Proposed Flow Injection Method Research Articles

Determination of lipoic acid by flow-injection and high-performance liquid chromatography with chemiluminescence detection

A new flow-injection (FI) and high performance liquid chromatography (HPLC) with chemiluminescence detection method has been proposed for the determination of α-lipoic acid (LA). The assay is based on the measurement of chemiluminescence (CL) produced during the reaction of α-lipoic acid with potassium permanganate in a sodium hexametaphosphate medium (pH 3). This reaction is accompanied by a weak CL, which is greatly increased in the presence of a formaldehyde solution. The proposed FI method allows the determination of LA over the range: 0.5–20μgmL−1 with LOD 4×10−3μgmL−1. An introduction of HPLC into the flow manifold improves selectivity of the method and allows the determination of LA in a complex sample. The chromatographic linear range is 2.5–30μgmL−1 with LOD 1.774μgmL−1. Chromatographic separation was achieved by isocratic elution (acetonitrile/potassium dihydrogen phosphate, pH 3, adjusted with phosphoric acid): 30/70 using a Cosmosil 5C18-MS-II (4.6mm×150mm I.D.) column at a flow rate of 1.0mLmin−1. The presented methods were utilized to determine the α-lipoic acid content in “Alfa-lipoic acid” capsules and in food products.

Flow injection analysis of BHA by NiHCF modified electrode

Abstract Flow injection method is described for the determination of the antioxidant tert-butylhydroxy anisole (BHA) based on its catalytic oxidation at a nickel hexacyanoferrate (NiHCF) surface modified graphite wax composite electrode fabricated using a new approach. The electrochemical characteristics of the modified electrode were studied using cyclic voltammetry. The voltammetric response of BHA at the modified electrode showed current densities remarkably higher than the bare graphite electrode and occurred at a reduced over voltage of 200 mV. Linear calibration graphs were obtained in the range of 1.2 × 10−6–1.07 × 10−3 M. A detection limit of 6 × 10−7 M was obtained with a correlation coefficient of 0.9983 based on S/N = 3. Reliable results were obtained by applying the proposed flow injection method to determine BHA spiked in dehydrated potato flakes. Results suggest that the developed method can be effectively employed for the determination of BHA from food samples.

Spectrophotometric flow injection procedure to indirect determination of paracetamol in pharmaceutical formulations using o-tolidine as reagent

A spectrophotometric flow injection method for the determination of paracetamol in pharmaceutical formulations is proposed. The procedure was based on the oxidation of paracetamol by sodium hypochloride and the determination of the excess of this oxidant using o-tolidine dichloride as chromogenic reagent at 430 nm. The analytical curve was linear in the paracetamol concentration range from 8.50 x 10-6 to 2.51 x 10-4 mol L-1 with a detection limit of 5.0 x 10-6 mol L-1. The relative standard deviation was smaller than 1.2% for 1.20 x 10-4 mol L-1 paracetamol solution (n = 10). The results obtained for paracetamol in pharmaceutical formulations using the proposed flow injection method and those obtained using a USP Pharmacopoeia method are in agreement at the 95% confidence level.

Spectrophotometric flow injection procedure to indirect determination of paracetamol in pharmaceutical formulations using o-tolidine as reagent

A spectrophotometric flow injection method for the determination of paracetamol in pharmaceutical formulations is proposed. The procedure was based on the oxidation of paracetamol by sodium hypochloride and the determination of the excess of this oxidant using o-tolidine dichloride as chromogenic reagent at 430 nm. The analytical curve was linear in the paracetamol concentration range from 8.50 x 10-6 to 2.51 x 10-4 mol L-1 with a detection limit of 5.0 x 10-6 mol L-1. The relative standard deviation was smaller than 1.2% for 1.20 x 10-4 mol L-1 paracetamol solution (n = 10). The results obtained for paracetamol in pharmaceutical formulations using the proposed flow injection method and those obtained using a USP Pharmacopoeia method are in agreement at the 95% confidence level.

Determination of catecholamines by flow-injection analysis and high-performance liquid chromatography with chemiluminescence detection

A chemiluminescence (CL) detection of catecholamines [norepinephrine (NE), epinephrine (E), dopamine (DA) and l-dopa (LD)] is described for the flow-injection (FI) and high-performance liquid chromatographic (HPLC) determination of these compounds. The detection method is based on the inhibition effect of catecholamines (CAs) on the CL reaction of luminol with iodine in the alkaline medium. The proposed FI method allows the determination of CAs in pharmaceutical preparations for the purpose of drug quality control. The calibration curves show good linearity in the concentration range of: 1.1–20.0 μg l −1 (NE), 0.5–5.0 μg l −1 (E), 0.6–9.0 μg l −1 (DA) and 0.6–10.0 μg l −1 (LD). The limits of detection (defined as a signal-to-noise ratio of 3) are: 0.34 μg l −1 (NE), 0.15 μg l −1 (E) and 0.18 μg l −1 (DA, LD). The HPLC procedure was successfully applied for the determination of catecholamines (NE, E, DA) in human urine after solid-phase extraction (SPE). In a simple run time CAs can be determined in 20 min. The chromatographic linear ranges are: 5.0–72.0 μg l −1 (NE), 5.0–48.0 μg l −1 (E) and 5.0–96.0 μg l −1 (DA). The limits of detection for three urinary CAs are: 0.71 μg l −1 (NE), 0.26 μg l −1 (E) and 0.73 μg l −1 (DA).

