Sensitivity enhancement of liquid chromatographic-direct chemiluminescence detection by on-line post-column solvent mediated pre-oxidative chemiluminescence

The compatibility of liquid chromatography solvents with oxidizing reagents frequently employed in direct chemiluminescence reactions is examined in this study. Various oxidizing reagents were examined for their response in hydro-organic and micellar mobile phases in both isocratic and gradient elution modes. Mild oxidants like hydrogen peroxide, periodate, cerium and hypochlorite were found to be completely compatible with common reversed phase HPLC solvents posing as no threat to the detection procedure. On the other hand, stronger oxidants like acidic permanganate were found to oxidize organic solvents towards the production of an intense light signal. Although several analytical applications can emerge from this finding, the conjunction of this system with reversed phase HPLC is impractical owing to a significant baseline increase which deteriorates the sensitivity of the analysis. A convenient solution to this problem is proposed based on the regulated on-line post-column pre-oxidation of the organic solvent (SPOC) with mild oxidants that have no influence on the final signal. The analytical utility of this new approach in the determination of organic compounds after chromatographic separation is demonstrated.     

Potentiometric measurement of ascorbate by using a solvent polymeric membrane electrode

A novel potentiometric method for the determination of ascorbate is described in this communication. It is based on ascorbate oxidation with permanganate which is continuously released from the inner reference solution of a ligand-free tridodecylmethylammonium chloride (TDMAC)-based polymeric membrane ion selective electrode (ISE). The ISE potential determined by the activity of permanganate ions released at the sample-membrane phase boundary is increased with the consumption of permanganate. The proposed membrane electrode is useful for continuous and reversible detection of ascorbate at concentrations in 0.1M NaCl ranging from 1.0 x 10(-6) to 1.0 x 10(-3)M with a detection limit of 2.2 x 10(-7)M.     

Determination of hypophosphite and phosphite by their reduction of iodine,iodate, or periodate and mercury(I) titration of iodide

A new potentiometric method is adopted for the accurate determination of hypophosphite and phosphite alone or in their mixtures using alcoholic iodine, iodate, and periodate. Hg(I) was used as titrant for the produced iodide in case of using alcoholic iodine as oxidant or for the excess added iodide in case of iodate and periodate as oxidants using silver amalgam as the indicatior electrode. The potential breaks which average 300 mV per 0.1 ml of titrant were sharp enough for precise determination of endpoints, and hence the high accuracy of the present method.     

Fluorimetric determination of cerium with paracetamol

A simple, sensitive and selective method for determination of cerium(IV), based on the oxidative reaction between cerium(IV) and paracetamol, has been developed. The fluorescent species is an oxidation product of paracetamol and has excitation and emission maxima at 265 and 360 nm, respectively. The fluorescent intensity of the system is linear over the range 2.0 x 10(-7)-8.0 x 10(-6)M Ce(IV). The method has been applied in the determination of Ce(IV) in synthetic mixtures and ores with good accuracy being achieved.     

Flow-injection fluorimetric determination of trimeprazine and trifluoperazine in pharmaceutical preparations

A flow-injection configuration for the fluorimetric determination of trimeprazine and trifluoperazine is proposed. The procedure is based on oxidation of the drugs by cerium(IV). The fluorescence of cerium(III) formed in the oxidation of trimeprazine or trifluoperazine is monitored. Lineal calibration graphs were obtained between 2 x 10(-7) and 1 x 10(-5)M for both trimeprazine and trifluoperazine with a sampling rate of 60 samples/hr. The relative standard deviations were over the ranges 0.78-1.16 and 0.84-0.97% for trimeprazine and trifluoperazine respectively. The applicability of the method to determination of trimeprazine and trifluoperazine was demonstrated by investigating the effect of potential interferences and by analysis of commercial pharmaceutical preparations.     

Kinetic spectrophotometric determination of the macrolide antibiotic josamycin in formulations

A simple kinetic spectrophotometric method was developed for the determination of josamycin in its dosage forms. The method is based on oxidation of the drug with alkaline potassium permanganate at room temperature for a fixed time of 20 min and measuring the produced green color at 611 nm. The absorbance-concentration plot is rectilinear over the range of 2-10 microg/mL (2.4 x 10(6)-1.2 x 10(-5)M) with minimum detectability of 1.0 microg/mL (1.2 x 10(-6)M). The determination of josamycin by fixed concentration and the rate-constant methods is also feasible with the calibration equations obtained, but the fixed-time method proved to be more applicable. The procedure was applied successfully to commercial tablets, and statistical analysis showed that the results compared favorably with those obtained by reference methods. The effect of sensitizers and surfactants on the performance of the proposed method was also studied. A proposal of the reaction pathway was presented.     

