Dagmar Obendorf

Selective and sensitive assay for the determination of benzodiazepines by high-performance liquid chromatography with simultaneous ultraviolet and reductive electrochemical detection at the hanging mercury drop electrode.

An isocratic chromatographic method for the simultaneous determination of 10 benzodiazepines is presented. The selectivity of the assay was optimized by variation of stationary phase, temperature, as well as ionic strength, composition and pH of the mobile phase and the dependence of the detector response on the applied potential was investigated. The best results with respect to resolution at moderate retention times were obtained with a mixture of 0.02 mol/l phosphate buffer (pH 6) and acetonitrile in a volume ratio of 55:45 (v/v) on a LiChrospher-100 RP-8ec column (150x4.6 mm I.D.). Considerable improvement of selectivity was achieved if the column temperature was kept constant at 12 degrees C. Two detection modes were applied, UV detection at 250 nm, inserted upstream to the electrochemical detector, and reductive electrochemical detection at the hanging mercury drop electrode at -1.4 V (vs. Ag/AgCl), which proved to be especially sensitive in case of nitrophenyl-containing benzodiazepine species. The elaborated assay showed to be linear up to at least 2 mg/l for each compound. Detection limits generally were in the range of 6.5-123 ng/ml (130 pg-2.46 ng on-column, using a 20-microl loop) with relative standard deviations between 1.1 and 8.6% depending on the actual benzodiazepine investigated.

Determination of naringin in grapefruit juice by cathodic stripping differential pulse voltammetry at the hanging mercury drop electrode

A highly sensitive cathodic stripping voltammetric method for the determination of naringin is presented. It is based on the formation and accumulation of two naringin–mercury complexes at the electrode surface, followed by reduction of the surface species during a differential pulse voltammetric scan. The cathodic stripping responses at −0.25 V and −0.42 V, are evaluated with respect to various experimental conditions, such as composition and pH of the supporting electrolyte, naringin concentration, accumulation potential and preconcentration time. The new method is suitable for the determination of naringin concentrations between 0.1 mg l−1 (1.72×10−7 mol l−1) and 40 mg l−1 (6.88×10−5 mol l−1). A 3σ limit of detection of 32 μg l−1 (55 nmol l−1) can be reached. The relative standard deviation (r.s.d.) is <1.5%. Recovery experiments yielded a mean recovery of 97% (r.s.d.=4.1%). The application of the procedure to the selective determination of naringin in grapefruit juice is demonstrated.

LITHIUM ORGANYL-POLYCHALCOGENOLATES

Abstract Lithium organyl-selenolates, RSeLi or tellurolates, RTeLi insert further equivalents of chalcogens with formation of selenenyl-selenolates, RSeSeLi or tellurenyl-tellurolates, RTeTeLi (R=Me, n-Bu, s-Bu, t-Bu, Ph). Mixed anions are only formed if the chalcogen insertion order corresponds with an increase of electronegativity, i.e.: RTeSeLi, RTeSLi, RSeSLi and n-BuTeSeSLi. All species have been characterized by 77-Se and 125-Te NMR-spectroscopy. Tellurenyl-tellurolates and tellurenyl-selenolates are stable at room temperature, selenenyl-selenolates and chalcogenyl-thiolates disproportionate. The chemical integrity of the chalcogenyl-chalcogenolates was checked by methylation with methyl triflate. The formed organyl-methyl-dichalcogenides exhibit symmetrization equilibria that are also obtained by mixing the symmetric compounds. Oxidation of tellurenyl-selenolates affords di(organyl-tellurenyl) selenides, (RTe)2Se (R=Me, n-Bu, t-Bu, Ph, p-MeC6H4, 2-thienyl) and (RTe)2Se2 for R=(Me3Si)3C. In solution...

Determination of trace levels of niguldipine in urine and blood by adsorptive stripping voltammetry at the hanging mercury drop electrode

A relatively simple electroanalytical procedure for the determination of niguldipine in biological samples is described. The technique involves adsorptive accumulation of the drug at the hanging mercury drop electrode (HMDE) followed by a differential-pulse polarographic determination of the preconcentrated species. The adsorptive stripping response is evaluated with respect to various experimental conditions, such as solvent composition and pH of the supporting electrolyte, accumulation potential and accumulation time. After a simple sample preparation, the method can be used for the determination of niguldipine in blood and urine. Interfering substances are simply removed by precipitation, adding a small amount of 5% ZnSO4 solution and ethanol to the urine or blood sample and centrifuging the mixture. A limit of detection of 6.7 ng per ml of urine and 41 ng per ml of blood is found with a mean recovery of 96% in urine and 71% in blood. The mean relative errors are 8.4% and 2.2%, respectively.

