Fluorimetric determination of total ascorbic acid by a stopped-flow mixing technique

A simple, rapid and automatic fluorimetric method for the determination of total ascorbic acid is described. The method makes use of the stopped-flow mixing technique in order to achieve the rapid oxidation of ascorbic acid by dissolved oxygen to dehydroascorbic acid, which then reacts with o-phenylenediamine to form a fluorescent quinoxaline. The initial rate and fluorescence signal of this system are directly proportional to the ascorbic acid concentration. The calibration graph was linear over the range 0.1-30 microg ml(-1) (kinetic method) and 0.25-34 microg ml(-1) (equilibrium method). The precision (% RSD) was close to 0.5%. The method has been used for the determination of ascorbic acid in pharmaceutical formulations, fruit juices, soft drinks and blood serum.     

Spectrophotometric method for the determination of ascorbic acid with iron (III)-1,10-phenanthroline after preconcentration on an organic solvent-soluble membrane filter

A solvent-soluble membrane filter is proposed for the simple and rapid preconcentration and spectrophotometric determination of ascorbic acid based on the reduction of 1, 10-phenanthroline (phen)-iron (III), which is collected on a nitrocellulose membrane filter as an ion-associate of the cationic complex of tri,phen-iron (II) [ferroin, Fe(phen)(2+)(3)] with an anionic surfactant (of dodecyl sulfate). The ion-associate collected is dissolved in a small volume of 2-methoxyethanol together with the filter. The colour intensity is measured at 510 nm against the reagent blank and is proportional to the content of ascorbic acid in the range 2.5-50 microg ascorbic acid in 5 ml of solvent with excellent reproducibility (RSD 3.2% for 200 microg 1(-1) ascorbic acid), the enrichment factor achieves 100-fold and detection limits better than 2.0 microg 1(-1) can be obtained. Diverse components of organic and inorganic compounds normally present in fruits, vegetable, beverages and urine do not interfere. The recoveries of the ascorbic acid added to the samples are quantitative.     

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.     

Kinetic spectrophotometric determination of ascorbic acid by reduction of toluidine blue

A simple kinetic method is described for the determination of ascorbic acid. The procedure is based on the reduction of toluidine blue with ascorbic acid. The rate of reaction is followed by measuring the decrease in absorbance of toluidine blue (lambda(max) = 600 nm) as a result of its decolorization upon reduction by ascorbic acid. Ascorbic acid in the range of 3-35 microg/ml was determined using slope and fixed time methods of analysis, while the variable time method allowed the determination of 5-50 microg/ml of ascorbic acid. The percent relative standard deviation of the method varied from 0.78 to 1.32% depending on the kinetic method used. The high sensitivity of the method also allows determination of low levels of ascorbic acid in some fruits and vegetables such as dew melon, water melon, parsley and coriander.     

Kinetic spectrofluorimetric determination of trace ascorbic acid based on its inhibition on the oxidation of pyronine Y by nitrite

A highly sensitive spectrofluorimetric method is proposed for the determination of trace amount of ascorbic acid using a new indication. The method is based on the inhibition of ascorbic acid on the oxidation of pyronine Y (PRY) by nitrite. The detection limit for ascorbic acid is 0.012 microg ml(-1), the linear range of the determination is 0.02-0.36 microg ml(-1). Analytical parameters, such as reagent concentration, pH, reaction temperature and time, were optimized. The relative standard deviations of eleven replication determinations of 0.12 and 0.24 microg ml(-1) ascorbic acid were 1.4 and 0.72%, respectively. This method has been used to determine ascorbic acid in pharmaceuticals, vegetables, fruits and soft drink with satisfactory results.     

Kinetic method for determination of ascorbic acid on flow injection system by using its catalytic effect on the complexation reaction of an ultra sensitive colorimetric reagent of porphyrin with Cu(II)

A kinetic method performed on a flow injection system is described for the determination of ascorbic acid by using its catalytic effect on the complexation reaction of Cu(II) with 5,10,15,20-tetrakis(4-N-trimethyl-aminophenyl)porphyrin. The characteristic spectrum of porphyrin (Soret band), which shows intense absorption around 400 nm (epsilon>2.0 x 10(5) cm(-1)M(-1)), was used first time for determining ascorbic acid. By incorporating the complexation reaction into a flow injection system, ascorbic acid could be determined either over a broad dynamic range of 0.1-1000 microg/ml or at a trace level below 5 ng/ml. Good repeatability was also achieved by testing a working standard of 0.1 microg/ml with 10 injections at a throughput of 35 h(-1), obtaining a relative standard deviation of 0.11%. Substances like amino acids, vitamins, sugars, organic acids and metal ions, showed no or little interference even present at high concentrations. The method was validated in the determination of ascorbic acid contents of some commercially available soft drinks by comparison with the official 2,6-dichloroindophenol method with reasonable agreement.     

