Flow Injection Manifold Research Articles

Flow injection biamperometric determination of captopril

An automated method to determine captopril (1-[(2 S)-3-mercapto-2-methylpropionyl]- l-proline) is proposed. A flow injection manifold based on the indirect biamperometric detection of the captopril by using Fe(III)/Fe(II) as an indicating redox system and a Z-shaped flow-cell configuration, was developed. The calibration curve is linear over the range 0.03–3.6 μg ml −1 of captopril. The relative standard deviation for the determination of 0.76 μg ml −1 of captopril is 0.97% ( n=12) and the sample throughput is 69 h −1. This method was applied to the determination of captopril to commercially available pharmaceutical preparations.

Design and fabrication of a low-cost flow-through cell for the determination of acetaminophen in pharmaceutical formulations by flow injection cyclic voltammetry

A low-cost electrochemical flow-through cell is designed and fabricated to use in conjunction with a flow injection (FI) system. This detector cell used a centrosymmetric radial flow thin-layer geometry with a stainless steel auxiliary electrode and a reference electrode (Ag/AgCl) without a salt bridge. The 5H pencil lead electrode used as a working electrode in the home-made cell is an extremely inexpensive electrode which performs as well as the expensive commercial glassy carbon electrode. Optimum conditions for determining acetaminophen using the proposed FI manifold was investigated. Appropriate volume of sample and/or standard solution containing acetaminophen in pH 2.2 Mcllvaine buffer solution was injected into the proposed FI system and mixed with the flowing stream of supporting electrolyte (pH 2.2 Mcllvaine buffer solution) at an optimum flow rate of 1 mlmin(-1). The cyclic voltammograms were recorded over the potential range from -0.5 to +2.0 V with a scan rate of 40 mVs(-1). Linear calibration curve over the range of 0.1-5 mM acetaminophen was established with the regression equation Y=3.68X+1.0157 (r(2)=0.9964). The recommended method has been applied to the determination of acetaminophen in 8 commercial pharmaceutical preparations. The percentage recoveries of the spiked acetaminophen in four tablet samples were ranging from 103 to 112 with the relative standard deviation in the range of 0.1-1.3%.

Continuous ultrasound-assisted extraction of iron from mussel samples coupled to a flow injection-atomic spectrometric system

Iron was extracted on-line from solid mussel samples by a simple continuous ultrasound-assisted extraction system (CUES) connected to a flow injection manifold, followed by on-line flame atomic absorption spectrometric determination. An experimental design was used for the optimisation of the CUES. The method allowed a total sampling frequency of 11 samples per hour, with a relative standard deviation for the complete procedure of 0.5% [for a sample containing 217.3 µg g−1 iron (dry mass)]. The detection limit was 0.7 µg g−1 (dry mass) for a sample amount of 30 mg. Accurate results were obtained by measuring TORT-1 certified reference material. The results demonstrated that this method was simple, quantitative and sufficiently sensitive for iron determination in mussel samples.

Static–dynamic pressurized hot water extraction coupled to on-line filtration–solid-phase extraction–high-performance liquid chromatography–post-column derivatization–fluorescence detection for the analysis of N-methylcarbamates in foods

A combination of static–dynamic modes of pressurized hot water extraction has been used for the extraction of N-methylcarbamates (namely, oxamyl, dioxacarb, metholcarb, carbofuran and carbaryl) from different fruits and vegetables. The selection of water as leaching agent provides a clean approach which avoids the use of organic solvents. A flow injection manifold coupled to the extractor has allowed the automation of the subsequent steps (namely, filtration, preconcentration, individual chromatographic separation, post-column derivatization and fluorescence detection) involved in the analytical process. Good recoveries, ranging from 80 to 104%, and precision, expressed as relative standard deviation, between 3.0 and 8.4% have been achieved by the proposed method.