Fabrication of herbicide biosensors based on the inhibition of enzyme activity that catalyzes the scavenging of hydrogen peroxide in a thylakoid membrane.

A novel herbicide biosensor with a thylakoid modified membrane electrode is presented. Thylakoid, isolated from spinach leaves, was entrapped in a membrane of poly (vinylalcohol) with the styrylpyridinium group (PVA-SbQ). The thylakoid membrane was fixed on the surface of a platinum electrode. It was found that the enzymes in thylakoid kept their activity for several months in the membrane. The oxidative current of hydrogen peroxide in a Tris-HCl buffer solution (pH 7.4) was detected at the modified electrode by a differential pulse voltammetric method. In the presence of herbicides, the oxidation current from the hydrogen peroxide decreased due to an inhibitor effect on the enzymes in thylakoid compared with that in the absence of the herbicides. The changes in the oxidation current at the electrode were proportional to the herbicide concentrations. The sensor could be used to detect herbicides in concentration ranges of 3 x 10(-9) - 1.5 x 10(-7) M for paraquat, 1 x 10(-8) - 3 x 10(-7) M for diuron, 4 x 10(-8) - 3 x 10(-6) M for prometryn, 5 x 10(-8) - 5 x 10(-6) M for atrazine and 1 x 10(-7) - 5 x 10(-6) M for ametryn, respectively. The enzyme activity on scavenging hydrogen peroxide in the modified PVA-SbQ membrane was examined.     

Application of inorganic oxidants to the spectrophotometric determination of ribavirin in bulk and capsules

Eight spectrophotometric methods for determination of ribavirin have been developed and validated. These methods were based on the oxidation of the drug by different inorganic oxidants: ceric ammonium sulfate, potassium permanganate, ammonium molybdate, ammonium metavanidate, chromium trioxide, potassium dichromate, potassium iodate, and potassium periodate. The oxidation reactions were performed in perchloric acid medium for ceric ammonium sulfate and in sulfuric acid medium for the other reagents. With ceric ammonium sulfate and potassium permanganate, the concentration of ribavirin in its samples was determined by measuring the decrease in the absorption intensity of the colored reagents at 315 and 525 nm, respectively. With the other reagents, the concentration of ribavirin was determined by measuring the intensity of the developed colored reaction products at the wavelengths of maximum absorbance: 675, 780, 595, 595, 475, and 475 nm for reactions with ammonium molybdate, ammonium metavanidate, chromium trioxide, potassium dichromate, potassium iodate, and potassium periodate, respectively. Different variables affecting the reaction conditions were carefully studied and optimized. Under the optimum conditions, linear relationships with good correlation coefficients (0.9984-0.9998) were found between the absorbance readings and the concentrations of ribavirin in the range of 4-1400 microg/mL. The molar absorptivities were correlated with the oxidation potential of the oxidants used. The precision of the methods were satisfactory; the values of relative standard deviation did not exceed 1.64%. The proposed methods were successfully applied to the analysis of ribavirin in pure drug material and capsules with good accuracy and precision; the recovery values were 99.2-101.2 +/- 0.48-1.30%. The results obtained using the proposed spectrophotometric methods were comparable with those obtained with the official method stated in the United States Pharmacopeia.     

Kinetic determination of traces of manganese in different materials by its catalytic effect on the methylene green-periodate reaction

The oxidation of Methylene Green by sodium periodate is a slow process. A kinetic method based on the catalytic effect of manganese(II) on this reaction in the presence of 1,10-phenanthroline as activator is described. The reaction is followed spectrophotometrically by measuring the decrease in the absorbance of the dye at 620 nm. Under the optimal experimental conditions [4 x 10(-5)M Methylene Green, 0.2M acetate buffer (pH 4), 2 x 10(-3)M 1,10-phenanthroline, 2.5 x 10(-3)M sodium periodate, 35 degrees ], manganese(II) between 0.2 and 30 ng ml is determined by the tangent method. The accuracy of the method and the influence of 44 foreign ions have been studied and an equation for the kinetics of the catalysed reaction is proposed. The procedure has been applied to the determination of manganese in water, milk and beer with excellent results.     

Optical detection of phenolic compounds based on the surface plasmon resonance band of Au nanoparticles

An indirect colorimetric method is presented for detection of trace amounts of hydroquinone (1), catechol (2) and pyrogallol (3). The reduction of AuCl4(-) to Gold nanoparticles (Au-NPs) by these phenolic compounds in the presence of cetyltrimethylammonium chloride (CTAC) produced very intense surface plasmon resonance peak of Au-NPs. The plasmon absorbance of Au-NPs allows the quantitative colorimetric detection of the phenolic compounds. The calibration curves derived from the changes in absorbance at lambda = 568 nm were linear with concentration of hydroquinone, catechol and pyrogallol in the range of 7.0 x 10(-7) to 1.0 x 10(-4)M, 6.0 x 10(-6) to 2.0 x 10(-4)M and 6.0 x 10(-7) to 1.0 x 10(-4)M, respectively. The detection limits were 5.3 x 10(-7), 2.5 x 10(-6) and 3.2 x 10(-7)M for the hydroquinone, catechol and pyrogallol, respectively. The method was applied satisfactorily to the determination of phenolic compounds in water samples and pharmaceutical formulations.     