Selective spectrophotometric determination of ascorbic acid in drugs and foods

A new analytical method was developed for the determination of ascorbic acid. The method is based on the reaction of ascorbic acid with 4-chloro-7-nitrobenzofurazane (NBD-Cl) in the presence of 0.2M sodium hydroxide, where a bluish green colour (lambda(max) 582 nm) is developed after dilution with 50% (v/v) aqueous acetone solution. Beer's law was obeyed in a concentration range of 5-20 microg ascorbic acid/ml with a good correlation coefficient (r = 0.9990). The method was found to be highly specific for the determination of ascorbic acid in the presence of dehydro-ascorbic acid, all other vitamins and minerals possibly present in multivitamin preparations, rutin, salicylamide, acetyl salicylic acid, paracetamol, caffeine, phenylephrine hydrochloride and dipyrone. Moreover, the proposed procedure was also successfully applied for the determination of ascorbic acid in some canned and fresh fruit juices, some vegetables and infant milk products without interference from coloured and other substances present in the plant extracts.     

Indirect determination of ascorbic acid by solid-phase spectrophotometry

Solid-phase spectrophotometry (SPS) technique, in the visible region, was used for the spectrophotometric determination of ascorbic acid based on the reducing effect on iron(III) ion, followed by formation of the iron(II)-ferrozine chelate. The chelate is easily sorbed on a dextran-type anion-exchange gel and the absorbance of the resin at 567 and 800 nm, packed in a 1 mm cell, is measured directly. The apparent molar absorptivity using 100 ml of sample was 2.1×10⁷ l mol⁻¹ cm⁻¹ and it allowed the determination of ascorbic acid in the range 5–90 ng ml⁻¹; the detection limit was 0.91 ng ml⁻¹ and the RSD 0.91% for a concentration of 50 ng ml⁻¹ of ascorbic acid (n=10). The proposed method permits a highly sensitive and selective determination of ascorbic acid without any preconcentration and it has been satisfactorily applied for its determination in fruit juices, pharmaceuticals, urine and conservative liquids.     

Indirect spectrophotometric determination of ascorbic acid with ferrozine by flow-injection analysis

A FIA indirect spectrophotometric determination of ascorbic acid was developed using its reducing action on Fe(III) in acidic medium and following the spectrophotometric determination of the reduced iron by using sodium 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-4',4'-disulphonate (ferrozine) as chromogenic reagent in buffered medium (pH 5.5) and monitoring the absorbance signal at 562 nm. A three-line manifold with two reaction coils was used: in the first reaction coil, ascorbic acid reduces Fe(III) to Fe(II); and in the second one, the complexation reaction is developed. The linear range of the method was 0.5-10 mug ml(-1) of ascorbic acid, the detection limit being 0.028 mug ml(-1). The proposed method was sensitive, rapid (sampling rate of 90 samples h(-1)) and reproducible (RSD 0.19%, n=10). Satisfactory results were obtained in the determination of ascorbic acid in pharmaceutical preparations, fruit juices and urine testifying the applicability of the method to real samples.     

Individual and simultaneous determination of uric acid and ascorbic acid by flow injection analysis

Flow injection methods for the individual and simultaneous determination of ascorbic acid and uric acid are proposed. A spectrophotometer and a miniamperometric detector are connected in sequence. The calibration graphs for uric acid obtained by measuring its absorbance at 293 nm and its current at +0.6 V are linear up to at least 80 and 70 mug/ml, respectively, with an rsd (n = 10) of 1 % for both methods at mid-range concentrations. The calibration graph for ascorbic acid with amperometric detection is linear up to 80 mg/l. with an rsd (n = 10) of 0.8% at 30 mg/l. The simultaneous determination of uric acid and ascorbic acid is based on measurement of the absorbance of uric acid at 393 nm and amperometric determination of both analytes at +0.6 V. The average relative errors of the analysis of binary mixtures of uric acid and ascorbic acid are 2.2 and 4.2%, respectively.     

Optimization of the separation of a group of antifungals by capillary zone electrophoresis

Two simple, rapid, and efficient methods for the analysis of seven antifungal compounds have been developed by capillary zone electrophoresis. Resolutions higher than 1.5 were obtained using 0.025 M phosphate buffer (pH 2.30) (analysis time close to 9 min) or 0.2 M formic acid (pH 2.15) (analysis time close to 6 min), with an applied voltage of 20 kV and a temperature of 30 degrees C. The highest sensitivity and selectivity can be obtained using phosphate buffer but the shortest analysis times are achieved in the formic system. The analytical characteristics of the optimized methods were investigated. The reproducibility obtained for migration times (RSD(n = 10) < or = 1.0%) and peak areas (RSD(n = 10) < or = 4.3%) was acceptable, but better reproducibilities were obtained when verapamil was used as internal standard (RSD(n = 10) < 0.4% for relative migration times and RSD(n = 10) < or = 2.2% for peak area ratios). The lowest limit of detection was obtained for clotrimazole (0.12 microg/ml) and the highest for fluconazole and voriconazole (0.90 microg/ml). The lowest and the highest limits of quantitation were, respectively, 0.40 microg/ml for clotrimazole and 3.00 microg/ml for fluconazole and voriconazole.     

