Quantitative analysis of urinary C-peptide by liquid chromatography-tandem mass spectrometry with a stable isotopically labelled internal standard

We describe the first results of a quantitative LC-tandem mass spectrometry method for urinary C-peptide with the use of [2H14]C-peptide as internal standard. LC was based on gradient elution of a Hypersil PEP C18 column. Mass spectrometry was performed in the negative electrospray ionization mode and by monitoring of the transitions at m/z 1514/1334 ([2H14]C-peptide) and 1507/1320 (C-peptide). For sample preparation, we applied ultrafiltration. The analytical performance of the method in terms of measurement precision gave an RSD of <2% (n=10). The overall imprecision was investigated from independent analysis of two urine samples in six-fold and resulted in an RSD<5%. The limit of detection, expressed as signal-to-noise ratio 3, was approximately 0.15 ng C-peptide injected. Analysis of 10 random urine samples from laboratory volunteers showed interference-free ion chromatograms at a signal-to-noise ratio of approximately 75 on average. The C-peptide concentrations calculated from quantification by the bracketing calibration technique ranged from 32 to 165 ng/ml.     

Synthesis and characterization of new organometallic complexes bonded stationary phases for high-performance liquid chromatography

Summary The preparation, characterization and potential liquid chromatographic applications of various organometallic iron complexes silica stationary phases are presented. These new supports are synthesized by covalently linking ferrocene, as well as some of its cationic derivatives, to appropriately derivatized silica support matrices. These columns exhibit moderate to high selectivity towards the separation of polycyclic aromatic hydrocarbons (PAHs). A charge transfer retention mechanism has been proposed. A comparison with a reference stationary phase, 3,5-dinitrobenzamide (DNB), to quantify the acceptor power of the new stationary bonded phases, is also reported. Finally, the effect of varying the derivatives of the bonded metallocene on PAHs retention is discussed.     

Origin of the saturation of the third-order optical nonlinear response of one-dimensional conjugated systems

The coupled quantum oscillator model is applied to the evaluation of conformational effects in thiophene and polyene oligomers. Thermochromism measurements on heptathiophene are presented and modeled. The effect of conformation is also studied in recently published nonlinear optical experiments on polyene oligomers. It is shown that the model accounts for a masterplot which summarizes the length dependence of the optical properties of conjugated molecules.     

Experimental evidence of coherence resonance in a time-delayed bistable system

We report on the experimental observation of coherence resonance in a bistable system with delay. Our system consists of a vertical-cavity surface-emitting laser subject to time-delayed optical feedback simultaneously from a long and from an extremely short external cavity. Coherence resonance is experimentally proven by analysis of the residence time distribution of the polarization mode-hopping regime and of the signal to noise ratio in the power spectrum.     

Highly birefringent and dichroic photonic crystal VCSEL design

We analyse the possibility to stabilise the polarisation of light emitted by photonic crystal vertical-cavity surface-emitting lasers (PC-VCSELs). We consider two different photonic crystal configurations in a GaAs/AlGaAs VCSEL: with elliptical holes and with selectively enlarged holes and a small central hole. Very large birefringence (∼400 GHz) and dichroism (∼7%) are demonstrated for the second PC-VCSEL design.     

Functional neural network analysis in frontotemporal dementia and Alzheimer's disease using EEG and graph theory

Background  

Although a large body of knowledge about both brain structure and function has been gathered over the last decades, we still have a poor understanding of their exact relationship. Graph theory provides a method to study the relation between network structure and function, and its application to neuroscientific data is an emerging research field. We investigated topological changes in large-scale functional brain networks in patients with Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD) by means of graph theoretical analysis of resting-state EEG recordings. EEGs of 20 patients with mild to moderate AD, 15 FTLD patients, and 23 non-demented individuals were recorded in an eyes-closed resting-state. The synchronization likelihood (SL), a measure of functional connectivity, was calculated for each sensor pair in 0.5–4 Hz, 4–8 Hz, 8–10 Hz, 10–13 Hz, 13–30 Hz and 30–45 Hz frequency bands. The resulting connectivity matrices were converted to unweighted graphs, whose structure was characterized with several measures: mean clustering coefficient (local connectivity), characteristic path length (global connectivity) and degree correlation (network 'assortativity'). All results were normalized for network size and compared with random control networks.     

Coherent anti‐Stokes Raman scattering in Raman lasers and Raman wavelength converters

In this paper recent research progress on the use of Coherent Anti‐Stokes Raman Scattering (CARS) in Raman lasers and Raman wavelength converters is reviewed. The latest insights in the physical nature and behavior of CARS are addressed, and the recent performance breakthroughs in CARS‐based Raman wavelength conversion are discussed. Based on the new findings regarding the behavior of CARS, a physical explanation for apparent inconsistencies in various experimental observations of Raman wavelength conversion is provided. To conclude it is shown that these recent insights also pave the way to the development of a novel CARS‐based mechanism for reducing the heat dissipation in Raman lasers.