Titrimetric determination of hydrogen peroxide in alkaline solution

Direct titration of hydrogen peroxide in alkaline bromide media has been accomplished with sodium hypochlorite. The relative standard deviation is 0.2%. A photometric end-point is recommended for the determination of 0.10-1.0 mequiv of peroxide. Larger samples are evaluated by use of Bordeaux Red as visual indicator. The hypochlorite procedure compares favourably with iodometry and permanganate in the analysis of commercial peroxides.     

Catalytic—kinetic determination of iodide,manganese and molybdenum with the use of an absorptiostat

The kinetic—catalytic determination of several catalysts by use of an absorptiostatic method is described. The determination of iodide in the range 0.5–5.0 $\text{μg}\text{45 ml}$ is based on its catalytic action on the cerium(IV)-arsenic(III) reaction. The catalytic action of manganese(II) on the reaction of malachite green with periodate was used for its determination in the range 1–10 $\text{μg}\text{45 ml}$. The determination of molybdenum(VI) in the range 10–100 $\text{μg}\text{40 ml}$ is based on its accelerating action on the oxidation of iodide with hydrogen peroxide.     

Flow injection determination of chromium(III) by pyrogallol chemiluminescence

A flow injection method is proposed for the determination of nanogram amounts of chromium(III) using a pyrogallol chemiluminescence system. It is based on its catalytic effect on the oxidation of pyrogallol with periodate at a neutral medium. The addition of 3-(N-morpholino)propanesulphonic acid to the reaction system increased the chemiluminescence signal for chromium(III). The present method allows the determination of 5-100ng/ml of chromium(III). The relative standard deviation of 2.2% (n = 10) was obtained at 20 ng/ml of chromium(III) and the detection limit (signal-to-noise ratio = 2) was 1 ng/ml with the sampling frequency of 25/hr.     

Simultaneous determination of dopamine, ascorbic acid and uric acid at poly (Evans Blue) modified glassy carbon electrode

A sensitive and selective electrochemical method for the determination of dopamine using an Evans Blue polymer film modified on glassy carbon electrode was developed. The Evans blue polymer film modified electrode shows excellent electrocatalytic activity toward the oxidation of dopamine in phosphate buffer solution (pH 4.5). The linear range of 1.0 x 10(-6)-3.0 x 10(-5) M and detection limit of 2.5 x 10(-7) M were observed in pH 4.5 phosphate buffer solutions. The interference studies showed that the modified electrode exhibits excellent selectivity in the presence of large excess of ascorbic acid and uric acid. The separation of the oxidation peak potentials for dopamine-ascorbic acid and dopamine-uric acid were about 182 mV and 180 mV, respectively. The differences are large enough to determine AA, DA and UA individually and simultaneously. This work provides a simple and easy approach to selectively detect dopamine in the presence of ascorbic acid and uric acid in physiological samples.     

Effect of oxidant type on the chemiluminescence intensity from the reaction of tris(2,2′-bipyridyl)ruthenium(III) with various organic acids

An investigation into the chemiluminescence of fourteen organic acids and tris(2,2′-bipyridyl)ruthenium(II) was undertaken. Particular emphasis was placed upon the method of production of the reagent, tris(2,2′-bipyridyl)ruthenium(III), with cerium(IV) sulfate, potassium permanganate, lead dioxide and electrochemical generation. Analytically useful chemiluminescence was observed when Ce(IV) or potassium permanganate were employed as oxidants. The kinetics of analyte oxidation was related to the intensity of the chemiluminescence emission, which increased by three orders of magnitude for tartaric acid after 40 h of oxidation.     