On-line flow injection solid sample introduction, leaching and potentiometric determination of fluoride in phosphate rock

A flow injection method with on-line solid sample dissolution was developed for the determination of fluoride in phosphate rock. The fluoride was selectively leached (98–102.4 % recovery) from a 50-mg powdered phosphate rock sample with 0.50 M citric acid. Using the zone sampling technique the fluoride in the buffered leachate was determined by injecting 87 μL into the carrier stream using a fluoride ion-selective electrode detector. The sensing element of the electrode was housed in a home-made sleeve-type flow-through cell. On-line solid sample digestion with 0.50 M citric acid at 55 °C resulted in minimum dissolution of interfering iron and aluminum ions with improved accuracy and calibration linearity. The incorporation of relatively high level of fluoride in the carrier stream (40 μg mL −1) facilitated the determination of high levels of fluoride in phosphate rock (up to 4.1%) with out the need for excessive on-line dilution. The optimized flow system was applied for the determination of fluoride in phosphate rocks samples and a reference material at a rate of nine samples per hour with a relative standard deviation ( n = 5) of 2.95–4.0 %. Comparison of the proposed flow injection method with the standard method, which involves steam distillation from sulfuric acid solution and manual titration with thorium nitrate, showed no evidence of bias at the 95% confidence level.

Direct olive oil anisidine value determination by flow injection

A flow injection (FI) spectrophotometric method for the determination of olive oil anisidine value (AV) is presented. A 40-l aliquot of 10% (w/v) olive oil samples in 2-propanol are injected in a p-anisidine in 2-propanol/acetic acid stream. The reagents are premixed in the FI system using a two-line manifold. Detection is achieved by monitoring absorbance continuously at 350 nm. The proposed FI method achieves a high analysis rate (110 samples/h), minimizes solvent consumption (2 ml 2-propanol and 0.4 ml acetic acid per analysis) and compares well with the manual spectrophotometric method (relative difference 0.5–6.8% for the analysis of 25 olive oil samples). Precision was found to be better than 1% relative standard deviation (R.S.D.; n=7). The linear range was 6–85 AV and is suitable for analysis of abused olive oils.

Simultaneous flow injection determination of ascorbic acid and cysteine using double flow cell

A spectrophotometric method was developed for the simultaneous determination of ascorbic acid (AA) and cysteine (Cys) by flow-injection (FI) system equipped with a double flow cell, which was installed in a double-beam spectrophotometer. Iron(III) is quantitatively reduced to iron(II) with AA or Cys in the presence of 1,10-phenanthroline (phen), producing an iron(II)–phen complex ( λ max=510 nm). It was found that copper(II) greatly accelerated the reduction reaction of iron(III) with Cys, while that with AA was little affected by copper(II). On the basis of the difference in the rate between the two reactions, AA and Cys can be determined in the ranges 2×10 −7 to 4×10 −5 M and 1×10 −6 to 9×10 −5 M, respectively at a rate of 45 samples per hour. The limits of detection ( S/ N=3) were 1×10 −7 M for AA and 2×10 −7 M for Cys. A batchwise method for the simultaneous determination of both reductants was also studied. The proposed FI method was successfully applied to the simultaneous determination of both analytes in pharmaceutical formulation.

Flow-injection spectrophotometric determination of cobalt(II) at low μg l −1 levels with 4-benzylpiperidinedithiocarbamate

A rapid, sensitive and reproducible flow-injection (FI) analysis method is described for the spectrophotometric determination of cobalt(II) using 4-benzylpiperidinedithiocarbamate as a chromogenic reagent. The method involves the measurement of the absorbance of the colored cobalt(II)-(4-benzylpiperidinedithiocarbamate) 2 complex at 640 nm. In the system, a microcolumn (MC) containing strong cation-exchange resin was placed between injection manifold and spectrophotometer to increase interactions between cobalt(II) and reagent, and for preconcentration of cobalt(II). The linear range of the proposed FI method was 0.6–100 μg l −1, and the limit of detection (signal/noise=3) was 0.1 μg l −1 for a 10 μl injection of cobalt(II) ion solution at a sampling rate of 60 h −1. The relative standard deviation (R.S.D.) for 0.6–100 μg l −1 cobalt(II) was <1.0% ( n=5). The precision and accuracy of the method were checked by analysis of certified reference metals, and the method was applied to river, sea and hot spring water samples.

