Fábio R.P. Rocha

Multicommutation in flow analysis: concepts, applications and trends

Multicommutation refers to flow systems designed with discrete computer-controlled commutators resulting in flow networks in which all the steps involved in sample processing can be independently implemented. The flow systems can be re-configured by the control software, presenting thus increased versatility, potential for automation and for minimization of both reagent consumption and waste generation. The main objective herein is to review the concept of multicommutation in order to permit a proper evaluation of the characteristics and potentialities of the related flow systems, to assist methodological implementation and to discuss similarities with other existing strategies. Implementation of tandem streams, controlled dilutions, wide-range determinations, sequential determinations, titrations and in-line separation/concentration are emphasized.

Peat as a natural solid-phase for copper preconcentration and determination in a multicommuted flow system coupled to flame atomic absorption spectrometry.

The physical and chemical characteristics of peat were assessed through measurement of pH, percentage of organic matter, cationic exchange capacity (CEC), elemental analysis, infrared spectroscopy and quantitative analysis of metals by ICP OES. Despite the material showed to be very acid in view of the percentage of organic matter, its CEC was significant, showing potential for retention of metal ions. This characteristic was exploited by coupling a peat mini-column to a flow system based on the multicommutation approach for the in-line copper concentration prior to flame atomic absorption spectrometric determination. Cu(II) ions were adsorbed at pH 4.5 and eluted with 0.50 molL(-1) HNO(3). The influence of chemical and hydrodynamic parameters, such as sample pH, buffer concentration, eluent type and concentration, sample flow-rate and preconcentration time were investigated. Under the optimized conditions, a linear response was observed between 16 and 100 microgL(-1), with a detection limit estimated as 3 microgL(-1) at the 99.7% confidence level and an enrichment factor of 16. The relative standard deviation was estimated as 3.3% (n=20). The mini-column was used for at least 100 sampling cycles without significant variation in the analytical response. Recoveries from copper spiked to lake water or groundwater as well as concentrates used in hemodialysis were in the 97.3-111% range. The results obtained for copper determination in these samples agreed with those achieved by graphite furnace atomic absorption spectrometry (GFAAS) at the 95% confidence level.

Green chemistry and the evolution of flow analysis. A review

Flow analysis has achieved its majority as a well-established tool to solve analytical problems. Evolution of flow-based approaches has been analyzed by diverse points of view, including historical aspects, the commutation concept and the impact on analytical methodologies. In this overview, the evolution of flow analysis towards green analytical chemistry is demonstrated by comparing classical procedures implemented with different flow approaches. The potential to minimize reagent consumption and waste generation and the ability to implement processes unreliable in batch to replace toxic chemicals are also emphasized. Successful applications of greener approaches in flow analysis are also discussed, focusing on the last 10 years.

An improved flow system for phenols determination exploiting multicommutation and long pathlength spectrophotometry.

A greener and sensitive procedure for spectrophotometric determination of phenols based on a multicommuted flow system with a 100cm optical path flow cell is presented. The method exploited the oxidative coupling of phenolic compounds with 4-aminoantipyrine in alkaline medium containing potassium hexacyanoferrate(III). Sensitivity was 80-fold higher than that achieved with a 1cm flow cell, making feasible the determination of phenols in the 10-100mugl(-1) range with a detection limit estimated as 1mugl(-1) phenol. The sampling rate and the coefficient of variation were estimated as 90 determinations per hour and 0.6% (n=10), respectively. The multicommutation approach allowed a 200-fold reduction of the reagent consumption in comparison with the reference batch method. Moreover, the chloroform extraction for analyte concentration is unnecessary in view of the increase in sensitivity. Recoveries within 93.3 and 106% were achieved for determination of phenol in natural and wastewater samples. Results agreed with the obtained by a reference method at the 95% confidence level.

Flow analysis strategies to greener analytical chemistry. An overview

An overview of the strategies adopted in flow analysis towards cleaner analytical methods is presented. The discussion deals with reagentless procedures, replacement of hazardous chemicals, strategies for waste minimization as well as on-line waste treatment or recycling. The potential of flow approaches such as sequential injection, multicommutation and monosegmented flow is emphasized. Automation, employment of solid-phase reagents and miniaturization are highlighted as alternatives for waste minimization.

