Multiple factor analysis in environmental chemistry

In environmental chemistry studies, it may be necessary to analyze data sets constituted by different blocks of variables, possibly of different types, measured on the same samples. Multiple factor analysis (MFA) is presented as a tool for exploring such data. The most important features of MFA are shown on a real environmental data set, consisting of two blocks of data, namely heavy metals and polycyclic aromatic hydrocarbons, measured for sediment samples. They are discussed and compared to principal component analysis (PCA). The usefulness of the weighting scheme used in MFA as a preprocessing step for other chemometric methods, such as clustering, is also highlighted.     

A case study in conformation design: learning by doing

Efforts are described to design simple, fully flexible but conformationally preorganised omega-hydroxy-nonanoic acids that could serve as the conformation controlling unit in analogues of the potent protein-kinase C activator aplysiatoxin. Such analogues are macrodilactones incorporating the designed omega-hydroxy-nonanoic acid and 3,4-dihydroxy-pentanoic acid, which contains the pharmacophoric groups. The design process (replacement of CH(2) groups by an oxygen atom, annelation of a six-membered ring and placement of alkyl substituents) of the omega-hydroxy-nonanoic acids was monitored by force-field calculations. In the end of this process simple analogues of aplysiatoxin are proposed in which the proper disposition of the pharmacophoric groups is secured by a conformationally flexible but preorganised template structure as part of the macrodilactone ring.     

Critical Comparison of Four Simple Adaptors for Flow Amperometric and Stripping Voltammetry with Mercury Drop Electrodes in Batch Cells

Electrochemical batch cells with mercury drop electrodes, MDEs, are readily available from a number of producers and widely used in electroanalysis for polarography, voltammetry and stripping voltammetry. To increase sample throughput and reduce reagent and sample consumption, the tendency is to couple the MDE with flow analysis systems (e.g., FIA, SIA or BIA). Many special flow cells for MDEs are described in the literature, but it is easier to convert existing commercial batch cells to flow operation. To assess the performance of such flow adaptors, four models were chosen because they are directly fitted to the glass capillary of any MDE, light enough not to impair the mechanical drop knocker and can be effortlessly built in the laboratory. They were all found adequate for flow amperometry and stripping voltammetry, with differences in sensitivity, flow rate dependence, response time, drop stability, tolerance to bubbles, uncompensated resistance and recontamination of disposed analyte from the electrolytic bath. Two of them, a simple L‐shaped PTFE tube and an upstream nozzle holder made from a disposable pipette tip, gather a larger set of desirable features.     

4-Hydroxy-2-quinolones 120. Synthesis and structure of ethyl 2-hydroxy-4-oxo-4H-pyrido-[1,2-a]pyrimidine-3-carboxylate

An improved method for the preparation and purification of ethyl 2-hydroxy-4-oxo-4H-pyrido[1,2-a]-pyrimidine-3-carboxylates is proposed. According to ¹H and ¹³C NMR spectroscopic data the synthesized compounds exist in DMSO solution in the 2-hydroxy-4-oxo form while in the crystal (at least for the case of the unsubstituted derivative) X-ray analysis shows that it occurs in the bipolar 2,4-dioxo form. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 864–876, June, 2007.     

Comparative study on separation of diastereomers by HPLC

Summary Reversed (RP-HPLC) and normal phase chromatographic (NP-HPLC) separations have been developed for diastereomers ofN-acyl-1-methyl-1,2,3,4-tetrahydo-β-carbolines which are acylated derivatives of simple natural β-carboline alkaloids. Separations of derivatives having different acyl moieties in theO,O-diacyl-tartaric acid ester subtituent differed remarkably. Little or no resolution in either NP-HPLC or RP-HPLC could be achieved with the diacetyl-tartrate derivative. Base-line separation by RP-HPLC but no separation by NP-HPLC was possible with the bulkier and more apolar dipivaloyl derivative. Retention order of the bis-benzoylated diastereomers was reversed and separation time increased dramatically by RP-HPLC. Good separation of the medium polarity, but rigid,N-camphanyl derivative by NP-HPLC has been achieved, whereas RP-HPLC could not be used for separation of these diastereomers. Separability of different diastereomers was highly dependent on polarity and rigidity of the derivatizingN-acyl moieties. Conformational analysis by molecular mechanics and comparison of the lowest energy conformational states of the diastereomers was applied to rationalise separation-retention behaviour of stereoisomers by RP-HPLC. Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001     

