Evolution of the commutation concept associated with the development of flow analysis

Evolution of the commutation concept has lead to the proposal and development of different generations of flow analyzers. Since the inception of the air-segmented flow systems till the availability of modern flow injection, sequential-injection and other flow-based analytical systems, a noteworthy improvement of the commutating devices has been noted.

Multi-functional manifold is described as a polyvalent approach for methodology implementation in a flow analyzer. It permits the investigation of mixing conditions under different flow patterns (unsegmented, segmented, monosegmented) with optional exploitation of the stopped-flow approach. For this purpose, spectrophotometric or turbidimetric measurements eventually affected by Schlieren noise were considered. Potentialities and limitations of the manifold are discussed in relation with methods based on relatively fast or slow chemical reactions. As applications, phosphate and chloride determinations in plant digests and natural waters were selected.

The manifold is characterized by high versatility and may work in connection with different flow configurations. Development will certainly lead to simple, versatile and miniaturized analyzers, able to run samples in a personalized fashion. In addition, random reagent selection, full automation, expansion of the analytical application range and increasing potentialities of the already existing methodologies are devised.     

Exploratory Analysis of Micrographic Teflon Images

 Some analytical techniques are able to produce images, making if possible to obtain a large amount of information. Working with images, the pixels can be treated like objects in a data matrix. In this way, a multi-way PCA of micrograph images was applied to the internal surface of a Teflon coil used for microwave sample digestion. This Teflon coil was used for 120 h and had been attacked with different acids. It was cut into 3 pieces (namely: initial, medium and central portions) in order to obtain 7 micrographs (2 for initial, 2 for medium and 3 for the central part). A micrograph of a piece from a new Teflon coil was used for comparison. With these 8 micrographs, it was possible to establish a three-dimensional arrangement. After multi-way PCA was applied to the Teflon surface images, was possible to group a great amount of information and to characterise different parts of this coil. Received March 3, 2000. Revision July 28, 2000.     

Theoretical and experimental analysis of the electromechanical behavior of a compact spherical loudspeaker array for directivity control

Sound directivity control is made possible by a compact array of independent loudspeakers operating at the same frequency range. The drivers are usually distributed over a sphere-like frame according to a Platonic solid geometry to obtain a highly symmetrical configuration. The radiation pattern of spherical loudspeaker arrays has been predicted from the surface velocity pattern by approximating the drivers membranes as rigid vibrating spherical caps, although a rigorous assessment of this model has not been provided so far. Many aspects concerning compact array electromechanics remain unclear, such as the effects on the acoustical performance of the drivers interaction inside the array cavity, or the fact that voltages rather than velocities are controlled in practice. This work presents a detailed investigation of the electromechanical behavior of spherical loudspeaker arrays. Simulation results are shown to agree with laser vibrometer measurements and experimental sound power data obtained for a 12-driver spherical array prototype at low frequencies, whereas the non-rigid body motion and the first cavity eigenfrequency yield a discrepancy between theoretical and experimental results at high frequencies. Finally, although the internal acoustic coupling affects the drivers vibration in the low-frequency range, it does not play an important role on the radiated sound power.     

A sensitive method for cadmium determination using an on-line polyurethane foam preconcentration system and thermospray flame furnace atomic absorption spectrometry.

A new sensitive and low cost method for cadmium determination at microg l(-1) levels that combines an on-line preconcentration system with the thermospray flame furnace atomic absorption spectrometry technique (TS-FF-AAS) is described in this work. Cadmium is preconcentrated from an acidic medium (pH 2.0) by forming a complex with ammonium O,O-diethyldithiophosphate (DDTP), which is then adsorbed onto polyurethane foam (PUF). The elution step is performed by using 80% (v/v) ethanol. The effects of the chemical and flow variables associated with the preconcentration were studied, such as the pH of formation of the Cd-DDTP complex, the DDTP concentration, the preconcentration and elution flow rate and the mass of adsorbent. The present method was operated in volume-mode (2 ml) and provided a linear range from 0.4 to 15.0 microg l(-1) with a sample throughput of 16 h(-1). The increase of power detection related to TS-FF-AAS by coupling the preconcentration system was confirmed by the enhancement of sensitivity (ca. 5 times), when compared to the value for TS-FF-AAS alone, thus achieving a low detection limit (0.12 microg l(-1)). The accuracy of the method was confirmed from analyses of spiked water samples and by the use of a reference technique (ETAAS). Certified biological materials were also used for the same purpose.     

