Andriy B. Vishnikin

Stepwise injection spectrophotometric determination of epinephrine

Simple, rapid and fully automated methods for the manual and automated spectrophotometric determination of epinephrine have been developed by using schemes of stepwise injection (SWIA) and sequential injection analysis (SIA) implemented in the same manifold. The determination is based on the formation of reduced form of 18-molybdodiphosphate heteropoly anion by its reaction with epinephrine. Using of the reaction vessel in the general SWIA configuration instead of a holding and reaction coil in the SIA manifold provides several essential advantages, including higher sensitivity and lower reagent consumption. The linear dependence of the analytical signal on the epinephrine concentration was preserved over the range of 1.5-30, 3.0-30, and 1.5-25μmolL(-1) by using of SWIA, SIA and spectrophotometric analysis, respectively. The relative standard deviation for the SWIA determination of 10μmolL(-1) epinephrine was 1.8% (n=10).

Highly sensitive sequential injection determination of p-aminophenol in paracetamol formulations with 18-molybdodiphosphate heteropoly anion based on elimination of Schlieren effect.

A highly sensitive, precise and automated method using sequential injection analysis to assay quantitatively low levels of the p-aminophenol (PAP) in paracetamol formulations has been developed. A solution containing PAP and paracetamol is injected into an acetate buffer carrier stream and merged on-line with 18-molybdodiphosphate heteropoly complex reagent to form a specific blue derivative that is subsequently detected spectrophotometrically at 820nm. The procedure has been optimized mainly with respect to measurement sensitivity. It is based on the leveling off of the refraction indices of the liquids mixed in the flow system by the careful matching of the refractive index of the reagent solution with that of the carrier and sample solutions. Amount as low as 0.5μmolL(-1) of PAP, which corresponds to the 0.001% of PAP in paracetamol tablets, can be reliably determined using the proposed method, which is clearly below the specification limits recommended for PAP determination in paracetamol drug and tablet formulations (50ppm or 0.005% (w/w)). The developed method was successfully applied to the analysis of paracetamol formulations spiked with PAP and determination of PAP content in Rapidol tablets past their expiration date both by proposed SIA and reference HPLC methods.

Determination of Ascorbic Acid with Wells-Dawson Type Molybdophosphate in Sequential Injection System

Two-electron reduced heteropolyblue is formed very fast at pH 3.75–4.75 in the reaction between Dawson-type molybdophosphate HPA and ascorbic acid (AsA) (ϵ814 = 1.15 × 104 L · mol−1 cm−1). Simple, fast, and direct SIA method for AsA determination was developed. Under found optimal conditions, linear calibration curve was obtained over the range 3 × 10−6–3.5 × 10−4 mol L−1 AsA, and detection limit (3 s) was 1 × 10−6 mol L−1. The proposed SIA method has high sample throughput of 60 h−1 and small reagent consumption. The procedure was applied to the analysis of pharmaceuticals and juices.

Using an Optical Probe as the Microdrop Holder in Headspace Single Drop Microextraction: Determination of Sulfite in Food Samples

A novel headspace single-drop microextraction method (HS-SDME) for determination of sulfite in the form of sulfur dioxide was developed. An optical probe was used as the droplet holder in the HS-SDME procedure, and the analytical signal (absorbance) was monitored online during the extraction process. The method is based on the conversion of sulfite to volatile sulfur dioxide by acidification of the analyzed solution. The liberated SO2 was absorbed by 25 μL of an aqueous mixed reagent solution placed on the optical probe tip and containing Fe(III), 1,10-phenantroline, and an acetic buffer solution of pH 5.6. During the extraction process, Fe(III) reduces to Fe(II) and the Fe(II) formed then reacts with 1,10-phenantroline to form a colored complex. Absorbance was measured at 510 nm. The calibration plot was linear in the range 0.032-0.320 mg L-1 of sulfite (as SO2), with a correlation coefficient of 0.9989. The limit of detection (LOD), calculated as three times the standard deviation of the blank test (n = 10), was found to be 8 μg L-1. The method was applied for analysis of real food samples, such as wine, jam, and juice.

