The atomic absorption spectrometric determination of lithium isotope abundances by direct measurement of the absorbance ratio

A specially designed amplifier allows direct measurement of the absorbance ratio. The unit simultaneously monitors the attenuation of both a natural-lithium and a ⁶Li-enriched hollow-cathode lamp, which are aligned in the optical axis of the instrument. The attenuations are converted to absorbances, the ratio of which is proportional to the relative isotope abundance. The stable signals obtained permit rapid determinations at natural abundance levels, with a relative standard deviation of 3%.     

Micellar liquid chromatography determination of B vitamins with direct injection and ultraviolet absorbance detection

A micellar reversed-phase liquid chromatographic procedure was developed for the control of five water-soluble vitamins, B (nicotinamide), B1 (thiamine), B2 (riboflavin), B6 (pyridoxine and pyridoxamine), in multivitamin pharmaceutical formulations (capsules, pills and syrups). Optimization procedure includes studies about the composition of the mobile phase (sodium dodecyl sulphate and the modifiers propanol, butanol or pentanol), flow-rate and temperature. Chromatographic analysis of all vitamins was carried out using a single mobile phase of 0.1 M SDS-4% (v/v) pentanol at pH 3, in a C18 column in isocratic mode, and UV-detection at 270, 290 and 325 nm. The flow-rates selected were 1.0 ml/min in the interval 0 to 6 min, and 2.0 ml/min until the end of the chromatogram and temperature was 45 degrees C. In the micellar liquid chromatographic system, the samples were injected without pretreatment, and the analysis time was below 12 min. Repeatabilities and intermediate precision were achieved according to ICH, and were below 5%. When the method is applied to real samples, the amount found with respect to the declared compositions were within the 91-105% range. These results were similar to those obtained with a conventional 60:40 (v/v) methanol-water mixture for some of the vitamins, but with the advantage of use a single mobile phase for the analyses of the five vitamins, with direct injection of the samples and reduced toxicity, flammability, environmental impact and cost of the micellar-pentanol solutions.     

Multistable polymer stabilised cholesteric liquid crystal: exceeding reflection limit in visible region

Cholesteric liquid crystal (CLC) with helically aligned molecules, demonstrates unique optical features. Conventionally, such structure reflects a maximum 50% of unpolarised light. Polymer-stabilised CLC was designed to exceed the limit by mixing liquid crystal (LC) and a photopolymerisable monomer of opposite chirality. The reflectance was carefully investigated and the result was 55.7% in the visible region at room temperature, where it exceeded the theoretical limit. Such phenomena were studied and theoretical model was discussed. Electro-optical test showed the driving voltage was of LC cell was slightly changed after polymerisation.     

Analysis of iso-alpha-acids and reduced iso-alpha-acids in beer by direct injection and liquid chromatography with ultraviolet absorbance detection or with mass spectrometry

A liquid chromatography (LC) method is described for the simultaneous analysis of iso-alpha-acids and reduced iso-alpha-acids in beer. Volatile mobile phase additives were selected to enable hyphenation to mass spectrometric (MS) operated in the atmospheric pressure chemical ionization (APCI) mode. Contrary to other recent LC optimization procedures for the same compounds, an alkaline pH was selected hereby improving peak shape and selectivity. Both UV and MS detection are sensitive enough to analyze beers without sample pre-concentration. All major bitter acids are separated within 65 min with exception of cis-dihydroisoadhumulone, which co-elutes with trans-isocohumulone. Due to the selectivity of the MS, these compounds could be differentiated according to their m/z value. The performance in terms of quantification of bitter acids by LC-UV and LC-MS are compared for standard solutions and a selection of 14 beers.     

Optical errors in a liquid chromatography absorbance cell

The shadowgraph method is used to visualize mobile phase flow through a liquid chromatography absorbance cell with "Z" flow geometry. It is shown that mixing is incomplete in the cell. Composition gradients persist throughout the elution of any sample. The gradients systematically change as the sample flows through the cell. Additionally, window wedge angles are measured in several cells. The implications for low noise absorbance detectors are discussed.     

Determination of quinolones in chicken tissues by liquid chromatography with ultraviolet absorbance detection

This paper presents an analytical method for the determination of quinolones in chicken tissues. The procedure involves pre-treatment by solid-phase extraction (SPE) and subsequent liquid chromatography (LC) with UV absorbance detection. Different SPE disposable cartridges and extractants of the tissue samples were tested, and various columns were systematically tested. The mobile phase was composed of acetonitrile and citric buffer at pH 4.5, with an initial composition of acetonitrile-water (12:88, v/v) and using linear gradient elution. Recoveries were 66-91% in the concentration range 30-300 microg kg(-1). The detector response was linear in this range. The limits of detection were 16-30 microg kg(-1). These values were lower than the maximum residue limits established by the European Union.     