Spectrophotometric determination of acyclovir and ribavirin in their dosage forms

Two simple, accurate, and reliable spectrophotometric methods have been developed for the determination of 2 antiviral drugs, acyclovir (ACV) and ribavirin (RBV), in their pharmaceutical formulations. These methods are based on oxidation of the 2 drugs with either cerium (IV) ammonium sulfate (Method A) or potassium persulfate (Method B). The products of oxidation in both methods are coupled with 3-methylbenzothiazolin 2-one hydrazone, producing a deep blue color with a maximum absorption wavelength at 630 nm. In Method A, the absorbance-concentration plots were linear over the ranges of 5-50 and 10-60 microg/mL with detection limits of 0.18 microg/mL (8 x 10(-7) M) and 0.63 microg/mL (2.58 x 10(-6) M) for ACV and RBV, respectively. In Method B, the ranges were 5-45 and 20-50 microg/mL with detection limits of 0.11 microg/mL (4.88 x 10(-7) M) and 1.40 microg/mL (5.73 x 10(-6) M) for the 2 drugs, respectively. The molar absorptivities were 4.1 x 10(3) and 3.65 x 10(3) L/mol/cm in Method A and 5.03 x 10(3) and 3.97 x 10(3) L/mol/cm in Method B for the 2 drugs, respectively. The proposed methods were applied successfully for the determination of the 2 drugs in their pharmaceutical formulations. The percentage recoveries +/- standard deviation were 99.57 +/- 0.86 and 100.82 +/- 0.46 for ACV; 99.41 +/- 1.08 and 100.35 +/- 1.03 for RBV. The results obtained were compared statistically with those given by official methods and showed no significant differences regarding accuracy and precision.     

Simultaneous kinetic spectrophotometric determination of periodate and iodate based on their reaction with pyrogallol red in acidic media by chemometrics methods

The univariate and multivariate calibration methods were applied for the simultaneous determination of iodate and periodate in water. The method is based on the reaction of periodate and iodate with pyrogallol red in sulfuric acid media. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of pyrogallol red at 470 nm. The calibration curve was linear over the concentration ranges of 0.1-12 and 0.1-14 μg ml⁻¹ for iodate and periodate, respectively. The experimental calibration matrix for partial least squares (PLS) and orthogonal signal correction (OSC-PLS) method was designed with 35 mixtures. The results obtained by the PLS and OSC-PLS were statistically compared. The effects of various cations and anions on iodate and periodate determination have been investigated.     

Electrocatalytic characteristics of a 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone modified carbon-paste electrode in the oxidation of ascorbic acid.

A carbon-paste electrode spiked with 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone was constructed by the incorporation of 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone in a graphite powder silicon oil matrix. It shown by cyclic voltammetry and double potential-step chronoamperometry, which this ferrocene derivative modified a carbon-paste electrode, can catalyze the ascorbic acid oxidation in an aqueous buffered solution. It has been found that under the optimum conditions (pH 7.00), the oxidation of ascorbic acid at the surface of this carbon paste modified electrode occurs at a potential of about 260 mV less positive than that of an unmodified carbon-paste electrode. The catalytic oxidation peak current was linearly dependent on the ascorbic acid concentration, and a linear calibration curve was obtained in the range of 6 x 10(-5) M-7 x 10(-3) M of ascorbic acid with a correlation coefficient of 0.9997. The detection limit (2sigma) was determined to be 6.3 x 10(-5) M. This method was also used for the determination of ascorbic acid in some pharmaceutical samples, such as effervescent tablets, ampoules and multivitamin syrup, by using a standard addition method. The reliability of the method was established by a parallel determination against the official method.     

A new liquid-membrane electrode for selective determination of perchlorate

A new liquid-membrane electrode which responds to perchlorate ion is described. It incorporates a 10(-2)M solution of the nitron-perchlorate ion-pair complex, in nitrobenzene, as a liquid membrane. The electrode exhibits near-Nernstian response for 10(-2)-2 x 10(-5)M perchlorate with an anionic slope of 56 mV pClO (4)). The response time is 20-90 sec, the working pH 2.5-8.5, the lower limit of detection 8 x 10(-6)M perchlorate and the selectivity for perchlorate relative to 27 inorganic and organic anions of different nature is reasonably high. Periodate, permanganate and thiocyanate, however, interfere. Determination of 2-1000 microg ml perchlorate in aqueous solutions shows an average recovery of 98.8% and a mean relative standard deviation of 1.9%. The electrode has been successfully used for direct potentiometric determination of the purity of perchlorate propellants and the solubility products of some sparingly soluble perchlorates.     

Peroxidase-catalysed luminol chemiluminescence method for the determination of glutathione

A luminol chemiluminescence (CL) method was developed for the determination of glutathione (GSH). GSH was indirectly determined by measuring the amount of hydrogen peroxide formed during the Cu(II)-catalysed oxidation of GSH with oxygen. The amount of hydrogen peroxide formed was continuously measured using the Arthromyces ramosus peroxidase-catalysed luminol CL reaction. The CL intensities at maximum light emission were linearly correlated with the concentration of GSH over the range 7.5 x 10(-7)-3.0 x 10(-5) M. The detection limit for GSH was about 10 times better than that of the spectrophotometric method using Ellman reagent.