11-Molybdobismuthophosphate—A new reagent for the determination of ascorbic acid in batch and sequential injection systems

The guanidinium salt of the new heteropolymolybdate 11-molybdobismuthophosphate Gua(6)PBiMo(11)O(40) (11-MBP) was synthesized, characterized and used as a reagent for batch spectrophotometric (SP) and sequential injection determination of ascorbic acid (AsA). When compared to other Keggins heteropolyanions, the reduction of 11-MBP with AsA is both fast and maximal within a pH range of 1.6-2.0. The stoichiometry of the reaction was determined using molar ratio and continuous variation methods and was shown to be 1:1. The molar absorptivity of the reduced form of 11-MBP was 6.0 x 10(3)L mol(-1)cm(-1) at 720 nm. The reaction is also specific for AsA. Only cysteine, hydroquinone and hydroxyacids were found to interfere with the reaction, while no interference was observed with the common reducing agents, including reducing sugars, catecholamines, nitrite, sulfite and iron(II) ions. Batch SP and sequential injection analysis (SIA) systems were developed for the determination of AsA, with calibration ranges of the SP methods at 2x10(-6)-8 x 10(-5)M for a 10mm cell and 5 x 10(-7)-3 x 10(-5)M for a 50mm cell and a limit of detection at 3 x 10(-7)M. The linear range of the SIA method was 6 x 10(-6)-5 x 10(-4)M, with a detection limit of 2 x 10(-6)M and a sample throughput of 15 h(-1). The proposed methods were successfully used for the determination of AsA in both pharmaceuticals and fruit juices, and the results were consistent with those provided by the 2,6-dichlorophenolindophenol method.

Simultaneous determination of rutin and ascorbic acid in a sequential injection lab-at-valve system

Graphical abstract Figure. No caption available. HighlightsSimultaneous determination of two reducing agents in pharmaceuticals.Improvement of precision and sensitivity in a sequential injection lab‐at‐valve system.Minimization of volume of effluents, reagent, and sample consumption.Dependence of spectrum of Dawson heteropoly blue on ratio of reducer to reagent. ABSTRACT A green, simple, accurate and highly sensitive sequential injection lab‐at‐valve procedure has been developed for the simultaneous determination of ascorbic acid (Asc) and rutin using 18‐molybdo‐2‐phosphate Wells‐Dawson heteropoly anion (18‐MPA). The method is based on the dependence of the reaction rate between 18‐MPA and reducing agents on the solution pH. Only Asc is capable of interacting with 18‐MPA at pH 4.7, while at pH 7.4 the reaction with both Asc and rutin proceeds simultaneously. In order to improve the precision and sensitivity of the analysis, to minimize reagent consumption and to remove the Schlieren effect, the manifold for the sequential injection analysis was supplemented with external reaction chamber, and the reaction mixture was segmented. By the reduction of 18‐MPA with reducing agents one‐ and two‐electron heteropoly blues are formed. The fraction of one‐electron heteropoly blue increases at low concentrations of the reducer. Measurement of the absorbance at a wavelength corresponding to the isobestic point allows strictly linear calibration graphs to be obtained. The calibration curves were linear in the concentration ranges of 0.3–24 mg L−1 and 0.2–14 mg L−1 with detection limits of 0.13 mg L−1 and 0.09 mg L−1 for rutin and Asc, respectively. The determination of rutin was possible in the presence of up to a 20‐fold molar excess of Asc. The method was applied to the determination of Asc and rutin in ascorutin tablets with acceptable accuracy and precision (1–2%).

Sequential injection determination of orthophosphate as ion associate of 12-molybdophosphate with Astra Phloxine

A simple and fast reaction between 12-molybdophosphate heteropoly anion and the polymethine dye Astra Phloxine was used for the development of a new SIA method for the determination of orthophosphate. The suggested method has higher sensitivity and a broader calibration range than existing SIA methods. It can be used to detect phosphate in the range from 0.2 to 10 μmol L(-1) with a detection limit of 0.1 μmol L(-1) and an acceptable throughput of 20 samples h(-1). The comparably low molybdate (5.6 mmol L(-1)) and dye (0.1 mmol L(-1)) concentrations led to an improvement in the stability of the base line. Inter-day reproducibility was better than 5%, while the intra-day RSD was in the range 0.8-3.5%. The method was used for the analysis of natural water samples and phosphorus-containing chemicals having a low content of orthophosphate.

Use of sequential injection analysis with lab-at-valve and an optical probe for simultaneous spectrophotometric determination of ascorbic acid and cysteine by mean centering of ratio kinetic profiles

Two new, simple, relatively fast and robust methods for the simultaneous kinetic determination of binary mixtures of ascorbic acid (Asc) and cysteine (Cys) were developed using the mean centering of ratio kinetic profiles method. The methods are based on the difference in the reaction rates of Asc and Cys with 18-molybdodiphosphate at pH 5.1. An optical probe as well as the sequential injection analysis lab-at-valve (SIA-LAV) method were used to carry out simultaneous kinetic analysis. The benefits of the mean centering of ratio kinetic profiles method were shown in comparison with other spectrophotometric kinetic methods. Asc and Cys can be determined in the concentration ranges 20-200 and 8-90u202fµmolu202fL-1 with the batch spectrophotometric method and 10-200 and 4-40u202fµmolu202fL-1 with the SI-LAV method, respectively. The method was successfully used to determine Asc and Cys in dietary supplements.