Flow Injection Spectrophotometr ic Catalytic Determination of Iodine in Milk.

A flow injection method based on the catalytic action of iodide on the color-fading reaction of the FeSCN2+ complex was proposed and applied to the determination of iodine in milk. At pH 5.0, temperature 32°C and measurements at 460 nm, the decrease in absorbancy of Fe3+-SCN (0.10 and 0.0020 mol l-1) in the presence of NO2- (0.3 mol l-1) is proportional to the concentration of iodide, with linear response up to 100.0 μg I- l-1. The detection limit was determined as 0.99 μg I- l-1 (3σ blank/slope); and the system handles 48 samples h-1. Organic matter was destroyed by a dry procedure carried out under alkaline conditions. Alternatively, the use of a Schoninger combustion after the milk dehydration was evaluated. The residue was taken up in 0.12 mol l-1 KOH solubilization. For typical samples, recoveries varied from 94.5 to 105%, based on the amounts of both organic matter destroyed. The accuracy of the method was established by using a certified reference material (IAEA A-11, milk powder) and a manual method. The proposed flow injection method is now applied as an indicator of milk quality in a Brazilian milk production system.

Spectrophotometric determination of cationic and anionic surfactants with anionic dyes in the presence of nonionic surfactants, part II: Development of batch and flow injection methods

On the basis of the changes in absorption spectra of azo dyes on the addition of an organic onium ion, spectrophotometric methods for the determination of organic onium salts and anionic surfactants were developed, and applied to flow injection method. Propyl orange (PO−) was used for the determination of organic onium ions. Pairs of PO− and Zeph+ (tetradecyldimethyl-benzylammonium ion) or PO− and nC18TMA+ (n-octadecyltrimethylammonium ion) were used for the determination of anionic surfactants. The determination range of organic onium ions were (0–3) × 10−5 M by a batch method and were (0–2) × 10−5 M by a flow injection method. The determination ranges of anionic surfactants were (0–2) × 10−5 M by the batch method, and were (0–5) × 10−5 M by the flow injection method, and the detection limit corresponding toS/N = 3 was 3 × 10−7 M by the flow injection method. By the proposed flow injection method, anionic surfactants in water samples were determined.

Determination of aluminium in kaolins by flow injection

A flow injection spectrophotometric method is described for the determination of aluminium in kaolins, based on its complexation with Alizarin Red S. The flow injection system permits a throughput of 120 samples per hour with a precision (relative standard deviation) of 1.31% and without any carryover. The aluminium contents in various clay extracts as determined by the proposed flow injection method were in good agreement with those determined by a well established classical method.

Spectrophotometric determination of ascorbic acid in pharmaceuticals by background correction and flow injection

Background correction has been shown to be an effective and indispensable modification in the spectrophotometric determination of ascorbic acid. The decomposition of ascorbic acid in pharmaceutical samples was carried out by incubation with sodium hydroxide to give products that were insensitive to ultraviolet light. The rapid oxidation in air of ascorbic acid, especially in dilute solutions, was avoided by the use of the flow injection principle for spectrophotometric determination and by employing a carrier stream of an anti-oxidizing nature consisting of 6 micrograms ml(-1) of 2-mercaptoethanol in 0.25% sulphuric acid. The optimized method with a single channel manifold made use of a carrier stream flow rate of 1.1 ml min(-1), an injection volume of 50 microl, a delay coil of 50 cm (0.5 mm i.d.) and detection at 245 nm. The throughput was at least 180 injections h(-1). The proposed flow injection method yielded results for the analysis of 0-20 micrograms ml(-1) of ascorbic acid that were 99-102% (relative standard deviation 0.6% or better) in agreement with those produced by comparable methods involving titration with iodine, chloranil or 2,6-dichlorophenolindophenol [4-(2,6-dichloro-4-hydroxyphenylimino)cyclohexa-2,5-dieno ne], and high-performance liquid chromatography. When the agreement was not good (as low as 14% with respect to the method being compared), this was traced to the presence of substances which are known to interfere in one or other of the methods of comparison.

Determination of pentachlorophenol by flow injection and amperometric detection with a glassy carbon electrode

A flow injection method is described for the determination of pentachlorophenol (PCP) based on its oxidation and amperometric detection with a glassy carbon electrode. The sample is injected into the carrier stream which is a 10 −2 M Britton-Robinson buffer solution of pH 7.0 and 1% ( v v ) methanol. Calibration plots are linear in the concentration ranges 1.0–10.0, 0.1-1.0, and 0.01-0.1 ppm of pentachlorophenol, and the detection limit (3 s b/m) is 3.7 ppb of this compound. Good results are obtained by applying the proposed flow injection method to determine pentachlorophenol in spiked drinking water.