Evaluation of transversal and longitudinal dispersion in a flow injection system by exploiting laser induced fluorescence: influence of flow-cell positioning

A fully mechanized set-up was built for the experimental determination of bi-dimensional dispersion with high spatial resolution (2400 μm²). Gravitational and wall effects in a single stream were evaluated by using time-based sampling and a micro-flow cell. Vertical upward and downward flows as well as horizontal flows were investigated. Ethylene glycol (MEG) and Rhodamine B in MEG were used as carrier and sample solutions, respectively. Longitudinal profiles were obtained by laser induced total fluorescence (LIF) at up to 19 transversal sites and combined to generate high-resolution bi-dimensional profiles. A two frontal maxima pattern was observed for all flows. The volumetric fraction of RB shape was highly stretched for downward flow and there was high asymmetry for horizontal flow. The sensitivity of three dispersion parameters was evaluated: maximum peak value, peak half-width at half-height, and peak area.Data modeling showed that the tanks-in-series was more sensitive to wall effects, had good adjustment with only one tank for upward and horizontal flow and needed two tanks for downward flow which was attributed to the latter having higher dispersion. A black box empirical modeling described better the gravitational effect and allowed to identify a parameter sensitive to upward and downward flow as well as hinting to two inner streams within the horizontal flow. It also pointed to a wall dispersion contribution of twice that of the liquid-liquid dispersion.     

Determination of lead in blood by hydrodynamic voltammetry in a flow injection system with wall-jet detector

Lead is one of the most widely distributed toxic heavy metals in the environment. It is a cumulative poison, affecting the brain and nervous system. The threshold between the normal lead level and the level where physiological effects become manifest is relatively narrow. It is therefore desirable to screen exposed populations in order to identify the danger in time. The lead concentration in the blood is a measure to the total amount of lead in the body. A fast, accurate and cheap method for the determination of lead in blood is therefore needed. The conventional method used to determine lead in blood is atomic absorption. Electrochemical methods like stripping voltammetry combine high analytical sensitivity with relatively low cost for the equipment; however, electrode preparation is critical for the success of an analysis, and highly skilled personnel are needed. We describe an automated electrochemical method, using flow injection analysis with a wall-jet detector. Lead is released from its binding site in the blood by ion exchange and quantified by stripping voltammetry with a mercury film electrode (MFE). The method allows for the detection of 0.05 ppm Pb(2+) with an accuracy of about 10%. Electrode poisoning by proteins from the blood can be effectively suppressed when a MFE modified with a Nafion(R)-membrane coating is used. Such modified electrodes can be activated in the solution without further treatment, and used for more than 100 analyses before they have to be replaced. A solid matrix MFE with a Nafion(R)-membrane and all necessary chemicals for mercury film formation and lead release has been developed. Such electrodes are discussed as disposable electrodes for a portable blood lead detector.     

Determination of nitrite by flow injection spectrophotometry using a home-made flow cell detector.

A simple, rapid, low cost and accurate determination using flow injection spectrophotometry with a home made flow cell detector, in combination with green LED and photodiode, had been developed for nitrite based on the Griess-Saltzman reaction, measuring the absorbance at 525 nm. The proposed system is able to monitor nitrite in freshwater samples at a throughput of 60 samples per hour with a relative standard deviation (%RSD) of less than 2.4% and a detection limit of <7 microg dm(-3). The calibration graph was linear between 0.010 - 1.0 mg dm(-3).     

Determination of cysteine in a pharmaceutical formulation by flow injection analysis with a chemiluminescence detector

A simple and convenient flow injection-chemiluminescence (FI-CL) method for the determination of cysteine is reported, based on a fast and strong CL in a basic luminol-cysteine-NaIO₄ solution. The linear range was 1.0×10⁻⁸ to 1.0×10⁻⁶ M with a detection limit (3s) of 5×10⁻⁹ M, which was 100 times more sensitive than previously reported CL methods. Singlet oxygen, hydroxyl radical and hydrogen peroxide were suggested to be produced in this reaction and were responsible for the CL of cysteine. This simple method has been successfully applied for the determination of cysteine in a pharmaceutical formulation.     

Digital radiation monitoring system with a personal computer

This paper describes a data processing system of monitors with a personal computer for a radioisotope laboratory. A monitoring system includes 6 area, 6 dust, and 3 gas monitors. Monitor signals through a multiplexer were fed to the personal computer which has analogue ports. Processed data were displayed on a color CRT, stored on a 5 inch mini floppy disk, and printed out as a report. We developed a data processing program with BASIC language. This system largely reduced manpower required to estimate radiation levels in the laboratory. The personal computer has sufficiently enough ability to construct a digital monitoring system for a small radioisotope laboratory.     

Determination of sulphur anions by flow injection with a molecular emission cavity detector

A molecular emission cavity detector is attached to a flow injection system for the determination of sulphide, sulphite and sulphate in the ranges 2–130, 3–150 and 5–250 ng S, respectively, in 3-μ1 samples. The sample throughput is abort 100 h^?1. Resolution of mixtures of these anions is also possible, based on the sequential appearance of their S₂ emission peaks. The use of a hydrogen peroxide carrier stream allows the determination of total sulphur.     

