Spectrophotometric determination of ascorbic acid in pharmaceutical preparations and fruit juices

Conditions were established for the determination of ascorbic acid using phsophovanadotungstic acid as reagent. The method was applied to the determination of ascorbic acid in pure form, pharmaceutical preparations and fruit juices. The method is sensitive (2-24 micrograms ml-1 of ascorbic acid) and rapid and tolerates the presence of common ingredients usually found in fruit juices. The results obtained with the proposed method showed good agreement with those given by the standard method.     

Determination of ascorbic acid content of some fruit juices and wine by voltammetry performed at pt and carbon paste electrodes

A method was developed for assessing ascorbic acid concentration in fruit juices and wine by differential pulse voltammetry. The oxidation peak for ascorbic acid occurs at about 530 mV (versus SCE) on a Pt strip working electrode and at about 470 mV on a carbon paste working electrode. The influence of the operational parameters like the pulse amplitude and the pulse period on the analytical signal was investigated. The obtained calibration graph shows a linear dependence between the peak height and ascorbic acid concentration within the range 0.31-20 mM with a Pt working electrode, and within the range 0.07-20 mM with a carbon paste working electrode. The equation of the calibration graph was y = 21.839x + 35.726, r2 = 0.9940, when a Pt strip electrode was used (where y represents the value of the current intensity measured for the peak height, expressed as μA and x the analyte concentration, as mM). R.S.D. = 2.09%, n = 10, C(ascorbic acid) = 2.5 mM. The equation of the calibration graph was y = 3.4429x + 5.7334, r2 = 0.9971, when a carbon paste electrode was used (where y represents the value of intensity measured for the peak height, expressed as μA and x the analyte concentration, as mM). R.S.D. = 2.35%, n = 10, C(ascorbic acid) = 2.5 mM. The developed method was applied to ascorbic acid assessment in fruit juices and wine. The ascorbic acid content determined ranged between 6.83 mg/100 mL juice for soft drinks (Fanta Madness) and 54.74 mg/100 mL for citrus (lemon) juices obtained by squeezing fruit. Different ascorbic acid concentrations (from standard solutions) were added to the analysed samples, the degree of recovery being comprised between 94.74 and 104.97%. The results of ascorbic acid assessment by differential pulse voltammetry were compared with those obtained by cyclic voltammetry. The results obtained by the two methods were in good agreement.     

A quick method for the simultaneous determination of ascorbic acid and sorbic acid in fruit juices by capillary zone electrophoresis

A method of quickly determining ascorbic acid and sorbic acid by capillary zone electrophoresis with ultraviolet detection was developed. The choice of background electrolyte, wavelength, injection time and applied voltage were discussed. Ascorbic acid and sorbic acid were well separated in 80 mmol L⁻¹ boric acid-5 mmol L⁻¹borax (pH = 8.0) in 5 min at the detecting wavelength of 270 nm. Under the optimum condition, the method has linear ranges of 2.54-352.00 mg L⁻¹ for ascorbic acid and 1.08-336.39 mg L⁻¹ for sorbic acid with the detection limit of 1.70 mg L⁻¹ for ascorbic acid and 0.54 mg L⁻¹ for sorbic acid, respectively. Other organic acids in fruit juices have no effect on the detection. This method is very feasible and simple and can be used to detect ascorbic acid and sorbic acid in fruit juices.     

A multi-pumping flow system for chemiluminometric determination of ascorbic acid in powdered materials for preparation of fruit juices

A multi-pumping flow-based procedure with chemiluminescent detection is proposed for the determination of ascorbic acid, AA, in fruit juices (powdered form). The method relies on the inhibitory effect of AA on the oxidation of luminol by hydrogen peroxide in alkaline medium. The system comprises several discretely actuated solenoid pumps as the only active components. It handles 100 samples per hour, and requires 96 μl of sample, 42 μg of luminol and 105 μg of potassium hexacyanoferrate(III) per determination. The analytical curve is linear up to about 11 mmol l^− 1 AA, and detection limit is 0.17 mmol l^− 1 AA. The system yields precise measurements (r.s.d. < 1%; n = 11), and recovery ranges from 94% to 106%. Results are in agreement with the reference method (AOAC) at the 95% confidence level.     

