Design and performance of a light-emitting diode detector compatible with a commercial capillary electrophoresis instrument

Indirect photometric detection in capillary electrophoresis (CE) has been predominantly performed in the UV region, in part due to a lack of suitable high-intensity and low-noise light sources in the visible spectral region. A new photometric detector based on light-emitting diodes (LEDs) as light sources and compatible with a commercially available CE instrument has been designed and constructed and its performance evaluated. The utility of this detector was successfully demonstrated by the indirect photometric detection of anions using a dye as probe and absorbance measured in the visible region. The detector exhibited very low baseline noise (around 0.03 mAU), stable output, and improved upper limit of detection linearity (502 mAU) compared with previously used LED detectors. The detector was tested for indirect detection of anions separated with an electrolyte containing 4 mM Orange G as the indirect detection probe, 10 mM histidine as an isoelectric buffer, and 0.05% hydroxypropylmethylcellulose to suppress the electroosmotic flow. Extremely low detection limits were obtained ranging from 0.16-0.36 microM (excluding chloride 0.56 microM), with separation efficiencies in the range of 154,000-274,000 theoretical plates.     

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.     

An NMR study of the freezing of emulsion-containing drops

Various nuclear magnetic resonance (NMR) techniques were used to monitor the freezing behaviour of suspended 2-mm-diameter drops. The drops were composed of hydrocarbon oils emulsified in either water or water/sucrose mixtures. As such they were good model systems for the study of spray freezing, sharing structural similarities with potential products such as ice cream. In particular, simple 1H NMR spectroscopy was used to monitor and individually quantify the freezing or solidification behaviour of the various constituent species of the drops. In addition, the effect of freezing on the emulsion droplet size distribution (and hence emulsion stability) was also measured based on NMR self-diffusion measurements. The effect of freeze/thaw cycling was also similarly studied. The nucleation temperature of the emulsion droplets was found to depend on the emulsion droplet size distribution: the smaller the droplets, the lower the nucleation temperature. Emulsion droplet sizing indicated that oil-in-sucrose-solution emulsions were more stable, showing minimal coalescence, whereas oil-in-water emulsions showed significant coalescence during freezing and freeze/thaw cycling.     

PHOTOCHEMISTRY OF URACIL. INTERSYSTEM CROSSING AND DIMERIZATION IN AQUEOUS SOLUTION

Abstract— Ultraviolet irradiation of ¹⁴C-uracil in aqueous solution results in the formation of hydrate and dimer photoproducts. The rate of dimerization increases with increasing uracil concentration, and decreases with increasing concentration of oxygen in solution. The kinetics are in agreement with a model previously proposed to account for the reactions, in which dimerization occurs by a reaction involving the triplet state of uracil, but hydration occurs from an excited singlet state. Oxygen reduces dimer formation by quenching the triplet. The quantum yield for intersystem crossing (ISC) to the triplet depends on the irradiation wavelength, increasing from 0.0014 at 280 nm to 0.016 at 230 nm. The ratio of rate constants for reaction of the triplet with oxygen and for dimerization is 1.1; the ratio of rate constants for triplet decay and for dimerization is 5.9 × 10^-5 M. The increase in ISC with photon energy suggests that ISC is favoured from excited vibrational levels. The quantum yield for hydration is about 0.002 at pH 4.5 for all wavelengths, but increases as the pH is decreased.     

PROPERTIES OF THYMINE DIMERS

Abstract— The photochemical and chemical properties of the four dimers of thymine have been studied. The extinction coefficients, reversal cross-sections and quantum yields for reversal are presented as a function of wavelength in the range 200–289 nm. At any wavelength, the dimers have different reversal cross-sections but also different extinction coefficients. The quantum yields for reversal are nearly the same for all four dimers, the values ranging from 0·6 to 0·9 over the wavelength range 200–289 nm. Titration curves for the four dimers show that for each dimer, two groups are involved with two pK's at about 10·5 and 12·2. Dimers A and B are stable at alkali and acid pH's. Dimers C and D are stable from pH 1 to 3 and unstable at other pH's, with thymine the main degradation product.     

