Square-wave adsorptive stripping voltammetry of menadione (vitamin K(3))

Menadione (vitamin K₃) undergoes a reversible two-electron transfer involving the quinone structure in acidic medium. As demonstrated by using cyclic voltammetry, the reduced form is more strongly adsorbed than the oxidized one. Stripping voltammetry of an adsorbed layer has been applied to the determination of this molecule after preconcentration of the reduced compound and scanning the potential towards less negative values. Adsorption, which is highly effective when stirring is used, approaches an equilibrium process in quiescent solution, as evidenced by a loss of part of the adsorbed material when stirring is stopped. A square-wave mode has been selected owing to its high sensitivity (the current is 20 times that for the differential pulse mode), but also to its high scan-rate, which minimizes the slow desorption process occurring during the scan. A concentration range from 2 × 10⁻¹⁰ to 5 × 10⁻⁷ M is easily investigated, the detection limit being 1.3 × 10⁻¹⁰ M. The influence of several operational parameters has also been considered.     

Applications of polarography and voltametry in analysis for drugs

Recent developments and applications of polarographic and voltammetric techniques in drug analysis are reviewed. Typical applications to pharmacologically active compounds are described.     

Electrochemical behaviour of some neuroleptics: Haloperidol and its derivatives

The electrochemical characteristics of Haloperidol and related compounds, representative neuroleptics of the butyrophenone family, have been investigated as a function of pH and concentration by direct-current, alternating-current and differential-pulse polarography and cyclic voltammetry at a hanging mercury drop electrode. A single cathodic wave representing an irreversible two-electron reduction is obtained, and its half-wave potential differs from that characteristic of aromatic ketone reduction. Adsorption processes disturb the wave behaviour and an adsorption prewave is observed at high concentrations. Quantitative measurements were successful in the concentration range 1 x 10(-4)-1 x 10(-6)M (0.4 mg/l.), the lower concentration representing the detection limit by differential-pulse polarography.     

Heritability of Stroop and flanker performance in 12-year old children

Background  

There is great interest in appropriate phenotypes that serve as indicator of genetically transmitted frontal (dys)function, such as ADHD. Here we investigate the ability to deal with response conflict, and we ask to what extent performance variation on response interference tasks is caused by genetic variation. We tested a large sample of 12-year old monozygotic and dizygotic twins on two well-known and closely related response interference tasks; the color Stroop task and the Eriksen flanker task. Using structural equation modelling we assessed the heritability of several performance indices derived from those tasks.     

The contribution of high frequencies to human brain activity underlying horizontal localization of natural spatial sounds

Background  

In the field of auditory neuroscience, much research has focused on the neural processes underlying human sound localization. A recent magnetoencephalography (MEG) study investigated localization-related brain activity by measuring the N1m event-related response originating in the auditory cortex. It was found that the dynamic range of the right-hemispheric N1m response, defined as the mean difference in response magnitude between contralateral and ipsilateral stimulation, reflects cortical activity related to the discrimination of horizontal sound direction. Interestingly, the results also suggested that the presence of realistic spectral information within horizontally located spatial sounds resulted in a larger right-hemispheric N1m dynamic range. Spectral cues being predominant at high frequencies, the present study further investigated the issue by removing frequencies from the spatial stimuli with low-pass filtering. This resulted in a stepwise elimination of direction-specific spectral information. Interaural time and level differences were kept constant. The original, unfiltered stimuli were broadband noise signals presented from five frontal horizontal directions and binaurally recorded for eight human subjects with miniature microphones placed in each subject's ear canals. Stimuli were presented to the subjects during MEG registration and in a behavioral listening experiment.     

Polarographic behaviour and hydrolysis of midazolam and its metabolites

The electrochemical behaviour of midazolam [7-chloro-5-(o-fluorophenyl)-3H-(2′- methyllimidazo) [1,5-a]-benzodiazepine was studied by polarography and cyclic voltammetry. The irreversible two-electron were is not strongly affected by the imidazole ring or the 5-o-fluorophenyl substituent, but the latter increases the rate of the hydrolysis in acidic media. Kinetic parameters are evaluated for midazolam and three of its hydroxylated metabolites. The hydrolysis is a first-order reaction initially but becomes second order. The 3-hydroxy matabolites are more easily hydrolyzed than midazolam. Midazolam (10^?4–10^?7 M) can be quanitified by using differential-pulse polarography; the detection limit is 6 × 10^?8 M.