Signal-to-noise ratio and aberration statistics in ocular aberrometry

We define a signal-to-noise ratio (SNR) for eye aberrometry in terms of the sensor geometry, measurement noise, and population statistics. The overall estimation error is composed of three main contributions: the bias in the estimated modes, the truncation error, and the error due to the noise propagation. This last term can be easily parametrized by the proposed SNR. We compute the overall error as well as the magnitude of its three components for a typical sensor configuration, population statistics, and different SNR. We show that there are an optimum number of Zernike aberration modes to be retrieved in each case.     

Classical theory of atom–surface scattering: The rainbow effect

The scattering of heavy atoms and molecules from surfaces is oftentimes dominated by classical mechanics. A large body of experiments have gathered data on the angular distributions of the scattered species, their energy loss distribution, sticking probability, dependence on surface temperature and more. For many years these phenomena have been considered theoretically in the framework of the “washboard model” in which the interaction of the incident particle with the surface is described in terms of hard wall potentials. Although this class of models has helped in elucidating some of the features it left open many questions such as: true potentials are clearly not hard wall potentials, it does not provide a realistic framework for phonon scattering, and it cannot explain the incident angle and incident energy dependence of rainbow scattering, nor can it provide a consistent theory for sticking. In recent years we have been developing a classical perturbation theory approach which has provided new insight into the dynamics of atom–surface scattering. The theory includes both surface corrugation as well as interaction with surface phonons in terms of harmonic baths which are linearly coupled to the system coordinates. This model has been successful in elucidating many new features of rainbow scattering in terms of frictions and bath fluctuations or noise. It has also given new insight into the origins of asymmetry in atomic scattering from surfaces. New phenomena deduced from the theory include friction induced rainbows, energy loss rainbows, a theory of super-rainbows, and more. In this review we present the classical theory of atom–surface scattering as well as extensions and implications for semiclassical scattering and the further development of a quantum theory of surface scattering. Special emphasis is given to the inversion of scattering data into information on the particle–surface interactions.     

Viscosities for Binary Mixtures of 1-Decanol, Hexane, and Diethylamine at 10, 25, and 40°C

Experimental viscosities provide information on the structure of liquids and are required in the design of processes, which involve fluid flow, mass transfer, or heat transfer calculations. This work reports experimental viscosity data of the binary mixtures: 1-decanol + hexane, 1-decanol + diethylamine, and hexane + diethylamine at 10, 25, and 40°C and atmospheric pressure for the whole range of compositions. The viscosities of the pure liquids and their mixtures were determined using Cannon Fenske viscometers thermostated at ±0.01°C. The estimated error in the measured viscosities was less than ±0.005 mPa-s. The dynamic viscosity and the excess energy of activation for viscous flow were also calculated. The equation of Redlich–Kister was used for fitting the excess properties of the binary mixtures. The excess viscosity shows positive deviations from ideal behavior for the mixtures 1- decanol + hexane and 1-decanol + diethylamine and a small negative deviation for the binary system hexane + diethylamine. The experimental results have been also used to test some empirical and semiempirical equations adopted previously to correlate viscosity composition data.     

Fourier transform infrared determination of caffeine in roasted coffee samples

A new procedure has been developed for the FT-IR determination of caffeine in roasted coffee samples. The method involves wetting the coffee samples with a 0.25 M aqueous NH₃ solution, extracting the caffeine with CHCl₃, and measuring absorbance at 1659 cm^–1 using a baseline established between 1900 and 830 cm^–1. The procedure proposed is fast, only requiring a total extraction time of 16 min for each sample, and provides a drastic reduction of the organic solvent consumed, from the 200 mL diethyl ether and 50 mL CHCl₃ required for each sample by the reference chromatographic UV-spectrometric determination to only 5 mL CHCl₃. The method provides a limit of detection of the order of 3 mg L^–1 caffeine and a relative standard deviation of 0.4% for 3 independent analyses of a sample containing 18.6%mg/g caffeine. The accuracy of the FT-IR procedure was evaluated from recovery experiments on spiked samples providing values from 94.4 to 100.1% and from the comparison of results found for a series of commercial samples, by both FT-IR and the official reference procedure. Received: 9 July 1999 / Revised: 23 September 1999 / /Accepted: 24 September 1999     

Electrospray mass spectrometry and supercritical fluid chromatography of methylated beta-cyclodextrins.