The amide rotational barriers in picolinamide and nicotinamide: NMR and ab initio studies

Pyridine carboxamides are a class of medicinal agents with activity that includes the reduction of iron-induced renal damage, the regulation of nicotinamidase activity, and radio- and chemosensitization. Such pharmacological activities, and the prevalence of the carboxamide moiety and the importance of amide rotations in biology, motivate detailed investigation of energetics in these systems. In this study, we report the use of dynamic nuclear magnetic resonance to measure the amide rotational barriers in the pyridine carboxamides picolinamide and nicotinamide. The activation enthalpies and entropies of DeltaH++ = 12.9 +/- 0.3 kcal/mol and DeltaS++ = -7.7 +/- 0.9 cal/mol K for nicotinamide and DeltaH++ = 18.3 +/- 0.4 kcal/mol and DeltaS++ = +1.3 +/- 1.0 cal/mol K for picolinamide report a substantial energetic difference for these regioisomers. Ab initio calculations of the rotational barriers are in good agreement with the experimentally determined values and help partition the 5.4 kcal/mol enthalpy difference into its major contributions. Of principal importance are the variations in steric interactions in the ground states of picolinamide and nicotinamide, superior pi electron donation from the pyridine ring in the transition state of nicotinamide, and an intramolecular hydrogen bond in the ground state of picolinamide.     

Imperfection sensitivity of coincident buckling systems

A two-degree of freedom structural system with both buckling loads equal or nearly so is examined, allowing for the presence of small imperfections specified by the values of two general imperfection parameters. A discrete generalized coordinates energy method is used, and the system considered is non-symmetric in the two modes of deflection. Conclusions regarding the types of behaviour possible are obtained by examination of all possible equilibrium path shapes. Comparison is made to behaviour of idealized systems. Change in buckling load value with imperfection size, or imperfection sensitivity, is established and imperfection sensitivity surface forms are sketched.     

Characterization of partially transesterified poly(beta-hydroxyalkanoate)s using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was used for the characterization of a partially transesterified poly(beta-hydroxyalkanoate), PHA, polymer produced by the bacterial strain Alcaligenes eutrophus using saponified vegetable oils as the sole carbon sources. The transesterification was carried out separately under acidic and basic conditions to obtain PHA oligomers weighing less than 10 kDa. The intact oligomers were detected in their cationized [M + Na](+) and [M + K](+) forms by MALDI-TOFMS. A composition analysis, using the MALDI-TOF spectra, indicate that the oligomers obtained via acid catalysis were terminated with a methyl 3-hydroxybutyrate end group, and those obtained by base catalysis had a methyl crotonate (olefinic) termination. In addition to HB (hydroxy butyrate), the oligomers were found to contain a small percentage of HV (hydroxy valerate). This was independently confirmed using gas chromatography/mass spectrometry (GC/MS). In comparison, the analysis of a commercial PHA polymer, transesterified under identical conditions, only showed the presence of HB, i.e. a pure PHB homopolymer. Copyright 1999 John Wiley & Sons, Ltd.     

Directed distance in digraphs: Centers and medians

The directed distance d_D(u, v) from a vertex u to a vertex v in a strong digraph D is the length of a shortest (directed) u - v path in D. The eccentricity of a vertex v in D is the directed distance from v to a vertex furthest from v. The distance of a vertex v in D is the sum of the directed distances from v to the vertices of D. The center C(D) of D is the subdigraph induced by those vertices of minimum eccentricity, while the median M(D) of D is the subdigraph induced by those vertices of minimum distance. It is shown that for every two asymmetric digraphs D₁ and D₂, there exists a strong asymmetric digraph H such that C(H) ? D₁ and M(H) ? D₂, and where the directed distance from C(H) to M(H) and from M(H) to C(H) can be arbitrarily prescribed. Furthermore, if K is a nonempty asymmetric digraph isomorphic to an induced subdigraph of both D₁ and D₂, then there exists a strong asymmetric digraph F such that C(F) ? D₁, M(F) ? D₂ and C(F) ∩ M(F) ? K. © 1993 John Wiley & Sons, Inc.