Determination of some phosphorus-containing compounds by flow injection with a molecular emission cavity detector

The use of molecular emission cavity analysis combined with a flow-injection system for the determination of nanogram amounts of some organophosphorus compounds and phosphorus oxo-anions is described. Sensitivit, calibration linearity, precision and detection limits were measured for each compound. Simultaneous determinations of some ternary mixtures of these compounds are also described. Sample throughput is 20 h^?1.     

Determination of some organophosphorus insecticides by flow injection with a molecular emission cavity detector

The determination is based on the measurement of HPO emission at 528 nm in a H₂/N₂ flame. Organophosphorus insecticides are extracted with hexane/dichloromethane at pH < 7, the solvent is evaporated, and the residue is dissolved in ethanol. Up to 2 μl of the solution is injected into the flow system. Mixtures of dicrotophos, dimethoate, malathion and parathion were determined.     

Peak width and reagent dispersion in flow injection analysis

Accurate equations are derived for relating peak width to injected concentration for single-line and merging-stream manifolds in which a well stirred mixing chamber is used to generate concentration gradients. The consequences of making an approximation to produce a linear relationship between peak width and the natural logarithm of the injected concentration are evaluated and shown to have little practical effect. The concept of reagent dispersion coefficient, D^R, is used in certain derivations and a relationship between this and the conventional dispersion coefficient is derived and investigated experimentally. The use of D^R to evaluate the likely performance of other flow-injection models is illustrated for the calculation of reagent-to-sample concentration ratios and the case of reversed f.i.a. (reagent injected into sample carrier stream). An extension of the usual peak-width method is in f.i.a. described; a low-dispersion coefficient manifold is used and the product concentration profile is monitored. The analytical information in the double peaks obtained is discussed and illustrated for the peak-width mode by the injection of copper(II) ions (1.6 × 10^?6?0.16 M) into a carrier stream of 10^?4 M EDTA. The single well stirred mixing chamber model is used as a basis for the evaluation of the results and is applied to discussion of other manifolds not containing a real mixing chamber, in particular for the calculation of peak base-widths.     

Control system of stockers in radioisotopes storage room using a personal computer

This paper describes a control system of stockers for radioactive source in storage room at laboratory for tracer experiment. The system is composed of a personal computer, a locker controller, three card readers, a monitor TV, and a video tape recorder (VTR). The personal computer controls other equipment with a registered user's number. When a user inserts an identification card into the card reader, the computer memorizes assigned gate number, the user's number and the time; it processes those data and prints out a document. The locker controller releases the electric key of user's locker which is designated by the computer. The VTR records the person entering into the storage room to identity if he uses his card. This system proved to effectively prevent intrusion into the storage room of an unregistered person or to use carelessly other user's source; in addition it can record precisely the stock of radioactive source.     

Oscillatory control of sample dispersion in a continuous flow system

A new strategy for the instrumental control of sample dispersion in continuous flow systems is presented. The method is based on shaking a loosely held straight reactor while the sample travels through the flow injection manifold. This external disturbance yields a sample transport more similar to the plug flow type because of the changes promoted on the flow pattern. Up to a three-fold increase in peak height, a comparable reduction in peak width and a more Gaussian peak profile are observed when the signals obtained with the shaken reactor are compared with those obtained with the same reactor but static. Improvements in the analytical performance as a function of different operational variables are shown for systems with or without a chemical reaction. Analytical implications and possible uses are discussed since this strategy allows the control of dispersion by simply selecting the frequency and amplitude of oscillation.     

Differential electrolytic potentiometry, a detector in flow injection analysis for oxidation-reduction reactions

For the first time, differential electropotentiometry (DEP) is coupled with the flow injection analysis (FIA) technique for detection of oxidation–reduction reactions, and is utilized for quantitative determination of vitamin C in pharmaceutical preparations using 1.0×10⁻³-M cerium(IV) in 0.50-M sulfuric acid as carrier. Two similar platinum electrodes were employed and polarized by a constant current. Optimization by the univariate method was carried out and the optimum conditions for current density, flow rate, sample size and concentration of sulfuric acid were 4 mA, 0.93 ml min⁻¹, 140 μl and 0.25 M, respectively. Vitamin C was determined in the concentration range 100–300 ppm with 0.9987 correlation coefficient and 1.9 standard deviation. The method was applied to the determination of vitamin C in pharmaceutical preparations and no excipient was found to pose any interference thus rendering the method suitable for determination of the drug in pharmaceutical preparations. The accuracy of the method was determined by comparison with the BP standard method.     

Determination of sulphite in wine by flow injection analysis with indirect electrochemical detection

A flow-injection method has been developed for the determination of total sulphite in wine samples. After hydrolysis of bound sulphite, sulphur dioxide is separated from the matrix by means of an in-line gas diffusion module. For detection indirect amperometry is used, with iodine as oxidizing reagent. The iodine can be generated in-line by merging and mixing iodate and iodide solutions, or alternatively, electrochemically from iodide. With the latter method the best results were obtained. The reproducibility of the peak heights is better than 2%. The linear range of the method can be regulated by adaptation of the current applied to generate the iodine reagent. Due to the low detection limits obtained (0.05 mg L^–1), wine samples can be strongly diluted before injection, which makes the sample pretreatment fast and simple. Good agreement has been found with the results obtained for the total sulphite concentration in different wine samples by titration.     

