Patrícia B. Martelli

Multicommutation in flow analysis. Part 2. Binary sampling for spectrophotometric determination of nickel, iron and chromium in steel alloys

Multicommutation flow systems for the spectrophotometric determination of nickel, iron and chromium in acid solutions of steel alloys employing a binary sampling approach are described. Dimethylglyoxime, salicylic acid and diphenylcarbazide were used as chromogenic reagents for nickel, iron and chromium, respectively. The flow networks were designed with active devices in order to provide facilities to handle reagent solutions employing a single pumping channel to propel all reagent solutions. Interference caused by iron on the nickel determination was suppressed on-line by using triethanolamine as masking agent. For sample concentrations ranging from 5 to 50 mg/l, the proposed method is characterized by a throughput of 60 determinations per hour and a relative standard deviation of about 1%. Consumptions of potassium peroxodisulphate and dimethylglyoxime solutions were 56 and 78 μl per determination, respectively. Iron and chromium were determined sequentially, with a reagent consumption of 80 μl and 26 μl, respectively. For sample concentrations ranging from 25 to 200 mg/l Fe and 20 to 60 mg/l Cr a throughput of 130 determinations per hour was achieved. Results compared well with those obtained by inductively coupled argon plasma atomic emission spectrometry.

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

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.

Automatic potentiometric titration in monosegmented flow system exploiting binary search

An automatic monosegmented flow system (MSFA) based on the binary search concept to perform potentiometric titration is proposed. A tubular hydrogen ion-selective electrode without inner reference solution, consisting of a conducting epoxy cylinder machined with an axial hole coated with tridodecylamine (TDDA) was employed as a sensor. The titration procedure was implemented by exploiting the binary search approach, after each analytical cycle data were evaluated to decide the variation of the titrant volumetric fraction to be inserted for the next cycle. The flow network comprised three-way solenoid valves controlled by a microcomputer running software in QUICKBASIC 4.5. Combination of binary search and MSFA resulted in an automatic system providing possibilities to perform flow titrations without any operator assistance. The main features of the proposed system were verified by titrating hydrochloric and acetic acid solutions and the feasibility of the approach was ascertained by analyzing vinegar, coke, lemon soda, isotonic, industrial and natural orange juice samples. Results were in agreement with those obtained with a conventional potentiometric titration, and no significant difference at 95% confidence level was observed. A 1% standard deviation (n=9) in results was also observed.

Automatic multicommutation flow system for wide range spectrophotometric calcium determination

An automatic flow system based on multicommutation concept is proposed to widen the linear concentration range for spectrophotometric calcium determination. The flow network was build up with three way solenoid valves to permit implementation of different sample processing conditions in order to achieve limited, medium and large dispersion degree without modification of the manifold configuration. Dilutions were carried out by changing both sampled volume and the analytical path length or applying zone sampling approach. The software was developed to control all steps of sample processing and to allow changing the manifold configuration in order to obtain suitable sample dilution. This condition was attained with up to 3 trials. A linear response from 0.250 to 1000 mg l ˇ1 , and a detection limit of 7 m gl ˇ 1 (99.7% confidence level) were achieved. The relative standard deviation was 0.83% (na10) or better. The sampling rate was ca. 60 h ˇ1 and 0.27 mg of the chromogenic reagent (3,3 0 -bis[N,N-bis(carboxymethyl)aminomethyl]-o-cresolphthalein) was consumed per determination. The procedure was applied to calcium determination in waters, plant materials, milk, antacid tablets, fertilizers and calcareous rocks. The results were in agreement with certified values or with those obtained with flame atomic absorption spectrophotometry at a 95% confidence level. # 1998 Elsevier Science B.V.

A multicommutation-based flow system for multi-element analysis in pharmaceutical preparations.

A flow system exploiting multicommutation and multidetection is proposed for sequential determinations in pharmaceutical preparations. The feasibilities were demonstrated by the determination of zinc, iron, copper, calcium and magnesium without changing the flow set-up. The gravitational flow of the solutions was exploited for addition of different chromogenic reagents and sample aliquots, thus avoiding the use of a propulsion unit. Transient signals at different wavelengths were measured simultaneously employing a fiber-optic multichannel spectrophotometer. Coefficients of variation of 1.0, 1.5, 1.4, 2.5 and 2.0% were obtained for iron, zinc, copper, calcium and magnesium, respectively. The mean sampling rate for the five species was 60 determinations per hour. In comparison to continuous reagent addition systems, the consumption was up to 160-fold lower. Results for pharmaceutical preparations agreed with those obtained by Flame atomic absorption spectrophotometry (FAAS) at the 95% confidence level.

