Reaction Coil Length Research Articles

Cold vapour generation and on-line trapping of cadmium species on quartz surface prior to detection by atomic absorption spectrometry

A quartz trap for on-line preconcentration of Cd species was designed. The cold vapour generation technique was used for the generation of Cd species. The trapping medium was formed by external heating of the inlet arm of a quartz T-tube. The generated analyte species were trapped on a quartz surface heated to the collection temperature, 350 °C, and the collected species were revolatilized when the trap was heated further to revolatilization temperature, 1000 °C, and hydrogen gas was introduced in the trapping medium. Two-level fractional factorial design and central composite design were used to optimize generation conditions in the flow injection mode. The results of the fractional factorial design demonstrated that the factors and their interactions were statistically significant. Three factors, length of reaction coil, carrier HCl concentration and NaBH4 concentration, were considered to be the most significant parameters in the optimization and their optimum values were found to be 30 cm, 0.3 M and 3% (m/v), respectively. Sea-water (BCR), tomato leaves (NIST 1573a) and oyster tissue (NIST 1566b) standard reference materials were analyzed to assess the accuracy of the proposed method. For a collection period of 3.0 min, i.e., 6.0 ml sample volume, the 3σ limit of detection was 1.8 pg ml−1; the enhancement factor for LOD was found to be 90 as compared with FI-HGAAS. The sample throughput rate was 12 h−1.

Catalytic Detection of Thiourea and Its Derivatives in HPLC Postcolumn Derivatization

A sensitive method for chromatographic determination of thiourea and its N‐derivatives has been developed. Thiourea, methyl‐, ethyl‐, dimethyl‐, diethyl‐, phenyl‐, allyl‐, acethylthiourea were eluted from high‐performance liquid chromatography C18‐bonded silica column by mobile phase containing 0.02 mol L−1 phosphate buffer solution (pH 5.0) and acetonitrile 20% (v/v). These compounds in the effluent were monitored by using photometric measurement of decreasing absorbance in postcolumn bromopyrogallol redhydrogen peroxide reaction catalyzed by each of thioureas. The optimum conditions for the catalytic reaction in static and flow modes were chosen. The conditions for the catalytic postcolumn reaction in the presence of thioureas were established by varying pH value, concentrations of bromopyrogallol red and hydrogen peroxide in the postcolumn‐reaction mixture and the reaction coil length. Careful choice of the reactor length and optimization of parameters for flow conditions was demonstrated to be very important for maximum analytical signal. It was shown, that the use of chromatographic separation with the catalytic detection allows increasing selectivity and sensitivity of determination of thiourea and its derivatives.

Sequential injection spectrophotometric determination of etilefrine hydrochloride

A simple, fast, economical and automated sequential injection spectrophotometric method for the determination of etilefrine hydrochloride is developed. The method is based on the condensation reaction of etilefrine hydrochloride with 4-aminoantipyrine in the presence of alkaline potassium hexacyanoferrate and the absorbance of the colored product measured at 503 nm. Aspiration order, flow rate, reaction coil diameter, reaction coil length, concentration of 4-aminoantipyrine and potassium ferricyanide, as well as aspiration volume of reagents and sample has been optimized. Using these optimized parameters, a linear relationship between the relative peak height and concentration was obtained in the range 1–20 mg l –1. The detection limit (as 3 σ value) was 0.1 mg l –1 and precision was 2.7% and 1.5% at 1 and 2 mg l –1, respectively. This method is superior over previously reported ones in terms of linear range, short analysis time, high sample throughput, excellent reagent economy and minimum waste generation.

Determination of Arsenic in Food Samples by Hydride Generation ? Atomic Absorption Spectrometry

A method for the determination of trace amounts of arsenic in food samples using flow injection analysis and atomic absorption spectrometry with hydride generation (FI-HG AAS) was developed. The parameters of the flow injection system and the hydride generation were optimized with respect to reagent concentrations, atomization temperature, injection volume, reaction coil length and carrier flow rate. The limits of detection and quantification were 0.34 µg L−1 and 1.2 µg L−:1, respectively, and the analytical curve is linear up to 30.0 µg L−1 arsenic. The relative standard deviation for 12 replicates varies between 5% for 4.0 µg L−1 As and 1.8% for 30.0 µg L−1 As, with an injection frequency of up to 135 h−1. Interferences from Ni(II), Cu(II), Fe(III), Cr(III), Mo(II), Bi(III), Se(IV), Se(VI), Sb(III) and Sb(V) could be masked with a mixture of ascorbic acid-KI in a 5.0 mol L−1 HCl solution. The accuracy of the proposed method was evaluated by using certified reference materials of biological samples, and the method was used to determine the content of arsenic in fish and coffee beans.

