Determination of amoxicillin in pharmaceuticals using sequential injection analysis and multivariate curve resolution

In this study, we report a method for quantifying amoxicillin in pharmaceuticals in the presence of interferents using sequential injection analysis (SIA) with a diode-array spectrophotometric detector and multivariate curve resolution with alternating least squares (MCR-ALS). With a suitable analytical sequence, we can use SIA to generate a pH gradient and, for each sample, obtain a data matrix. We used augmented matrices to resolve the system and obtain the spectra and concentration profiles of the components in the sample.We studied what are the effects of imposing trilinearity at the resolution stage, how to choose the species that will be used for quantification (acid, basic or the sum of the two), and which is the most suitable concentration of the reference standard. Once the optimum conditions were established, we performed the quantification in three amoxicillin-containing pharmaceuticals (flubiotic, augmentine, and clamoxyl). With this method, determination is quick, the reactants and instrumentation are inexpensive, and pretreatment of the sample is unnecessary.     

Use of multivariate curve resolution for determination of chromium in tanning samples using sequential injection analysis

We report a method for determining total chromium in tanning samples using sequential injection analysis (SIA) with a diode-array spectrophotometric detector. With a suitable analytical sequence CrO₄^2– is converted to Cr₂O₇^2– inside the tubes of the SIA system, after total oxidation of chromium(III). A data matrix is obtained and analysed by several chemometric techniques based on multivariate analysis: principal components analysis, simple-to-use interactive self-modelling mixture analysis, and multivariate curve resolution-alternating least-squares. We studied several samples from different stages of a tanning process. Two of these samples were easily oxidized but the others needed more extreme conditions. The analytical sequence prepared, which was based on obtaining a pH gradient and used H₂SO₄ as reagent, is valid and independent of the level of oxidation needed for the sample. We established a calibration model and evaluated the figures of merit. In some samples we found interferents. With this method the amounts of chromium in each sample were quantified and the results were statistically similar to those obtained by use of the reference method, atomic absorption spectrometry.     

Trimethylsilyl-substituted diazoalkanes : I. Trimethylsilyldiazomethane

Trimethylsilyldiazomethane was prepared by the action of aqueous KOH on nitroso-N-(trimethylsilylmethyl)urea. The spectroscopic properties of this stable, greenish-yellow liquid which can be isolated by gas chromatography are discussed. Its reaction with acetic acid gives the expected CH₃CO₂CH₂SiMe₃ in addition to SiC cleavage products, CH₃CO₂CH₃ and CH₃CO₂SiMe₃. Products of the 1,3-dipolar addition of Me₃SiCHN₂ to activated olefins were not very stable, and only the adduct with acrylonitrile was isolated as a pure material Trimethylsilyldiazomethane undergoes Me₃SiCH transfer to olefins, giving trimethylsilyl-substituted cyclopropanes, in the presence of CuCI in benzene, but other products are formed as well. Thus such a reaction with cyciohexenegave anti-7-trimethylsilylnorcarane (65%), syn-7-trimethylsilylnorcarane (7%), cis- and trans-l,2-bis(trimethylsilyl)ethylene (9% and 13%, respectively), and an unidentified Me₃SiCH trimer (2.3%).     

Exploiting sequential injection analysis with lab-on-valve and miniaturized potentiometric detection: Epinephrine determination in pharmaceutical products

Miniaturized potentiometric units were constructed, evaluated and incorporated in a SIA-LOV manifold in order to the control of pharmaceutical analysis. The method validation was done with ephinefrine determinations in commercial pharmaceutical products. The optimization procedures were directed at potential versus epinephrine concentration. This approach was achieved by selecting 60 cm of reactor and a flow rate of 11 μL s⁻¹ and injecting 78 μL of epinephrine standard solutions in a 1.0 × 10⁻³ mol L⁻¹ IO₄⁻ solution. A linear range was found for epinephrine concentrations between 2.0 × 10⁻⁴ and 2.5 × 10⁻³ mol L^−l with a slope of 35528 mV L mol^−l and r² = 0.997. Under these conditions the analytical results for the commercial pharmaceutical formulations of 0.908 and 0.454 mg mL⁻¹, respectively, with a R.S.D. of 0.34 for both, was obtained. Through down scaling periodate-selective electrode it was possible to benefit from the recognized advantages of the lab-on-valve sequential-injection based systems, namely regarding equipment portability, reduced consumption of the sample and the reagents and the reduction of effluent waste. Furthermore, the new periodate electrode configuration is easy to achieve in common laboratories and enables the implementation of low volume detection cell where the electrical noise, that is frequently presented in potentiometric based procedures, is significantly reduced.     

A versatile set up for implementing different flow analysis approaches: Spectrophotometric determination of nickel in steel alloys

An arrangement capable of implementing the four principal types of flow analysis processes, monosegmented flow analysis (MSFA), flow injection analysis with multicommutation and binary sampling (FIA-MBS), flow injection analysis with sandwich sampling (FIA-SS) and sequential injection analysis (SIA) is described. The core of the flow manifold is a six-way solenoid valve that is assembled together with three three-way solenoid valves in order to provide a versatile flow network. Software was written in VisualBasic 3.0 to give a friendly working structure allowing the user to easily choose the flow variables and the kind of flow system. The reliability of the flow set up for implementation of the four flow analysis systems was evaluated by means of the spectrophotometric determination of nickel in steel alloys, based on the formation of a colored complex with dimethylglyoxime (DMG). The performances of the four different flow methodologies were compared. The reagent consumptions per determination were 4.0 mg of triethanolamine, 6.0 mg of potassium persulfate and 0.6 mg of DMG. When the flow set up was instructed through the software to implement MSFA, FIA-MBS, and FIA-SS approaches, a sampling frequency of 40 samples/h was obtained, while 30 samples could be processed per hour in the SIA mode. The precisions, evaluated as the relative standard deviation of ten determinations were 0.7%, 1.6%, 1.8% and 3.1% for the MFSA, FIA-MBS, FIA-SS and SIA systems, respectively. The results for determination of nickel in steel alloys presented good agreement with the reference method (ICP OES), showing no significant difference at a confidence level of 95%.     