Adsorptive stripping voltammetry of nicardipine at a HMDE; determination of trace levels nicardipine in blood and urine

The redox behaviour of nicardipine, a 1,4-dihydropyridine calcium antagonist, has been studied in different media on mercury, glassy carbon, gold and platinum electrodes using various voltammetric techniques. A highly sensitive adsorptive stripping voltammetric method for the determination of nicardipine based on adsorption of the drug onto mercury, followed by differential pulse voltammetric determination of the surface species, is described. All factors (pH, supporting electrolyte, accumulation potential and time, etc.) influencing adsorption as well as voltammetric response are discussed. The application of adsorptive stripping voltammetry at the hanging mercury drop electrode (HMDE) to the determination of trace levels of nicardipine in human urine and blood is illustrated, without an extraction procedure being necessary prior to the voltammetric measurement. A limit of detection of 4.8 ng per ml urine and 34 ng per ml blood is found with a mean recovery of nicardipine in urine and blood of 97%. The mean relative error does not exceed 6.5%.

Preparation and nuclear magnetic resonance spectroscopy of [Ag2{µ-CH2(PPh2)2}3][O3SCF3]2, a disilver(I) complex with three bridging CH2(PPh2)2 ligands

[Ag2(µ-dppm)3][O3SCF3]2[dppm = bis(diphenylphosphino)methane] has been prepared and characterized by 31P n.m.r. spectroscopy. Variable-temperature n.m.r. studies revealed that the Ag–P bonds are kinetically stable on the n.m.r. time-scale below 233 K. At ambient temperature there is intermolecular ligand exchange whereas at intermediate temperatures intramolecular inter-metal exchange of the phosphorus atoms of the dppm ligands occurs.

Neuartige Synthesen monosubstituierter Nickelocene als Zugang zu mehrkernigen Metallocensystemen

Abstract The nickelocene derived triple- and mono-decker salts CpNiCpNiCp + BF 4 − and NiCp + BF 4 − were utilized as “Jonas”-type synthons for capping monosubstituted cyclopentadienylides R-Cp − (R-CP − = (η 5 -C 5 H 5 )Co I [(η 4 -1,3-C 5 H 5 )-5- exo -Cp − ] (Ia) and − Cp-Cp − , fulvalenedianion (IIa) to form the corresponding binuclear neutral metallocenes (η 5 -cyclopentadienyl) [η 4 -(-5- exo -nickelocenyl)-1,3-cyclopentadiene] Co I (I) and binickelocene (II). Side reactions were investigated and a new by-product, the betainic cobaltoceniumyl-3-triphenyl-methyl-cyclopentadienide (III) was isolated. All new compounds were characterized by MS, IR, and partially by CV and NMR. The novel homodinuclear system binickelocene (II) exhibits diamagnetism.

Preparation and NMR spectroscopy of [Hg{(μ-PEt2)Cr(CO)5}3]−

Abstract The new tetranuclear phosphido-bridged compound [Hg{(μ-PEt 2 )Cr(CO) 5 } 3 ] − has been obtained by reaction of [Hg{(μ-PEt 2 )Cr(CO) 5 } 2 ] with Li[Cr(CO) 5 PEt 2 ]. The coordination of [Cr(CO) 5 PEt 2 ] − results in an unprecedented increase in J (Hg,P), whereas an increase in the number of phosphorus ligands coordinated to mercury is usually accompanied by a decrease in the magnitude of J (Hg,P). This anomaly is interpreted in terms of CrHg donor acceptor interactions.

Voltammetric study of some metallocene-substituted (η4-1,3-cyclopentadiene-5-exo-yl) cobalt complexes; ([CpCo(η4-C5H5R)], R = −C C-(η5−C5H4)FeCp; -(η5−C5H4)FeCp; -(η5C5H4)NiCp)

Abstract The redox behavior of three metallocene substituted (η 5 -cyclopentadienyl) (η 4 -1,3-cyclopentadien-5-exo-yl) cobalt complexes have the general formular [CpCo(η 4 −C 5 H 5 R)], where R = −CC -(η 5 −C 5 H 4 )FeCp (3), -(η 5 −C 5 H 4 )FeCp (4) or -(η 5 −C 5 H 4 )NiCp) (5), was studied by cyclic voltammetry and related techniques. Oxidation occurs by an ECE (E = electrochemical step, C = chemical step) process. The character of the chemical step has been probed by cyclic voltammetry in solvents of varying donor ability. In the case of (4) and (5), voltammetric data obtained in solvents of low donor ability, like dichloromethane or nitromethane, indicate formation of bimetallocenes by oxidative deprotonation, whereas the chemical step after oxidation of (3),(4) or (5) in acetonitrile is addition of solvent molecules to the cation, leading to decomposition of the complexes and formation of an electroactive solvent adduct of a cyclopentadienyl cobalt complez [η 5 −CpCo(L) 3 ] + .

The reactions of Cr(CO)5PRH2 (R = Ph or cyclohexyl) complexes with mercury(II) and silver(I) compounds

Abstract The μ 2 -PRH and μ 3 -PR bridged complexes Cr(CO) 5 [μ 3 -P(c-Hex)]{Hg[μ 2 -P(c-Hex) 2 ]Cr(CO) 5 } 2 , [Hg{(μ 2 -PRH)Cr(CO) 5 } 3 − , (R = c-Hex, Ph) and [Ag{(μ 2 -PRH)Cr(CO) 5 } n ] 1− n n = 1,2) have been prepared, and characterized by 31 P and 199 Hg NMR spectroscopy (c-Hex = cyclohexyl).