Boaventura F. Reis

Multicommutation in flow analysis. Part 1. Binary sampling: concepts, instrumentation and spectrophotometric determination of iron in plant digests

Abstract A novel strategy to handle sample and reagent introduction in continuous flow systems is proposed. Basic features such as the effect of flow cell volume, analytical pathlength and peristaltic pump pulsation on the precision of measurement were studied. The manifold was based on a set of three-way solenoid valves controlled by a microcomputer using software written in QuickBASIC. It could also control the rotation speed of the peristaltic pump. By sampling slugs of sample and reagent solutions synchronized with pulsation of the peristaltic pump, aliquots with volumes as low as 2 μl could be sampled with a relative standard deviation

Simultaneous determination of nitrate and nitrite by flow injection analysis

Abstract An automatic method for the simultaneous determination of nitrate and nitrite by flow injection analysis is described. Nitrate is reduced to nitrite with a copperized cadmium column. Nitrite is diazotized and coupled with N -(l-naphthyl)ethylenediammonium dichloride. The merging zones approach is used to minimize reagent consumption. The injector system is arranged so that two peaks are obtained, one corresponding to nitrite and the other to nitrite plus nitrate. A sampling rate of about 90 samples per hour is possible; the precision is better than 0.5% for nitrite in the range 0.1–0.5 mg l t and 1.5% for nitrate in the range 1.0–5.0 mg l t

Merging zones in flow injection analysis

Abstract A double proportional injector is described, and an improvement of the continuous flow-injection method is discussed. Sample and reagent are both injected into an inert carrier stream; the reagent is consumed only in the presence of the sample, and can otherwise be recovered continuously. The main characteristics of the flow injection system, e.g. high sampling rate and good accuracy, precision and sensitivity, are maintained. The proposed system is compared with other flow-injection systems suggested earlier. The determination of phosphate in plant material by the molybdenum blue flowinjection method was chosen to demonstrate the feasibility of the system; the consumption of ascorbic acid is only 9% of that in the original method.

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.

Multi-pumping in flow analysis: concepts, instrumentation, potentialities

Abstract A novel strategy for the implementation of flow-based analytical procedures using several micropumps is proposed. The pumps are switched individually or in combination, in order to create a pulsed flowing stream through the analytical path, and are the only active devices acting simultaneously as liquid propelling units, sample insertion ports and commuting elements. Configuration and control of the flow system are then greatly simplified. The micropumps produce distinct stroke volumes at distinct pulse frequencies with high reproducibility ensuring the attainment of very stable flow rates. This leads to an enhanced versatility that enables the utilisation of different approaches for sample management including step-wise variable sample volume, binary sampling and merging zones without reconfiguration of the system hardware. In contrast to the typical flow systems, the proposed one is characterised by a pulsed flow ensuring a fast sample/reagent mixing that contributes to improve the reaction development—thus sensitivity—even in situations of limited dispersion. The basic features and the performance of the proposed strategy are evaluated in the spectrophotometric determination of Cr(VI) in natural waters with 1,5-diphenylcarbazide.

Zone-sampling processes in flow injection analysis

Abstract In flow injection analysis, it is possible to select a small portion of a dispersed zone and inject it into another carrier stream. The potentialities of such zone-sampling processes are discussed, and the main factors involved are investigated by spectrophotometric measurements. The usefulness of zone sampling in routine work is demonstrated in the atomic absorption spectrometry of potassium in plant digests. The simple system, which provides automatic dilution, allows analysis of 120 samples per hour, although the required sample dispersion factor was 0.0076; the results agreed with those obtained by conventional a.a.s. after hundredfold manual sample dilution.

Merging zones in flow injection analysis: Part 2. Determination of Calcium, Magnesium and Potassium in Plant Material by Continuous Flow Injection Atomic Absorption and Flame Emission Spectrometry

Abstract A flow-injection analysis system is described for the automatic determination of calcium, magnesium and potassium in plant material by atomic absorption and flame emission Spectrometry. Flow rates and damping factors were studied; the sample undergoes a dispersion of ca. 40-fold with automatic incorporation of lanthanum when necessary. The proposed method allows 300 determinations per hour with a typical relative standard deviation of 0.5% and a reagent consumption of only 500 μg of lanthanum per determination, which is about 1% the usual amount. The results for plant digests agree with those obtained by the standard a.a.s. procedure.

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.

Flow-injection spectrophotometric determination of paracetamol in tablets and oral solutions.

A flow injection method is proposed for the determination of paracetamol in pharmaceutical dosage forms. The method is based on the nitration of paracetamol with sodium nitrite, and the absorption of the reaction product is measured at 430 nm in alkaline medium. Unlike other colorimetric methods used for determination of paracetamol, this method does not require the use of heat. The influence of several operating parameters is studied. The method was applied to the determination of paracetamol in oral solutions and in tablets, alone or associated with caffeine. When the results were compared with those obtained by the official HPLC method (USP 24) the relative differences found were from 0.4 to 2.3%, with relative standard deviations below 1%.

flow injection systems wiht inductively-coupled argon plasma atomic emission spectrometry: Part 2. The generalized standard addition method

Abstract In multicomponent analysis by inductively-coupled argon plasma emission spectrometry, the generalized standard addition method is useful in overcoming matrix and/or spectral interferences. As this method requires many standard additions, it becomes cumbersome when done manually if many elements are to be determined. By using a flow injection system, the standard addtion manipulations required can be significantly simplified and completed within a shorter period of time with much less sample material. A flow injection system with merging zones is used to demonstrate the method for analyses of a Ni/Cu/Zn standard and of alloys. The reproducibility of measurements of the injected sample, with or without additions of standards is always better than 99%. The results obtained compare well with those obtained by the manual procedure and also with those obtained by atomic absorption spectrometry.