A very sensitive flow system for the direct determination of copper in natural waters based on spectrophotometric detection

A very sensitive flow injection method with spectrophotometric detection has been developed for the on-line determination of copper in natural waters. The method exhibits a limit of detection three times lower than the most sensitive direct spectrophotometric method previously described and then allows the direct and simple in situ determination of copper in most natural waters. The method was based on the measurement of the absorbance of the coloured complex formed by copper with the chromogenic reagent di-2-pyridyl ketone benzoylhydrazone (dPKBH) in an alkaline medium. This complex presents stoichiometry 1:2 (Cu:dPKBH), and exhibits maximum absorbance at 370 nm. The manifold used was very simple, and consisted of two channels. The first one contained the sample while the second one contained the colorimetric reagent (3.3×10 −4 M dPKBH in 10% ethanol), in a 1.6×10 −2 M phosphate buffer solution at pH 8. The performance of the system was optimised by using both univariate and modified simplex methodologies. When modified simplex was used, the best signal was obtained for a sample injection volume of 529 μl, a reaction coil length of 1.29 m, and a reagent flow rate of 4.8 ml min −1. Under optimum conditions, the response was linear up to 3 mg l −1 copper, the equation of the straight line being y=0.314 x+5.2×10 −4 ( r 2=0.998). The method allowed a sampling frequency of 40 samples per hour and exhibited a precision of 2.11% (as R.S.D., n=11). The limit of detection was 4.6 μg l −1 (calculated as 3 s b/ m, where s b is the standard deviation of the y-intercept and m represents the slope of the straight line), and was therefore more sensitive than all the direct continuous methods reported previously. The method was successfully applied to the analysis of real water samples, with an average relative error of 5.32%.

Indirect spectrophotometric determination of p-aminobenzoic acid in sunscreen formulations by sequential injection analysis

A simple indirect sequential injection spectrophotometric method for the determination of the UV-filter p-aminobenzoic acid (PABA) in sunscreen formulations is proposed. The determination is based on the reaction of PABA with hypochlorite in acidic medium and the subsequent measurement of the residual chlorine by using the well-known reaction with o-tolidine. The experimental variables involved in the sequential injection analysis (SIA) system (sandwich arrangement, volumes of sample and reagents, propulsion flow rate, reaction coil length) and adequate concentrations of reagents were studied. The SIA method has a linear range up to 20 μg ml −1 (3 s y / x / b detection limit) of 1.0 μg ml −1 and a measurement throughput of 55 injections h −1. Sunscreen cosmetics containing PABA were analyzed by the proposed method and the results were validated by comparison with those obtained by a liquid chromatographic procedure used as reference. The analytical features of the proposed method make it suitable for quality control of final products.

Bismuth determination in environmental samples by hydride generation–electrothermal atomic absorption spectrometry

A hydride generation procedure, via flow injection, coupled to electrothermal atomic absorption spectrometry was optimised for Bi determination in sea water and hot-spring water and acid extracts from coal, coal fly ash and slag samples. The effects of several variables such as hydrochloric acid and sodium tetrahydroborate concentrations, hydrochloric acid and sodium tetrahydroborate flow rates, reaction coil length, trapping and atomisation temperatures, trapping time and the Ar flow rate have been investigated by using a 2 9*3/128 Plackett–Burman design. From these studies, certain variables (sodium tetrahydroborate concentration and trapping time) showed up as significant, and they were optimised by a 2 2+star central composite design. In addition, a study of the bismuthine trapping and atomisation efficiency from graphite tubes (GTs) permanently treated with uranium, tantalum, lanthanum oxide, niobium, beryllium oxide, chromium oxide and tantalum carbide were investigated. The results obtained were compared with those achieved by iridium and zirconium-treated GTs. The best analytical performances, with characteristic mass of 35 pg and detection limit of 70 ng l −1, were achieved by using U-treated GTs. Accuracy were checked using several reference materials: 1643d (Trace Elements in Water), TM-24 (Reference Water), GBW-07401 (Soil) and 1632c (Trace Elements in Coal).