Spectrophotometric determination of thorium in standard samples and monazite sands based on the floated complex of thorium with N-hydroxy-N,N′-diphenylbenzamidine and thorin

A selective and sensitive spectrophotometric method for the determination of Th(IV) has been based on the reaction with thorin and subsequent extraction of the red-orange coloured complex with N-hydroxy-N,N'-diphenylbenzamidine (HDPBA) in benzene as floated complex at pH 2.2. The complex in ethanol exhibits a maximum absorbance at 495 nm, with a molar absorptivity of 6.0x10(4) l mol(-1) cm(-1), with a Sandell's sensitivity of 3.9x10(-3) microg cm(-2). The method follows Beer's law up to 3.0 microg Th(IV) ml(-1). None of the common cations and anions tested interfere. The detection limit of the method is 0.04 microg Th(IV) ml(-1), the RSD (n=10) is 1.4%. The method has been successfully employed for the determination of thorium in various standard and monazite samples.     

Optimization of a chemical modifier in the determination of selenium by graphite furnace atomic absorption spectrometry and its application to wheat and wheat flour analysis.

A method for the determination of total selenium in wheat and wheat flour using graphite furnace atomic absorption spectrometry (GFAAS) with palladium/ascorbic acid as a chemical modifier was studied. The effects of nickel nitrate, palladium/ascorbic acid, and palladium/magnesium nitrate as chemical modifiers on the sensitivity in the determination of selenite, selenate and selenomethionine by GFAAS were compared. The palladium/ascorbic acid modifier was used for the determination of total selenium in wheat and wheat flour, because the oxidation states of the selenium ion are not important in the determination. The detection limit was estimated to be 1 microg L(-1) (calculated as 3sigma of the blank); the calibration curve was linear for the concentration range 5 - 50 microg L(-1) and the recovery range was 96.66 - 101.80%. The optimal ashing and atomizing temperatures were 1300 degrees C and 2250 degrees C, respectively. The proposed method was successfully applied to the determination of total selenium in wheat and wheat flour.     

AQC柱前衍生化RP-HPLC法测定蒜氨酸及其有关物质的含量

采用6-氨基喹啉-N-(羟基琥珀酰亚胺基)氨基甲酸酯(6-aminoquinolyl -N- Hydroxysuccinimide Carbamate ,AQC)为柱前衍生化试剂,建立了AQC柱前衍生化RP-HPLC法测定蒜氨酸及其有关物质的含量。该衍生化方法反应瞬间完成,衍生化产物稳定。色谱条件为：Kromasil C18柱（250mm×4.6mm,5mm）,流动相A为0.1%乙酸铵(含0.03%乙酸),流动相B为水-乙腈（40∶60）,线性梯度洗脱,流速1.0ml/min,检测波长248nm。蒜氨酸在1.1719~1500μg /ml浓度范围内线性关系良好（r=0.9998）, 日内、日间精密度良好(RSD <1.8%,n=5), 加样回收率为99.1%（RSD1.9%,n=5）,检测限为3ng,该方法准确、方便、快速。     

Spectrophotometric determination of iron with ferrozine by flow-injection analysis

Two methods for the determination of iron by normal FIA and reversed FIA were developed using sodium 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-4',4'-disulphonate (ferrozine). The reagent formed a chelate with Fe(II) in hexamethylentetramine buffered medium at pH 5.5. In one previous reaction coil Fe(III) was reduced to Fe(II) by ascorbic acid and in the other reaction coil the complexation reaction was developed. The linear range of the determination was 0.5-6 and 0.1-5 mug ml(-1) of iron for normal FIA and reversed FIA respectively. The proposed method was sensitive (detection limit 0.012 and 0.010 mug ml(-1)), rapid and reproducible (RSD 0.3 and 0.28%). The method was satisfactorily applied to the determination of iron in waste water, toadstool tissue, potato leaves, human hair and bauxites at a sampling rate of 90 and 50 samples h(-1) for normal FIA and reversed FIA respectively.     