The use of ion exchange resin for reagent immobilization and concentration in flow systems: determination of nickel in steel alloys and iron speciation in waters

Neste trabalho, foi investigado o emprego de persulfato de potassio imobilizado em resina anionica AG1-X8, como agente oxidante, na determinacao espectrofotometrica de niquel em ligas metalicas, e na especiacao de Fe (III)/Fe (II) em aguas de rio, utilizando-se sistema de analise em fluxo. O diagrama de fluxo foi desenvolvido para que a oxidacao da especie de interesse e o recondicionamento da resina pudessem ser efetuados em linha. A determinacao de niquel baseou-se no metodo da dimetilglioxima, apresentando uma frequencia analitica de 80 determinacoes por hora. Com este sistema, obteve-se um baixo consumo de reagentes, 10 mg (200 mL) de persulfato de potassio e 8,0 mg (133 mL) de dimetilglioxima por determinacao. Para uma solucao de amostra contendo 21,5% (m/m) de Ni, encontrou-se um desvio padrao relativo de 1% (n = 10). Aplicando-se o teste-t pareado, obteve-se uma concordância com 95% de confianca entre os resultados do sistema proposto e os obtidos com espectrofotometria de absorcao atomica com chama. Na determinacao de Fe (III), utilizou-se tiocianato de potassio como reagente cromogenico, alcancando-se uma frequencia analitica de 60 determinacoes por hora. O metodo apresentou um consumo de reagentes de 3,5 mg (70 mL) persulfato de potassio e de 5,4 mg (270 mL) de tiocianato de potassio por determinacao. O desvio padrao relativo calculado para uma solucao de amostra contendo 3,17 mg L-1 de Fe (III) foi de 1,5% (n = 10). Os resultados empregando-se o metodo proposto para Fe (II) foram comparados com aqueles obtidos com o metodo espectrofotometrico, baseado na reacao com 1,10 - fenantrolina, apresentando uma concordância com nivel de confianca de 95%.

Detecting and circumventing sources of inaccuracy in flow analysis

Mechanized procedures for chemical analyses are susceptible to systematic errors usually difficult to trace. This holds also for flow analysis that although accepted worldwide may sometimes yield inaccurate results. The present work focused on the development of a simple system able to detect and circumvent sources of inaccuracy. The versatility inherent in the multicommuted flow analyzers was exploited for real-time characterization of the source of inaccuracy and overcoming the problem for every assayed sample. Specific situations where the analytical results are more susceptible to inaccuracy are emphasized.

A flow system with a conventional spectrophotometer for the chemiluminescent determination of lactic acid in yoghurt

A flow-based analytical procedure for lactate determination in yoghurt by chemiluminescence using a conventional UV-Vis spectrophotometer as detector is described. The radiation source was switched off. The flow cell was machined in acrylic with a 1 mm path length and a 80 mm(2) surface and was positioned 2 mm distal of the photodetector (100 mm(2) sensible area) in order to improve detection. The flow network computer-controlled comprised a set of three-way solenoid valves assembled to implement the multicommutation approach. The chemiluminescence was obtained by using the reaction of luminol with hydrogen peroxide, catalyzed by hexacyanoferrate (III) after enzymatic reaction with lactate. The lactate oxidase enzyme was immobilized on porous silica beads (glass aminopropyl, SIGMA). The signal generated by the spectrophotometer reaction was read by the microcomputer and stored as a function of time for further treatment. Immobilization condition, enzyme concentration, temperature, pH, stability of the enzymatic reactor, and flow rates were investigated. The feasibility of the system was ascertained by analyzing a set of yoghurt samples. Results were in agreement with those obtained by a conventional method (Boehringer UV-Kit), and no significant difference at 95% confidence level was observed. A linear response within 10-125 mg l(-1)l-lactate, a 1.9% standard deviation (n=10), and an analytical throughput of 55 determinations per hour were achieved.

Trial measurements in flow analysis

Trial measurements allow analytical results to be obtained by matching the sample volume with the dynamic range of the detector. In flow analysis, implementation of this approach can be accomplished by carrying out sequential injections or using concentration gradients. Trial measurements are performed using an increasing concentration sequence and the results are compared with a previously set threshold value. Once this value is surpassed, the remainder of the sample by-passes the detector and goes to waste. The potential and limitations in unsegmented and monosegmented flow systems are discussed. Improved systems for the automated determination of manganese in rocks by atomic absorption spectrometry are proposed. The typical measurement precision of flow injection was maintained and no baseline drift was observed. Results were in agreement with those obtained by inductively coupled plasma atomic emission spectrometry.

Simultaneous in-line concentration for spectrophotometric determination of cations and anions

A flow system is proposed for simultaneous in-line concentration of cations and anions. A sliding-bar commutator was employed to insert an anion and a cation exchange column into a flowing sample stream for serial retention of the analytes. In the injector alternative position, different solutions flowed through the columns for parallel elution of the species in different analytical paths. Three-way solenoid valves allowed the intermittent reagent introduction in the sample zones. Signals were measured by employing two flow-through LED-based detectors. The simultaneous retention of the sample zones in coiled reactors can be also performed to increase the residence time and the analyte conversion rate. The analytical potentiality was demonstrated by the in-line concentration of ammonium and phosphate followed by spectrophotometric detection. For a 90 s loading time, the sampling rate was estimated as 40 determinations per hour, which is three-fold higher than the obtained without performing the tasks simultaneously. Enrichment factors of 8.0 and 18 were estimated for phosphate and ammonium, respectively, yielding detection limits of 1 mg L-1 PO43- and 1 mg L-1 NH4+ (99.7% confidence level). The reagent consumption was lower than 2 mg per determination. Results for freshwater samples agreed with the obtained by reference APHA procedures at the 95% confidence level.