Joseph Hubert

An atmospheric pressure waveguide-fed microwave plasma torch: the TIA design

We report theoretical and experimental investigations of an atmospheric pressure, microwave plasma torch with axial gas injection (torche a injection axiale design). It is a waveguide-based structure comprising both waveguide and coaxial elements serving the purpose of wavemode conversion and impedance-matching. The device includes features common to various waveguide-fed torches disclosed previously by other authors, but not yet modelled. Our paper provides a simple equivalent circuit description of the torch operation that accounts for its impedance-matching, power transfer to plasma and tuning characteristics, as verified experimentally. From the outcome of the model and using our experimental results, we introduce new features in the torch design that enable one to optimize its performance. We also examine ways of simplifying its structure and operation.

Prediction of retention times for aromatic acids in liquid chromatography

Abstract The chromatographic behaviour of 30 aromatic acids was examined in a system of fine particle macroporous polystyrene-divinylbenzene copolymers and acetonitrile-water mixtures at different pH as eluents. The acids were benzoic, phenylacetic, cinnamic, mandelic, naphthoic and hippuric acids, and their hydroxy and/or methoxy derivatives. At low pH, the logarithm of the capacity ratios of these acids was linearly related to the logarithm of their partition coefficients in the octanol-water system calculated after Rekker. By combining the above result and the dissociation constant of the acid, it was possible to predict the retention times of the acids at a given pH of the eluent.

Selectivity of a phenyl-bonded silica gel☆

The selectivity of a phenyl-bonded silica gel was examined as the difference between the capacity ratios of five groups of compounds in acetonitrile- and tetrahydrofuran-water mixtures, in pure tetrahydrofuran and in n-hexane. The five groups were polyaromatic hydrocarbons, alkylbenzenes, halogenated benzenes, aliphatic alcohols and alkanes. In the reversed-phase mode, the polyaromatic hydro- carbons were retained more than the other groups, as expected, but selectivity was clearly observed in the chromatographic behavior of the chlorobenzenes. The use of the π energy effect obtained from the Van der Waals volume and the delocalization energy of solutes was tested for the optimization of reversed-phase liquid chromatography.

Retention versus van der waals volume and π energy in liquid chromatography

A system in which only the size and the π energy of the molecules may be involved, is examined by reversed-phase liquid chromatography using octadecyl bonded silica gels as the packing. If hydrogen bonding and Coulombic forces are negligible, the retention of molecules depends upon their size, and the presence of π electrons enhances the selectivity. Therefore, the difference between the logarithm of the capacity ratios of alkanes and polycyclic aromatic hydrocarbons (PAH) was defined as the π energy effect and log k′ (PAH) = log k′ (Van der Waals volume) — log k′ (π energy effect). The π energy effect calculated for alkylbenzenes and chlorobenzenes from the capacity ratios obtained on different packings in acetonitrile—water mixturs was constant. Therefore, the prediction of the retention time of these compounds was also possible from their Van der Waals volumes and π energy. The solvent effects of tetrahydrofuran and n-hexane are also discussed.

Use of Chemometrics and Laser-Induced Breakdown Spectroscopy for Quantitative Analysis of Major and Minor Elements in Aluminum Alloys

In the present work, quantitative analysis of major and minor elements in aluminum alloys is investigated using chemometrics and laser-induced plasma spectroscopy with a commercially available laser-induced breakdown (LIBS) spectrometer. Multivariate calibrations use the entire signal matrix for all elements in a single multivariate regression model. This enables accounting for the correlation between variables often referred to as matrix effects in conventional univariate modeling. Modeling the entire signal matrix improves robustness over traditional univariate calibration since it can compensate for matrix effects. Several nonlinear data pretreatment methods have been used to correct for nonlinear behaviors of the analytical signals prior to performing the multivariate calibration. The use of multivariate calibration in combination with cubic implicit nonlinear data pretreatment showed the most accurate results. The accuracy reported with the developed multivariate calibration is better than 5% for the major alloying elements. Based on the results obtained, the use of chemometrics and laser-induced plasma spectroscopy have been successfully applied to the quantitative analysis of major and minor alloying elements in aluminum.

