A.O. Jacintho

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.

Ion exchange in flow injection analysis: Determination of Ammonium Ions at the μg 1-1 level in Natural Waters with Pulsed Nessler Reagent

Abstract Ion exchange is incorporated in flow injection analysis by using a resin column in the sample loop of an electronically operated proportional injector. Effects of sample aspiration rate, sampling time, eluting agent concentration, pumping rate of the sample carrier stream, resin column size and sample acidity, were investigated to develop a preconcentration procedure for ammonium ion determination in natural waters at the μg 1-1 level with pulsed Nessler reagent. The possibility of sample buffering before the adsorption step was studied. The proposed method is characterized by a precision of about 2%, a sampling rate of about 40 samples per hour. and a reagent consumption of 40 μl per sample, and is almost free of interferences. Recoveries from 95 to 105% were achieved in analysing rain-water samples with ammonium contents of less than 200 μg 1-1. Alternative flow diagrams and the injector command unit are discussed.

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.

Flow injection systems with inductively-coupled argon plasma atomic emission spectrometry: Part 1. Fundamental Considerations

Abstract Flow injection systems with inductively-coupled argon plasma atomic emission spectromeiry are proposed. Effects of flow rates, injected volumes and mixing Coil lengths are investigated and conditions for the measurement of the flow injection transient signal are discussed. The peak profile measured with the spectrometer corresponds well with the estimate of the true sample zone distribution near the inlet of the spectrometer made by a zone-sampling process; thus the plasma is not a limiting factor in the proposed systems. For plant analysis, the system provides nearly zero sample dispersion and so the inherent sensitivity of the spectrometric method is preserved. The results obtained for 10 elements in the NBS Orchard Leaves reference material (SRM 1571) are in good agreement with the certified values. For determinations of calcium and magnesium in dolomitic limestones, cadmium is used as internal standard and so the merging zones configuration is employed. The proposed system provides medium sample dispersion and permits about 100 samples to be analysed per hour. Relative standard deviations of 1.34% and 1.23% were calculated for the calcium and magnesium data, respectively. The analytical results compare favorably with those obtained by normal i.c.p. spectrometry with pneumatic sample aspiration, after manual sample dilution.

Zone trapping in flow injection analysis : Spectrophotometric determination of low levels of ammonium ion in natural waters

Abstract In flow injection analysis, a selected portion of a processed sample zone can be removed and, after a predetermined period of time reintroduced into the same carrier stream. This zone-trapping technique, easily applied, is suitable for methods based on relatively slow chemical reactions in which sensitivity or sampling rate is critical. It has some advantages over the stopped-flow and intermittent-flow approaches. The technique was studied for a model system without chemical reaction, and was then applied in a sensitive spectrophotometric method for the determination of ammonium ion in natural waters, based on a modified Berthelot reaction. In zone trapping, the main portion of the reacting sample zone is retained in a 38°C water bath so that about 80% complete reaction is achieved without limiting the sampling rate. The proposed method is suitable for 90–100 measurements per hour with a relative standard deviation of 0.5% for a typical sample (0.15 mg l−1 ammonium). Beers law is followed up to 1 mg l−1 and the detection limit is 5 μg l−1. Interferences of metals are overcome with EDTA. The results for waters agree well with those obtained by a standard manual procedure.

A simple procedure for standard additions in flow injection analysis : Spectrophotometric Determination of Nitrate in Plant Extracts

Abstract The standard additions method with constant sample volumes is readily achieved in a flow injection system if zone sampling is used to select segments with different concentration levels from the dispersed zone of a known standard, and these segments are merged with the sample zone. This system requires a single standard solution, is suitable for routine large-scale analyses, and does not need complex equipmnt. The number of sequence and level of additions are chosen in accordance with the required sampling rate, the speed of the method and the mean analyte content in the samples. Details of system design, stability and potential are discussd. As an applicaion, this system was used in the spectrophotometric determination of nitrate in plant extracts where the standard additions method is required to overcome the matrix effect. The results, based on twelve additions per sample (about 100 measurements per hour) agree with those obtained by the corresponding manual procedure.

Merging zones in flow injection analysis: Part 5. Simultaneous determination of aluminium and iron in plant digests by a zone-sampling approach

Abstract A flow injection procedure is proposed for the simultaneous determination of aluminium and iron in plant material. The method is based on a flow configuration involving zone sampling and merging zones. Aluminium is determined spectrophotometrically with eriochrome cyanine R as reagent and iron by atomic absorption spectrometry. The advantages of this method over other procedures already reported are discussed. The effects of reagent composition for the aluminium determination are described in detail. The zone-sampling approach permits an easier pH control in the aluminium determinations so interferences caused by variations in sample acidity are avoided without the need for very concentrated buffers. The merging zones configuration greatly reduces the consumption of reagents. The proposed method permits the analysis of about 120 samples (240 determinations) per hour, with good precision (r.s.d.

Merging zones in flow injection analysis: Part 6. determination of calcium in natural waters, soil and plant materials with glyoxal bis(2-hydroxyanil)

Abstract A flow injection procedure is proposed for the spectrophotometric determination of calcium in natural waters, soil extracts and plant digests, employing glyoxal bis(2-hydroxyanil) as the colour-forming reagent. The necessary dissociation of this reagent, which is rather slow, proceeds outside the analytical path, and the merging-zones approach is used for reagent addition. Composition of reagents, dissociation time of the colourforming reagent, ethanol content in the carrier streams and interferences are described. In the analysis of plant and soil materials, zone sampling is required for initial sample dilution. The proposed systems are very stable and permit a sampling rate of 180 determinations per hour. Relative standard deviations are less than 1%. The results compare well with those obtained by inductively-coupled argon-plasma atomic emission spectrometry.

Catalytic determination of molybdenum in plants by flow-injection spectrophotometry with ion-exchange separation

The catalytic effect of molybdenum n the oxidation of iodide by hydrogen peroxide is utilized. A cation-exchange resin column is incorporated into the system to remove interfering ions; during sample processing, the interfering ions are eluted towards waste. Plant materials are ashed and solutions are injected. The proposed system can handle about 40 samples per hour with molybdenum contents in the 1.0–40.0 μg 1−1 range. Results are precise (r.s.d. usually <%) and in agreement with those obtained by graphite-furnace atomic absorption spectrometry.

Spectrophotometric flow injection determination of chloride in ethanol

Abstract A flow injection procedure is described for the spectrophotometric determination of chloride in ethanol, based on the mercury(II) thiocyanate—iron(III) reaction. Effects of reagent composition and ethanol content of the sample are investigated in detail. The proposed system can analyse 120 samples of ethanol (94–100% v v ) per hour, with a relative standard deviation lower than 1%, when the chloride content ranges from 0.1–6.0 ppm. Recoveries of ca. 96% are found.