A simple simultaneous flow injection method based on phosphomolybdenum chemistry for nitrate and nitrite determinations in water and fish samples

A direct spectrophotometric flow injection method for the simultaneous determination of nitrite and nitrate has been developed. The method is based on the oxidation of a phosphomolybdenum blue complex by the addition of nitrite and the decrease in absorbance of the blue complex is monitored at 820 nm. The injected sample is split into two segments. One of the streams was directly reacted with the above reagent and detected as nitrite. The other stream was passed through a copperised cadmium reductor column where reduction of nitrate to nitrite occurs, and the sample was then mixed with the reagent and passed through the cell of the spectrophotometer to be detected as nitrite plus nitrate. The conditions for the flow injection manifold parameters were optimised by experimental design and the concentration of nitrite and nitrate was determined in the linear range from 0.05 to 1.15 μg ml −1 nitrite and 0.06 to 1.6 μg ml −1 nitrate with a detection limit of 0.01 μg ml −1 for nitrite and 0.025 μg ml −1 for nitrate. The method is suitable for the simultaneous determination of nitrite and nitrate in fish and water samples with a sampling rate of 25±2 sample per hour.

Coupling continuous subcritical water extraction, filtration, preconcentration, chromatographic separation and UV detection for the determination of chlorophenoxy acid herbicides in soils

Subcritical water extraction has been coupled with filtration, preconcentration and chromatographic analysis for the determination of acid herbicides in different types of soil. Two experimental designs were used for the optimization of the leaching step. The use of water as extractant in the continuous mode at a flow-rate of 1 ml/min and 85 °C was sufficient for quantitative extraction of the analytes. A static extraction time was unnecessary for reducing the extraction time to 1 h. A minicolumn containing C 18-Hydra as sorbent proved an excellent material for the quantitative preconcentration of the herbicides prior to individual chromatographic separation. A flow-injection manifold was used as interface for coupling the four steps, thus allowing automation of the whole analytical process. Recoveries of the target analytes ranged between 94.2 and 113.1%, and repeatabilities, expressed as relative standard deviations, were between 0.61 and 6.83%.

Rapid spectrophotometric determination of fosfestrol following on-line hydrolysis by alkaline phosphatase using flow injection and chasing zones

A new flow injection (FI) method is reported for the spectrophotometric determination of fosfestrol (diethyl-stilbestrol (DES) diphosphate) in pharmaceutical formulations. The proposed method is based on the on-line hydrolysis of the analyte by alkaline phosphatase (Al-Pase) using a “chasing zones” FI manifold. The orthophosphate ions, thus, generated are determined spectrophotometrically ( λ max=690 nm) using the molybdenum blue approach. The chemical and FI variables affecting the enzymatic reaction were investigated. The proposed method is very precise ( s r=1.1% at 1×10 −4 mol l −1 fosfestrol, n=12), fast (allowing up to 40 samples h −1 to be analyzed) and has a determination range of 2×10 −5 to 2×10 −4 mol l −1, with a satisfactory 3 σ detection limit of 5×10 −6 mol l −1. The method was shown to provide accurate determinations of the fosfestrol concentration in a pharmaceutical formulation, giving relative errors, e r, of +0.6 and −0.5% compared to the value stated by the supplier (Asta Medica Inc.) and the concentration derived using a method recommended by the United States Pharmacopoeia XXI, respectively. In addition, the average recoveries of known amounts of the analyte ranged between 99.2 and 101.2%.

Flow injection determination of copper in mussels by flame atomic absorption spectrometry after on-line continuous ultrasound-assisted extraction

Copper was extracted on-line from solid mussel samples by a simple and rapid continuous ultrasound-assisted extraction system (CUES). The CUES is connected to a flow injection manifold, which permits the on-line flame atomic absorption spectrometric determination of copper. The manifold is simple and the copper signal was obtained for a volume of 250 μl of acid leachate injected into an ultrapure water carrier stream. An experimental design was used for the optimization of the continuous leaching procedure. Compared to off-line ultrasonic-assisted extraction methods, sonication time is reduced by factors of 6–12, the leaching takes place at room temperature (20 °C), and the analysis time is reduced because centrifugation was not necessary to separate the liquid phase. The method allowed a total sampling frequency of 11 samples h −1, with a relative standard deviation for the complete procedure of 2.7% (for a sample containing 2.0 μg g −1 copper (wet mass, n=11). The limit of detection was 0.06 μg g −1 (wet mass) for 30 mg of sample. The analytical procedure was verified for a reference standard material (TORT-1). The analytical procedure was applied to mussel samples from Galicia (Spain).