Radical production in the radiolysis of liquid benzene

Iodine scavenging techniques have been used to examine the role of the phenyl radical in the radiolysis of benzene with γ-rays. Biphenyl, the main product in the radiolysis of neat benzene, and iodobenzene yields were determined as a function of iodine concentration. The yield of biphenyl in neat benzene is found to be 0.075 molecules/100 eV and independent of dose up to 1 Mrad. The addition of 0.1 mM iodine increases the biphenyl yield to about 0.4 molecules/100 eV. This increase is thought to be due to a reaction of iodine with the phenyl radical–benzene adduct, which apparently has a very long lifetime. Further addition of iodine leads to a decrease in biphenyl to about 0.09 molecules/100 eV at 30 mM iodine. This decrease is accompanied by an increase in iodobenzene from 0.04 to 0.32 molecules/100 eV. It appears that iodine can effectively compete with benzene for scavenging phenyl radicals.     

Fourier transform Raman spectroscopy in the visible region

Low-resolution (ca. 4cm^–1) Raman spectra of various liquids and solids, excited by an argon laser at 448 and 514.5 nm, were recorded using a BOMEM DA3.02 commercial interferometer. These spectra were compared with those obtained in the same experimental conditions on a conventional dispersive spectrometer. It is concluded that for low-resolution Raman studies with excitation in the visible region, the dispersive method is superior to the interferometric technique.     

Substituent effects in the benzene dimer are due to direct interactions of the substituents with the unsubstituted benzene

The prevailing views of substituent effects in the sandwich configuration of the benzene dimer are flawed. For example, in the polar/pi model of Cozzi and co-workers (J. Am. Chem. Soc. 1992, 114, 5729), electron-withdrawing substituents enhance binding in the benzene dimer by withdrawing electron density from the pi-cloud of the substituted ring, reducing the repulsive electrostatic interaction with the nonsubstituted benzene. Conversely, electron-donating substituents donate excess electrons into the pi-system and diminish the pi-stacking interaction. We present computed interaction energies for the sandwich configuration of the benzene dimer and 24 substituted dimers, as well as sandwich complexes of substituted benzenes with perfluorobenzene. While the computed interaction energies correlate well with sigmam values for the substituents, interaction energies for related model systems demonstrate that this trend is independent of the substituted ring. Instead, the observed trends are consistent with direct electrostatic and dispersive interactions of the substituents with the unsubstituted ring.     

Evaluation of a direct extraction/liquid scintillation counting technique for the measurement of uranium in water

An extraction procedure utilizing alpha liquid scintillation was evaluated for the rapid determination of uranium in aqueous environmental samples. The extraction efficiency of the system was measured under varying chemical conditions including pH. The procedure was evaluated against a traditional radiochemical technique using both laboratory prepared control samples and actual groundwater. Finally, the possibility of obtaining isotopic information from the liquid scintillation spectra was also investigated using a curve fitting routine.     

Determination of benzene in gasoline using direct injection-mass spectrometry

A high-speed determination of benzene in gasoline samples using a non-separative method based on direct injection into the mass spectrometer is proposed. The results obtained are very similar to those provided with fast GC-MS.The calibration set was made up of gasoline samples in which the benzene was determined chromatographically and samples of gasoline subjected to a process of evaporation - until the complete disappearance of the original benzene - to which known concentrations of this compound had been added. A PLS1 multivariate calibration model was constructed. Cross-validation was used to select the optimum number of PLS components. The prediction capacity of the model was checked with an additional group of gasoline samples that had not been used either in the construction or in the validation of the model.With the direct injection method proposed here it was possible to analyse 24 samples over a period of 1 h. The direct injection method is rapid, simple and - in view of the results - highly suitable for the determination of benzene in gasoline samples.     

Molecular translational-motion correlation time in liquid benzene

A model-free method is used for determining dipole-dipole nuclear-spin interaction correlation times for liquids from the spin-lattice relaxation times on the basis of the molecular shape, the spin positions in the molecules, and the spin distribution in the liquid. An estimate has been made of the proton relative-motion correlation time for liquid benzene.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 4, pp. 504–506, July–August 1987.     

Photoelectrochemical studies on gold electrodes with surface oxide layers: Part I. Photocurrent measurement in the visible region

By illumination with visible light at wavelengths of 340–730 nm, anodic photocurrents were observed at a gold electrode in contact with a 0.2 M NaClO₄ aqueous solution (pH 6.0) in the potential range from +1.0 to +2.1 V vs. RHE. Based on the results of potentiodynamic and photocurrent measurements, the following three characteristic potential ranges have been distinguished: (1) +1.0–+1.4 V, where possibly low-coverage surface oxides (or chemisorbed OH) other than Au₂O₃ are present; (2) +1.5–+1.8V where the predominant surface oxide is Au₂O₃ up to ca. 1 nm in thickness; and (3) above +1.9 V, where a thicker surface layer, absorbing mainly in the UV wavelength range, is formed on the Au₂O₃ under-layer. The photocurrent quantum yield at the Au₂O₃ layer 0.3–0.8 nm thick was estimated to be on the order of 20% at 490 nm, assuming an absorption coefficient of 10⁵ cm^?1.     