Flow-injection analysis of iron in tap water using its catalytic effect on the oxidation of an azo compound

1-Amino-8-hydroxy-7-(o-hydroxy-p-methylphenylazo)-3,6-naphthalenedisulfonic acid (p-CH3) reacts with KIO4 in an acid solution to form a colorless azoxy compound. The reaction was enhanced with iron(III). Therefore, a flow-injection analysis (FIA) of iron was investigated by measuring the decrease in the absorbance of p-CH3. The flow system used had three channels. Each carrier (distilled water), KIO4 solution as a oxidizing agent and reagent(p-CH3) solution was pumped at 1.0 cm3/min. A sample solution was injected manually into the carrier stream. Then, a p-CH3 solution containing sample was mixed with a KIO4 solution. Under the optimum conditions (temperature, 50°C; p-CH3, 1.2×10−4 M; KIO4, 1.0×10−5 M; HCl, 0.3 M; reaction coil length, 9 m, i.d. 1 mm; injection volume of sample, 1.0 cm3), Fe(III) could be determined in the concentration range of 1.4 to 16 ppb (ng cm−3) by monitoring at 556 nm. The detection limit of Fe(III) was 0.48 ppb (S/N=5). Also, 15 samples could be analyzed per hour. Mn(II), Cr(VI) and V(V) at more than 0.8-fold, 5-fold and 50-fold concentrations of Fe(III)(8 ppb) showed positive interferences, respectively. However, these metal ions did not interfere with the determination of iron in tap water. As a result, the proposed method was applied to the determination of iron in tap water without any masking agents. The results of a trace iron determination in tap water showed a good agreement with the values obtained by ICP-AES.

A sequential-injection system for spectrophotometric determination of p-aminobenzoic acid in sunscreens.

A sequential injection method is proposed for spectrophotometric determination of p-aminobenzoic acid (PABA) in cosmetic formulations. The method is based on diazotization of the analyte, coupling with 8-hydroxyquinoline, and the subsequent formation of a colored product. The experimental conditions used (coupling reagent, sandwich arrangement, volumes aspirated, propulsion flow rate, reaction coil length) were studied. Response of the sequential injection method were linearly dependent on concentrations up to 25 micro g mL(-1) and the detection limit was 2 micro g mL(-1). Throughput was 51 measurements per hour and a complete cycle, including three measurement per sample and a washing step, resulted in a sample throughput of 13.8 h(-1). Sunscreen formulations containing PABA were analyzed by the proposed method and validated by use of an HPLC procedure. The two methods gave comparable results. Yields ranged between 97-103% and good precision (RSD between 2 and 6%) was obtained. The accuracy, speed, and selectivity of the method make it suitable for safe application for quality-control of final products.

Comparative study of the micellar enhanced spectrophotometric determination of β-lactamic antibiotics by batch and flow injection analysis using a multisimplex design

A study has been made on the spectrophotometric determination of the β-lactamic antibiotics, amoxicillin and ampicillin, in micellar media using Cu ions as catalyst. Batch and flow injection approaches were compared. Multisimplex design was used to determine the optimal values of the flow injection analysis (FIA) system. Chemical (buffer, pH and Cu(II) concentrations) and physical (flow rate, temperature and reaction coil length) variables were considered. The analytical performance characteristics were as follows: the detection limits for batch and flow-through systems were 2.5×10 −7 and 2×10 −6 M, respectively, and a relative standard deviation less than 1% was found for both methods. The proposed FIA methodology was satisfactorily applied to the determination of the antibiotics in pharmaceutical formulations.

EXTRACTION SPECTROPHOTOMETRIC DETERMINATION OF LIDOCAINE USING FLOW INJECTION ANALYSIS

A new flow-through method (flow injection analysis, FIA) for the lidocaine assay, using extraction of ion – associate with bromocresol green, was proposed. The optimum reaction and flow-through conditions (reagent concentrations, pH, flow rates of reagents, injected volume of sample, length of reaction and extraction coil, total flow rate through the separator, withdrawal rate of organic phase) were found. The Lambert-Beer' law is obeyed in the concentration range of 1.4–16.2 μg ml−1.

