Ana C.B. Dias

Multi-pumping flow systems: an automation tool

Multi-pumping flow systems (MPFS) are one of the most recent developments in terms of the design, conception and implementation of continuous flow methodologies, for sample and reagent handling and for the automation of analytical procedures. Based on the utilisation of multiple solenoid micro-pumps they enable the configuring of fully automated and easily controlled and operated analytical systems since all the fundamental operations involved in carrying out a sample analysis, including sample insertion, reagent addition and signal measurement could be carried out by the same manifold component, reducing the number of system parts and minimising its control or the occurrence of mal-functions. On the other hand, micro-pumps actuation produce a pulsed flow characterised by a chaotic movement of the solutions, which contributes to a fast sample/reagent homogenisation with low axial dispersion yielding improved analytical signals. The combination of such advantageous features resulted in simple, compact, versatile, fast, low-cost analytical procedures, exhibiting low reagent and low sample consumption, reducing the production of undesirable wastes and minimising operator intervention.

A critical examination of the components of the Schlieren effect in flow analysis

The Schlieren effect is a consequence of light inflexions altering the spectrophotometric measurements that are due to the formation of optical artefacts such as mirror and lenses inside the flowing sample. It may influence signal-to-noise ratio, sample throughput and measurement repeatability in spectrophotometric flow-based procedures. It was critically examined, in order to provide guidelines for system design. Results demonstrate that the magnitude of the Schlieren effect is governed by the combined influence of two components. The first is related to loss of light by reflection at sharp liquid interfaces and prevails under poor mixing conditions. In this situation the transmitted light becomes more intense and somewhat erratic. The second is related to light refraction in regions with pronounced refractive index gradients, and prevails under good mixing conditions. It can lead to positive or negative modifications of the transmitted light, the direction being dependent on the relevance of these components. The importance of the solubility of the chemical species involved, the possibility of interactions with the inner wall of the tubing, and the feasibility of dual-wavelength spectrophotometry for compensating purposes are discussed.

Multi-pumping flow system for spectrophotometric determination of bromhexine

A novel flow-based procedure involving the multi-pumping approach was developed for the spectrophotometric determination of bromhexine in pharmaceutical preparations. The method is based on reaction with 3-methyl-2-benzothiazolinone hydrazone (MBTH) and Ce(IV). Several solenoid micro-pumps are present in the manifold in order to provide improved system control. Critical tasks in continuous flow analysis, sample-reagent introduction and solution propelling, are then efficiently carried out. Moreover, the pulsed flow inherent to the mini-pumps ensures good mixing conditions, thus improving the reaction zone homogenisation. This aspect becomes particularly attractive for the implementation of analytical procedures involving several consecutive reactions as it happens with the proposed procedure. Linear calibration plots (r > 0.995, n = 6) were obtained for bromhexine concentrations up to 400 mg l -1 . Detection limit was 2 mg l -1 and the sampling rate was about 45 samples h -1 Results are precise (R.S.D. < 1.5%; n = 10) and in agreement with those furnished by the reference procedure involving a potentiometric titration.

Single reaction interface in flow analysis

The dual or multiple reaction interface concept, commonly associated to the distinct flow techniques, was replaced by a single interface concept, which do not no rely on the utilisation of a well-defined and compelling sample volume but only on mutual penetration of sample and reagent zones at a single reaction interface where both sample and reagent met together prior to detection. In the proposed approach basic principles of flow analysis, such as controlled dispersion and reaction zone formation, are not influenced by sample and reagent volumes but determined exclusively by the extension of the overlap of two adjoining quasi-infinite zones enhanced by multiple flow reversals and the pulsed nature of the flowing streams. The detector is positioned at the core of the flow manifold (not in the conventional terminal position), and repetitive flow reversals enable interface manipulations, including multi-detection of the entire reaction interface or the monitoring of the evolution of a pre-selected interface zone by using suitable reversal cycle times. The implementation of the developed approach was facilitated due to the configuration simplicity and operational versatility of multi-pumping flow systems. Its performance was evaluated by monitoring processes involving two or four-solution reaction interfaces.