Automated direct assay system for the measurement of sex steroid hormones in serum using high-performance liquid chromatography

An automated direct assay system using high-performance liquid chromatography was developed for the simultaneous measurement of estradiol, estrone, progesterone, 17 alpha-hydroxyprogesterone, 20 alpha-hydroxyprogesterone, testosterone and androstenedione in biological fluids. A comparison between the values measured by this method and by radioimmunoassay revealed good correlation for estradiol (r = 0.938, p less than 0.001) and progesterone (r = 0.903, p less than 0.001). Estradiol and estrone could be analysed above the level of 250 pg/ml, and progesterone, 17 alpha-hydroxyprogesterone, 20 alpha-hydroxyprogesterone, testosterone and androstenedione could be analysed above the level of 5.0-7.5 ng/ml. The method was applied to the clinical appraisal of placental function and maturation of ovarian follicles.     

A feasibility study on the use of visible and short wavelengths in the near-infrared region for the non-destructive measurement of wine composition

The aim of this study was to explore the capability of spectroscopy in the visible (Vis) and short wavelength near-infrared (NIR) regions for the non-destructive measurement of wine composition in intact bottles. In this study we analysed a wide range of commercial wines obtained in Australia in different types of bottles (e.g. colours, diameters and heights), including different wine styles and varieties. Wine bottles were scanned in the Vis-NIR region (600–1,100 nm) in a monochromator instrument in transflectance mode. Principal component analysis (PCA) and partial least-squares (PLS) regression were used to interpret the spectra and develop calibrations for wine composition. Due to the relatively small number of samples available full cross-validation (leave-one-out) was used as validation. The coefficient of correlation in calibration and the standard error of cross-validation (SECV) were 0.67 (SECV: 0.48%), 0.83 (SECV: 4.01 mg L⁻¹), 0.70 (SECV: 28.6 mg L⁻¹) and 0.50 (SECV: 0.15) for alcohol content, total SO₂, free SO₂ and pH, respectively, in the set of wine samples analysed. These preliminary results showed that the assessment of wine composition by Vis and short wavelengths in the NIR is possible for either qualitative analysis (e.g. low-, medium- and high-quality grading), or for screening of composition during bottling and storage. Although low accuracy and precision were obtained for the chemical parameters routinely analysed in wine, calibration models for the chemical parameters were considered acceptable for screening purposes in terms of the standard errors obtained.     

Optofluidic differential spectroscopy for absorbance detection of sub-nanolitre liquid samples

We present a novel optofluidic differential method for carrying out absorbance spectroscopy of sub-nanolitre volumes of liquid samples on a microfluidic chip. Due to the reduction of liquid volume, the absorbance detection in microfluidics is often hindered by either low sensitivity or complex fabrication. To address this issue, we introduced an optofluidic modulator which can be easily integrated into a PDMS (polydimethylsiloxane) based microfluidic chip. The modulator was controlled by the fluid pressure and the absorbance spectrum of the analyte was obtained by taking differential measurements between the analyte and reference medium. An advantage is that this method doesn't need a complicated fabrication step. It is compatible with conventional microfluidic chips and measurements can be carried out on a normal transmission microscope. The performance of the device was tested by measuring solutions containing methylene blue, with concentrations as low as 13 μM.     

Multiresidue determination of fluoroquinolones in milk by column liquid chromatography with fluorescence and ultraviolet absorbance detection

Column liquid chromatography with fluorescence (FLD) and UV-diode array detection (UV-DAD) was used for the simultaneous determination of ciprofloxacin (CIPRO), enrofloxacin (ENRO), marbofloxacin (MARBO), danofloxacin (DANO) and sarafloxacin (SARA) residues in milk, using norfloxacin (NOR) as internal standard. Two solid-phase extraction (SPE) cartridges, were evaluated for sample clean-up and preconcentration, Strata X, based on a modified styrene-divinylbenzene polymer, and Strata Screen A, a mixed anion exchanger/C8 reversed-phase sorbent. The fluoroquinolones (FQs) were separated on a polar endcapped column (AQUA C18). The recoveries for raw milk spiked with the antibiotics at three concentrations close to the maximum residue limit (MRL), were 80-103% for ENRO, CIPRO and DANO, with relative standard deviations (R.S.D.) lower than 6.6%. SARA recoveries were 70% (R.S.D. = 7%) and values in the order of 95% (R.S.D. = 1.5%) were obtained for MARBO at the MRL level. The quantification limits ranged from 2.4 to l0 ng ml(-1) and are below the MRL established for these drugs by the European Union. The method was successfully applied to the analysis of ENRO and its metabolite CIPRO in an incurred milk sample.