 50 microm inner diameter. Both measures would appear to be counterproductive. Optimisation of the probe concentration with regard to signal-to-noise ratio resulted in an electrolyte of 4 mM Orange G, 0.05% HPMC buffered at pH 7.7 by the addition of 10.0 mM histidine isoelectric buffer. Very high separation efficiencies of 128 000-297 000 plates were made possible by the relatively high probe concentration. Combined with excellent detection sensitivity, even with the introduction of hydrodynamic flow and a reduced optical path length, these measures resulted in limits of detection ranging from 0.216 to 0.912 microM with a deuterium lamp light source (248 nm) and from 0.147 to 0.834 microM with a 476 nm blue light-emitting diode (LED) light source. Reproducibility over 30 consecutive runs without changing the electrolyte was excellent, with relative standard deviation (RSD) values of 0.14-0.80% for migration time, 1.27-3.36% for peak area and 0.88-5.12% for peak heights. The optimised electrolyte was used for the analysis of inorganic anions in air filter samples, providing good agreement with results obtained by ion chromatography.     

Poly(arylene sulfides) with pendant cyano groups as high-temperature laminating resins. II

Poly(phenylene sulfides) containing various amounts of pendant cyano groups were synthesized from m-benzenedithiol and the corresponding amounts of p-dibromobenzene and 3,5-dichlorobenzonitrile. The polymers prepared by the use of 10, 15, 20, and 25% of the nitrile-containing dichloro compound were slightly off-white with melting ranges below 100°C and had inherent viscosities of about 0.15 dl/g in hexamethylphosphoric triamide at 30°C. The polymers prepared from m-benzenedithiol and the stoichiometric amounts of 2,4-dichlorobenzonitrile or 3,5-dichlorobenzonitrile looked similar to those described above, yet they possessed much higher melting ranges. The poly(phenylene sulfide) prepared by the use of 2,4-dichlorobenzonitrile had an inherent viscosity of 0.06 dl/g while the polymer prepared from the 3,5-dichloro isomer had an inherent viscosity of 0.38 dl/g. All the polymers listed above were crosslinked by heating alone or in the presence of anthracene-9,10-bisnitrile oxide to give black resinous polymers that were insoluble in hexamethylphosphoric triamide in which the original polymers dissolved quite readily.     

Selectivity control in the separation of aromatic amino acid enantiomers with sulphated beta-cyclodextrin

Control of selectivity in the enantiomeric separation of three aromatic amino acids (phenylalanine, tyrosine and tryptophan) is demonstrated by electrokinetic capillary chromatography utilising temperature variations coupled with the use of sulphated-beta-cyclodextrin (s-beta-CD) as a pseudostationary phase. The concentration of s-beta-CD and temperature were used as experimental variables to control the observed selectivity. A double-coated capillary was used and proved very robust with reproducibility of migration times being <2.0% R.S.D. between runs and <2.6% on using a new capillary. The system was modelled successfully using an artificial neural network (ANN) comprising one input layer, two hidden layers and one output layer. The model accurately described the observed separations with a correlation coefficient of 0.999 being observed between predicted and observed migration times. Selectivity optimisation was achieved using the normalised resolution product and minimum resolution criteria, with both providing optima at different experimental conditions. The selectivity changes observed also allowed the estimation of electrolyte temperatures within the capillary at high operating currents (>100 microA). Using a 50 microm i.d. capillary and an electrolyte comprising 20 mM phosphate and 15 mM s-beta-CD, a temperature of 52 degrees C was calculated within the capillary at an applied voltage of +30 kV.     

Potentiometric flow-injection determination of copper-complexing organic ligands with a copper-wire indicating electrode

The potentiometric response of a copper-wire indicator electrode in a flow-injection system with a phosphate-buffered carrier stream can be used to determine copper-complexing ligands; glycine, histidine, L-cysteine, EDTA, ethylenediamine, triethylenetetramine, dopamine and imidazole are discussed. The electrode response is shown to give peak potentials with a Nernstian relationship to total injected ligand concentration over limited ranges, depending on the stoichiometry, stability, and oxidation state of the copper complexes formed. Galvanostatic measurements showed that complex formation with Cu²⁺ or Cu⁺ or mixed species can be responsible for the response characteristics. The effects of adding 0.1 M sodium chloride to the carrier stream are generally beneficial, particularly in obtaining sharper responses. Detection limits can be improved to about 10^?5 M by adding about 10^?5 M Cu²⁺ to the carrier stream, but the linear range of Nernstian response is then narrow.     