Flow systems exploiting in-line prior assays

An expert sequential injection system involving a prior assay is proposed for spectrophotometric determination of phosphate and eventually zinc in soil extracts. The result of phosphate determination is the basis for a concentration-oriented decision regarding to the need or not for zinc determination. Zinc was only determined if a threshold value (peak height corresponding to 5.0mgl(-1)P) was surpassed. The methods involved formation of molybdenum blue and the Rhodamine 6G/ammonium thiocyanate/Zn(2+) ternary complex. Variations in the threshold value were < 2% during 4h operating periods, false responses were not verified, and the analytical time was reduced in about 30%. Precise results (R.S.D. <3%P and < 1% Zn) in agreement with spectrophotometry and flame atomic absorption spectrometry were obtained. The innovation permits faster information processing, as well as a reduction in the number of measurements, number of analytical steps, laboratorial time, and consumption of sample and reagents, thus waste generation.

Multipumping Flow Systems: An Alternative Approach to Sample Handling in Spectroscopy Measurements

Abstract Multipumping flow systems (MPFSs) are one of the most recent developments in terms of flow‐based analytical methodologies. They differ from conventional flow‐based modalities such as flow injection analysis (FIA) and sequential injection analysis (SIA) in several aspects, including manifold components and configuration, operational mode, and flow hydrodynamics. This paper presents an overview of MPFSs and discusses their basic features, including methodological implementation, operational characteristics, analytical performance, potential, and limitations. Applications to different types of samples involving different detection techniques are discussed.

A Multipumping Flow System for In Vitro Screening of Peroxynitrite Scavengers

Peroxynitrite anion is a reactive nitrogen species formed in vivo by the rapid, controlled diffusion reaction between nitric oxide and superoxide radicals. By reacting with several biological molecules, peroxynitrite may cause important cellular and tissue deleterious effects, which have been associated with many diseases. In this work, an automated flow-based procedure for the in vitro generation of peroxynitrite and subsequent screening of the scavenging activity of selected compounds is developed. This procedure involves a multipumping flow system (MPFS) and exploits the ability of compounds such as lipoic acid, dihydrolipoic acid, cysteine, reduced glutathione, oxidized glutathione, sulindac, and sulindac sulfone to inhibit the chemiluminescent reaction of luminol with peroxynitrite under physiological simulated conditions. Peroxynitrite was generated in the MPFS by the online reaction of acidified hydrogen peroxide with nitrite, followed by a subsequent stabilization by merging with a sodium hydroxide solution to rapidly quench the developing reaction. The pulsed flow and the timed synchronized insertion of sample and reagent solutions provided by the MPFS ensure the establishment of the reaction zone only inside the flow cell, thus allowing maximum chemiluminescence emission detection. The results obtained for the assayed compounds show that, with the exception of oxidized glutathione, all are highly potent scavengers of peroxynitrite at the studied concentrations. (Journal of Biomolecular Screening 2007:875-880)

Fully Automated Spectrophotometric Method for the Determination of Buspirone in Pharmaceutical Preparations

Abstract A fully automated kinetic method for the spectrophotometric determination of buspirone in pharmaceutical preparations is proposed. The analytical system was implemented by using multiple solenoid micropumps for sample and reagents insertion, which led to improved operational simplicity and low reagent consumption. A three‐way solenoid valve enabled the establishment of two parallel reactors and the exploitation of the fixed time approach for differential kinetics. Linear calibration plots for buspirone concentrations of up to 60 mg L−1 were obtained, with a detection limit of 2.8 mg L−1. The sampling rate was about 55 samples per hour.

Spectrophotometric flow-injection determination of zinc in plant digests based on a spot test

A spot test was implemented in a flow-injection system for the spectrophotometric determination of zinc in digests of plant materials. It is based on the influence of Zn(2+) on the oxidation rate of 1-naphthylethylenediamine (NED) by hexacyanoferrate(III) under acidic conditions. In order to control the precipitate formation and to maintain the resulting suspension, a micellar medium was established by adding Triton X-100. The proposed system handles about 65 samples per hour, meaning 72mug NED and 9.0mg K(3)[Fe(CN)(6)] per determination. Baseline drift is usually <0.01 absorbance per hour and the analytical signals for 0.5-2.5mgl(-1) Zn range within ca. 0.07-0.45 absorbance. Linearity of the analytical curve is fair (r>0.999, n=6) and detection limit was estimated as 0.2mgl(-1) Zn. Results are precise (R.S.D.<1%, n=10) and in agreement with flame atomic absorption spectrometry and with certified values of standard reference materials.