Differential electrolytic potentiometry, a detector in flow injection analysis for precipitaton reactions

The application of differential electrolytic potentiometry as a detection system in flow injection analysis for precipitation reactions is described. Different combinations of electrodes were investigated. The optimum conditions for the current density and the flow rate were elucidated. In the case of chloride, an Ag/AgCl-Pt pair was found to be successful. For iodide a combination of Ag-Pt electrodes was found to give good results. The relation between the concentration of analyte and the measured signal was found to be linear.     

Sequential flow injection determination of chlorine species using a triiodide-selective electrode detector.

A flow injection analysis (FIA) method for the determination of four residual chlorine species, namely combined available chlorine (CAC), free available chlorine (FAC), total available chlorine (TAC) and chlorite (ClO2-) was developed using a flow-through triiodide-selective electrode as a detector. An important strategy of speciation studies utilized the kinetic discrimination of reactions between the CAC and FAC with Fe2+, which was applied to the speciation of FAC, CAC and TAC. The speciation of available chlorine species and chlorite (an oxychlorine species) was achieved by using the same set-up, but using flow streams of different pH. The effects of the pH of the carrier stream, the flow rate and the sample volume were studied. The method exhibited linearity from 2.8 x 10(-6) to 2.8 x 10(-4) M active chlorine (expressed as OCl-) with a detection limit of 1.4 x 10(-6) M. The selectivity of the method was studied by examining the minimum pH for the oxidation of iodide by other oxidants, and also by assessing the potentiometric selectivity coefficients. The proposed method was successfully applied to the determination of chlorine species in tap water, and disinfecting formulations where good agreement occurred between the proposed and standard methods were found.     

Determination of ethanol by air-stream separation with flow injection and electrochemical detection at a nickel oxide electrode

Ethanol is separated by an air stream from a 1-ml sample with collection in 1 ml of 0.2 mol l^-1 sodium hydroxide. Measurement is by voltammetry at a tubular, catalytic nickel oxide electrode with 30-μl sample injected into a continuous flow stream. Relative standard deviation for repetitive measurements is 1.8% for synthetic samples. Serum samples extracted for 100 s and then measured yielded linear results for concentrations of 2 × 10^-4–5 × 10^-2 mol l^-1 ethanol (0.001–0.23%), with relative standard deviation of 2.0%. The time per determination was about 2 min.     

Automatic determination of insolubles in lubricating oils by flow injection analysis employing an LED-photometer detector

A flow injection system is presented for the determination of the insolubles content in used lubricating oil samples. The system is based on the injection of an aliquot of the sample in a stream of organic solvent where it is dispersed, and measurement of the scattered radiation (measured as apparent absorbance) in the visible range (λ = 640 nm). An LED-based photometer was used for this purpose. The whole system including sample injection and data acquisition was controlled by a personal computer. Calibration curves exhibited good linearity (h = 0.415 ± 0.016C + 0.00 ± 0.03, r² = 0.9995, confidence level of 95%) in the range up to 2.68% (insolubles in pentane). Detection and quantification limits were respectively 0.07% and 0.16% (w/w). The method was validated by analysis of 25 real samples by the proposed method and the FTIR method finding high correlation. Waste generation and reactive consumption is much less than in the official method (ASTM D-893). The proposed method employs 25 mL of kerosene per sample while the official method employs 200 mL of pentane.     

Sample preconcentration by continuous flow extraction with a flow injection atomic absorption detection system

The flow manifold described allows automatic extraction of metal ions in aqueous samples into 4-methyl-2-penthanone with ammonium pyrrolidinedithiocarbamate as an extracting agent. The organic extract is led into the loop of an injector situated in an integrated feed system of an atomic absorption spectrometer. No dispersion of the injected organic extract plug, 110 μl, occurs in the aqueous feed stream and the resulting signal from the spectrometer is a peak. An increase in sensitivity of 15–20 is achieved for copper, nickel, lead and zinc in comparison with direct aspiration of the aqueous samples. The sampling frequency is 40 h^?1 and the consumption of 4-methyl-2-pentanone is typically 0.3 ml min^?1. The detection limit for copper is about 1 μ l^?1.     

Pharmaceutical applications of a high-performance flow injection system

The flow-injection method is applied as a means of decreasing the work load associated with repetitive control determinations. The system described utilizes the design concepts used in post-column reaction with spectrophotometric detection in high-performance liquid chromatography. This equipment allows for minimal set-up and change-over time between samples and also provides the high accuracy and precision required in a testing laboratory. The system is applied to dissolution samples of hydrochlorothiazide and timolol maleate, and to combination tablets of hydrochlorothiazide, methyldopa and amiloride. Samples can be quantified at rates of up to 1200 h^?1.