Solid-phase fluorescence spectroscopy for the determination of acetylsalicylic acid in powdered pharmaceutical samples

A rapid, simple and rugged procedure without requiring any prior sample treatment was developed for the determination of acetylsalicylic acid (ASA) in tablets formulations by solid-phase fluorescence spectroscopy. The method was carried out on powdered samples, consisting of an active substance dispersed in lactose, maize starch, talc and magnesium stearate. Previous knowledge of the sample bulk composition is needed for proper application of the method. Wavelengths for maximum excitation and emission were 288 and 318 nm, respectively, and the fluorescence intensity was linear with ASA concentration within the 50-170 mg g⁻¹ range. Detection and quantification limits were 2.2 and 7.3 mg g⁻¹, and the analytical frequency was 200 h⁻¹. For a typical sample, the relative standard deviation of results was estimated as 2.3% (n = 10). Accuracy was assessed by comparing the analytical results obtained with the proposed method with those related to a reference method recommended by British Pharmacopoeia: no differences between the methods were found at the 95% confidence level.     

Simultaneous spectrophotometric determination of ethinylestradiol and levonorgestrel by partial least squares and principal component regression multivariate calibration

Two spectrophotometric methods for the determination of Ethinylestradiol (ETE) and Levonorgestrel (LEV) by using the multivariate calibration technique of partial least square (PLS) and principal component regression (PCR) are presented. In this study the PLS and PCR are successfully applied to quantify both hormones using the information contained in the absorption spectra of appropriate solutions. In order to do this, a calibration set of standard samples composed of different mixtures of both compounds has been designed. The results found by application of the PLS and PCR methods to the simultaneous determination of mixtures, containing 4–11 μg ml⁻¹ of ETE and 2–23 μg ml⁻¹ of LEV, are reported. Five different oral contraceptives were analyzed and the results were very similar to that obtained by a reference liquid Chromatographic method.     

Asymmetric synthesis of (R)-S-(1,2,4-triazol-3-yl)cysteines by nucleophilic addition of triazolethiols to a NiII complex with a chiral dehydroalanine Schiff base

An efficient method was developed for the asymmetric synthesis of (R)-S-(1,2,4-triazol-3-yl)cysteines by the addition of 3,4-disubstituted 1,2,4-triazole-5-thiols at the electrophilic C=C bond in a Ni^II complex of a Schiff base of dehydroalanine with (S)-N-(N-benzylprolyl)aminobenzophenone. The stereoselectivity of the formation of diastereomeric complexes with the (S,R) configuration under conditions of thermodynamic control of the nucleophilic addition exceeds 94%. Acid treatment of the reaction mixtures afforded enantiomerically pure (R)-S-hetarylcysteines (ee >98%).     

Biocatalysis and biorecognition in nonaqueous media. Some perspectives in analytical biochemistry

Biocatalysis and, to a lesser extent, biorecognition in non-aqueous media (including organic solvents as well as supercritical fluids and gases) constitute at present an exciting research area which has already demonstrated its biotechnological potential in numerous, varied applications. Less attention, however, has been paid to its analytical possibilities, even though many advantages have been postulated and a wide range of poorly water-soluble analytes are present in samples (or waste materials) from food and drink, petrochemical, pharmaceutical, military and other industries. The main approaches, developed in recent years to exploit the use of enzymes, antibodies or antibody mimics in water-restricted environments for analytical purposes, as well as possible future directions are briefly discussed.