On-line preconcentration of niobium(V) and tantalum(V) as 4-(2-pyridylazo) resorcinol-citrate ternary complexes in geological samples by ion interaction high-performance liquid chromatography

4-(2-Pyridylazo) resorcinol (PAR) and citrate were used as pre-column complexing agents for the determination of Nb(V) and Ta(V) as ternary complexes in geological samples. Aliquots of 2 ml of the standard and sample solutions containing the Nb(V) and Ta(V) complexes were loaded onto a concentrator column (C18, 0.4 cm x 4.6 mm) with a carrier mobile phase comprising 20% (v/v) methanol and containing 5 mM acetic acid, 5 mM citric acid and 10 mM tetrabutylammonium bromide (TBABr), pH 6.5 at 2 ml/min for 2 min, with the effluent being directed to waste. An automatic switching valve was then switched to flush both complexes from the concentrator column onto a C18 analytical column using a mobile phase comprising 32% (v/v) methanol and containing 5 mM acetic acid, 5 mM citric acid and 3 mM TBABr, pH 6.5 for 2.5 min. The switching valve was then switched back to the original position, and cleaned with methanol for 7 min to eliminate unwanted species still adsorbed to the concentrator column. This procedure prevented later eluting compounds from reaching the analytical column, which reduced the overall run time. The detection limits of Nb(V) and Ta(V) (determined at a signal-to-noise ratio of 3, detection wavelength of 540 nm and a 2-ml sample volume) were 0.012 and 0.039 ppb for Nb(V) and Ta(V), respectively. Recoveries of Nb(V) and Ta(V) were 99.4 and 96.2%, respectively. The HPLC results obtained from the reference granite and basalt samples agreed well with inductively coupled plasma MS and certified values, but the HPLC method yielded slightly low values of the Nb/Ta ratio.     

Elution mechanism in electrostatic ion chromatography with histidine as an isoelectric ampholytic mobile phase

Electrostatic ion chromatography (EIC) using a zwitterionic stationary phase (formed by coating a C18 material with a hydrophobic zwitterionic surfactant) was studied with a mobile phase comprising an aqueous solution of histidine at the pH of its isoelectric point, together with non-suppressed conductometric detection. Anions and cations were found to be eluted as separate peaks, unlike the elution behaviour observed on the same system when pure water was used as mobile phase. An explanation was suggested in terms of protonation equilibria of the overall uncharged histidine to form small amounts of histidine cations and anions in the mobile phase which could act as counterions for analyte anions and cations. This suggestion was supported by measured pH changes occurring in the bands of eluted analyte anions (a decreased pH compared to the mobile phase) and cations (an increased pH compared to the mobile phase). The analytical potential of this type of EIC is discussed.     

New Acylated Saponins from Polygala myrtifolia

The ten new acylated presenegenin (=(2β,3β,4α)‐2,3,27‐trihydroxyolean‐12‐ene‐23,28‐dioic acid) glycosides 1 – 10 have been isolated by successive MPLC from the roots of Polygala myrtifolia L. as five inseparable mixtures of the trans‐ and cis‐4‐methoxycinnamoyl derivatives, i.e., myrtifoliosides A₁/A₂ ( 1 / 2 ), B₁/B₂ ( 3 / 4 ), C₁/C₂ ( 5 / 6 ), D₁/D₂ ( 7 / 8 ), and E₁/E₂ ( 9 / 10 ). Their structures were elucidated mainly by extensive spectroscopic experiments, including 2D NMR techniques, as 3‐O‐(β‐D ‐glucopyranosyl)presenegenin 28‐{O‐β‐D ‐galactopyranosyl‐(1→3)‐O‐β‐D ‐xylopyranosyl‐(1→4)‐O‐[D ‐apio‐β‐D ‐furanosyl‐(1→3)]‐O‐α‐L ‐rhamnopyranosyl‐(1→2)‐O‐[α‐L ‐arabinopyranosyl‐(1→3)]‐4‐O‐(trans‐4‐methoxycinnamoyl)‐β‐D ‐fucopyranosyl} ester ( 1 ) and its cis‐isomer 2 , 3‐O‐(β‐D ‐glucopyranosyl)presenegenin 28‐{O‐β‐D ‐galactopyranosyl‐(1→3)‐O‐β‐D ‐xylopyranosyl‐(1→4)‐O‐[D ‐apio‐β‐D ‐furanosyl‐(1→3)]‐α‐L ‐rhamnopyranosyl‐(1→2)‐4‐O‐(trans‐4‐methoxycinnamoyl)‐β‐D ‐fucopyranosyl} ester ( 3 ) and its cis‐isomer 4 , 3‐O‐(β‐D ‐glucopyranosyl)presenegenin 28‐{O‐β‐D ‐galactopyranosyl‐(1→3)‐O‐β‐D ‐xylopyranosyl‐(1→4)‐O‐α‐L ‐rhamnopyranosyl‐(1→2)‐4‐O‐(trans‐4‐methoxycinnamoyl)‐β‐D ‐fucopyranosyl} ester ( 5 ) and its cis‐isomer 6 , 3‐O‐(β‐D ‐glucopyranosyl)presenegenin 28‐{O‐D ‐apio‐β‐D ‐furanosyl‐(1→3)‐O‐[β‐D ‐xylopyranosyl‐(1→4)]‐O‐α‐L ‐rhamnopyranosyl‐(1→2)‐4‐O‐(trans‐4‐methoxycinnamoyl)‐β‐D ‐fucopyranosyl} ester ( 7 ) and its cis‐isomer 8 , and 3‐O‐(β‐D ‐glucopyranosyl)presenegenin 28‐{O‐α‐L ‐arabinopyranosyl‐(1→3)‐O‐[β‐D ‐xylopyranosyl‐(1→4)]‐O‐α‐L ‐rhamnopyranosyl‐(1→2)‐4‐O‐(trans‐4‐ methoxycinnamoyl)‐β‐D ‐fucopyranosyl} ester ( 9 ) and its cis‐isomer 10 .     