Preconcentration of Cd(II) and Pb(II) Using Humic Substances and Flow Systems Coupled to Flame Atomic Absorption Spectrometry

This paper demonstrates the potential of two natural adsorbents, vermicompost and humic acid, for preconcentration of cadmium(II) and lead(II) using flow systems coupled to flame atomic absorption spectrometry (FAAS). The procedure involves the adsorption of Cd(II) or Pb(II) on these materials (using columns containing 25mg) and subsequent elution for determination by FAAS. Cadmium(II) was preconcentrated for 4min (flow rate of 4.0mLmin^–1) and eluted with 220µL of 3.0molL^–1 HNO₃. Under these conditions, preconcentration factors of 46 and 27 were obtained for vermicompost and humic acid, respectively. Except when using 1.0molL^–1 nitric acid (for humic acid), all conditions for lead(II) preconcentration were identical to those for cadmium(II), and preconcentration factors of 62 and 83 were obtained when vermicompost and humic acid, respectively, were used. The systems were stable with only slight variations in the slopes of the analytical curves (ca. 5% after 8h working period). The long-term stability shows that a minimum of 120 and 100 cycles, respectively, can be run using the same masses of vermicompost or humic acid. The detection limits for Cd(II) were 0.4 and 0.8µgL^–1 for vermicompost and humic acid respectively, while the detection limits for Pb(II) were 8.8 and 12.1µgL^–1, also for vermicompost and humic acid. The accuracy of the methods was checked by using spiked and real (certified and reference) samples. Due to the concomitant sorption of other metals leading to variable slopes for lead and cadmium determination, it was necessary to adopt the standard addition method for calibration purposes.     

Revisiting proteus: do minor changes in lectin structure matter in biological activity? Lessons from and potential biotechnological uses of the Diocleinae subtribe lectins

Significant differences in function have been observed among lectins structurally similar to concanavalin A, but their high homology with this widely used lectin has kept them in obscurity. The observation of large differences in the potency of many of these Diocleinae lectins as stimulators of Interferon-g production by human peripheral blood mononuclear cells has lead to a major effort to unravel their chemical structure and biological activity. Modeling studies of some of these lectins reveal conformational changes in side chains of some residues involved in the carbohydrate-binding site, with possible effects on the ability of these proteins to recognize specific carbohydrate structures. Additionally, all them constitute in fact a mixture of isolectins, which in different proportions could lead to diverse effects. The present review of the biological actions of Diocleinae lectins includes several in vitro and in vivo immunological findings, as well as their effects on insect growth and reproduction. In these systems Diocleinae lectins proved to be quite diverse in their potency. Such diversity in the biological activity of highly related proteins recalls the origin of the name protein: like Proteus, the capability of assuming various forms is the essential feature of this class of molecules.     

Acid extraction and cloud point preconcentration as sample preparation strategies for cobalt determination in biological materials by thermospray flame furnace atomic absorption spectrometry

A thermospray flame furnace atomic absorption spectrometer (TS-FF-AAS) was employed for Co determination in biological materials. Cobalt presents a high atomization temperature and consequently poor sensitivity is obtained without changing its thermochemical behavior. The effect of different complexing agents on sensitivity was evaluated based on the formation of Co volatile compounds. A cloud point procedure was optimized for Co preconcentration for further improvement of sensitivity. Samples were treated with 1 mol l^− 1 hydrochloric acid solution for quantitative extraction of Co without simultaneous extraction of Fe, since it is a strong interferent. After the extraction and preconcentration steps, a sample volume of 150 μl was introduced into the hot Ni tube using air as carrier at a flow-rate of 0.4 ml min^− 1. The best sensitivity was attained using ammonium pyrrolidinedithiocarbamate (APDC) and Triton X-114 was employed for implementation of the cloud point procedure. The detection limit obtained for Co was 2.1 μg l^− 1 and the standard deviation was 5.8% for a solution containing 100 μg l^− 1 (n = 10). Accuracy was checked using two certified reference materials (tomato leaves and bovine liver) and results were in agreement with certified values at a 95% confidence level. Employing the developed procedure, Co were quantified in different biological materials (plant and animal tissues). The proposed method presents suitable sensitivity for cobalt determination in the quality control of foods.     

