Maria Fernanda Giné

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

Simultaneous preconcentration of copper, zinc, cadmium, and nickel in water samples by cloud point extraction using 4-(2-pyridylazo)-resorcinol and their determination by inductively coupled plasma optic emission spectrometry.

A procedure for simultaneous separation/preconcentration of copper, zinc, cadmium, and nickel in water samples, based on cloud point extraction (CPE) as a prior step to their determination by inductively coupled plasma optic emission spectrometry (ICP-OES), has been developed. The analytes reacted with 4-(2-pyridylazo)-resorcinol (PAR) at pH 5 to form hydrophobic chelates, which were separated and preconcentrated in a surfactant-rich phase of octylphenoxypolyethoxyethanol (Triton X-114). The parameters affecting the extraction efficiency of the proposed method, such as sample pH, complexing agent concentration, buffer amount, surfactant concentration, temperature, kinetics of complexation reaction, and incubation time were optimized and their respective values were 5, 0.6 mmol L(-1), 0.3 mL, 0.15% (w/v), 50 degrees C, 40 min, and 10 min for 15 mL of preconcentrated solution. The method presented precision (R.S.D.) between 1.3% and 2.6% (n=9). The concentration factors with and without dilution of the surfactant-rich phase for the analytes ranged from 9.4 to 10.1 and from 94.0 to 100.1, respectively. The limits of detection (L.O.D.) obtained for copper, zinc, cadmium, and nickel were 1.2, 1.1, 1.0, and 6.3 microg L(-1), respectively. The accuracy of the procedure was evaluated through recovery experiments on aqueous samples.

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.

Monitoring trace metals in urban aerosols from Buenos Aires city. Determination by plasma-based techniques

A study was undertaken, within the framework of a 3 years national project, to assess the content of 13 elements in airborne particulate matter collected in representative zones of the metropolitan area of Buenos Aires. The sampling strategy followed consisted in collecting simultaneously 67 samples of PM10 particulate matter in 9 sampling sites covering an area of about 30 km2 during one week. The collection was performed on ash-free fibre-glass filters using high volume samplers. A combination of aqua regia and perchloric acid was used for leaching metals from filters. Key elements, namely Al, Ca, Cu, Fe, Mn, Mo, Ni, Pb, S, Sb, Sn, Zn and Zr, were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS) at micro g g(-1) and ng g(-1) levels. Analyte concentration varied from 130 ng g(-1)(Mo) to over 30%(Ca). Multivariate statistical analysis was performed on the data set including the measured elemental compositions for the monitored period. The atmospheric concentration found for Pb confirms the decreasing levels of this element since the introduction of unleaded gasoline in 1995: 88 ng m(-3)(2001) < 220 ng m(-3)(1997) < 3900 ng m(-3)(1994). The average S concentration above 3 microg m(-3) is somehow unexpectedly high for Buenos Aires since the relatively low S content of liquid fuels and the massive usage of natural gas imply low emissions of this element from combustion activities. To the best of our knowledge, S concentrations are reported for the first time for this city.

Automated flow analysis system based on multicommutation for Cd, Ni and Pb on-line pre-concentration in a cationic exchange resin with determination by inductively coupled plasma atomic emission spectrometry

Abstract A flow system for on-line pre-concentration of Cd, Ni and Pb and determination by inductively coupled plasma atomic emission spectrometry is presented. The manifold comprised a set of three-way solenoid valves, which were controlled by a microcomputer running software written in Quick Basic 4.5 by means of an A/D board. Since the flow network was based on the multicommutation concept pre-concentration and elution steps could be simultaneously performed in several columns in parallel with significant analytical throughput improvement. Comparing obtained results with certified values, no significant difference at 95% confidence level was observed. Other valuable features such as an analytical throughput of 90 determinations per hour, relative standard deviation

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.

Cloud-point extraction of molybdenum in plants and determination by isotope dilution inductively coupled plasma mass spectrometry

Molybdenum in plant digests was accurately determined by cloud point extraction and quantification by isotope dilution inductively coupled plasma mass spectrometry. Plant digests spiked with 98Mo received 8-hydroxyquinoline and the surfactant Triton X-100, attaining an enrichment factor close to 70. The 98Mo/95Mo measurements were characterized by RSDs from 0.5 to 1%. Instrumental mass discrimination of 1.4% was experimentally determined and used to correct the results. Concentrations found in plant reference materials were in agreement at the 95% significant level with certified values. A limit of detection of 0.8 ng g−1 was calculated based on the blank uncertainties and calibration curve on both isotopes.

Determination of cadmium in biological materials by tungsten coil atomic absorption spectrometry

A simple, low cost tungsten coil furnace was evaluated for electrothermal atomic absorption spectrometry (ETAAS). The open atomizer was resistively heated by using a power supply with a voltage feedback circuit. The analysis time using a manually injected sample volume of 10 µl was 41 s. Acceptable precision (relative standard deviation <3%, n= 12 at 1.0 ng ml–1 of cadmium) was obtained for cadmium values ranging from 0.5 to 6.0 ng ml–1. The characteristic mass for peak height absorbance was 0.5 pg. Results compare well with certified values of different biological materials digested with nitric acid. For certain reference materials better accuracy was obtained from data based on integrated absorbance.

Confluent streams in flow injection analysis

Abstract The addition of a confluent stream increases the mean length of the sample zone and simultaneously decreases the involved concentrations. Simple equations describing these effects under ideal mixing conditions are proposed. The effects of the confluent stream on the overall sample dispersion may be more or less compensated, depending on the decrease in the post-confluence dispersion. When this compensation is almost total, a paradoxical situation occurs, in which the recorded peak height becomes practically unaffected by variations in the flow rate of the merging stream. In this situation, the peak width approaches a limiting value which is independent of the injected volume and of the flow rate of the confluent stream. Limiting values for the peak width and for the mean length of the sample zone are postulated. When the post-confluence dispersion is negligible, the effects of the confluence appear in the recorded peak. The confluent stream addition reduces the overlap between successive sample zones. The site of the confluence may be an important dispersion factor. A saturation index is proposed and practical implications are discussed.