Flow monitoring of NADH consumption in bioassays based on packed-bed reactors bearing NAD +-dependent dehydrogenases : Determination of acetaldehyde using alcohol dehydrogenase

An enzymatic method for the individual or simultaneous determination of pyruvic acid and acetaldehyde is described. Alcohol dehydrogenase (ADH) was immobilized onto aminopropyl-modified controlled pore glass, which was then used for the construction of packed-bed (PB) reactors. ADH catalyses the reduction of acetaldehyde to ethanol, in the presence of the coenzyme NADH, which is oxidized to NAD +. Photometric measurements in a fully automated flow injection (FI) manifold are used to monitor the decrease of NADH absorbance at 340 nm. The possibility of pyruvate measurements, by combining the above mentioned system with soluble pyruvate decarboxylase (PyDC) is also demonstrated. PyDC catalyses the decarboxylation of pyruvate to acetaldehyde. Analytical parameters such as the buffering system, working pH, flow rate, sample size, and NADH concentration were studied. The interference of various compounds present in real samples was also investigated. Linear calibration graphs over the ranges 0.08–1.25 and 0.04–0.4 mM acetaldehyde were constructed in the presence of 50 mM succinate pH 7.5 and 50 mM phosphate pH 7.0 buffering systems, respectively. A linear calibration graph over the range 0.08–1.25 mM pyruvate was also constructed in the presence of 50 mM succinate pH 7.5 buffer solution. The reactors remain active for more than 6 months under specified storage conditions. The maximal sample throughput is 30 h −1 and the R.S.D. of the method is 0.9% for 0.2 mM acetaldehyde ( n=6). The suitability of the proposed method for real samples was tested by recovery studies.

On-line ion-exchange matrix separation and inductively coupled plasma mass spectrometric determination of trace impurities in high-purity aluminium

The feasibility of using an anion-exchange resin for on-line separation of trace amounts of Fe, Cu, Mn, Mg, Cr, Ni, Zn, Co, Pb, and Cd from pure aluminium was investigated. A test portion of 50 mg of aluminium drillings was dissolved with 6 ml of a buffer solution, 2.64% m/v NaOH–0.91% m/v KCl (pH = 13.4 ± 0.1). An aliquot of 500 µl of the solution obtained was passed through a micro-column, included in a flow injection (FI) manifold, packed with a 2 + 1 mixture of Metalfix Chelamine and Hyphan Cellulose (the micro-column volume was about 150 µl). The retained trace elements were eluted in the back-direction with 200 µl of a 3 + 1 mixture of 1 mol l−1 HCl–1 mol l−1 HNO3 and then determined using flow injection inductively coupled plasma mass spectrometry. The operating parameters and operational scheme of the manifold of the FI on-line ion-exchange assembly are given. The accuracy of the proposed method was tested by analysing a pure aluminium standard sample (Aluminium Pechiney, CODE 566). Recoveries from 50 mg test portions of a high-purity aluminium (99.999%) spiked with 2.5 and 25 ng each of the studied analytes were close to 100%, and the relative standard deviations ranged from 1 to 3%. Determination limits (10s) were in the ng g−1 range.

Reversed flow-injection manifold for the spectrophotometric determination of captopril based on its inhibitory effect on the Co(II)–2,2′-dipyridyl-2-pyridylhydrazone complex formation

The present work reports a new, simple and rapid reversed flow-injection (r-FI) method for the accurate and precise spectrophotometric determination of captopril (CPL) in pharmaceutical formulations. The method is based on the inhibitory effect of CPL on the complex formation of Co(II) with 2,2′-dipyridyl-2-pyridylhydrazone (DPPH). The chemical and FI variables were studied and optimized. The calibration graph was linear in the range 0–250 mg l −1 CPL, at a sampling rate of 60 injections per hour. The method was found to be very precise [ s r=0.8% at 100.0 mg l −1 CPL ( n=12)] and the 3 σ detection limit ( c L=2.5 mg l −1) was quite satisfactory. Its application to commercially available pharmaceuticals produced excellent results, with a mean relative error of e r<1.0%.