A portable and low cost equipment for flow injection chemiluminescence measurements

A compact, reliable and low cost flow injection chemiluminescence system is described. The flow system consists of a set of solenoid micro-pumps that can dispense reproductive micro-volumes of solutions. The luminometer was based on a coiled cell constructed from polyethylene tubing that was sandwiched between two large area photodiodes. The whole equipment costs about US

Estratégias para aumento de sensibilidade em espectrofotometria UV-VIS

750 and weights ca. 3 kg. Equipment performance was evaluated by measuring low concentrations of hydrogen peroxide by oxidation of luminol and for the determination of ammonium, based on its inhibition of the luminescence provided by the reaction of luminol and sodium hypochlorite. Linear responses were achieved within 1.0-80 micromol L(-1) H(2)O(2) and 0.6-60 micromol L(-1) NH(4)(+) with detection limits estimated as 400 nmol L(-1) H(2)O(2) and 60 nmol L(-1) NH(4)(+) at the 99.7% confidence level. Coefficients of variation were 1.0 and 1.8%, estimated for 20 micromol L(-1) H(2)O(2) and 15 micromol L(-1) NH(4)(+) (n=20), respectively. Reagent consumption of 55 microg luminol, effluent volume of 950 microL per determination and sampling rate of 120 samples per hour were also achieved.

Flow-injection solid-phase spectrophotometry for the determination of zinc in pharmaceutical preparations

Spectrophotometry is one of the most widespread analytical techniques due to its simplicity, reliability, and low-cost instrumentation for both direct measurements and coupled to other techniques or processes such as chromatography, electrophoresis and flow analysis. However, the application is often limited by sensitivity. This article describes some advances that greatly improve the performance of spectrophotometric measurements, especially in order to increase sensitivity, including the employment of liquid-core waveguides and solid-phase spectrophotometry.

An improved flow system for spectrophotometric determination of anions exploiting multicommutation and multidetection

A flow system exploiting solid-phase spectrophotometry is proposed for the determination of zinc in pharmaceutical preparations. The chromogenic reagent 1-(2-tiazolylazo)-2-naphthol (TAN) was immobilized on C18 bonded silica loaded into a homemade flow cell with 1 mm optical path and 78 mm 2 cross section. The flow cell was designed in such a way that the sensitivity is enhanced and the attenuation of the radiation beam due to scattering and absorption by the solid material is minimized. The flow cell was placed in the spectrophotometer so that the radiation beam was focused on the overall surface containing the adsorbing material. Reagent immobilization was performed on-line and allowed to work for at least one month. Analyte reaction, retention and detection were performed simultaneously, followed by elution with hydrochloric acid. The apparent molar absorptivity was estimated as 2.0210 5 l mol ˇ1 cm ˇ1 and the procedure allowed the determination of zinc in the 0.04‐4.0 mg l ˇ1 range with a coefficient of variation of 3.3% (na10). A sample throughput of 45 determinations per hour and a detection limit of 10 m gl ˇ1 (99.7% confidence level) were achieved. The results agreed at a 95% confidence level with those found by inductively coupled plasma atomic emission spectrometry. # 1999 Elsevier Science B.V. All rights reserved.

A green analytical procedure for sensitive and selective determination of iron in water samples by flow-injection solid-phase spectrophotometry

A multicommutated flow system is proposed for the determination of anions in water samples. The flow set up was assembled with a set of computer-controlled three-way solenoid valves in order to manage the addition of different reagents by binary sampling. An optical-fiber CCD-array spectrophotometer with a tungsten-halogen lamp was employed for multidetection. Water samples were used as carrier and the chromogenic reagents were intermittently added, allowing the sequential determination of nitrate, nitrite, chloride and phosphate with or without in-line concentration by ion exchange. In-line concentration of the analytes was performed during the signal measurements of the other species. In this way, a 180 s loading time was implemented without impairing the sampling rate (estimated as 50 determinations per hour). Under the proposed conditions, the procedure can be used for samples containing 30–300 gl −1 N-NO2 − , 0.1–1.0 mg l −1 N-NO3 − , 1.0–10 mg l −1 Cl − , and 0.05–2.5 mg l −1 P-PO4 3− . Detection limits were estimated as 6 gl −1 N-NO2 − ,4 0 gl −1 N-NO3 − , 400 gl −1 Cl − and 30 gl −1 P-PO4 3− at 99.7% confidence level. Coefficients of variation were estimated (n = 20) as 1.6, 2.2, 2.3 and 1.5% for nitrite, nitrate, chloride and phosphate, respectively. The reagent consumption was reduced from 3- to 40-fold and from 20- to 760-fold regarding the conventional FIA systems and batch procedures, respectively. Results for river water samples agreed with those obtained by single-analyte FIA procedures at the 95% confidence level.