Electrocatalytic detection of streptomycin and related antibiotics at ruthenium dioxide modified graphite--epoxy composite electrodes

The application of ruthenium dioxide (RuO2) modified electrodes to the electrocatalytic detection of the saccharide-related antibiotics streptomycin, novobiocin and neomycin, at low fixed potentials, was investigated. The RuO2-modified graphite - epoxy composite electrodes give extremely stable and reproducible catalytic oxidation currents for these antibiotics at potentials as low as +0.2 V (versus Ag - AgCl). Rapid quantification at the micromolar level is therefore possible. Standard calibration graphs for streptomycin and neomycin yielded slopes of 4.43 and 0.08 nA microM-1 over the linear ranges of 1.5 x 10(-6) - 2.5 x 10(-4) and 1 x 10(-5) - 2 x 10(-3) M, respectively. Owing to its catalytic oxidation by the RuIII - RuIV couple, rather than the RuIV - RuVI transition (which catalyses the oxidation of streptomycin and neomycin), novobiocin could be detected at a lower (+0.2 V) potential, with a sensitivity of 1.31 nA microM-1. Detection limits of 1.5, 6.0 and 10 microM were obtained for streptomycin, novobiocin and neomycin, respectively. These catalytic surfaces can be renewed (by polishing), with a surface-to-surface reproducibility of 6.5% for the detection of 5 x 10(-5) M streptomycin. The analytical application of RuO2-modified carbon paste electrodes to the analysis of these antibiotics by flow injection was investigated, with a view to liquid chromatographic separation with electrochemical detection applications.     

Analysis of terbutaline in human plasma by high-performance liquid chromatography with electrochemical detection using a micro-electrochemical flow cell.

A high-performance liquid chromatographic method is described for the determination of terbutaline in human plasma in the range 1-35 ng/ml. Detection was achieved using a carbon fibre micro-electrochemical detector and a column-switching system. The microelectrode cell has advantages over conventional glassy carbon electrode-based detection systems in that it is easy to prepare, flexible in its operation and suffers less trouble from problems such as air bubbles and leaks. Furthermore, it has a better detection limit for terbutaline (0.8 ng/ml) to that obtained using a conventional glassy carbon electrode flow detector (2 ng/ml). Sample clean-up was by on-line solid-phase extraction with column switching, providing a method which was sensitive and reproducible, where the mean overall coefficient of variation was 5.60% and drug recovery in excess of 86% at the concentration levels studied.     

Asymmetric synthesis of β-substituted Baylis-Hillman products via lithium amide conjugate addition

A three-step protocol for the asymmetric synthesis of a range of β-substituted Baylis-Hillman products has been developed. This procedure involves the diastereoselective conjugate addition of lithium (R)-N-methyl-N-(α-methylbenzyl)amide to an α,β-unsaturated ester to generate an N-protected β-amino ester in high de. Subsequent asymmetric aldol reaction via deprotonation with LDA, transmetallation with B(OMe)₃ and addition of an aldehyde gives a range of syn-aldol products in moderate to high de. Purification of the syn-aldol products to homogeneity followed by tandem N-oxidation and Cope elimination gives the desired β-substituted Baylis-Hillman products in good yield and high de and ee.     

A spectrometric,separation and voltammetric study of the complexation reactions of bromazepam with iron(ii),copper(ii) and cobalt(ii)

Bromazepam, in the form of a cationic iron(II) chelate, can be determined spectrophotometrically at 588 nm with a limit of detection of ca. 10^-6 M. When this chelate is ion-paired with perchlorate, it can be extracted into organic solvents such as 1,2-dichloroethane and 4-methyl-2-pentanone, and determined by atomic absorption spectrometry with a limit of detection of 1.5 × 10^-5 M bromazepam at the iron resonance 248.3-nm line. Ion-pairs involving the Fe(II), Cu(II) and Co(II) chelates and perchlorate can be separated by h.p.l.c. using a C₁₈ reverse-phase column and a mobile phase of 4:1 water—methanol, with a u.v. detector at 242 nm. This approach allowed for the determination of iron(II) ions in aqueous solution with a limit of detection of 10^-8 M. The h.p.l.c. method could also be used to quantify bromazepam spiked in plasma in the concentration range 1–10 μg ml^-1, following extraction of bromazepam from plasma and subsequent formation of the iron(II) ion-pair. Copper(II) forms a labile chelate with bromazepam in pH 4.8 acetate buffer which, when subjected to differential pulse voltammetry at the hanging mercury drop electrode, gives rise to a catalytic phenomenon which can be utilised for the determination of bromazepam in the concentration range 10^-5–10^-9 M.     