Speciation of chromium (VI) and total chromium determination in welding dust samples by flow-injection analysis coupled to atomic absorption spectrometry

We have studied the speciation of chromium (VI) in stainless-steel welding dusts. The approach used for the analysis of Cr(VI) and total Cr relies on a flow-injection analyzer (FIA) equipped with two different sequential detectors. The system measures Cr(VI). by colorimetry (with 1,5-diphenyl carbohydrazide) and total chromium content by flame atomic absorption spectroscopy (AAS). The extraction of the samples of welding-fume dusts is achieved in a buffer solution (acetic acid and sodium acetate at pH 4). This extraction procedure gives a 96% recovery of chromium (VI). The FIA-AAS system that has been described is also more sensitive, has a lower detection limit (0.005 mug ml(-1)) and gives a better precision (< 1%) than other equivalent systems that have been previously described.

High nitrogen atom yield downstream of an atmospheric pressure flowing Ar-N2 microwave discharge

A large yield of nitrogen atoms can be achieved at atmospheric pressure, from an Ar-N2 surface-wave-induced microwave discharge with half the microwave power needed with a pure N2 discharge. The dissociation of N2 molecules into atomic nitrogen in this Ar-N2 discharge is close to 22% at 0.3% N2 concentration and decreases with increasing nitrogen concentration. The maximum nitrogen atom yield per watt is reached at 3% of N2. Such a low power and simple nitrogen atom source, with a typical flow rate of 10 l min-1, is of interest for achieving metal nitriding at atmospheric pressure.

Hydrophobicity and chromatographic behaviour of aromatic acids found in urine

Abstract In reversed-phase liquid chromatography, the capacity ratios of urinary compounds were related with their hydrophobicity calculated by Rekkers hydrophobic fragmental constants. The retention behaviour of these compounds differed from that of non-ionizable compounds whose retention time can be predicted in different mixtures of acetonitrile and water as eluent and on an octadecyl silica packing from their calculated hydrophobicity. However, the calculated values for hydrophobicity and/or those derived from the results obtained for non-ionizable compounds could be useful to analyze the metabolites of acidic compounds in urine. The retention behaviour of all the compounds on gradient elution is also discussed.

Determination of pentachlorophenol and its oil solvent in wood pole samples by SFE and GC with postcolumn flow splitting for simultaneous detection of the species

An alternative approach is described for the measurement of pentachlorophenol (PCP) and its oil solvent in wood samples by supercritical fluid extraction (SFE) and gas chromatography (GC). The determination is achieved over a single chromatographic run using postcolumn flow splitting for simultaneous ECD/FID detection of the SFE extracted species. First, PCP and oil components are quantitatively extracted from a 0.3-g wood sample using 10% MeOH/CO(2) supercritical fluid at 0.65 g/mL and 120 °C. An aliquot of the SFE solution is then mixed with 10 mL of a buffered aqueous phase at pH 9.4. After PCP is acetylated by the addition of 500 μL of acetic anhydride, it is followed by its extraction with 2.00 mL of hexane along with oil. Then, 0.5 μL of supernatant organic phase is injected into the GC for a selective and simultaneous determination of the species. The method has a linear response over 3 orders of magnitude for both species with a linear regression correlation coefficient higher than 0.98 (95% confidence limit) and an absolute detection limit of 60 ng of PCP and 80 μg of oil per 0.1-g wood sample. The precision (relative standard deviation) is 4% for PCP and 1% for oil as established for a typical average concentration sample. The accuracy of the SFE GC-ECD/FID combined technique for PCP and oil was assessed by analyzing wood samples collected from newly and in-service PCP/oil-impregnated red pine poles.

Liquid Chromatographic Behavior of Nitrogen Compounds

Abstract Chromatographic behavior of nitrogen compounds differed from others. The column efficiency was poor for the compounds and sometimes solutes were not eluted out from a column. Therefore, the elution volume of alkylamines, anilines, pyridines, pyrazines, quinolines and aminopolyaromatic hydrocarbons was measured on a methacrylate gel and octadecyl bonded silica gels in pH controlled acetonitrile/water mixtures. The solvent effect on the dissociation constant differs from that obtained for aromatic acids. The values in acetonitrile/water mixtures are smaller than those obtained in 100% water. The linear relation between log P and log k′ values is obtained in eluents of pH 7 where the retention of these compounds is maximized. Some hydrophobic fragmental constants are proposed from this result. Prediction of retention time of these compounds from their log P values can be done in the individual groups on octadecyl bonded silica gels in pH controlled acetonitrile/water mixtures.