Implementation of suitable flow injection/sequential injection-sample separation/preconcentration schemes for determination of trace metal concentrations using detection by electrothermal atomic absorption spectrometry and inductively coupled plasma mass spectrometry

Various preconditioning procedures encomprising appropriate separation/preconcentration schemes in order to obtain optimal sensitivity and selectivity characteristics when using electrothermal atomic absorption spectrometry (ETAAS) and inductively coupled plasma mass spectrometry (ICPMS) are presented and discussed. Demonstrating that such pretreatments advantageously are to be performed in flow injection (FI) or sequential injection (SI) manifolds, where all appropriate unit operations can be effected under enclosed and strictly controlled conditions, procedures based on ion-exchange, precipitation/coprecipitation, adsorption, liquid–liquid extraction, extraction/back-extraction and hydride generation are reviewed. Emphasising the use of SI-systems, special attention is paid to the use of the newly introduced approach termed the lab-on-valve (LOV), where all necessary manipulations required prior to detection are effected in a microconduit placed on top of an SI selection valve.

Microwave-assisted water extraction of acid herbicides from soils coupled to continuous filtration, pre-concentration, chromatographic separation and UV detection

A microwave-assisted extraction procedure using water as extractant has been performed for the extraction of acid herbicides (namely, bentazone, 2,4-D, trichlopyr, 2,4,5-T and 2,4,5-Tp) from different types of soil. Two experimental designs were used for the optimization of the leaching step. The selection of water as extractant provided a clean approach by avoiding the use of organic solvents. The use of water also enhanced the extraction efficiency due to the high interaction of the microwave irradiation with polar solvents. The time required for total removal of the target compounds was 35 min. The recoveries yield were from 87.64 to 106.14% with a repeatability (expressed as relative standard deviation) ranging between 1.34 and 9.24%. A within-laboratory reproducibility, also expressed as relative standard deviation, varied from 3.97 to 10.41%. A flow-injection manifold allowed automation of the whole process by hyphenating the steps subsequent to extraction (namely, filtration, preconcentration, chromatographic separation and UV detection) for the determination of the analytes.

Solvent extraction flow-injection manifold for the simultaneous spectrophotometric determination of free cyanide and thiocyanate ions based upon on-line masking of cyanides by formaldehyde

This study reports a sensitive solvent extraction flow-injection (FI) method for the simultaneous spectrophotometric determination of free cyanide and thiocyanate in human saliva and pralidoxime solutions. Cyanide and thiocyanate form colored ( λ max=540 nm) ternary complexes with copper and 2,2′-dipyridyl-2-quinolylhydrazone (DPQH) that are extractable into chloroform. The determination of thiocyanates in the presence of cyanides is accomplished after on-line masking of the latter with formaldehyde through a binary inlet static mixer (BISM). Total thiocyanates and cyanides are determined in a second run, without the use of the masking agent. The proposed method allows the determination of the analytes in the range of 0–4 mg l −1 thiocyanates and 0–3 mg l −1 cyanides, with the 3 σ detection limits being 0.007 and 0.004 mg l −1, respectively. The precision of the method ( s r<1.0% at 1 mg l −1 CN − or SCN −, n=12 in both cases) and the sampling rates were quite satisfactory (60 injections per hour). The method was applied to the analysis of human saliva and pralidoxime solutions and gave recoveries in the range of 98.0–102.2% for both analytes whereas the mean relative error was e r=1.7%.

Development of the continuously variable volume reactor for flow injection analysis: Part 1. Design, capabilities and testing

A new apparatus for mixing sample and reagent in flow injection analysis (FIA) is described. The continuously variable volume reactor (CVVR) replaces the conventional mixing coil in a flow injection (FI) manifold to provide mixing and dilution. A linear actuator motor allows control of the chamber volume via LabVIEW software. The chamber volume can be incremented in steps of 1μl over the range 68–1704μl. In addition, the chamber has an integral variable-speed stirring unit that is also under computer control. Experiments were performed to evaluate the dispersion characteristics of this new device, evaluate the volume reproducibility, and understand the mixing characteristics. Use of the chamber is shown in the determination of iron(II) in pond water, and in NIST SRM 1643d with excellent results and a detection limit of 3.7μg/l iron(II). Advantages of the CVVR and future research activities using the device are discussed.

Integrated luminometer for the determination of trace metals in seawater using fluorescence, phosphorescence and chemiluminescence detection

The paper describes an integrated luminometer able to perform fluorescence (FL), room temperature phosphorescence (RTP) and chemiluminescence (CL) measurements on seawater samples. The technical details of the instrumentation are presented together with flow injection (FI) manifolds for the determination of cadmium and zinc (by FL), lead (RTP) and cobalt (CL). The analytical figures of merit are given for each manifold and results are presented for the determination of the four trace metals in seawater reference materials (NASS-5, SLEW-2) and Scheldt estuarine water samples.