Five commercial dimethylated beta-cyclodextrin (DM-beta-CD) samples were analysed by electrospray (or ionspray) mass spectrometry (ESI-MS) and supercritical fluid chromatography (SFC) with evaporative light scattering detection. A silica and a nitro-bonded silica were selected using CO2-methanol-acetonitrile-water and CO2-methanol as mobile phase, respectively. An extensive optimisation scheme was performed for mobile phase selection. Both SFC systems were used for analyses of complex DM-beta-CD samples. Peak identifications were made using off-line ESI-MS. Commercial DM-beta-CDs are impure mixtures of homologues and isomers and analysis reveals that every manufacturer produces a different mixture.     

Electrothermal Atomic Absorption Determination of Chromium in Sediments

An electrothermal atomic absorption spectrometric method has been developed to determine total chromium concentration in natural sediment samples. Samples were mineralized by dry ashing at 550°C and fused at 1100°C for 30 min with lithium metaborate. Dissolved samples were injected in graphite tubes, mineralized at 1100°C, and atomized at 2600°C. Standard addition calibration made unnecessary the use of chemical modifiers. The method provided a limit of detection from 23 to 45 ng liter⁻¹. The relative standard deviation for chromium determination was between 3 and 14%. Results obtained for a standard reference material agree well with the certified value. A series of natural samples were analyzed by the developed procedure and also directly by neutron activation analysis without any pretreatment and comparable results were obtained by both procedures.     

MCDP: an advanced tool to simulate comb‐like polymers

A strategy to study polymeric systems with ordered structures, and in particular comb‐like polymers, is presented. These are dense systems for which atomistic simulations with conventional methods are difficult or even impracticable. The strategy, which has been incorporated into a computer program named MCDP, is based on a Configuration Bias Monte Carlo algorithm and a method to investigate the structure of crystalline polymers using force‐field calculations. To obtain a maximum efficiency, the MCDP computer program has been optimized and parallelized. The ability of MCDP to investigate ordered polymers have been tested by simulating two complex systems: (1) the crystal structure of poly(4‐methyl‐1‐pentene), and (2) the biphasic structure of poly(α‐octyl‐β‐L‐aspartate), a comb‐like polyamide derived from β‐amino acids. The results obtained from MCDP simulations demonstrates the efficiency and reliability of this method to study both the NVT and NPT behavior of ordered dense polymers. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 162–171, 2001     

Enthalpy relaxation in polymethyl(α-n-alkyl)acrylates: Effect of length of alkyl chain

In this work, we have investigated by DSC the structural relaxation of amorphous polymethyl(α-n-alkyl)acrylates in which it is possible to change the length of the alkyl chain. We have evaluated the Narayanaswamy parameter, x, which controls the relative contribution of temperature and of structure to the relaxation time, the apparent activation energy, Δh*, and the nonexponentiality parameter, β, of the stretched exponential response function. The results suggest that x increases while Δh* decreases and β remains constant as the length of the side chain increases. This allows us to comment on the effect of chemical modification on the relaxation kinetics. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 583–593, 1998     

Acetylenic Cyclophanes as Fullerene Precursors: Formation of C60H6 and C60 by Laser Desorption Mass Spectrometry of C60H6(CO)12

A logical precursor of macrocycle C ₆₀ H ₆ , cyclophane C₆₀H₆(CO)₁₂ ( 1 ) represents a building block in a possible total synthesis of C₆₀. In Fourier transform ion cyclotron resonance laser desorption mass spectroscopic experiments in the negative-ion mode, 1 fragments to C₆₀H₆ ( 2 ) under successive loss of CO. Further loss of six H atoms and rearrangement gives C₆₀ ions with a fullerenic structure.