Disposable electrochemical flow cells for catalytic adsorptive stripping voltammetry (CAdSV) at a bismuth film electrode (BiFE)

Catalytic adsorptive stripping voltammetry (CAdSV) has been demonstrated at a bismuth film electrode (BiFE) in an injection-moulded electrochemical micro-flow cell. The polystyrene three-electrode flow cell was fabricated with electrodes moulded from a conducting grade of polystyrene containing 40% carbon fibre, one of which was precoated with Ag to enable its use as an on-chip Ag/AgCl reference electrode. CAdSV of Co(II) and Ni(II) in the presence of dimethylglyoxime (DMG) with nitrite employed as the catalyst was performed in order to assess the performance of the flow cell with an in-line plated BiFE. The injection-moulded electrodes were found to be suitable substrates for the formation of BiFEs. Key parameters such as the plating solution matrix, plating flow rate, analysis flow rate, solution composition and square-wave parameters have been characterised and optimal conditions selected for successful and rapid analysis of Co(II) and Ni(II) at the ppb level. The analytical response was linear over the range 1 to 20 ppb and deoxygenation of the sample solution was not required. The successful coupling of a microfluidic flow cell with a BiFE, thereby forming a “mercury-free” AdSV flow analysis sensor, shows promise for industrial and in-the-field applications where inexpensive, compact, and robust instrumentation capable of low-volume analysis is required.     

Development and validation of a sequential-injection method with chemiluminescence detection for the high throughput assay of the total antioxidant capacity of wines

This work reports a sequential-injection analysis (SIA) method with chemiluminescence (CL) detection for the rapid assay of the total antioxidant capacity (TAC) in wines. The method exploited the Co(II)-catalysed CL reaction of luminol with hydrogen peroxide in alkaline medium. Zones of sample, hydrogen peroxide, catalyst (Co(II) solution) and alkaline luminol were sequentially aspirated into the holding coil of the SIA manifold. Then, the flow was reversed and the stacked zones were directed to the CL detector. As the zones overlapped, antioxidants in the samples scavenged a portion of hydrogen peroxide and the decrease in the CL intensity was monitored and related to the TAC. The chemical and geometric conditions were studied and the method was validated in terms of linearity, accuracy (trueness and precision), matrix effects, signal additivity and robustness. The reproducibility of the method (expressed as the between-days % relative standard deviation) was between 2.5 and 3.4% and the trueness (expressed as the % recovery in wines spiked with gallic acid) was in the range 96.7-97.3%. The sampling frequency was 60 samples h⁻¹. The proposed SIA-CL method was compared with the DPPH method and the Folin-Ciocalteau (FC) method for the analysis of 25 wine samples.     

Computation of partial molar properties using continuous fractional component Monte Carlo

ABSTRACT

An alternative method for calculating partial molar excess enthalpies and partial molar volumes of components in Monte Carlo (MC) simulations is developed. This method combines the original idea of Frenkel, Ciccotti, and co-workers with the recent continuous fractional component Monte Carlo (CFCMC) technique. The method is tested for a system of Lennard–Jones particles at different densities. As an example of a realistic system, partial molar properties of a [NH₃, N₂, H₂] mixture at chemical equilibrium are computed at different pressures ranging from P = 10 to 80 MPa. Results obtained from MC simulations are compared to those obtained from the PC-SAFT Equation of State (EoS) and the Peng–Robinson EoS. Excellent agreement is found between the results obtained from MC simulations and PC-SAFT EoS, and significant differences were found for PR EoS modelling. We find that the reaction is much more exothermic at higher pressures.     