Continuous ultrasound-assisted extraction of hexavalent chromium from soil with or without on-line preconcentration prior to photometric monitoring.

A continuous ultrasound-assisted extractor was coupled to a photometric detector in order to obtain a fully automated approach for the determination of CrVI in soil. The use of a flow injection (FI) manifold as interface between the extractor and the photometric detector allowed the monitoring of CrVI after extraction in a continuous manner. The coloured complex formed between 1,5-diphenylcarbazide (DPC) and CrVI was used as recommended in EPA method 7196A because it is one of the most sensitive and selective reactions for CrVI determination. A preconcentration minicolumn packed with a strong anion-exchange resin was placed between the extractor and the detector, providing a more sensitive method. The linear dynamic ranges were 1-10 and 0.25-7.5 mg l-1 for the methods without (method A) and with preconcentration (method B), respectively. The limits of detection were 4.52 ng for method A and 1.23 ng for method B. Both methods were applied to a natural contaminated soil and the results obtained agreed well with those obtained by the reference EPA method 3060A. The influence of different amounts of CrIII in the samples was also studied and the results showed that the proposed methods did not disturb the original species distribution.

Flow injection screening and semiquantitative determination of polycyclic aromatic hydrocarbons in water by laser induced spectrofluorimetry — chemometrics

A continuous method is proposed here for the screening and subsequent semiquantitative determination, if required, of polycyclic aromatic hydrocarbons (PAHs) in water. The experimental arrangement is based on the use of a flow injection (FI) manifold that enables preconcentration of the analytes on a C 18 minicolumn before their automatic introduction into a flow-cell for detection. An Nd:YAG laser at 355 nm is used as excitation source and fluorescence emission is detected by a charge coupled device. Multivariate information is analysed by soft independent modelling of class analogy (SIMCA) and partial least squares regression type 1 (PLS-1), in order to obtain qualitative and semiquantitative information.

Immunochromatographic flow-injection method for detection of human serum immunoglobulin G antibodies to Helicobacter pylori

This study reports on the development of an immunochromatographic flow-injection (FI) method for the quantitation of human serum IgG antibodies to Helicobacter pylori. Patients’ sera were injected into the carrier stream of a FI manifold incorporating an H. pylori immuno-adsorbent reactor. The immuno-adsorbent was prepared by covalently linking H. pylori antigens to periodate oxidized Sepharose CL-4B. Any H. pylori antibody present in the serum is bound to the immuno-sorbent. The bound antibody was quantified by injecting into the carrier stream anti-human IgG peroxidase conjugate followed by TMB/H2O2 as the enzyme substrate. The FI system was optimized with respect to antigen loading (1.0mg/ml gel), bioreactor length (3.0cm), enzyme conjugate (0.2mg/ml) and sample loop size (250μl). One hundred clinical samples were analyzed for their H. pylori antibody concentrations and the results evaluated with respect to their receiver–operator characteristics (ROC). When a cut-off absorbance of between 0.7 and 0.75 was used for positive and negative samples a specificity (>95%), a sensitivity (>90%)and an overall accuracy (>94%) were obtained.

Water soxhlet extraction assisted by focused microwaves: a clean approach.

A prototype of a Soxhlet extractor that enables the use of water as extractant and application of focused microwaves on the cartridge zone has been designed and checked. The approach consists of a single unit in which the shortening of the distillation glassware allows reception of the water vapor in a refrigerant connected to the top of the sample cartridge vessel, its condensation, and dropping on the solid sample. When the water into the cartridge has reached a given level, irradiation with focused microwaves during an optimized time accelerates the leaching of the target analytes. Then, a valve is actuated and the extract is driven to the distillation flask where the leached species are concentrated as new vapor is formed and sent to the refrigerant and then to the cartridge. This new hybrid discontinuous-continuous approach has been used for leaching acid herbicides from different types of soil. The time required for total removal of the target compounds was 48 min. A flow injection manifold made posible the integration of the subsequent steps (preconcentration, chromatographic separation, UV detection) for the determination of the analytes, the recoveries of which range from 105.24 to 96.44%.