Mixed-mode electrokinetic chromatography of aromatic bases with two pseudo-stationary phases and pH control

The electrokinetic chromatographic (EKC) separation of a series of aromatic bases was achieved utilising an electrolyte system comprising an anionic soluble polymer (polyvinylsulfonic acid, PVS) and a neutral beta-cyclodextrin (beta-CD) as pseudo-stationary phases. The separation mechanism was based on a combination of electrophoresis, ion-exchange interactions with PVS, and hydrophobic interactions with beta-CD. The extent of each chromatographic interaction was independently variable, allowing for control of the separation selectivity of the system. The ion-exchange and the hydrophobic interactions could be varied by changing the PVS and the beta-CD concentrations, respectively. Additionally, mobilities of the bases could be controlled by varying pH, due to their large range of pKa values. The separation system was very robust with reproducibility of migration times being <2% RSD. The two-dimensional parameter space defined by the two variables, [beta-CD] and %PVS, was modelled using a physical model derived from first principles. This model gave very good correlation between predicted and observed mobilities (r2=0.999) for the 13 aromatic bases and parameters derived from the model agreed with the expected ion-exchange and hydrophobic character of each analyte. The complexity of the mathematical model was increased to include pH and this three-dimensional system was modelled successfully using an artificial neural network (ANN). Optimisation of both the two-dimensional and three-dimensional systems was achieved using the normalised resolution product and minimum resolution criteria. An example of using the ANN to predict conditions needed to obtain a separation with a desired migration order between two of the analytes is also shown.     

High-speed simultaneous ion-exclusion/cation-exchange chromatography of anions and cations on a weakly acidic cation-exchange resin column

The simultaneous ion-exclusion/cation-exchange separation column packed with a polymethacrylate-based weakly acidic cation-exchange resin of 3 microm particle size was used to achieve the simultaneous high-speed separation of anions and cations (Cl(-), NO3(-), SO4(2-), Na(+), K(+), NH4(+), Ca(2+) and Mg(2+)) commonly found in environmental samples. The high-speed simultaneous separation is based on a combination of the ion-exclusion mechanism for the anions and the cation-exchange mechanism for cations. The complete separation of the anions and cations was achieved in 5 min by elution with 15 mM tartaric acid-2.5 mM 18-crown-6 at a flow-rate of 1.5 ml/min. Detection limits at S/N=3 ranged from 0.36 to 0.68 microM for anions and 0.63-0.99 microM for cations. This method has been applied to the simultaneous determination of anions and cations in several environmental waters with satisfactory results.     

Crystal structure of cis-[Rh(biq)2Cl2]Cl·3H2O: Solid-state characterization and crystal packing analysis

The title compound, cis-[Rh(biq)₂Cl₂]Cl·3H₂O (biq = 2,2′-biquinoline) crystallized in the monoclinic space group P2 ₁ /n with a = 11.231(2) Å, b = 20.895(4) Å, c = 14.081(3) Å, β = 94.76(3)°, V = 3293.0(11) ų, D _c = 1.565 g cm⁻³, μ = 0.806 mm⁻¹, F(000) = 1576 and Z = 4. It contains a monomeric [Rh(biq)₂Cl₂]⁺ cation, a chloride ion and three molecules of H₂O. The rhodium(III) ion is hexa coordinated forming a distorted octahedral arrangement. The mean Rh(III)–N distance for the four Rh(III)–N bonds is 2.0625 Å. The two chloride atoms are bonded in a cis configuration [Rh(III)–Cl bond distances are 2.329(3) and 2.341(4) Å]. The structure shows a curling stacks of cationic complexes interacting via offset-face-to-face (OFF) π–π aryl interaction motif. Water molecules and chloride ions are hydrogen bonded (H₂O···H–OH and Cl···H–OH) and links the curling stacks by hydrogen bonding via Rh–Cl···H–OH interactions.