Determination of Gallic acid by differential pulse polarography: Application to fruit juices

Phenolic compounds possess antioxidant, antitumor, antimutagenic, antibacterial and anticarcinogenic properties. Gallic acid (2,3,4-trihydroxybenzoic acid) is a natural phenolic compound; therefore, determination of trace gallic acid is very important. In this work, a novel, sensitive and reliable method was developed using differential pulse polarography. The most suitable buffer system was found to be Britton-Robinson (B-R) buffer, pH 10.0. The gallic acid peak in this medium appeared at about ?160 mV, it responded well to standard additions, and high reproducibility was obtained. The calibration graph was linear in the concentration range of gallic acid from 1.0–50.0 μM with a correlation coefficient of 0.999. The limit of detection (LOD) and limit of quantification (LOQ) were obtained as 0.3 and 1.0 μM, respectively. The proposed method was successfully applied to the determination of gallic acid in fruit juices. The influences of some other commonly found inorganic and organic salts on the determination were also examined. Some interferiences were eliminated by using complexing agents, e.g. EDTA.     

Extraction spectrophotometric method for the determination of ascorbic acid in pharmaceutical preparations, urine and fruit juices with potassium iodate

An indirect determination of ascorbic acid is based on extraction of the iodine produced by reduction of potassium iodate. Beer's law is obeyed over the range 0.02-0.36 mg/ml ascorbic acid.     

Determination of ascorbic acid in soft drinks, preserved fruit juices and pharmaceuticals by flow injection spectrophotometry: Matrix absorbance correction by treatment with sodium hydroxide

Two flow injection systems for the spectrophotometric determination of ascorbic acid at 245 nm have been described. On treatment with sodium hydroxide a fraction of the ascorbic acid was decomposed into substances, which do not absorb in UV region, and the decrease in signal measured. This was directly related to the amount of ascorbic acid present. The calibration graph was linear over the range 1-25 and 1-50 microg/ml in the two methods with a correlation coefficient of 0.9981 and 0.9994, respectively. The detection limit (2sigma) was 0.5 and 0.2 microg/ml, respectively. The RSD for 1 microg/ml standard was 2.5 and 1.8% (n = 6) in the two methods, and the sampling throughput 30/hr. The methods permitted the use of 6 microg/ml of 2-mercaptoethanol as an anti-oxidant and stabilizer for ascorbic acid, which is difficult to handle at its microg/ml level. Upon matrix absorbance correction, spiked samples that are known to contain UV-absorbing substances produced an average recovery of 101% with a RSD of 1.2%. The methods were used for the rapid and simple determination of ascorbic acid in soft drinks, preserved fruit juices and pharmaceuticals and the results thus produced compared with those obtained by previously checked methods involving titration with iodine, chloranil 2,6-dichlorophenolindophenol, and HPLC. When there was a disagreement between the results, this was traced to the presence of substances which are known to interfere in comparison methods.     

Determination of L-ascorbic acid in fruit and vegetable juices by flow injection with immobilised ascorbate oxidase

Ascorbate oxidase was immobilised on cyanogen bromide activated-Sepharose 4B and incorporated in a flow-injection system with amperometric detection at a glassy carbon electrode at +0.6 V. On passage through the immobilised ascorbate oxidase a fraction of the L-ascorbic acid was converted into dehydroascorbic acid and the decrease in signal was measured. This could be directly related to the amount of L-ascorbic acid present. The calibration graph was linear over the range 0-400 ng ml(-1) with a correlation coefficient of 0.9994. The detection limit (2 sigma) in phosphate buffer (0.08 M, pH 5.5) was 4.0 ng ml(-1). The relative standard deviation for a 200 ng ml(-1) standard was 1.0% (n = 10) and the sampling throughput was 30 samples h(-1). The method was used for the simple and rapid determination of L-ascorbic acid in fruit and vegetable juice.     

Determination of pesticides in fruit and fruit juices by chromatographic methods. An overview

In order to combat a variety of pests, pesticides are widely used in fruits. Several extraction procedures (liquid extraction, single drop microextraction, microwave-assisted extraction, pressurized liquid extraction, supercritical fluid extraction, solid-phase extraction, solid-phase microextraction, matrix solid-phase dispersion, and stir bar sorptive extraction) have been reported to determine pesticide residues in fruits and fruit juices. The significant change in recent years is the introduction of the Quick, Easy, Cheap, Effective, Rugged, and Safe (QuEChERS) methods in these matrices analysis. A combination of techniques reported the use of new extraction methods and chromatography to provide better quantitative recoveries at low levels. The use of mass spectrometric detectors in combination with liquid and gas chromatography has played a vital role to solve many problems related to food safety. The main attention in this review is on the achievements that have been possible because of the progress in extraction methods and the latest advances and novelties in mass spectrometry, and how these progresses have influenced the best control of food, allowing for an increase in the food safety and quality standards.     

Sequential injection titration with spectrophotometric detection for the assay of acidity in fruit juices.