Sequential injection spectrophotometric determination of oxybenzone in lipsticks.

A sequential injection (SI) procedure for the spectrophotometric determination of oxybenzone in lipsticks is reported. The colorimetric reaction between nickel and oxybenzone was used. SI parameters such as sample solution volume, reagent solution volume, propulsion flow rate and reaction coil length were studied. The limit of detection was 3 microg ml(-1). The sensitivity was 0.0108+/-0.0002 ml microg(-1). The relative standard deviations of the results were between 6 and 12%. The real concentrations of samples and the values obtained by HPLC were comparable. Microwave sample pre-treatment allowed the extraction of oxybenzone with ethanol, thus avoiding the use of toxic organic solvents. Ethanol was also used as carrier in the SI system. Seventy-two injections per hour can be performed, which means a sample frequency of 24 h(-1) if three replicates are measured for each sample.

Development of an Offline Noncompetitive Flow Immunoassay for the Determination of Interleukin-8 in Cell Samples

A noncompetitive flow immunoassay system (FIA) for the analysis of interleukin-8 (IL-8) in cell samples was developed. Affinity interaction assays based on offline incubation of excess labeled antibodies and antigen (IL-8) were carried out. The residual unbound labeled antibody was trapped in an immunoaffinity column with immobilized IL-8 while the immunocomplex, labeled antibody/IL-8, was detected by a fluorescence detector. Two fluorophores, FLUOS and Cy5.5, were conjugated with IL-8 antibody. Optimization and comparison between the two fluorescent labeled antibodies were performed with regard to pH, antibody concentration, flow rate, injection volume, and association time. Additionally, a horseradish peroxidase enzyme label was used for the conjugation to the anti-IL-8. The enzyme substrate reaction was optimized with respect to temperature and length of the substrate reaction coil. The detection limits were found to be 200 amol using the FLUOS-labeled anti-IL-8 and 1 fmol using the Cy5.5 fluorescence label. The developed FIA technique was applied for the analysis of IL-8 in cell samples. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was used to identify IL-8 in the cell samples.

Spectrophotometric determination of thiocyanate by sequential injection analysis

A sequential injection analysis (SIA) system was developed for the spectrophotometric determination of thiocyanate as Fe(SCN) 2+. The physical parameters (flow rate, sample and reagent volumes, holding coil and reaction coil internal diameter and length) which influence dispersion in a SIA system were optimized. The effect of the reagent concentration and nitric acid concentration (in the reagent solution and carrier stream, respectively), were also studied. Reagent consumption was effectively reduced by introducing the reagent as a zone into the nitric acid carrier stream, compared to flow injection analysis where the reagent itself is the carrier stream. The system is fully computerized and able to monitor thiocyanate in samples at 24 samples per hour with a relative standard deviation of <1.20% for samples and standards with thiocyanate concentrations above 10.0 mg l −1. The calibration graph is linear from 2.0 to 150.0 mg l −1 with a 3 σ detection limit of 1.1 mg l −1. The system has been applied to the determination of thiocyanate in process solutions and waste waters.

Determination of heparin using flow injection analysis with spectrophotometric detection

A study has been made of the conditions for the reaction of heparin with thiazine dyes in both static and flow-through systems. Factorial design was used to determine the significance of the individual flow-through experimental parameters and their optimal values were found: at pH = 6, c dye = 8 × 10 −5 mol l −1, the optimum overall flow rate (dye : buffer = 1 : 1) is of 4.2 ml min −1, volume injected of 500 μl, reaction coil length of 20 cm. Methods were developed for the flow-through spectrophotometric determination of heparin. The concentration ranges of calibration curves are 0–10 and 0–12 μg ml −1, respectively, for individual dyes, the limit of detection of about 0.1, limit of determination of about 0.3 μg ml −1.

