Raquel P. Sartini

Crystal seeding in flow-injection turbidimetry : determination of total sulfur in plants

Reproducible addition of on-line produced suspensions is proposed to improve supersaturation conditions in flow turbidimetry. This crystal seeding leads to a simplification in system design, and an improvement in sampling rate and/or sensitivity in procedures usually limited by rate of turbidity formation. The feasibility of the approach is demonstrated in developing a turbidimetric flow-injection procedure for determination of total sulphur in plant materials based on lead sulphate precipitation after adding a confluent stream with lead phosphate nucleant. Effects of reagent concentrations, acidity, temperature and timing were investigated, and timing and ethanol content manifested themselves as the most relevant parameters. 400 samples may be run per hour, special procedures for system washing are not needed, surfactant is not required and base-line drift is not observed after operation periods of 8 h. Results are precise (R.S.D. < 0.005 for 5-25 mg S l -1 ) and in agreement with those obtained by the usual flow-injection procedure based on barium sulphate.

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

Abstract 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.

Multicommutation in flow analysis. Part 4. Computer-assisted splitting for spectrophotometric determination of copper and zinc in plants

Abstract An improved splitting process in flow analysis accomplished by means of a computer controlled three-way solenoid valve is proposed. This valve is located at the splitting site, and defines the flow pattern and the portions of the processed sample delivered to each emergent stream. As an application, the simultaneous spectrophotometric determination of copper and zinc in plant digests was selected. The method is based on metal complexation with cyanide, differential kinetics of decomplexation, and reaction with Zincon. The proposed system is very stable and handles 45 samples per hour which corresponds to 0.27 mg Zincon per determination. Precise measurements (r.s.d. = 0.7% and 1.7% for first and second peaks) are obtained within 0.00–1.00 mg 1 −1 Cu and 0.00–2.00 mg 1 −1 Zn. Detection limits are 0.05 and 0.04 mg l −1 for Cu and Zn. Results are in agreement with standard reference materials.

Determination of cadmium in foodstuffs and plant materials by flow injection-spectrophotometry including ion exchange

A flow-injection system with a minicolumn packed with a strongly basic anion exchanger (AG1-X8 resin, 200–400 mesh) is proposed for the spectrophotometric determination of cadmium in foodstuffs, plants and similar based on the formation of a cadmium iodide-Malachite Green (MG) associate. Improved sensitivity and selectivity are attained since cadmium is in-line concentrated as chlorocomplexes whereas lead, copper and other potential interferents are discarded. Reagent compositions, flow rates, commutation times, mean available time for chlorocomplexes formation, and conditions for concentration and elution were investigated. A 2.0 M NaCl-0.1 M HC1 solution was elected for chlorocomplexes formation, and a 90-s concentration time was set. The eluant, also sample carrier stream, was a 2.0 M NaNO3 plus 0.1 M HNO3 solution. The proposed system handles about 25 samples per hour (2.50–25.0 μg Cd l−1), consuming ca. 8 ml of sample, 0.25 mg MG and 0.41 g KI per determination. The detection limit is 0.11 μg Cd l−1. Results are precise (R.S.D. 2.72 and 2.26% for 5.12 and 19.7 μg Cd l−1) and in agreement with certified values of standard reference materials. Recoveries within 99.1 and 111.2% were found. With two resin minicolumns, the applicability of the system is expanded, and samples solubilized with nitric and perchloric acids can also be run. The first minicolumn stands for in-line perchlorate removal before chlorocomplex formation, and the second one for cadmium concentration. Although more complex, similar system performance and results quality relatively to the system with a single minicolumn are attained.

Flow analysis with accuracy assessment

Abstract The concept of accuracy assessment is exploited in flow analysis. The resulting system is based on multi-commutation because multiple sample processing is involved. The manifold includes different quasi -independent methods and permits recovery tests to be performed on every sample. Results are then intrinsically more accurate and additional information on the sample matrix is provided. The feasibility of the approach is demonstrated in the large scale determination of chloride in river water samples presenting high variability in matrix composition and/or in acidity. The spectrophotometric mercury thiocyanate-Fe(III) method and the turbidimetric method based on the precipitation with silver ions are implemented in a single manifold and a 40.00 mg l −1 Cl − intermittent stream is added in-line to each sample for producing recovery data. Analysis of the four recorded analytical signals permits evaluation and eventual compensation of matrix effects. The final result is calculated after considering the results obtained by two different methods, and the eventual correction of matrix effects. The proposed system is very robust, handles 25 samples per hour and yields precise results between 0.50 and 10.0 mg l −1 Cl − . Potentialities and limitations of the approach are discussed.

Spectrophotometric determination of total nitrogen in plant materials using a flow-injection system with an AgCl(s) reactor

Abstract An improved flow-injection system with an AgCl (s) reactor is proposed for the spectrophotometric determination of total nitrogen in Kjeldahl digests of plant materials. After sample injection, the established sample plug is alkalinised and allowed to flow through the mini-column where Ag + ions are displaced as the soluble [Ag(NH 3 ) 2 ] + complex. Bromopyrogallol red and o -phenanthroline are then added and a coloured ternary complex is formed under neutral conditions. The system is very stable, and baseline drift is usually −1 . The analytical curve is linear up to 6.00% (w/w) N (dry basis), regression coefficient has been estimated as >0.999 ( n =6) and detection limit is 0.2% (w/w) N. Accuracy was confirmed by running standard reference materials and typical samples already assayed by conductimetry.

Spectrophotometic flow injection determination of lead in port wine using in-line ion-exchange concentration

A flow injection system with in-line ion exchange is proposed for the spectrophotometric determination of lead in Port wine. A Chelex-100 resin mini-column (200–400 mesh) is used for lead concentration, and the chromogenic reaction is based on the formation of a ternary complex between lead, Malachite Green and iodide. Forty-five digested wine samples can be run per hour (25–500 µg l–1 Pb) and results are comparable to those obtained by ETAAS. Other features are the measurement precision (RSDs lower than 2.6%), a detection limit of 12 µg l–1 and a sampling rate of 45 h–1.