A critical evaluation of high performance liquid chromatography-electrospray ionisation-mass spectrometry and capillary electrophoresis-electrospray-mass spectrometry for the detection and determination of small molecules of significance in clinical and forensic science

A critical review of applications for the period 2000-2003, taken from the Web of Knowledge database, of the techniques high performance liquid chromatography-electrospray ionisation-mass spectrometry (HPLC-ESI-MS) and capillary electrophoresis-electrospray ionisation-mass spectrometry (CE-ESI-MS) to the detection and determination of small molecules of significance in clinical and forensic science is presented. The molecules of mass less than 500 Da are chosen according to selected structural classes in which they give ESI signals primarily as [M+H](+) ions although other ions such as [M-H](-), [M+Na](+) and [M+NH(4)](+) are also reported. The structural classes are drugs with amine-containing side chains, drugs with N-containing saturated ring structures, 1,4-benzodiazepines, carbohydrates, benzimidazoles, other heterocycles, sulphonylureas, anthracyclines, sulphonamides, penicillins, cephalosporins, tetracyclines, nitrocatechols, steroids, flavonoids, oxazaphosphorines, cannabinols, and miscellaneous molecules. Details are given on the fragmentations, where available, that these ionic species exhibit in-source and in ion-trap, triple quadrupole and time-of-flight mass spectrometers. The review then gives a critical evaluation of these recent HPLC-ESI-MS and CE-ESI-MS analytical methods for the detection and determination of small molecules of clinical and forensic significance. Analytical information on, for example, sample concentration techniques, HPLC and CE separation conditions, recoveries from biological media and limits of detection (LODs) are provided.     

A polarographic study of the hydrolysis of 1,4-benzodiazepines and its analytical applications

The mechanism of hydrolysis of flurazepam (Dalmane) and six of its metabolites was investigated in mildly acidic solution (pH 0–2) by differential pulse polarography. Simultaneous determinations of the “parent” compound(s) and hydrolytic degradation product(s) are possible because of the different reduction potentials. The kinetic results can be explained if the hydrolytic reaction is considered reversible; this is important for evaluation of the hydrolysis and absorption of 5-(o-fluorophenyl)-1,4-benzodiazepines in the stomach. The rate constants and the pK_a values corresponding to protonation of the azomethine groups are shown to be correlated. Appropriate kinetic data for other 1,4-benzodiazepines make it possible to evaluate the effects of certain substituents on the rate of hydrolysis and on the peak potentials of the “parent” compounds and their hydrolytic degradation products. The results of the kinetic investigations can be used for the identification of an isolated 5-(o-fluorophenyl)-1,4-benzodiazepine or identification of any 1,4-benzodiazepine studied here which exhibits some degree of acid hydrolysis within 24 h.     

Determination of kinetic parameters for the inhibitory effects of organic sulphides on an amperometric peroxide biosensor in non-aqueous media

The amperometric detection of some organic sulphides in non-aqueous media using a horseradish peroxidase modified platinum electrode is reported. The performance of the electrode is based on the inhibitory effect of sulphides on the activity of peroxidase. The electrode responds rapidly to micromolar concentrations of the sulphides with linearities extending up to 0.4, 0.25 and 0.8 mmol/dm(3) for thiourea, ethylenethiourea, and mercaptoethanol, respectively. The current as peroxide concentration approaches infinity in the absence of any inhibitor (I(max)), and the apparent inhibition constant (K'(i)) for each inhibitor were evaluated. I(max)/K'(i), i.e. the percentage inhibition of sensor response per millimolar of each inhibitor, was calculated and used as a measure of the performance of the sensor in the determination of the organic sulphides. From these values the sensitivity trend of the biosensor for the organic sulphides is ethylenethiourea > thiourea > mercaptoethanol. The kinetic effects of other operational parameters such as mediator concentration, water content of the solvent and working potential were also evaluated to assess analytical performance.