库仑滴定法测定Vitamin C片及黄瓜、青椒中抗坏血酸的含量

建立了以1.0 mol.L-1碘化钾和1.0 mol.L-1醋酸混合液作为电解液的恒电流库仑滴定法测定抗坏血酸含量的方法。结果表明,方法的回收率在98.3%~100.8%之间,RSD在0.21%~0.56%(n=3)。该法可直接用于V itam in C片及黄瓜、青椒等样品中抗坏血酸含量的测定,方法简便灵敏,结果令人满意。     

Determination of astaxanthin in Haematococcus pluvialis by first-order derivative spectrophotometry

A highly selective, convenient, and precise method, first-order derivative spectrophotometry, was applied for the determination of astaxanthin in Haematococcus pluvialis. Ethyl acetate and ethanol (1:1, v/v) were found to be the best extraction solvent tested due to their high efficiency and low toxicity compared with nine other organic solvents. Astaxanthin coexisting with chlorophyll and beta-carotene was analyzed by first-order derivative spectrophotometry in order to optimize the conditions for the determination of astaxanthin. The results show that when detected at 432 nm, the interfering substances could be eliminated. The dynamic linear range was 2.0-8.0 microg/mL, with a correlation coefficient of 0.9916. The detection threshold was 0.41 microg/mL. The RSD for the determination of astaxanthin was in the range of 0.01-0.06%; the results of recovery test were 98.1-108.0%. The statistical analysis between first-order derivative spectrophotometry and HPLC by T-testing did not exceed their critical values, revealing no significant differences between these two methods. It was proved that first-order derivative spectrophotometry is a rapid and convenient method for the determination of astaxanthin in H. pluvialis that can eliminate the negative effect resulting from the coexistence of astaxanthin with chlorophyll and beta-carotene.     

Chemiluminescence flow sensor for the determination of ascorbic acid with immobilized reagents

A novel flow sensor based on chemiluminescence (CL) for the determination of ascorbic acid has been proposed. The analytical reagents, luminol and ferricyanide, were both immobilized on an anion-exchange resin column. The CL signal produced by the reaction between luminol and ferricyanide, which were eluted from the column through sodium phosphate injection, was decreased in the presence of ascorbic acid. The CL emission intensity was linear with ascorbic acid concentration in the range 0.01-0.8 mug ml(-1); the detection limit was 5.5 x 10(-3) mug ml(-1). The whole process, including sampling and washing, could be completed in 1 min with a relative standard deviation of less than 5%. The sensor could be reused more than 100 times and has been applied successfully to the analysis of ascorbic acid in pills and vegetables.     

Simultaneous determination of dopamine and carvedilol in human serum and urine by first-order derivative fluorometry.

A sensitive and selective method for simultaneous determination of carvedilol and dopamine was described. The emission wavelengths of carvedilol and dopamine were at 354 nm and 314 nm with the excitation at 290 nm, respectively. The determination of carvedilol and dopamine by normal fluorometry was difficult because the emission spectra of carvedilol and dopamine were overlapped seriously. The first derivative peaks of carvedilol and dopamine were at 336 nm and 302 nm, respectively. The linear regression equations of the calibration graphs of carvedilol and dopamine were C = 0.000557H-0.00569 and C = 0.00438H-0.0812, with the correlation coefficients were 0.9953 and 0.9988, respectively. The liner range for the determination of carvedilol was 0.002 microg ml(-1) to 0.02 microg ml(-1), and 0.05 microg ml(-1) to 0.6 microg ml(-1) for dopamine. The detection limits were 1 ng ml(-1) for carvedilol and 0.04 microg ml(-1) for dopamine, respectively. The relative standard derivative (RSD) of 4.38% and 4.35% was observed for carvedilol and dopamine, respectively. The recovery of carvedilol was from 95.00% to 106.7% in human serum and from 97.50% to 105.0% in urine sample. The recovery of dopamine was from 100.0% to 102.5% in human serum and from 97.50% to 105.0% in urine sample. This method is simple and can be used for determination of carvedilol and dopamine in human serum and urine sample with satisfactory results.     

Spectrophotometric determination of aluminum with m-carboxyphenylfluorone, a novel chemical probe, and its application.

A highly spectrophotometric method for the determination of aluminum was developed. This method used the color reaction between m-carboxyphenylfluorone (MCPF) as a novel chemical probe and aluminum in the presence of a surfactant, poly(N-vinylpyrrolidone) (PVP, K-90) (0.03 - 1.40 microg of aluminum in a final volume of 10 ml at 561 nm). The proposed method showed excellent sensitivity (molar absorptibity of 1.70 x 10(5) l mol(-1) cm(-1)) and reproducibility (within-day precision: RSD = 0.35% n = 6, between-day precision: RSD = 0.44% n = 6). Linearity was achieved over the range 3 - 140 microg L(-1) with a correlation coefficient of 0.9999, and the effects of foreign substances were low.