Rotational isomerism—XIII : Rotational isomerism in furfuraldehyde

The equilibrium between the cis and trans isomers of furfuraldehyde has been studied by measuring the stereospecific coupling constants between the aldehyde and ring protons in various solvents. These values when combined with the observed values of the couplings in the individual conformers at low temperatures give the energy difference between the cis and trans forms in these solvents. These values together with other previous measurements demonstrate the large solvent effect on this equilibrium. E(cis)-E(trans) is +1·5 kcals/mole in the vapour, much smaller in non-polar solvents (+0·3 kcals/mole in CF₂Cl₂), ca 0 in CHCl₃ and becomes negative in polar media (−1·0 kcals/mole in the pure liquid). This solvent dependence is given a quantitative explanation in terms of a known theory of solvation.     

Metal carbonyl-trifluorophosphine systems : X. Mono-and triruthenium carbonyls

Triruthenium dodecacarbonyl reacts with high pressure PF₃ or with PF₃/CO mixtures to yield two series of compounds. At high PF₃ pressures, the mononuclear species Ru(PF₃)_x(CO)_5−x (x = 5, 4 and 3) are the main products with only traces of Ru(PF₃)₂(CO)₃ and Ru(PF₃)(CO)₄ being found. With high carbon monoxide pressure and less PF₃, the species Ru₃(PF₃)_y(CO)_12−y (y = 0–6) are the main products. It is seen that Ru₃(CO)₁₂ can have up to six CO groups replaced sequentially before the cluster is broken followed by the formation of the mononuclear species. The evidence shows that the Ru(PF₃)_x(CO)_5−x species are stereochemically non-rigid.     

Mechanisms involved in stannane generation by aqueous tetrahydroborate(III): Role of acidity and l-cysteine

The role played by acidity (0.01–5 mol L^− 1 HNO₃) and l-cysteine (0.1–0.2 mol L^− 1) in the formation of stannane by reaction of Sn(IV) solution with aqueous tetrahydroborate(III) (0.05–0.2 mol L^− 1), has been investigated by continuous flow hydride generation coupled with atomic absorption spectrometry using a miniature argon–hydrogen diffusion flame as the atomizer. Different mixing sequences and reaction times of the reagents were useful in the identification of those processes which contribute to the generation of stannane in different reaction conditions, both in the absence and in the presence of l-cysteine. The lack of stannane generation at high acidities is due to the formation of Sn substrates and hydridoboron species which are unreactive. The capture of the stannane in solution, following its ionization to SnH₃⁺ from already formed stannane, does not play any role. While the presence of l-cysteine, does not affect the generation efficiency at lower acidities, it expands the optimum range of acidities for stannane generation to higher values. This effect can be addressed to both the buffering capacity of l-cysteine and to the formation of Sn-(l-cysteine) complexes, while the formation of (l-cysteine)–borane complexes do not play a significant role. Formation of Sn-(l-cysteine) complexes also appears to be useful for stabilization of tin solution at low acidities values.     

Rapid automated assay of anti-oxidation/radical-scavenging activity of natural substances by sequential injection technique (SIA) using spectrophotometric detection

A PC-controlled sequential injection analysis (SIA) system equipped with a spectrophotometric diode-array detector is used for rapid monitoring and evaluation of antioxidation/radical scavenging activity of biological samples. The automated method is based on the known reaction of stable 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) with antioxidants in organic or aqueous-organic media resulting in bleaching of DPPH due to its quenching by the interaction with the analytes. The decrease of the absorbance of DPPH (compared to blank experiment carried out with water–ethanol 1:1 instead of the test solution) measured at 525 nm is related to concentration of an antioxidant in the test solution. With the optimised SIA procedure it is possible to detect down to micromolar concentrations of model antioxidants such as ascorbic acid, caffeic acid, (+)-catechin, (–)-epicatechin and rutin and to evaluate the concentration of these antioxidants in the micromolar to millimolar range. The sample throughput is 45 h^–1. Thanks to its rapidity and sensitivity, the proposed SIA method is suitable for performing routine screening tests for the presence of various antioxidants in large series of lyophilised herbal or mushroom extracts (the amount of sample needed for the analysis is several milligrams).     

Etudes photochimiques—XIV : Photoreactions des methylisothiazoles

Photorearrangement of methylisothiazoles proceeds via a zwitterion and the amount of rearranged products increases with solvent polarity. When irradiated in the presence of primary amines, 3-methylisothiazole which is the less isomerised, and 5 methylisothiazole give charge transfer complexes. The amine cation radical is hydrolysed to an aldehyde. 4-Methyl isothiazole does not lead to a charge transfer complex.     

Elemental fluorine : Part 15. Selective direct fluorination of quinoline derivatives

Direct fluorination of various quinoline derivatives in acidic reaction media gives fluorinated quinoline products arising from electrophilic substitution processes.     

Flechteninhaltsstoffe—XCIII : Struktur der (−)-Placodiolsäure

The structure of(−)-placodiolic acid, a new dibenzofurane derivative of the lichen Lecanorarubina (Vill.) Ach. has been established as (−)-isousnic acid isomethoxide (I).