On the improvement of solid-phase extraction room-temperature phosphorimetry for the analysis of polycyclic aromatic hydrocarbons in water samples

This article presents considerable improvements in SPE-RTP methodology for the analysis of polycyclic aromatic hydrocarbons in water samples. Bulky glassware and vacuum pump were removed from the previous extraction procedure [E.D. Hagestuen, A.D. Campiglia, Talanta 49 (1998) 547] by processing 10 ml of sample with syringes and stainless steel filter holders. The oven-drying step was substituted by air sample drying, which employed positive pressure to remove water excess from the extraction membrane. As a result, analysis times from 8 to 10 min per sample were obtained. No costs in levels of detection were observed. Limits of detection at the pg ml(-1) level were estimated for phenanthrene, pyrene, benzo(k)fluoranthene, benzo(a)pyrene, and chrysene. The feasibility of identifying single components in PAH mixtures was investigated. Binary and tertiary mixtures were resolved by appropriate choice of excitation and emission wavelengths. The advantage of using excitation-emission matrix analysis was demonstrated for a six-component mixture.     

Evaluation of Medicinal Plant Decomposition Efficiency Using Microwave Ovens and Mini-Vials for Cd Determination by TS-FF-AAS

Sample preparation is an important step in the analytical process because it can introduce different error sources. Its efficiency is checked based on the quality and reliability of the data obtained, and the time spent on this task. This work evaluates medicinal plant decomposition parameters using polypropylene mini-vials heated by microwave radiation, applying the single vessel concept. As an example, sample mass amount (5 mg), mixture of oxidant agents (200 µL conc. HNO₃+150 µL 30% v/v H₂O₂), microwave time and power were evaluated by heating 24 mini-vials in both closed-vessel (CV) and focused (F) microwave ovens. In order to achieve the best-optimised condition for sample preparation, cadmium and residual carbon determinations were carried out for all experiments. The residual carbon (between 0.30 and 0.45%) was determined by CHN elemental analysis and the cadmium concentration (up to 0.80 µg g⁻¹) by thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) in three different medicinal plants. The accuracy of the proposed methods was assessed using certified reference materials (rye grass, BCR 281 and bovine liver, NIST 1577b) as well as conventional microwave assisted decomposition. The new method used a minimum sample amount (5 mg) and reagent volume (ca. 400 µL) and only 4 and 10 min of microwave sample decomposition for CV and F ovens, respectively. In addition, the capacity of both microwave ovens was increased up to 4 times while contamination risks were inherently reduced with the single vessel concept.     

Determination of Cd and Pb in food slurries by GFAAS using cryogenic grinding for sample preparation

A simple method combining slurry sampling after cryogenic grinding and the use of a permanent modification of the integrated platform inside the transversely heated graphite atomizer (THGA) was proposed for the determination of Cd and Pb in foods. Potentialities of the cryogenic grinding were evaluated for grinding different materials of difficult homogenization such as high fat and high fiber tissues. Animal and vegetal samples were cut into small pieces and ground in liquid nitrogen for 2 min. Slurries were prepared directly in the autosampler cup after cryogenic grinding by transferring an exact amount of homogeneous powdered material (5-20 mg) to the cup, followed by 1.00 mL of 0.2% (v/v) HNO3 containing 0.04% (v/v) Triton X-100 and sonication for 30 s, before transferring into the platform previously coated with 250 microg W and 200 microg Rh. Use of a tungsten carbide-rhodium permanent modifier combined with NH4H2PO4 conventional modifier improves tube lifetime and increases the pyrolysis temperature for Cd. Homogeneity tests, carried out by comparing the between- and within-batch precision for each kind of sample, showed no significant differences at the 95% confidence level, indicating good homogeneity for 5-20 mg masses. Detection limits were 3.3 ng g(-1) Cd and 75 ng g(-1) Pb for 1% m/v slurries. Results for determination of Cd and Pb in foods slurries were in agreement with those obtained with digested samples, since no statistical differences were found by the paired t-test at the 95% level.