Rapid enzymatic chemiluminescent assay of glucose by means of a hybrid flow-injection/sequential-injection method

This work reports a hybrid flow-injection analysis (FIA)/sequential-injection analysis (SIA) method for the rapid enzymatic assay of glucose with soluble glucose oxidase (GOD). The method relies on the sequential injection of segments of the sample and of a solution of enzyme by means of a multi-port selection valve in a flowing water stream. As the two zones are swept downstream, they overlap and merge so that the glucose in the sample is enzymatically oxidised. The generated hydrogen peroxide is merged with an alkaline luminol solution and the chemiluminescence (CL) intensity is monitored and related to the glucose concentration in the sample. The linear range of the method for glucose determination is 0.01-1 mmol L⁻¹, the relative standard deviation is 3.9% at the 0.08 mmol L⁻¹ level (n = 8), the limit of detection at the 2σ level is 4 μmol L⁻¹ glucose and the injection rate is 80 h⁻¹. The method was applied to the analysis of energy drinks and honey with relative errors in glucose determination in the range 100 ± 4.3%. The advantages of the proposed method are the wide linear range, the simple instrumentation used, the low consumption of sample and reagents, the elimination of catalysts and immobilised enzymes and the high sample throughput.     

Application of ternary complexes in pharmaceutical analysis

Summary The formation of a ternary complex of iron(III) with isoniazid-2-hydroxybenzaldehyde hydrazone(INSH) in a cetyltrimethylammonium bromide(CTAB) micellar medium, leads to a simple, sensitive and accurate spectrophotometric microdetermination of the iron(III), either in pure aqueous solutions or, particularly, in various anti-anaemic formulations. The apparent molar absorptivity of this complex and Sandell's sensitivity at 386 nm were 2.08×10⁴ lmol^–1 cm^–1 and 2.68 ngcm^–2, respectively. The calibration graph which was traced according to the regression line equation, A=3.70×10^–1 C+1.09×10^–3 (r=1.0000; n=28), was rectilinear for 100 ppb to 3.0 ppm of iron(III). The accuracy and the precision of the method could be considered as very satisfactory, since the mean CV% was 0.202 in the range 0.5–3.0 g of iron(III) per ml. The results obtained for iron(III) by the new method and by the o-phenanthroline method, were compared statistically by means of the Student t-test and the variance ratio F-test; no significant difference was found.Parts I and II see [6] and [7]     

Sequential injection method for the direct spectrophotometric determination of bismuth in pharmaceutical products

A spectrophotometric method is reported for the determination of bismuth in pharmaceutical products using sequential injection analysis. Methylthymol blue (MTB) was used as a color forming reagent and the absorbance of the Bi(III)-MTB complex was monitored at 548 nm. The various chemical and physical variables that affected the reaction were studied. A linear calibration graph was obtained in the range 0.0-75.0 mg l⁻¹ Bi(III) at a sampling frequency of 72 h⁻¹. The reagent consumption was considerably reduced compared to conventional flow injection systems, as only 150 μl of MTB were consumed per run. The precision was very satisfactory (s_r=0.5%, at 50.0 mg l⁻¹ Bi(III), n=12) and the limit of detection, c_L, was 0.250 mg l⁻¹. The developed method was applied successfully to the analysis of various pharmaceutical products containing Bi(III). The relative errors e_r, were <1.5% in all cases and were evaluated by comparison of the obtained results with those found using atomic absorption spectrometry as the reference method.     

Determination of methimazole and carbimazole by flow-injection with chemiluminescence detection based on the inhibition of the Cu(II)-catalysed luminol-hydrogen peroxide reaction

This paper reports an indirect flow-injection (FI) method for the determination of the anti-hyperthyroid drugs methimazole and carbimazole in pharmaceuticals. The method was based on the inhibition that these thioimidazole drugs caused on the Cu(II)-catalysed chemiluminescence (CL) reaction between luminol and H₂O₂. The CL reaction was induced on-line and injection of the sample produced negative peaks as a result of the Cu(II) complexation by the analytes. The height of the FI peaks was proportional to the drug concentration in the sample. The linear range was 2-100 and 3-120 mg l⁻¹ for methimazole and carbimazole, respectively. The relative standard deviation was 1.9% for methimazole and 2.1% for carbimazole at the 50 mg l⁻¹ level (n=10). Common excipients present in pharmaceutical tablets were found not to interfere with the analysis. The method was applied to the determination of methimazole and carbimazole in pharmaceutical formulations with recoveries in the range 100±4%.