A simple sequential injection analysis (SIA) with spectrophotometric detection for an assay of acidity in fruit juice was investigated. An alkaline reagent (sodium hydroxide), a sample and an indicator (phenolphthalein) were first aspirated and stacked as adjacent zones in a holding coil. With flow reversal through a reaction coil to the detector, zone penetration occurred, leading to a neutralization reaction that caused a decrease in the color intensity of the indicator being monitored for absorbance at 552 nm. The effects of various parameters were studied. Linear calibration graphs for acidities of 0.2 - 1.0 and 0.5 - 2.5% w/v citric acid as a standard, with a relative standard deviation of 1% (acidity of 0.3 - 0.6% w/v as citric acid, n=11) and a sample throughput of 30 samples h(-1), were achieved. The developed method was validated by a standard titrimetric method for assaying the acidity of fruit juice samples.     

Polarographic determination of sorbic acid in fruit juices and soft drinks

A simple differential-pulse polarographic method using a laboratory-built hanging mercury drop electrode as the working electrode was developed for the determination of sorbic acid in fruit juices and soft drinks. Sorbic acid was extracted from the samples with diethyl ether. After reduction of the ethereal solution to a small volume by direct evaporation, the residual ether was dissolved in the supporting electrolyte (25 ml of acetonitrile + 1 ml of 0.06 M acetic acid + 0.8 g of tetraethylammonium bromide). Peak current was measured at -1.7 V. The working range of the method, without dilution or pre-concentration of the samples, was from 4 to 229 p.p.m. for the original juice and drink samples. The validity of the method was confirmed by parallel determinations using the method of the Association of Official Analytical Chemists and by recovery tests on a large variety of juice samples. Satisfactory recoveries and agreement in results from the two methods were obtained. The recovery and precision (relative standard deviation) of the method were 97 +/- 4 and 100 +/- 3%, respectively, for blackcurrant juice for five determinations.     

Uncertainty and traceability in an assay method (ascorbic acid in juices)

We discus two of the most important aspects for ensuring the quality of the measurements in an assay method. These parameters are: measurement uncertainty and traceability of the measuring method. To be able to exemplify these parameters, we took as an example the measuring of ascorbic acid in commercial juices to clearly understand all the implicit metrological requirements in an assay method. To do this, it was necessary to meticulously analyze every step of the ascorbic acid measuring process via implementation of the practical application of the AOAC 967.21 Official Method for determining ascorbic acid, which is a titrimetric method based on the ISO 10012 norm. The uncertainty and traceability were studied and applied to control the process variability on the assay method. All this allows us to show the importance of implementing a measurement control system on an assay method, thus guaranteeing repeatable and traceable measurements, and ensuring that the measurements have been done accurately.     

Determination of thiabendazole in fruit juices by a new monoclonal enzyme immunoassay

A competitive, indirect enzyme-linked immunosorbent assay (ELISA) for thiabendazole has been developed and applied to the analysis of fruit juices spiked with this fungicide. The immunoassay is based on a new monoclonal antibody derived from a hapten functionalized at the nitrogen atom in the 1-position of the thiabendazole structure. To our knowledge, such a structure has not been previously used to obtain antibodies to thiabendazole. The I50 value and the detection limit of the ELISA for standards were 0.2 and 0.05 ng/mL, respectively. Fruit juices were analyzed by diluting samples in assay buffer, without extraction or cleanup. Samples were not even centrifuged or filtered to remove fruit pulp. Under these conditions, the immunoassay was able to accurately determine thiabendazole down to 1 ng/mL in orange and grapefruit juices, down to 5 ng/mL in banana juice, and down to 20 ng/mL in apple and pear juices. Sensitivity differences of the ELISA were caused by the minimum dilution required by each juice to minimize matrix effects: 1/10 for orange and grapefruit juices, 1/50 for banana juice, and 1/100 for apple and pear juices. In an attempt to further increase the sensitivity of the immunoassay for matrixes showing the strongest interferences, apple and pear juices spiked with thiabendazole at low levels (1-20 ng/mL) were extracted with ethyl acetate before analysis. This simple procedure entailed a significant reduction of matrix effects, which in fact allowed us to determine accurately as low as 5 ng/mL thiabendazole in apple and pear juices. Irrespective of whether samples were analyzed by the direct dilution method or after extraction, the simplicity, sensitivity, and sample throughput of this monoclonal immunoassay makes it a very convenient method for the routine monitoring of thiabendazole residues in fruit juices.     

Determination of phenolic acids in fruit juices by isocratic column liquid chromatography

A simple and rapid analytical method of five phenolic acids, gallic, chlorogenic, caffeic, ellagic and ferulic acid, which are naturally occurring bioactives, were determined in fruit juices by isocratic LC using photodiode array UV detection. The sample was pre-treated by solid-phase extraction (a combination of Sep-Pak Plus tC18 and Bond Elut PSA).