Study and application of a method for the determination of metallic elements by ICP-AES with preconcentration on an active carbon-silica gel microcolumn in a FI system

Using active carbon-silica gel as adsorbent and sodium diethyldithiocarbamate (NaDDTC) as chelating reagent in a flow injection system, an ICP-AES method has been developed for preconcentration and determination of trace metallic elements in high purity rare earth oxides. The experimental parameters such as pH, flow rate, amount of adsorbent, length of reaction coil and eluent acidity were optimized. At pH 4.6 Al,Cr,Cu,Fe,Pb,V,Zn could be preconcentrated, and subsequently determined by ICP-AES. This method can eliminate matrix effects. Its enrichment factors were 8.1–12.6 with detection limits in the ng/mL range and RSD of 2.3–5.0. This method was applied to the analysis of high purity of La2O3 and Eu2O3 with satisfactory results.

Fluorimetric flow-injection analysis of total amounts of aldehydes in auto exhaust gas and thermal degradation emission gas with cyclohexane-1,3-dione

A simple flow-injection (FI) spectrofluorimetric method for the assay of total volatile aldehydes in auto exhaust gas and emission gas from thermal degradation was developed. Aldehydes, such as formaldehyde, acetaldehyde, propionaldehyde and n-butyraldehyde, reacted with cyclohexane-1,3-dione (CHD) to form more strongly fluorescent compounds. A two-channel flow system was assembled. Distilled water and 0.02% CHD were delivered at 0.75 ml min −1. The optimum conditions were pH 5 (2.2 M CH3COONH4-CH3COOH buffer solution), reaction temperature 70 °C, reaction coil length 0.5 mm i.d. × 7 m, cooling coil length 2 m, sample size 60 μ1, excitation and emission wavelengths, 376 nm and 452 nm. Aldehydes in sample gas (10 1) were collected by passing the gas at a flow rate of 0.5 1 min −1 through two impingers connected in series. 10 ml of methanol was used as an absorbent and diluted sample solution was injected into the carrier stream. The calibration graph was linear in the range 100–1000 ppb. The detection limit was 30 ppb and a sampling frequency of 30 h−1 was attained. Relative standard deviation for 10 standard formaldehyde solutions (500 ppb) was 1.5%. This rapid and simple FI method was applied to the determination of the total amount of aldehydes, calculated as formaldehyde, in auto exhaust gas and emission gas from the thermal degradation of polymers.The method is useful for monitoring aldehyde emissions and investigating the removal effect of aldehydes from various sources.

Continuous-flow spectrophotometric determination of amino acids with 1,2-naphthoquinone-4-sulphonate reagent

A completely continuous-flow method for amino acid determination is proposed. The spectrophotometric detection is based on the reaction between amino acids and 1,2-naphthoquinone-4-sulphonic acid (NQS) in basic sodium carbonate-sodium hydroxide medium (pH 10). The reaction was developed on-line because NQS solutions are unstable in this medium. Phenylalanine (Phe) was chosen as a model amino acid to develop the method. The influence on the absorbance of different variables (pH, buffer and reagent concentrations, temperature, reaction coil length and flow-rate) were studied. For Phe at 470 nm, the calibration graph was linear up to 1.5 × 10 −4 M, the reproductibility was 2.5% and the limit of detection was 2 × 10 −6 M. The sample throughout was 40 h −1. Measurements of reaction product spectra were carried out in the range 290–590 nm. Spectral differences for nineteen amino acids derivatives were studied by using principal component analysis and cluster analysis. On this bases a spectral classification can be made.

Spectrophotometric determination of oxytetracycline by flow injection

A flow injection spectrophotometric method is proposed for the determination of oxytetracycline. The method is based on the reaction of this drug with iron(III) in the presence of sulphuric acid to form a coloured complex. Less than 10 µg ml–1 of the drug (defined as the amount of the drug that gave a signal of twice the background noise) can be determined by measuring the absorbance at 435 nm of the complex formed. Parameters affecting the absorbance such as the acid concentration, sample volume and the reaction coil length were all studied in order to optimize them. The method was applied to the determination of oxytetracycline in real samples and some pharmaceutical preparations. The relative standard deviation was 1.7% for 80 µg ml–1(for ten replicate injections). The sample throughput is 17 samples h–1. The effect of some foreign species was also investigated.

Control of dispersion and variation of reaction coil length in flow injection analyzers by flow reversals

Control of dispersion and variation of reaction coil length in flow injection analyzers by flow reversals